Abstract

To comprehend emotional prosodic cues in speech is a critical function of human social life. However, it is common in everyday communication that conflicting information in emotional prosody and semantic content co-occur. Here, we sought to specify brain regions involved in conflict monitoring of these interfering communication channels. By means of functional magnetic resonance imaging, we obtained signal increases in the right dorsal anterior cingulate cortex and right superior temporal gyrus (STG) and superior temporal sulcus when participants listened to incongruous compared with congruous sentences. Moreover, valence-specific effects were found in the left inferior frontal gyrus and left STG for happily intoned sentences expressing a negative content. The left caudate nucleus along with the thalamus was active when angrily intoned sentences were coupled with positive semantic content. Our results suggest a brain network that monitors conflict in emotional prosody and emotional semantic content comprising of medial prefrontal areas that have previously been associated with cognitive conflict processing. Furthermore, our study extends the knowledge of these processes by suggesting valence-specific differences of emotional conflict processing.

Introduction

In spoken language, emotionally salient information can be provided by variations in speech melody (emotional prosody) or by emotional semantics (verbal emotional content). According to Grandjean et al. (2006), the term “prosody” refers to all suprasegmental changes in the course of a spoken utterance, whereas “emotional prosody” is implemented through modifications of pitch over time (Bänziger and Scherer 2005).

The 2 main features of human communication are not always coherently linked the same way. Sometimes, we prefer to mask our emotional stance and keep speech melody as neutral as possible in order to appear noninvolved, while expressing highly emotional statements. Other times, we may add information via speech melody to statements, which are semantically neutral. In general, it is a common rather than an exceptional instrument of everyday language to use irony, sarcasm, or banter. Their common feature is that what is said (i.e. the semantic content) can be opposed to how it is said (i.e. prosody). In social interactions, the ability to detect a possible divergence of emotional prosody and emotional semantics is crucial in order to avoid psychosocial difficulties and misunderstandings (Ross and Monnot 2008), and to be able to make predictions about other people's behavior. Hence, the motivation of our study was to examine the effects of incongruence in human emotional speech.

In recent years, the number of neuroimaging studies regarding emotional prosody has steadily increased. It was demonstrated that a specific brain network consisting of mid and posterior superior temporal cortex as well as inferior and orbitofrontal cortex is associated with the processing of information provided by speech melody (Ethofer et al. 2006; Wildgruber et al. 2006). However, it has been hypothesized (Kotz et al. 2006; Schirmer and Kotz 2006) that vocal emotional comprehension is based on a number of subprocesses. Although basic acoustic analysis is mediated by bilateral superior temporal gyri, the integration of emotionally significant acoustic cues is associated with the anterior superior temporal sulcus (STS).

In order to infer emotional judgments from these auditory perceptions and to evaluate their emotional category, frontal areas come into play. Particularly, the inferior frontal cortex (and adjacent orbitofrontal cortex) has been found activated during the late evaluation step. However, it is an open question whether these areas are also involved when emotional prosody and semantics transfer conflicting information, as suggested by some recent functional magnetic resonance imaging (fMRI) studies (Schirmer et al. 2004; Mitchell 2006). Concerning brain regions that are specifically involved in the word-level interaction of emotional prosody and semantics, Schirmer et al. (2004) found signal increases in the bilateral inferior frontal gyrus (IFG). These activations were modulated by gender indicating a higher sensitivity for emotional conflict in speech for women. No reliable sex-specific effects can be concluded though, as most studies investigating emotional prosodic effects tested variable numbers of male and female participants (Schirmer et al. 2004; Mitchell 2006).

