Abstract

The mediodorsal nuclei of thalamus (MD), prefrontal cortex (PFC), and nucleus accumbens core (NAc) form an interconnected network that may work together to subserve certain forms of behavioral flexibility. The present study investigated the functional interactions between these regions during performance of a cross-maze–based strategy set-shifting task. In Experiment 1, reversible bilateral inactivation of the MD via infusions of bupivacaine did not impair simple discrimination learning, but did disrupt shifting from response to visual cue discrimination strategy, and vice versa. This impairment was due to an increase in perseverative errors. In Experiment 2, asymmetrical disconnection inactivations of the MD on one side of the brain and PFC on the other also caused a perseverative deficit when rats were required to shift from a response to a visual cue discrimination strategy, as did disconnections between the PFC and the NAc. However, inactivation of the MD on one side of the brain and the NAc contralaterally resulted in a selective increase in never-reinforced errors, suggesting this pathway is important for eliminating inappropriate strategies during set shifting. These data indicate that set shifting is mediated by a distributed neural circuit, with separate neural pathways contributing dissociable components to this type of behavioral flexibility.

Introduction

It is well established that different subregions of the mammalian prefrontal cortex (PFC) mediate the ability to adapt behavior in response to changing environmental contingencies. For example, attentional set shifting is a type of behavioral flexibility that requires an organism to disengage from a once relevant set of stimulus dimensions and begin responding to a previously irrelevant set to perform optimally. In humans, this executive function is assessed using the Wisconsin card sort task (WCST), a neuropsychological task that is critically dependent on the integrity of the dorsolateral PFC in humans (Lombardi and others 1999). Patients with damage to the PFC acquire an initial sorting rule readily during performance of the WCST, but when the rule is shifted without warning, patients fail to adapt their responding to the negative feedback (Stuss and others 2000). Similarly, lesions of the dorsolateral PFC in primates or the medial PFC in rats impair set shifting on an intradimensional/extradimensional shifting task (Dias and others 1997; Birrell and Brown 2000). Moreover, manipulation of the medial PFC in rats also impairs the ability to shift from one discrimination strategy to another using a maze-based set-shifting procedure (Ragozzino and others 1999; Stefani and others 2003; Floresco, Magyar, and others 2006). In contrast, the orbital PFC in rats and primates plays an essential role in reversal learning, a simpler form of behavioral flexibility, whereas lesions to the medial PFC in rodents or the dorsolateral PFC in primates are without effect (Dias and others 1997; Birrell and Brown 2000; McAlonan and Brown 2003; Kim and Ragozzino 2005).

It is becoming increasingly apparent that different subcortical afferents of the PFC contribute to behavioral flexibility mediated by the frontal lobes. For example, both the dorsal and ventral regions of the striatum have been implicated in different forms of set shifting. Patients with pathology of the dorsal striatum display impairments in extradimensional shifts (Owen and others 1993; Lawrence and others 1999), and functional imaging studies have shown increased activation of the caudate nucleus specifically during the reception of negative feedback (Monchi and others 2001). In keeping with these findings, inactivation of the dorsomedial striatum in rats disrupts shifting from one discrimination strategy to another, impairing selectively the ability to maintain new strategies (Ragozzino and others 2002). Recent findings in our laboratory have also implicated the nucleus accumbens core (NAc) in strategy set shifting. The NAc receives a dense projection from the medial PFC (Brog and others 1993; Gabbott and others 2005), and inactivation of this nucleus also impairs strategy set shifting in a manner distinct from that observed following manipulations of the PFC or dorsal striatum. Inactivation of the NAc (but not shell) caused a pattern of errors that indicated a failure to maintain new strategies as well as elimination of inappropriate response alternatives during a strategy shift (Floresco, Ghods-Sharifi, and others 2006), similar to “loss of set” errors, made by Parkinsonian patients performing a set shift (Gauntlett-Gilbert and others 1999). Thus, impairments induced by inactivation of the dorsomedial striatum or NAc are qualitatively different from the perseverative impairment one sees with manipulations of the PFC, indicating that these regions may play functionally distinct roles in the shifting of behavioral strategies.

Another subcortical region that shares projections with both the PFC and the NAc are the mediodorsal nuclei of the thalamus (MD). The PFC is reciprocally connected with MD (Groenewegen 1988; Condé and others 1995), and afferents from the MD also terminate in the NAc (Berendse and Groenewegen 1990). Given this anatomical connectivity, it is not surprising that this diencephalic region has also been implicated in different types of behavioral flexibility. Functional imaging studies have revealed that performance of the WCST causes significant activation of the MD during the receipt of negative feedback, indicating that this region of the thalamus may flag the need to initiate an adaptive strategy shift (Monchi and others 2001). Schizophrenic individuals also show deficits in strategy set shifting (Pantelis and others 1999), which has been linked to the observed loss of cells from the MD in schizophrenic individuals relative to controls (Popken and others 2000). Additionally, Korsakoff's amnesiacs, whose lesions encompass the MD (Joyce 1987), show perseverative impairments on the WCST (Oscar-Berman and others 2005). Studies in experimental animals have also implicated the MD as an important nucleus for regulating different forms of behavioral flexibility. For instance, lesions of the MD disrupt reversal learning under certain conditions, although these effects are variable and are dependent on a number of factors. These include the type of stimuli used or whether the animal is subjected to a single or multiple reversals (Tigner 1974; Means and others 1975; Chudasama and others 2001; but see Beracochea and others 1989). However, neurotoxic lesions of the MD cause a robust increase in perseverative-type errors when rats were required to shift response rules from a match to a nonmatch to place strategy on a working memory task, in a manner similar to rats with lesions of the medial PFC (Hunt and Aggleton 1998; Dias and Aggleton 2000). This latter finding indicates that the MD may be particularly important in situations that require strategy shifting.

The data reviewed above suggest that the MD is part of a neural circuit that facilitates behavioral flexibility mediated in part by the PFC and NAc. However, the specific contribution that the MD makes to this executive function remains to be explored fully. Furthermore, given that the MD projects to both the PFC and the NAc, it is unclear whether this nucleus interacts with one or both of these regions to mediate behaviors such as strategy set shifting. Thus, in Experiment 1, we investigated the role of the MD in strategy set shifting known to be mediated by the medial PFC and NAc (Ragozzino and others 1999; Floresco, Ghods-Sharifi, and others 2006). Rats with reversible bilateral inactivations of the MD performed 1 of 2 strategy set shifts: either the acquisition of an egocentric response and a subsequent shift to a visual cue–based strategy or the reverse. In Experiment 2, we used reversible asymmetrical disconnection lesions to investigate the possible routes of serial information transfer between the MD, NAc, and PFC during set shifting.

Experiment 1A and B

Materials and Methods

Subjects

Male Long-Evans rats (Charles River Laboratories, Montreal, Canada) weighing 280–360 g at the beginning of the experiment were used. Rats were individually housed in plastic cages in a temperature controlled room (20 °C) on a 12-h light–dark cycle. Immediately following surgery, all rats were restricted to 85% of their free-feeding weight, with free access to water for the duration of the experiment.

Apparatus

A four-arm cross-maze was used, made of 1.5 cm thick plywood and painted white. Each arm was 60 cm long and 10 cm wide, with 20 cm high walls on each arm and with cylindrical food wells (2 cm wide × 1 cm deep) drilled into the end of each of the arms, 2 cm from the end wall. Four table legs attached to the ends of each arm elevated the maze 70 cm above the floor. Removable pieces of white opaque plastic (20 × 10 cm) were used to block the arms of the maze to form a “T” configuration. The maze resided in a room measuring 3.4 × 3.4 m.