Mitchell (2006) used an experimental setup, in which participants had to judge the type of emotional prosody. Sentences were either congruent with the semantic content, incongruent with the semantic content, or were low-pass filtered in a way that precludes comprehension of semantic meaning (“prosody-only” condition). By comparing the activation of either congruent or incongruent sentences with activity related to the prosody-only condition, no clusters in the dorsal anterior cingulate cortex (dACC) and the dorsolateral prefrontal cortex (DLPFC) were observed. Generally, both regions have been often reported in cognitive control tasks (MacDonald et al. 2000; Durston et al. 2003; Kerns et al. 2004; Egner and Hirsch 2005). Although, the results confirm that the left IFG together with the bilateral superior and middle temporal gyri and the basal ganglia are associated with decoding of emotional prosody in a conflicting semantic context, the absence of dACC and DLPFC activation leads to the assumption that monitoring of auditory emotion conflict may be processed differently compared with the processing of cognitive conflict in response selection paradigms.

The basic principle of conflict tasks is that they require participants to respond to one stimulus dimension while ignoring another conflicting dimension. Incongruence in prosody and semantics share with some conflict tasks that 2 stimulus features provide contradicting information. One feature is task relevant, and the other feature is task irrelevant. According to the dimensional overlap model (Kornblum and Stevens 2002), these kinds of conflict tasks would be classified as “Stroop-like” because their common characteristics are that they provide interfering information between 2 stimulus features as in the classic color-word Stroop task, in which participants are instructed to respond to the ink color of a presented word while ignoring the word meaning (Stroop 1935; MacLeod 1991). During incongruent trials (e.g., the word BLUE printed in red), cognitive conflict is behaviorally indicated by lower response accuracy and slower reactions. According to the conflict-monitoring hypothesis (Botvinick et al. 2004), the dACC and associated regions of the medial wall play an important role in the detection of a response conflict between simultaneously active, competing representations. When conflicting response alternatives are activated, the ACC signals the need for conflict resolution, while the DLPFC is engaged to reduce conflict in the following trials.

Studies investigating cognitive conflict usually present nonemotional information streams comprising an irrelevant as well as a relevant stimulus dimension. Only recently, there have been some attempts to study brain activity of conflict in the emotional domain (Etkin et al. 2006; Haas et al. 2006, 2007; Egner et al. 2008). Note that it is important to differentiate those studies, which examined interference effects between related affective dimension (Etkin et al. 2006; Mitchell 2006; Egner et al. 2008) from studies in which emotional information serves as a distracter (Blair et al. 2007; Luo et al. 2007; Mitchell et al. 2008). In the study of Etkin et al. (2006) as well as in their follow-up study (Egner et al. 2008), participants were presented with pictures of emotional faces. They had to decide whether the facial expression was happy or fearful while trying to ignore emotionally congruent or incongruent affective words (“HAPPY,” “FEAR”) projected on top of the faces. Although the authors found distinct neural networks in dependence of the emotionality of the distracting information, activity in the dACC was associated with conflict monitoring irrespective of the emotional content of the irrelevant information. Therefore, the authors proposed that the dACC serves as a general conflict detection system whenever there is interference between task-relevant and task-irrelevant information. In contrast, the rostral ACC (rACC) was involved in emotional conflict resolution. It was concluded that this area acts comparably to the DLPFC in conflict resolution in the cognitive domain, as the rACC is strongly connected to the amygdala and therefore serves as a modulator of emotional responsiveness. Although supporting evidence exists for the rACC to resolve conflict of stimuli capable of generating emotional reactions (Whalen et al. 1998, 2006; Etkin et al. 2006; Egner et al. 2008), further studies will have to show to what extent this activity depends upon functional connections with the amygdala. Special focus should also be given to how these studies handled and analyzed sequence effects as resolution processes usually occur on the trial following conflict (Egner 2007).

Here, we address open questions concerning the role of the dACC in conflict monitoring of emotional information as well as the emotional prosody network during the processing of incongruent information. Specifically, is the ACC really not involved in the detection of conflict between prosodic and semantic information as suggested by Mitchell (2006)? We address this question by measuring brain activity while participants judge the emotional prosody of acoustically presented sentences (happy, angry, and neutral), which either have an incongruent or congruent semantic content (e.g., “She left him standing outside the door”; negative semantic content spoken with happy prosody [incongruent] vs. angry prosody [congruent]). This was achieved by the presentation of sentence-level prosody rather than examination of word-level prosody (Schirmer et al. 2004). In contrast to Etkin et al. (2006), this task also provides socially communicative validity.

Second, we address the question whether different emotional categories are processed in different brain networks. Some recent studies have provided evidence for valence-specific processing of prosodic information (Van Lancker Sidtis et al. 2006; Rodway and Schepman 2007; Rymarczyk and Grabowska 2007). We sought to achieve this by dissociating brain activation of 2 emotional categories (happy, angry).

As mentioned above, superior parts of the temporal lobe have been associated with early processes of emotional prosodic processing (Schirmer and Kotz 2006). There are consistent findings that relate the superior temporal gyrus (STG) to the analysis of spectrotemporal information (Hickok and Poeppel 2007). We hypothesize that STG increases in activation in response to emotionally spoken sentences compared with neutral sentences. Based on recent neuroimaging findings, we also predict a rightward lateralization during prosodic processing (Kotz et al. 2003; Mitchell et al. 2003; Grandjean et al. 2005; Belin 2006; Ethofer et al. 2006; Beaucousin et al. 2007; Wiethoff et al. 2008).

Furthermore, a link between prosodic speech perception and subcortical activations is frequently described in imaging (Kotz et al. 2003; Mitchell 2006) and patient studies (Blonder et al. 1989; Cancelliere and Kertesz 1990; Paulmann et al. 2008). Patients with basal ganglia impairment often suffer from aprosodic syndromes. Therefore, we expect that processing and resolution of emotional prosodic conflict will occur with involvement of the basal ganglia, presumably dorsal striatal regions, which have previously been reported (Kotz et al. 2003; Mitchell 2006).

Materials and Methods

Participants

Twenty right-handed, normal-hearing participants (10 female) were recruited from the university student population of the University of Magdeburg; the age ranged from 22 to 35 years (mean age 24.9; standard deviation, SD = 3.7). All participants were native German speakers, and none reported any current or past serious medical or neurological condition. They received an MR safety screening and gave informed consent prior to the experiment. The protocol was approved by the local ethics committees of the University of Magdeburg and the Medical School Hannover.

Auditory Paradigm

The stimulus material consisted of 96 German sentences that were spoken by a professional actress. All stimuli were taped with a digital audio tape recorder, digitized at a 16-bit/44.1-kHz sampling rate, and were rated according to their valence (Kotz et al. 2003; Kotz and Paulmann 2007). The sentences were divided into 3 categories of semantic content: thirty-two sentences had a positive semantic content (e.g., “Sie hat ihre Prüfung bestanden.”; “She has passed her exam.”), a negative semantic content (e.g., “Sie hat ihm das Herz gebrochen.”; “She broke his heart.”), or a neutral semantic content (“Sie hat das Buch gelesen.”; “She read the book.”). Each sentence was recorded 3 times: spoken with positive (happy/P), negative (anger/N), or neutral (X) emotional prosody. This resulted in a total of 288 auditory stimuli distributed over 9 experimental conditions (see Fig. 1) (XX, XN, XP, NN, NX, NP, PP, PX, PN; first letter: prosody; second letter: semantic content). Thus, in one-third of the sentences, semantic content and emotional prosody were incongruent. The sentences had an average duration of about 1.6 s. Behavioral ratings were acquired in order to assure that participants correctly identified sentences as neutral, happy, or angry. Participants responded to prosody by pressing 1 of 3 buttons on a custom-built response box with their right hand as fast and correctly as possible. Half of the participants responded with the right-hand index finger to positive valence and with their right-hand ring finger to negative valence; the other half had the reverse finger assignment, whereas all of the subjects had to press with the right-hand middle finger in response to neutral valence. Auditory stimuli were delivered to participants using a high-frequency shielded transducer system. This transmission system included a piezoelectric loudspeaker enabling the transmission of strong sound pressure levels (∼105 dB) with excellent attenuation characteristics. These loudspeakers were embedded in tightly occlusive MR-compatible headphones allowing unimpeded conduction of the stimulus. Participants wore noise protection earplugs to provide additional noise attenuation. Auditory stimulation was adjusted during a short training session at comfortable listening level, which was exactly identical for all subjects. After the scanning, all participants reported an optimal sound quality and an optimal understanding of all sentences.