Surgery

Rats were anesthetized with 100 mg/kg xylazine and 7 mg/kg ketamine hydrochloride and implanted with 23 gauge bilateral stainless steel guide cannulae into the MD (flat skull—anterior–posterior [AP]: −2.9 mm from bregma, medial–lateral [ML]: ±0.7 mm from midline, dorsal–ventral (DV): −4.9 mm from dura). The coordinates were based on the atlas of Paxinos and Watson (1998). Four jeweler's screws were implanted surrounding the cannulae and secured in place with dental acrylic. Thirty gauge obdurators flush with the end of the guide cannulae remained in place until the rats were given infusions. Each rat was given at least 7 days to recover from surgery prior to behavioral training. During this recovery period, animals were food restricted and were handled for at least 5 min per day.

Microinfusion Procedure

Infusions into the MD were made through 30 gauge injection cannulae extending 0.8 mm below the guide cannulae. The injection cannulae were attached by a polyethylene tube to a 10-μL syringe. Saline or the local anesthetic bupivacaine hydrochloride (0.75%; Abbott Laboratories Saint Laurent, Quebec, Canada) was infused at a rate of 0.5 μl per 72 s by a microsyringe pump (Sage Instruments Model 341). Injection cannulae were left in place for an additional 1 min to allow for diffusion. Each rat remained in its home cage for a further 10 min prior to behavioral testing.

Ten minutes before both the response discrimination learning and the strategy set shift, rats received a microinfusion. For both Experiments 1A and B, each rat was assigned to 1 of 3 infusion treatment groups, 1) Day 1-saline and Day 2-saline, 2) Day 1-saline and Day 2-bupivacaine, and 3) Day 1-bupivacaine and Day 2-saline. Group 1 served as the control group, group 2 determined whether MD inactivation impaired strategy set shifting, and group 3 determined whether inactivation of the MD would affect the acquisition or consolidation of discrimination learning that may yield behavioral differences during the set shift.

Maze Familiarization Procedure

The familiarization and strategy set-shifting procedures have been described previously (Ragozzino 2002; Floresco, Ghods-Sharifi, and others 2006; Floresco, Magyar, and others 2006). Before the first day of familiarization to the maze, the rats were pre-exposed to 10–20 of the reward pellets they would receive in the maze. On the first day of familiarization, rats were placed in the center of the cross-maze, which had each arm baited with 5 pellets: 2 in each well and 3 down the length of the arm. A rat was placed in the maze and allowed to freely navigate and consume the food pellets for 15 min. If a rat consumed all 20 pellets prior to 15 min, it was removed from the maze and placed in a holding cage, the maze was rebaited with 20 additional pellets, and the rat was placed back in the center of the maze. The second day of familiarization was the same as the first day, except there were only 3 pellets in each arm: 2 in each well, and 1 on each arm. On subsequent days of maze familiarization, only 4 pellets were placed on the maze, 1 in each well. Additionally, a black laminated piece of poster board (9 × 20 × 0.3 cm), which served as the visual cue, was placed in a random arm and rotated between rebaitings of the maze. Rats were picked up and placed at the start of an arm, allowed to traverse the arm of the maze, consume the pellet, and were immediately picked up and placed at the beginning of another baited arm. Rats continued this procedure daily until they depleted the maze 4 or more times in 15 min. Rats required an average of 4.58 ±0.3 days of familiarization (range, 3–14 days) to reach this criterion.

After the rat had achieved familiarization criterion, the turn bias for the rat was determined. The white opaque plexiglas insert was placed at the entrance of one of the arms, forming a “T” configuration. A rat was placed in the stem arm and allowed to turn left or right to obtain a food pellet. In one of the choice arms, the visual cue insert was placed on the floor. After a rat chose an arm and consumed a food pellet, it was picked up, placed in the stem arm, and allowed to make the next choice. If the rat chose the same arm as the initial choice, it was returned to the stem arm until it chose the remaining arm and consumed the food pellet. After choosing both arms, the rat was returned to the holding cage, the Plexiglas barrier and visual cue were moved to different arms, and a new trial commenced. Thus, a trial for the turn-bias procedure consisted of entering both choice arms and consuming the food pellets. The turn that a rat made first during the initial choice of a trial was recorded and counted toward its turn bias, and the direction (right or left) that a rat turned 4 or more times over 7 total trials was considered its turn bias. After determining the turn bias, a rat's obdurators were removed from the guide cannulae and 2 injection cannulae extending 0.8 mm past the guide cannulae were inserted for 1 min, but no solution was injected at this time. This procedure was performed to familiarize the animal to the 2 infusions they would receive over the next 2 days of testing. Response (Experiments 1A and 2) or visual cue (Experiment 1B) discrimination training commenced on the following day.

Experiment 1A, Day 1: Response Discrimination Learning

A detailed discussion of the merits of this type of set-shifting task and comparisons with other set-shifting tasks used in rodents has been addressed previously (Floresco, Ghods-Sharifi, and others 2006; Floresco, Magyar, and others 2006). For this discrimination, the rat was required to always turn in the direction opposite its turn bias (left or right), regardless of the location of the visual cue placed in one of the arms (See Fig. 1A, upper panel). Over the course of training, 1 of 3 start arms were used, discouraging the use of an allocentric spatial strategy to locate the food. On Day 1 of training, a rat was started from the arms designated west, south, and east (W, S, and E, respectively). The location of these arms relative to the spatial cues in the room was varied across animals, so that the maze was placed in 1 of 4 possible orientations. For every trial, the visual cue was placed in one of the choice arms so that over every consecutive set of 12 trials it was placed an equal number of times in each choice arm, with no more than 2 appearances in a row in any one arm. The order of the start location for each trial, as well as the position of the visual cue, was determined pseudorandomly and taken from a preset sequence that was identical for each animal. On an individual trial, the rat was placed in the stem arm and required to make the appropriate turn in order to receive a food pellet. Between trials, a rat was placed back in the holding cage on a bench adjacent to the maze. The intertrial interval was ∼15 s. A rat continued to receive training trials until it reached a criterion of 10 correct consecutive choices. There was no limit on the number of trials a rat was allotted to reach this criterion. After the rat achieved this acquisition criterion, it received a probe trial; this consisted of starting the rat from the fourth arm (north, N) that was never a start arm prior to acquisition criterion. During probe trials, the visual cue was inserted in the arm opposite to the direction that the rat was required to turn. If a rat correctly turned the same direction as was required during training, then response discrimination training was completed. If a rat made an incorrect turn, response training was continued until a rat made an additional 5 correct choices consecutively, at which time another probe trial was administered. This procedure was continued until a rat made a correct choice on the probe trial. The following measures were taken for each rat and used for data analysis: 1) trials to criterion, defined as the total number of test trials completed before a correct choice on the probe trial was made and 2) probe trials, defined as the total number of probe trials an animal required to get one correct. The total time it took to complete training was also recorded.

Figure 1.

Example of the strategy set-shifting task used in Experiments 1A and 2. The arrows in the maze represent the correct navigation pattern to receive reinforcement. (A) During initial response discrimination training on Day 1 (upper panels), in this example, the rat was started from the south (S), west (W), and east (E) arms and always had to make a 90° turn to the right to receive food reinforcement. A black and white striped visual cue was randomly placed in one of the choice arms on each trial but did not reliably predict the location of food during response training. During the set shift on Day 2 (lower panels), the rat is required to use a visual cue discrimination strategy. Here, the rat was started from the same arms but had to always enter the arm with visual cue, which could require either a right or left turn. Thus, the rat must shift from the old strategy and approach the previously irrelevant cue in order to obtain reinforcement. (B) Examples of the 3 types of errors that rats could make during the set shift. See Materials and Methods for details.

Figure 1.

Example of the strategy set-shifting task used in Experiments 1A and 2. The arrows in the maze represent the correct navigation pattern to receive reinforcement. (A) During initial response discrimination training on Day 1 (upper panels), in this example, the rat was started from the south (S), west (W), and east (E) arms and always had to make a 90° turn to the right to receive food reinforcement. A black and white striped visual cue was randomly placed in one of the choice arms on each trial but did not reliably predict the location of food during response training. During the set shift on Day 2 (lower panels), the rat is required to use a visual cue discrimination strategy. Here, the rat was started from the same arms but had to always enter the arm with visual cue, which could require either a right or left turn. Thus, the rat must shift from the old strategy and approach the previously irrelevant cue in order to obtain reinforcement. (B) Examples of the 3 types of errors that rats could make during the set shift. See Materials and Methods for details.