Figure 1.

Activation maps of incongruence- and valence-related activity.

Figure 1.

Activation maps of incongruence- and valence-related activity.

Magnetic Resonance Imaging (MRI) Data Acquisition

Images were acquired employing a standard head coil with a Siemens Trio 3-T scanner (Erlangen, Germany) at the ZENIT (Zentrum für neurowissenschaftliche Innovation und Technologie, University Clinic of Magdeburg, Germany). Functional images were recorded axially along the anterior commissure-posterior commissure plane (AC–PC) plane with a T2*-weighted gradient-echo echoplanar imaging (EPI) sequence (repetition time TR = 3500 ms, echo time TE = 30 ms, flip angle = 90°, field of view (FoV) 192 × 192 mm2, matrix 64 × 64, 3 × 3 ×5 mm in-plane resolution) with 18 slices of 4-mm thickness interspersed with 1-mm in-between gaps (7 slices extended ventrally from the AC–PC plane). Slices were acquired in an interleaved manner. One session was recorded with 2 short breaks of stimulation (2 × 20 s) during which scanning continued, resulting in a total of 658 volumes. All participants were instructed to keep still during the pauses of stimulation and to shut their eyes throughout the session. The first 3 volumes were discarded to allow for magnetic saturation effects. For structural image acquisition, we used a T1-weighted sequence in the same orientation as the functional sequence to provide detailed anatomical images aligned to the functional scans (192 1-mm slices, TR 2500 ms, TE 4.77 ms, and FoV of 256 mm). High-resolution structural images were also acquired for the purpose of cross-subject registration.

The experiment applied an event-related design comprising a mean interstimulus interval of 7000 ms with additional jitter randomly chosen from a distribution ranging from 0 to 2 s. Forty-eight null events were included in the design.

Data Analysis

Functional MRI data were analyzed with SPM5 (Welcome Department of Cognitive Neurology, London). As the slices of each volume were not acquired simultaneously, a timing correction procedure was used. All volumes were realigned to the 10th volume, unwarped to remove variance caused by movement-by-field-inhomogeneity interactions, normalized to a standard EPI template, and smoothed with a Gaussian kernel of 8-mm full-width at half-maximum to account for anatomical differences between subjects and to allow statistical inference using Gaussian Random Field theory. For the first level, data, were high-pass filtered (128 s), and an autoregressive function (AR-1) was employed to estimate for temporal autocorrelation in the data and to correct the degrees of freedom accordingly. Trials were modeled as stick functions in SPM at the end of each sentence. The sentence structure was arranged in a way that the identification of the emotional semantic content could not be completed before the end of the sentence. Even if prosodic cues provided antecedent information, conflict effects between emotional semantic content and prosody did not occur before both information types have been processed and emotional valence was detected. Regressors of event-related blood oxygen level–dependent (BOLD) responses were modeled in each participant for correctly identified sentences from each trial type using a standard hemodynamic response function. For incongruent trials following incongruent trials as well as for error trials (incorrectly identified sentences) we modeled separate regressors. In line with previous findings of trial sequence effects (Gratton et al. 1992; Kerns et al. 2004; Ullsperger et al. 2005; Etkin et al. 2006; Egner 2007), we separated incongruent trials with regard to their occurrence in the experiment. These studies have demonstrated that the degree to which task-irrelevant information interferes with task-relevant information processing depends upon the order of trial sequence. Following the definitions of Etkin et al. (2006), we separated incongruent trials preceded by a congruent trial and considered them as events during which their neural activity reflects conflict monitoring, and therefore high conflict and low regulation.