Experiment 1A, Day 2: Shift to Visual Cue

The acquisition of this discrimination required the animal to cease the use of a response strategy and instead use a visual cue–based strategy to obtain food reward (Fig. 1A, lower panel). The day following successful acquisition of the response discrimination, rats were trained to enter the arm that contained the visual cue, the location of which was pseudorandomly varied in the left and right arms such that it occurred in each arm with equal frequency for every consecutive set of 12 trials. The same training procedure, start arms and criteria to complete the visual cue version, were used as described in the response version. For probe trials, the visual cue was placed in the arm opposite that the rat had been trained to enter during response discrimination training.

Errors committed during the set shift were broken down into 3 error subtypes to determine whether treatments altered the ability to either shift away from the previously learned strategy (perseverative errors) or acquire and maintain the new strategy after perseveration had ceased (regressive or never-reinforced errors). A “perseverative error” was scored when a rat made an egocentric response as required on the response version (e.g., turned right) on trials that required the rat to turn in the opposite direction to enter the arm containing the visual cue (Fig. 1B, left panel). Six of every 12 consecutive trials required the rat to respond in this manner (i.e., enter the arm opposite of the previously learned turn direction). As described in previous studies (Dias and Aggleton 2000; Ragozzino and others 1999; Ragozzino 2002; Floresco, Ghods-Sharifi, and others 2006; Floresco, Magyar, and others 2006), these types of trials were separated into consecutive blocks of 4 trials each. Perseverative errors were scored when a rat entered the incorrect arm on 3 or more trials per block of 4 trials where the rat was required to enter the arm opposite of the previously learned turn direction. Once a rat made less than 3 perseverative errors in a block for the first time, all subsequent errors of this type were no longer counted as perseverative errors because at this point the rat was choosing an alternative strategy at least half of the time. Instead, these errors were now scored as “regressive” errors (Fig. 1B, left panel). The third type of error, termed “never-reinforced errors,” was scored when a rat entered the incorrect arm on trials where the visual cue was placed in the same arm that the rat had been trained to enter on the previous day (Fig. 1B, right panel). For example, during training on Day 1, a rat might be required to turn right. During the shift on Day 2, a rat must now enter the arm with the visual cue, and for half of the trials, the cue was in the right arm. In this situation, a never-reinforced error was scored when a rat entered the left arm (i.e., a choice that was not reinforced on either Day 1 or Day 2). Regressive and never-reinforced errors are used as an index of the animals' ability to maintain and acquire a new strategy, respectively.

Experiment 1B: Visual Cue to Response Set Shift

For these experiments, rats were initially trained on the visual cue version of the task on Day 1 followed by testing on the response version on Day 2. All other aspects of the testing procedure were identical to those described above. On the shift to the response version, the same measures were assessed as those for Experiment 1, where rats were required to shift from a response to a visual cue strategy. However, perseverative and regressive errors were analyzed from the trials in which a rat was required to turn in the arm opposite to that of the visual cue. Ten minutes before each test day, rats received a microinfusion of either saline or bupivacaine, as described above.

Histology

Upon completion of behavioral testing, the rats were sacrificed in a carbon dioxide chamber. Brains were removed and fixed in a 4% formalin solution. The brains were frozen and sliced in 50-μm sections prior to being mounted and stained with cresyl violet. Placements were verified with reference to the neuroanatomical atlas of Paxinos and Watson (1998).

Statistical Analysis

Separate analyses of variance (ANOVAs) were conducted on trials to criterion data from the response acquisition on Day 1. Trials to criterion data obtained from Day 2 were analyzed using a 2-way between/within-subjects ANOVA, with treatment as the between-subjects factor and choice Type as the within-subjects factor. Multiple comparisons were conducted using Dunnett's test. Trials per minute, as well as the number of probe trials required to reach criterion, were analyzed using separate 1-way ANOVAs.

Results

Experiment 1A: Effects of Bilateral MD Inactivation on a Response to Visual Cue Set shift

Histology.

Figure 2 illustrates the location of the cannula tips in the MD. All animals with placements encroaching on habenula or subthalamic nuclei were excluded. Rats receiving inactivations of the habenula were unimpaired on the strategy set shift, though those animals with subthalamic placements generally took more trials to criterion than saline-treated animals.

Figure 2.

Schematic of coronal sections of the rat brain showing the placements of the cannulae tips for all rats that received infusions of bupivacaine into the MD. Brain sections adapted from Paxinos and Watson (1998). Numbers correspond to millimeters from bregma.

Figure 2.

Schematic of coronal sections of the rat brain showing the placements of the cannulae tips for all rats that received infusions of bupivacaine into the MD. Brain sections adapted from Paxinos and Watson (1998). Numbers correspond to millimeters from bregma.

Day 1: response discrimination.

The results for the acquisition of the response strategy on Day 1 are shown in Figure 3A. There were no significant difference between groups in total trials to reach the criterion (F2,17 = 3.11, not significant [NS]). Additionally, neither there were any differences in the number of probe trials required to reach the criterion (F2,17 = 0.14, NS) nor were there differences in the number of trials completed per minute between groups (F2,17 = 0.07, NS). Thus, inactivation of the MD does not impair learning of a simple response discrimination.

Figure 3.

Experiment 1: Bilateral inactivation of the MD disrupts shifting from a response to a visual cue–based strategy (AC) and vice versa (DF). Data are expressed as means + standard error of mean. (A) Trials to criterion on acquisition of a response discrimination on Day 1 by rats receiving infusions of saline (white, black bars) or bupivacaine into the MD (hatched bar). Inactivation of the MD did not impair response learning. (B) Trials to criterion on the shift to visual cue discrimination strategy on Day 2 following infusions of either saline (white and hatched bar) or bupivacaine into the MD (black bar; single star denotes P < 0.05 significantly different from saline/saline). (C) Analysis of the type of errors committed in Experiment 1A during the set shift on Day 2. Inactivation of the MD prior to the set shift (black bars) significantly increased the number of perseverative errors (left) but did not affect regressive (middle) and never-reinforced (right) errors (double stars denote P < 0.01 significantly different from saline/saline, white bars). (D) Trials to criterion on acquisition of the visual cue discrimination on Day 1 by rats receiving either infusions of saline (white, black bars) or bupivacaine into the MD (hatched bar). Inactivation of the MD did not impair visual cue discrimination learning. (E) Trials to criterion on the shift to the response discrimination on Day 2 following infusions of either saline (white and hatched bars) or bupivacaine into the MD (black bar; double stars denote P < 0.01 significantly different from saline/saline). (F) Again, MD inactivation had a significant effect on perseveration (left), and in this case, never-reinforced errors (right), but did not increase the number of regressive errors (middle) during the shift on Day 2 (double stars denote P < 0.01, single star denotes P < 0.05 significantly different from the same type of errors made by saline/saline-treated rats).

Figure 3.

Experiment 1: Bilateral inactivation of the MD disrupts shifting from a response to a visual cue–based strategy (AC) and vice versa (DF). Data are expressed as means + standard error of mean. (A) Trials to criterion on acquisition of a response discrimination on Day 1 by rats receiving infusions of saline (white, black bars) or bupivacaine into the MD (hatched bar). Inactivation of the MD did not impair response learning. (B) Trials to criterion on the shift to visual cue discrimination strategy on Day 2 following infusions of either saline (white and hatched bar) or bupivacaine into the MD (black bar; single star denotes P < 0.05 significantly different from saline/saline). (C) Analysis of the type of errors committed in Experiment 1A during the set shift on Day 2. Inactivation of the MD prior to the set shift (black bars) significantly increased the number of perseverative errors (left) but did not affect regressive (middle) and never-reinforced (right) errors (double stars denote P < 0.01 significantly different from saline/saline, white bars). (D) Trials to criterion on acquisition of the visual cue discrimination on Day 1 by rats receiving either infusions of saline (white, black bars) or bupivacaine into the MD (hatched bar). Inactivation of the MD did not impair visual cue discrimination learning. (E) Trials to criterion on the shift to the response discrimination on Day 2 following infusions of either saline (white and hatched bars) or bupivacaine into the MD (black bar; double stars denote P < 0.01 significantly different from saline/saline). (F) Again, MD inactivation had a significant effect on perseveration (left), and in this case, never-reinforced errors (right), but did not increase the number of regressive errors (middle) during the shift on Day 2 (double stars denote P < 0.01, single star denotes P < 0.05 significantly different from the same type of errors made by saline/saline-treated rats).

Day 2: shift to visual cue.

The results for the set shift to visual cue strategy are shown in Figure 3B. This analysis revealed a significant main effect of treatment (F2,17 = 14.27, P < 0.001). Additionally, there was a significant main effect of choice type (F1,17 = 87.98, P < 0.001), as rats made significantly more correct choices than errors over the course of the task. There was no choice × treatment interaction (F2,17 = 0.36, NS). Dunnett's test showed that rats receiving inactivation of the MD before the set shift on Day 2 (n = 7) took significantly more trials to reach criterion than rats receiving saline (n = 7, P < 0.05), whereas rats receiving inactivations on Day 1 did not differ from controls (n = 6).

The number of errors committed during the strategy set shift was analyzed separately using a mixed between/within-subjects ANOVA, with treatment as the between-subjects factor and the 3 types of errors measured as the within-subjects factors. A main effect for treatment (F2,17 = 13.69, P < 0.001) and error type was observed (F2,32 = 16.00, P < 0.001), as well as a significant error × treatment interaction (F4,34 = 4.35, P < 0.01). Simple main effects analyses and multiple comparisons revealed that this interaction was due to the fact that inactivation of the MD prior to the set shift increased the number of perseverative errors relative to controls (F2,17 = 9.56, P < 0.005 and Dunnett's, P < 0.001; see Fig. 3C). Rats receiving inactivations of the MD on Day 1 and saline on Day 2 were indistinguishable from control rats. Furthermore, there was a noticeable increase in never-reinforced errors in rats receiving inactivations of the MD prior to the set shift, although this effect did not achieve statistical significance (F2,17 = 1.65, NS). There was no significant difference between groups in the number of regressive errors (F2,17 = 0.29, NS). Additional analyses of the number of probe trials required before reaching criterion and the number of trials completed per minute (see Table 1) revealed no significant differences between groups (all F values < 1.02, NS). Thus, inactivations of the MD caused an increase in perseverative errors on this type of set-shifting task, in a manner similar to PFC manipulations (Ragozzino and others 1999; Stefani and others 2003; Floresco, Magyar, and others 2006).

Table 1.

Means ± standard error of mean trials completed per minute for Experiments 1 and 2

 Experiment 1A (response to cue) Experiment 1B (cue to response) Experiment 2 (response to cue) 
 Saline/saline Bupi/saline Saline/bupi Saline/saline Bupi/saline Saline/bupi Saline Unilateral Ipsilateral PFC–NAc MD–NAc MD–PFC 
Day 1 1.33 ± 0.07 1.29 ± 0.10 1.24 ± 0.15 1.72 ± 0.14 1.60 ± 0.07 1.75 ± 0.12 1.60 ± 0.09 1.52 ± 0.11 2.23 ± 0.19 1.48 ± 0.22 1.59 ± 0.10 1.89 ± 0.11 
Day 2 1.37 ± 0.10 1.56 ± 0.07 1.46 ± 0.14 1.66 ± 0.08 1.50 ± 0.07 1.72 ± 0.10 1.83 ± 0.11 1.69 ± 0.10 2.43 ± 0.25 2.00 ± 0.09 1.51 ± 0.07 2.10 ± 0.10 
 Experiment 1A (response to cue) Experiment 1B (cue to response) Experiment 2 (response to cue) 
 Saline/saline Bupi/saline Saline/bupi Saline/saline Bupi/saline Saline/bupi Saline Unilateral Ipsilateral PFC–NAc MD–NAc MD–PFC 
Day 1 1.33 ± 0.07 1.29 ± 0.10 1.24 ± 0.15 1.72 ± 0.14 1.60 ± 0.07 1.75 ± 0.12 1.60 ± 0.09 1.52 ± 0.11 2.23 ± 0.19 1.48 ± 0.22 1.59 ± 0.10 1.89 ± 0.11 
Day 2 1.37 ± 0.10 1.56 ± 0.07 1.46 ± 0.14 1.66 ± 0.08 1.50 ± 0.07 1.72 ± 0.10 1.83 ± 0.11 1.69 ± 0.10 2.43 ± 0.25 2.00 ± 0.09 1.51 ± 0.07 2.10 ± 0.10 

Note: bupi, bupivacaine.

Experiment 1B: Effects of MD Inactivation on a Visual Cue to Response Set Shift

Day 1: visual cue discrimination.

The results for the acquisition of the visual cue–based strategy on Day 1 are shown in Figure 3D. A 1-way ANOVA showed no significant difference between groups in total trials to reach the criterion (F2,20 = 0.182, NS). Additionally, neither there were any differences found in the number of probe trials required to reach the criterion (F2,20 = 0.13, NS) nor were there differences in the number of trials taken per minute between groups (F2,20 = 0.50, NS). Thus, inactivation of the MD does not impair the acquisition of a simple visual cue–based discrimination. One rat receiving bupivacaine on Day 1 and saline on Day 2 was excluded from the analysis as a statistical outlier, with its trials to reach criterion on Day 2 more than 2 standard deviation (SD) away from the mean of the group with that data point inclusive (mean = 52.0, SD = 34.18; excluded rat = 180 trials).

Day 2: shift to response.

The results from the shift to a response-based strategy are shown in Figure 3E. The analysis of these data revealed a significant main effect of treatment (F2,20 = 4.56, P < 0.05), a main effect of choice (F2,20 = 116.66, P < 0.001). Dunnett's test indicated that rats receiving inactivations of the MD before the set shift on Day 2 (n = 8) took significantly more trials to reach criterion than rats receiving saline (P < 0.01, n = 8), though rats receiving Day 1 inactivation and Day 2 saline did not differ from controls (n = 7).

Analysis of the errors committed during the set shift revealed a significant main effect of treatment (F2,20 = 4.73, P < 0.05), error type (F4,40 = 31.72, P < 0.001), and error × treatment interaction (F2,40 = 3.29, P < 0.05, Fig. 3F). Subsequent 1-way ANOVAs for each error type revealed no significant treatment effect for regressive errors (F2,20 = 0.57, NS). However, as in Experiment 1, a significant treatment effect was observed for perseverative errors (F2,20 = 4.84, P < 0.05). Pairwise comparisons indicated that this was due to an increase in perseverative errors for animals receiving MD inactivations prior to the set shift on Day 2 relative to controls (P < 0.005), and animals receiving Day 1 inactivations did not differ from controls. In this experiment, we also observed a significant main effect of treatment for never-reinforced errors for this strategy set shift (F2,20 = 3.71, P < 0.05). Additional analyses of the number of trials completed per minute and the number of probe trials required to complete the strategy set shift show no significant treatment effects (all F values < 2.42, NS). Thus, Experiment 1B both confirms the results of Experiment 1A and indicates that the MD may play a role in the generation of alternative strategies, as evidenced by the increase in never-reinforced errors.

Experiment 2: Effects of Asymmetrical Disconnection Inactivations of the PFC, NAc, and MD on a Response to Visual Cue Set Shift

The results from Experiment 1 indicate a role for the MD in set shifting. However, the MD sends projections to both PFC and NAc, which have been implicated in mediating this form of behavioral flexibility (Ragozzino and others 1999; Floresco, Ghods-Sharifi, and others 2006; Floresco, Magyar, and others 2006). In Experiment 2, we use an asymmetrical disconnection inactivation procedure to delineate the routes of information transfer within this thalamic–cortical–striatal circuit. Disconnection lesions are used to identify functional components of a circuit where information is transferred serially from one structure to another structure in the same hemisphere, on both sides of the brain in parallel. Furthermore, the design assumes that dysfunction will result from blockade of neural activity at the origin of a pathway in one hemisphere and the termination of the efferent pathway in the contralateral hemisphere. It follows that a unilateral inactivation at either site should have no effect on behavior, as the unaffected brain regions in the contralateral hemisphere will serve to compensate. We have used this reversible disconnection procedure previously to delineate the routes of information transfer between brain regions for both working memory and decision making processes (Floresco and others 1997, 1999; Floresco and Ghods-Sharifi 2006). As inactivation of the MD, NAc (Floresco, Magyar, and others 2006), or PFC (Ragozzino and others 1999) disrupted performance on both response to visual cue and visual cue to response set shifts while having no effect on the initial learning of either discrimination, in this experiment, we only tested animals performing a response to visual cue set shift.

Materials and Methods

Subjects and Surgery

Three groups of male Long-Evans rats (290–380 g) were implanted with 2 sets of bilateral cannula. One group of rats was implanted with one pair of cannula in the MD and a second pair in the prelimbic region of the PFC (flat skull: AP = +3.0 mm, ML = ± 0.7 mm from bregma, and DV = −2.7 mm from dura). A second group was implanted with one pair of cannula in the MD and a second pair in the NAc (flat skull: AP = +1.6 mm, ML = ± 1.8 mm from bregma, and DV = −6.0 mm from dura). A third group of rats was implanted with one pair of cannula in the PFC and a second pair in the NAc. The cannulation into the NAc at these coordinates transects the corpus callosum, effectively eliminating any contralateral projections from the PFC to the NAc (Gorelova and Yang 1997). This point is important as disconnection lesions are most effective where there are no contralateral projections between brain regions (Floresco and others 1999; Dunnett and others 2005). Thirty gauge obdurators flush with the end of the guide cannulae remained in place until the injections were made. Each rat was given at least 7 days to recover from surgery before testing. The location of acceptable infusion placements in all brain regions tested in Experiment 2 is presented in Figure 4.

Figure 4.

Schematic of coronal sections of the rat brain showing the placements of the cannulae tips for all rats that received double cannulations. Numbers beside each plate correspond to millimeters from bregma. (A) Location of cannulae tips (black circles) for all rats used for data analysis receiving MD–PFC infusions, (B) PFC–NAc infusions or (C) MD–NAc infusions. For clarity, figures represent the asymmetrical infusion procedure; all rats received infusions in either the left or the right hemisphere that was counterbalanced across animals.

Figure 4.

Schematic of coronal sections of the rat brain showing the placements of the cannulae tips for all rats that received double cannulations. Numbers beside each plate correspond to millimeters from bregma. (A) Location of cannulae tips (black circles) for all rats used for data analysis receiving MD–PFC infusions, (B) PFC–NAc infusions or (C) MD–NAc infusions. For clarity, figures represent the asymmetrical infusion procedure; all rats received infusions in either the left or the right hemisphere that was counterbalanced across animals.

Maze Familiarization Procedure and Response to Visual Cue Set-Shifting Procedure

The familiarization response discrimination training and set-shifting procedures were identical to that used in Experiment 1A.

Microinfusion Procedure

All animals received mock infusions (injection cannula inserted with no flow) following the turn bias procedure on the last day of maze familiarization, as well as before response discrimination training on Day 1. The day after acquisition of the response discrimination, and before the set shift on Day 2, all animals received injections in the opposite hemisphere of that which received the mock infusion the previous day. Unilateral inactivations of MD and PFC were induced with microinfusions of bupivacaine as in Experiment 1. However, for inactivations of the NAc, we used the γ-aminobutyric acid (GABA) agonists baclofen and muscimol (Sigma-Aldrich Canada, Ontario, Canada) (75 ng/μL of each drug dissolved in saline and infused at a volume of 0.3 μL). This procedure was employed because projections from brain regions that terminate in the NAc shell pass through the NAc (Gorelova and Yang 1997). The use of GABA agonists inactivates cell bodies in the NAc but leave fibers of passage relatively intact (McFarland and Kalivas 2001; Floresco, Ghods-Sharifi, and others 2006).

In total, 6 groups of rats were tested, 3 of which received disconnection treatments as follows: 1) a unilateral bupivacaine infusion into the PFC in combination with bupivacaine infused into the contralateral MD, 2) a unilateral bupivacaine infusion into the PFC in combination with a contralateral infusion of baclofen/muscimol into the NAc, and 3) a unilateral bupivacaine infusion into the MD in combination with a contralateral infusion of baclofen/muscimol into the NAc. Two initial control groups were formed, one group received unilateral inactivations with a saline infusion into the contralateral structure as follows: 1) a unilateral baclofen/muscimol into NAc/saline into the contralateral MD or PFC, 2) unilateral bupivacaine into MD/saline into NAc or PFC, and 3) unilateral bupivacaine into the PFC/saline into the contralateral NAc or MD. The saline control group was composed of the following combinations of asymmetrical bilateral infusions: 1) unilateral infusions of saline into both MD and NAc, 2) unilateral infusions of saline into both PFC and NAc, and 3) unilateral infusions of saline into the MD and PFC (See Table 2). The hemisphere (left or right) used for the injection was also counterbalanced across animals within groups. With respect to combined ipsilateral inactivation control treatments, numerous studies using disconnection designs have shown that ipsilateral lesions of 2 interconnected structures in the same hemisphere do not impair behavior relative to the effect of crossed, disconnection lesions (Olton and others 1982; Warburton and others 2000; Chudasama and others 2003; Dunnett and others 2005). Nevertheless, because PFC–MD and PFC–NAc disconnections significantly increased the number of trials to criterion during the set shift (see Results), we included a third control group. This consisted of 6 rats that received unilateral inactivations of both the PFC and either the MD or the NAc in the same hemisphere.

Table 2.

Summary of treatment groups used in Experiment 2

Cannulation n Treatment Group 
MD–PFC MD–bupi, PFC–bupi Disconnection 
 MD–saline, PFC–bupi Unilateral PFC 
 MD–bupi, PFC–saline Unilateral MD 
 MD–saline, PFC–saline Saline 
 MD–bupi, PFC–bupi Ipsilateral inactivations 
PFC–NAc MD–bupi, PFC–bupi Disconnection 
 PFC–saline, NAc–bac/mus Unilateral NAc 
 PFC–bupi, NAc–saline Unilateral PFC 
 PFC–saline, NAc–saline Saline 
 PFC–bupi, NAc–bac/mus Ipsilateral inactivations 
MD–NAc MD–bupi, NAc–bac/mus Disconnection 
 MD–saline, NAc–bac/mus Unilateral NAc 
 MD–bupi, NAc–saline Unilateral MD 
 MD–saline, NAc–saline Saline 
Cannulation n Treatment Group 
MD–PFC MD–bupi, PFC–bupi Disconnection 
 MD–saline, PFC–bupi Unilateral PFC 
 MD–bupi, PFC–saline Unilateral MD 
 MD–saline, PFC–saline Saline 
 MD–bupi, PFC–bupi Ipsilateral inactivations 
PFC–NAc MD–bupi, PFC–bupi Disconnection 
 PFC–saline, NAc–bac/mus Unilateral NAc 
 PFC–bupi, NAc–saline Unilateral PFC 
 PFC–saline, NAc–saline Saline 
 PFC–bupi, NAc–bac/mus Ipsilateral inactivations 
MD–NAc MD–bupi, NAc–bac/mus Disconnection 
 MD–saline, NAc–bac/mus Unilateral NAc 
 MD–bupi, NAc–saline Unilateral MD 
 MD–saline, NAc–saline Saline 

Note: bupi, bupivacaine; bac/mus, baclofen/muscimol.

Results

Saline and Unilateral Inactivation Control Groups

There were no differences in the number of trails to reach criterion during the set shift on Day 2 between rats receiving saline infusion combinations into the PFC–MD, PFC–NAc, or MD–NAc, so their data were combined to form one control group (F2,9 = 0.99, NS, n = 12). A mixed ANOVA was performed on the total trials to reach criterion during the strategy set shift for rats with unilateral inactivations, with the brain region (NAc, PFC or MD) and side of infusion as 2 between-subjects factor. Neither were there differences between groups on the number of trials to reach criterion for rats receiving unilateral inactivations (PFC n = 6, NAc n = 6, MD n = 6 [F2,12 = 0.35, NS]) nor was there a main effect of side of infusion (F1,12 = 1.76, NS) nor a side × brain region interaction (F2,12 = 0.82, NS) so the data from all rats receiving unilateral inactivations were combined to form a second control group (Fig. 5B, inset). Lastly, there were no differences in performance between rats that received ipsilateral inactivations of the PFC and MD (n = 3) or PFC and NAc (n = 3; F1,4 = 0.15, NS), so their data were combined to form the third control group.

Figure 5.

Experiment 2: The effects of disconnection of the MD–PFC, MD–NAc, and PFC–NAc pathways on shifting from a response to a visual cue–based strategy. Data are expressed as means + standard error. (A) Trials to criterion on acquisition of a response discrimination on Day 1 by rats receiving mock infusions. As there were no differences between animals receiving unilateral (Uni: MD, NAc, or PFC, inset) infusions on Day 2, they were combined to form one control group for analysis. (B) Trials to criterion on the shift to the visual cue discrimination on Day 2 following infusions of saline (white bar), unilateral inactivations (Uni, gray bars), ipsilateral inactivations (Ipsi, striped bar), or disconnections of the MD–PFC (hatched bar), PFC–NAc (black bar), or MD–NAc (cross-hatched bar). Neither saline infusions nor unilateral inactivations impaired rats' ability to shift strategy. However, rats receiving disconnections of the MD–PFC and PFC–NAc pathways showed significant increases in total trials to reach criterion relative to saline control group (double stars denote P < 0.01 vs. saline control group). (C) Schematic representing the 3 types of asymmetrical disconnection treatments used in Experiment 2.

Figure 5.

Experiment 2: The effects of disconnection of the MD–PFC, MD–NAc, and PFC–NAc pathways on shifting from a response to a visual cue–based strategy. Data are expressed as means + standard error. (A) Trials to criterion on acquisition of a response discrimination on Day 1 by rats receiving mock infusions. As there were no differences between animals receiving unilateral (Uni: MD, NAc, or PFC, inset) infusions on Day 2, they were combined to form one control group for analysis. (B) Trials to criterion on the shift to the visual cue discrimination on Day 2 following infusions of saline (white bar), unilateral inactivations (Uni, gray bars), ipsilateral inactivations (Ipsi, striped bar), or disconnections of the MD–PFC (hatched bar), PFC–NAc (black bar), or MD–NAc (cross-hatched bar). Neither saline infusions nor unilateral inactivations impaired rats' ability to shift strategy. However, rats receiving disconnections of the MD–PFC and PFC–NAc pathways showed significant increases in total trials to reach criterion relative to saline control group (double stars denote P < 0.01 vs. saline control group). (C) Schematic representing the 3 types of asymmetrical disconnection treatments used in Experiment 2.

Day 1: Response Discrimination

The results for the acquisition of the response strategy on Day 1 following mock infusions are shown in Figure 5A. A 1-way ANOVA showed no significant difference between groups in total trials to reach criterion for the response discrimination (F5,54 = 0.53, NS). Additionally, there were no differences between groups on the number of probe trials required to reach the criterion or the number of trials completed per minute (all F values < 1.45, NS). Thus, all rats in all groups required an equivalent number of trials to learn a response discrimination.

Day 2: Shift to Visual Cue

The results for the set shift to a visual cue–based strategy are shown in Figure 5B. The analysis revealed a significant main effect of treatment (F5,54 = 3.82, P < 0.005), as well as a main effect of choice (F1,54 = 269.81, P < 0.001), and no significant treatment × choice interaction (F5,54 = 1.20, NS). Dunnett's test indicated that rats receiving MD–PFC (n = 9) and PFC–NAc (n = 7) disconnections before the set shift on Day 2 required significantly more trials to reach criterion than rats receiving saline (both P < 0.001). Although disconnection of the MD and NAc also increased the number of trials required to reach criterion, this increase did not achieve statistical significance relative to saline (P = 0.14; n = 8). Importantly, neither unilateral inactivations of each region nor combined ipsilateral inactivations of the PFC–MD or PFC–NAc increased the number of trials to reach criterion, as these control groups did not differ from saline-treated animals.

A mixed ANOVA on the errors committed during the set shift, with group as the between-subjects variable, and the 3 types of errors as levels of the within-subjects factor revealed a main effect of group (F5,54 = 3.73, P < 0.005) and a main effect of error (F2,108 = 5.59, P < 0.001) and most pertinently, a significant group × error type interaction (F10,108 = 4.87, P < 0.001). Subsequent simple main effects analyses and Dunnett's test revealed that rats receiving MD–PFC disconnections made significantly more perseverative errors than saline-treated controls (P < 0.05). Similarly, the rats that received PFC–NAc disconnections also made significantly more perseverative errors (P < 0.001) relative to saline-treated controls (Fig. 6A). Neither the unilateral or ipsilateral inactivation control groups nor the MD–NAc disconnection group differed from saline-treated controls in total perseverative errors. Furthermore, there were no differences between any groups on the number of regressive errors committed during the set shift (F5,54 = 0.61, NS, Fig. 6B). In contrast with these findings, analysis of never-reinforced errors revealed that rats receiving MD–NAc disconnection made significantly more never-reinforced errors, relative to saline controls (P < 0.001; Fig. 6C); all other groups did not differ from saline-treated rats.

Figure 6.

Analysis of the type of errors committed during the set shift in Experiment 2. (A) MD–PFC (hatched bar) and PFC–NAc (black bar) disconnections resulted in a significant increase in perseverative errors, whereas unilateral inactivations (Uni, gray bar), ipsilateral inactivations (Ipsi, striped bar), and MD–NAc disconnections (cross-hatched bar) were without effect compared with saline treatments (white bar; single and double stars denote P < 0.005, 0.01, vs. saline control group, respectively). (B) None of the disconnection treatments or unilateral/ipsilateral inactivations altered the number of regressive errors. (C) Only MD–NAc disconnections resulted in an increase in never-reinforced type errors relative to saline controls (double stars denote P < 0.01 vs. saline control group).

Figure 6.

Analysis of the type of errors committed during the set shift in Experiment 2. (A) MD–PFC (hatched bar) and PFC–NAc (black bar) disconnections resulted in a significant increase in perseverative errors, whereas unilateral inactivations (Uni, gray bar), ipsilateral inactivations (Ipsi, striped bar), and MD–NAc disconnections (cross-hatched bar) were without effect compared with saline treatments (white bar; single and double stars denote P < 0.005, 0.01, vs. saline control group, respectively). (B) None of the disconnection treatments or unilateral/ipsilateral inactivations altered the number of regressive errors. (C) Only MD–NAc disconnections resulted in an increase in never-reinforced type errors relative to saline controls (double stars denote P < 0.01 vs. saline control group).

Additional analyses showed an effect of group for the number of trials completed per minute during the strategy set shift (F5,54 = 4.91, P < 0.01). Multiple comparisons revealed that rats in the ipsilateral inactivation control group were actually faster to complete trials than saline-treated rats, but no other groups differed from the saline control group (see Table 1). Analysis of the number of probe trials required to reach criterion showed no differences between groups (F5,54 = 0.21, NS). Thus, disconnections between the PFC and MD, PFC and NAc, and the MD and NAc all disrupted shifting from a response to a visual cue–based strategy, but in a dissociable manner. Disconnections between the PFC and either the MD or the NAc caused a selective increase in perseverative responding, whereas similar manipulations between the MD and NAc selectively increased never-reinforced errors.

Discussion

The present data delineate a role for the MD in shifting between behavioral strategies and demonstrate that a distributed neural network incorporating the PFC, MD, and NAc mediate dissociable components of set shifting. Disconnecting the serial flow of information from the MD to the PFC or the PFC projection to the NAc resulted in a perseverative deficit, impairing the ability of rats to inhibit the use of a previously relevant strategy or signal the need for a shift. However, disconnections between the MD to the NAc resulted in a unique deficit, where animals did not perseverate, but instead displayed a selective increase in never-reinforced errors, indicating a failure to eliminate inappropriate strategies during the set shift. Notably, neither unilateral inactivation of any of the targeted structures nor ipsilateral inactivations impaired behavior, further supporting the contention that the impairments were due specifically to functional disconnection and not due to additive effects of unilateral inactivations.

Bilateral MD Inactivations

Bilateral inactivation of the MD impaired shifting between response and visual cue–based discrimination strategies, without affecting initial discrimination learning, indicating that the MD is specifically involved in shifting between strategies. These impairments were due to an increase in the number of perseverative errors, which implies a disruption in the ability to disengage from a previously relevant strategy. In addition, these treatments also increased the number of never-reinforced errors, although this effect only achieved statistical significance on the more difficult visual cue to response shift. This latter finding indicates that the MD also may play a role in eliminating inappropriate response alternatives based on negative feedback, particularly in situations when animals are required to perform more difficult shifts.

The strategy set-shifting task used in the present study required rats to cease attending to a previously relevant stimulus dimension (e.g., turn direction) and attend to a newly relevant stimulus (e.g., the visual cue) in order to obtain reward. However, given that the same stimuli were used during the initial discrimination and during the set shift, it may be argued that this task also assesses a form of reversal learning. In this regard, a number of studies have assessed the effects of MD lesions on different types of reversals. Tigner (1974) reported that MD lesions induced deficits in both the initial learning and reversal of brightness and tactile discriminations, although no effect was seen for spatial reversals. Using a more rule-based reversal in the operant chamber, Beracochea and others (1989) failed to find any effect of MD lesion on reversals. On the other hand, MD lesions did not impair learning of an initial spatial reversal but did disrupt subsequent reversals on a T-maze task (Means and others 1975), whereas other studies failed to find any effect on serial spatial reversals performed in an operant chamber (Neave and others 1993). With respect to reversal learning for visual stimuli, Chudasama and others (2001) demonstrated that excitotoxic lesions of the MD did impair reversals of visually cued stimulus–reward associations but that this effect only emerged on latter serial reversals while having a minimal effect on the initial reversal. Taken together, it is apparent that the MD may play a more prominent role in serial reversal learning for visual or spatial discriminations but may not be as important for initial reversal learning. Given that rats in the present study were only required to shift strategies once, it is unlikely that the impairments induced by MD inactivation reported here were due to a disruption of processes related to reversal learning.

Although the role of the MD in reversal learning is unclear, lesions of this nucleus do cause robust perseverative deficits when rats are required to shift from a matching to a nonmatching to place strategy on a T-maze task (Hunt and Aggleton 1998), indicating that this form of behavioral flexibility is critically dependent on the integrity of the MD. A similar perseverative deficit was observed in the present study, and this type of impairment is similar to what has been reported following medial PFC manipulations (Ragozzino and others 1999; Ragozzino 2002; Floresco, Magyar, and others 2006). This finding was not unanticipated, considering the strong reciprocal connections between the MD and the medial PFC (Condé and others 1995; Gabbott and others 2005). A number of studies have demonstrated that lesions or inactivations of the MD can induce similar behavioral deficits to those observed following PFC manipulations. These include different types of delayed response tasks in both primates and rats (Harrison and Mair 1996; Floresco and others 1999; de Zubicaray and others 2001; Funahashi and others 2004; Bailey and Mair 2005) and familiarity estimates in humans (Zoppelt and others 2003). The present findings indicate that the MD also plays a key role in set shifting and indicate that this region may interact with the PFC to facilitate this form of behavioral flexibility; a hypothesis tested directly in Experiment 2.

MDPFC Disconnections

Asymmetrical bupivacaine infusions into the MD and PFC, which disconnected the flow of information between these structures, increased the total trials to reach criterion on a visual cue discrimination during the strategy set shift. This deficit was due to a selective increase in perseverative errors, indicating that this thalamocortical pathway is necessary for successful disengagement from a previously relevant strategy. The prelimbic region of the medial PFC has been shown to have a role in maintaining an established outcome–action association in working memory for the purpose of guiding future behavior (Corbit and Balleine 2003). In that study, rats with lesions to the prelimbic PFC were trained to differentially respond for sucrose solution and sugar pellets, one of which was subsequently experimentally devalued with a specific satiety manipulation. Lesioned rats failed to differentiate between the devalued and nondevalued rewards and globally depressed responding. These results indicate that the PFC is crucial for the incorporation of specific reward information into behavior (Corbit and Balleine 2003). Furthermore, lesions of the MD reveal a similar deficit (Corbit and others 2003). When these findings are viewed in light of the present data, they suggest that the MD and the PFC work in concert to suppress the use of previously established rules based on irrelevant reinforcement contingencies; a contention supported by the present findings that disconnection between the MD and PFC impaired the suppression of a previously relevant strategy during the set shift.

Further insight into the specific roles that the MD and PFC may play in mediating behavioral flexibility comes from functional imaging studies in humans. Thalamic activation has been linked to performance of the WCST, where activation of the MD occurred specifically when the participants received negative feedback after an incorrect choice but not positive feedback (Monchi and others 2001). These findings suggest that increased activity in the MD serves as a signal for the necessity to shift set. In contrast, activation of the dorsolateral PFC occurred during both positive and negative feedback, indicating the PFC is critically involved in comparing online feedback with past events (Monchi and others 2001). Along similar lines, MD neurons recorded from awake, behaving rabbits learning a discriminative avoidance task display earlier and more robust development of discriminative activity during reversal of stimulus-reinforcement contingencies when compared with similar activity of PFC neurons (Orona and Gabriel 1983). Thus, changes in reinforcement contingencies are associated with an initial alteration in the discriminative activity of MD neurons, which subsequently can modulate PFC neural activity to facilitate changes in response strategies. As such, even though inactivation of either the PFC (Ragozzino and others 1999) or the MD (present study) caused a similar perseverative deficit during the set shift, each of these regions may mediate different cognitive operations that facilitate shifting from one strategy to another. It follows that disconnection of the MD and PFC would deprive the PFC of information related to the absence of reward (mediated initially by the MD) after entering an incorrect arm during the set shift. This in turn would impair the ability to extinguish responding in accordance with the previously relevant strategy so that a new strategy might be initiated (Lebron and others 2004).

PFC–NAc Disconnections

As was observed following MD–PFC disconnections, asymmetrical inactivations of the PFC and the NAc also impaired the ability to shift from one discrimination strategy to another, causing a selective increase in perseverative errors. However, these manipulations did not increase the number of regressive or never-reinforced errors, which are used as an index of an animal's ability to maintain and establish a new strategy during the shift (Ragozzino 2002; Ragozzino and others 2002; Floresco, Ghods-Sharifi, and others 2006; Floresco, Magyar, and others 2006). This pattern of errors is interesting, in light of the effect of bilateral inactivations of the NAc on this task (Floresco, Ghods-Sharifi, and others 2006). Specifically, inactivation of the NAc increased regressive and never-reinforced errors but did not affect perseverative responding. We have interpreted these findings to indicate that the NAc is not involved in the suppression of previously acquired strategies, but instead facilitates the maintenance of a new strategy, potentially working in conjunction with the dorsomedial striatum (Ragozzino and other 2002; Floresco, Ghods-Sharifi, and others 2006).

Although both perseverative and regressive errors entail a choice consistent with the previously acquired, but now incorrect, strategy, a key difference is when these errors occur in the choice sequence. Regressive errors occur later in the choice sequence when rats use the previous strategy on fewer than 50% of trials where the correct response is opposite to that required using the initial rule. Thus, perseverative and regressive errors are inexorably related as an animal cannot maintain a novel strategy unless it has first shifted from the previously rewarded strategy. We have previously proposed that the PFC and NAc play distinct roles during different stages of set shifting. Specifically, PFC plays an essential role early in set shifting, suppressing the use of a previously correct strategy (Ragozzino and others 1999), whereas once the appropriate new strategy has been ascertained, maintenance of the novel strategy is likely mediated by parallel corticostriatal circuits that include NAc (Floresco, Ghods-Sharifi, and others 2006). The finding that bilateral inactivation of the NAc increased regressive but not perseverative errors (Floresco, Ghods-Sharifi, and others 2006), whereas PFC–NAc disconnection increased perseverative responding suggests that this corticostriatal circuit facilitates both the suppression of previously established response rules and the maintenance of novel strategies. The increase in perseverative errors following PFC–NAc disconnections may be attributed to disruption of the transfer of updated information from the PFC regarding the success or failure of current strategies to the NAc for maintenance of a novel discrimination strategy. This hypothesis is consistent with previous studies investigating the role of this pathway in the context of a 5-choice serial reaction time task (Christakou and others 2004). In that study, disconnection of the PFC–NAc pathway caused a deficit in the integration of reward information into the behavioral repertoire. Likewise, asymmetrical inactivations between the PFC and the NAc also impaired working memory performance on a delayed response version of the radial arm maze task (Floresco and others 1999). Taken together, these data support the contention that interactions between the PFC and NAc play a crucial role in the maintenance of behavioral strategies in response to changes in reward contingencies, as well as updating responding based on trial-unique reward information. These include sustained attention (Christakou and others 2004), working memory (Floresco and others 1999), and, as demonstrated here, set shifting.

MDNAc Disconnections

Disconnection inactivations of the MD to the NAc resulted in a unique pattern of errors during the set shift, distinct from that induced by either the MD–PFC or PFC–NAc disconnections. Specifically, we observed a selective increase in never-reinforced type errors, with no concomitant increase in perseverative or regressive errors. As opposed to perseverative/regressive errors, never-reinforced errors are scored when an animal makes choice that was incorrect during both initial discrimination training and during the shift and are used as an index of how quickly animals are able to parse out ineffective strategies. During the set shift, intact rats learn quickly that the previously correct strategy is no longer appropriate and engage alternative strategies until they find the optimal solution. Never-reinforced errors may be interpreted as an attempt to use alternative strategies, perhaps a reversal of the previously acquired rule (e.g., always turn left instead of right). Intact rats make relatively few errors of this type and learn that these strategies do not lead to reward reliably. In contrast, bilateral inactivation of the NAc caused a disproportionate amount of never-reinforced errors (as well as regressive errors), indicating that the NAc also mediates the elimination of inappropriate response options, enabling the reorganization of behavior to obtain reward in an optimal manner (Floresco, Ghods-Sharifi, and others 2006). From the present data, it is apparent that this thalamostriatal pathway plays a specialized role in establishing novel strategies, facilitating the elimination of ineffective response options when reward contingencies have changed. To our knowledge, this is the first characterization of a functional role for this pathway. In contrast, disconnections of this pathway did not impair working memory assessed using a delayed response variant of the radial arm maze (Floresco and others 1999), indicating that this pathway does not mediate working memory functions subserved by the PFC. Yet, the increase in never-reinforced errors observed in the present study indicates information transfer from the MD to the NAc is necessary for the successful elimination of inappropriate strategies based on reward feedback. Together, these findings highlight the differences in the functional neural circuitries that underlie different executive functions (i.e., working memory and behavioral flexibility) mediated by the frontal lobes.

When viewed collectively, the data from Experiment 2 further clarify the functional roles for parallel thalamo-cortico-striatal circuits involved in the overall process of strategy set shifting and provide important information about the functions of specific subcircuits that mediate dissociable components of behavioral flexibility. Based on the present data, in addition to previous findings, we propose the following. Early in the set shift, increased activity in the MD encodes information about changes in reward contingencies and subsequently relays this information to the PFC (Fig. 7A). In turn, the PFC serves to suppress the use of a previously correct, but now irrelevant, strategy, a process facilitated by dopaminergic inputs to this region (Ragozzino and others 1999; Ragozzino 2002; Floresco, Magyar, and others 2006). Once perseveration has ceased and the animal engages new strategies, the activity in the MD–NAc pathway facilitates new learning by curtailing the use of inappropriate strategies (Fig. 7B). Subsequently, when the appropriate strategy has been ascertained, maintenance of the novel strategy is mediated by corticostriatal circuits that include the PFC and NAc (Fig. 7C).

Figure 7.

Schematic of some of the neural circuitry that mediates strategy set shifting. See Discussion for details.

Figure 7.

Schematic of some of the neural circuitry that mediates strategy set shifting. See Discussion for details.

Conclusions

Behavioral flexibility is a composite of different processes: suppression of irrelevant strategies, acquisition and generation of novel strategies, and maintenance of effective strategies. The present data emphasize that shifting from one behavioral strategy to another is mediated by a distributed neural circuit and clarify the contributions that different cortical subcortical regions make to this form of behavioral flexibility. Our findings that dissociable components of different phases of strategy set shifting are mediated by distinct neural circuits complement the findings of human functional imaging studies, with different patterns of activation of the PFC, thalamus, or striatum associated with different cognitive requirements of the WCST (Monchi and others 2001). Here, we establish that parallel pathways originating from the MD and terminating in both the PFC and NAc make distinct contributions to behavior when an organism must change its behavior in response to dynamic environmental demands. A functional projection from the MD to the PFC is important for signaling the need to shift strategies, and the projection to the NAc is important for eliminating inappropriate response strategies. Furthermore, the PFC projection to the NAc facilitates the integration of reward information and behavioral consequences into behavioral responses geared toward the anticipation of future reward.

It is interesting to note that there is evidence of compromised MD–PFC circuitry in schizophrenia, as this disorder is associated with reductions in the number of neurons and overall volume of the MD (Andreasen and others 1994; Young and others 2000; Dorph-Petersen and others 2004). Additionally, schizophrenic individuals show deficits in tasks assessing behavioral flexibility that are rooted in impaired concept formation (Pantelis and others 1999) and often display perseverative responding in attentional set-shifting paradigms (Elliott and others 1998). When viewed in light of the present findings, it is possible that the deficits in behavioral flexibility observed in schizophrenia may be due in part to a compromised projection originating in the MD and terminating in the PFC and NAc. Collectively, these findings further elucidate the neural circuitry that regulates behavioral flexibility in the normal brain and provide important insight into the neural pathology that underlies executive dysfunction in some psychiatric disorders where impairment in behavioral flexibility is a prominent symptom.

This work was supported by Discovery Grant from the Natural Sciences and Engineering Research Council of Canada to SBF. SBF is a Canadian Institutes of Health Research New Investigator, a Michael Smith Foundation for Health Research Scholar, and the recipient of a National Alliance for Research on Schizophrenia and Depression Young Investigator Award. We are indebted to Sarvin Ghods-Sharifi and Desirae Haluk for their assistance with behavioral testing. Conflict of Interest: None declared.

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