Incongruent trials preceded by another incongruent trial were considered as events during which their neural activity reflects conflict resolution, and therefore low conflict and high regulation. There were only 9 of the latter trials. We modeled these high-regulation trials as a separate regressor, but did not subject them to any further analyses due to the low number of events. The 6 realignment parameters (3× translations, 3× rotations) were also included as nuisance variables in the design matrix. Subsequently, to test hypotheses about regionally specific condition effects, the estimates were compared using linear contrasts. The resulting set of voxel values for each contrast constituted and SPM {T} map. Random effects analysis was the performed using t-test analysis across the participants on the contrast images of interest.

For our a priori regions of interest (ROIs), we used a statistical threshold of P < 0.001 uncorrected for multiple comparisons and 10 voxels spatial extent, which protects against false-positive results and provides an equivalent correction for multiple comparisons (Forman et al. 1995). Our ROIs comprised the right and left superior and middle temporal cortex; medial, middle, and inferior prefrontal cortex; and the anterior cingulate cortex because these areas have been associated with either emotional prosody processing or conflict monitoring in previous imaging studies (e.g., Kotz et al. 2003; Schirmer and Kotz 2006; Egner et al. 2008). In order to report only strong effects, we used the same threshold of P < 0.001 uncorrected for multiple comparisons and 10 voxel spatial extent for our search volume in the caudate nuclei despite of the known difficulty to detect subgyral activations compared with cortical activations (Phelps et al. 2004; Etkin et al. 2006). Anatomical ROIs were marked using the Automated Anatomical Labeling software implemented in the WFU pickatlas (Tzourio-Mazoyer et al. 2002; Maldjian et al. 2003). To validate participants’ correct and erroneous responses, we conducted a t-test, in which we compared brain activation maps for all false against all correct responses. A very conservative statistical threshold was applied (False Discovery Rate, FDR-corrected P < 0.01, k > 300) due to the fact that there were on average 20 error trials per participant. The FDR correction (Genovese et al. 2002) controls the expected proportion of the rejected hypotheses that are falsely rejected. Coordinates of activated voxels were converted from MNI to Talairach space with the mni2tal-transform developed by Matthew Brett (www.mrc- cbu.cam.ac.uk/Imaging/mnispace.html).

Subsequently, the Talairach and Tournoux atlas was used to label activation clusters (Talairach and Tournoux 1988). Nevertheless, original, unconverted MNI coordinates are reported in the activation tables for reasons of accuracy (Chau and McIntosh 2005). The description of the methods is consistent with the guidelines suggested by Poldrack et al. (2008).

Results

During the fMRI experiment, participants' accuracy in correctly identifying the emotional prosody of sentences was high in all conditions (repeated measures analysis of variance with factors congruence and valence, all P > 0.32). The overall error rates were smaller than 2% (congruent: 1.9% [standard error of the mean, SEM 0.5]; incongruent: 1.6% [SEM 0.3]). As participants were able to quickly identify prosody, that is, before the end of a given sentence, but conflict was not induced before they had heard the complete sentence and identified both prosody and semantics, the present design does not provide a behavioral measure for conflict. In order to acquire such a measure, we conducted a second experiment with the same set of stimuli. Twenty participants (mean age 24.9; SD = 3.7; range 22–35 years; and 10 females) were instructed to rate the emotional semantic content of the sentences rather than emotional prosody. Behavioral data indicate strong conflict effects between emotional semantic content and emotional prosody. There was an significant conflict-induced slowing effect of reaction times for the incongruent conditions (NP + PN) compared with the congruent conditions (NN + PP) of 63 ms (t(19) = 3.7; P < 0.001). Subjects made significantly more errors during incongruent trials compared with congruent trials (t(19) = 2.8; P < 0.01). We could thus show that the incongruent sentences reliably induce emotional interference.

As is listed in Table 1, cerebral responses obtained for prosodic sentences as compared with neutrally spoken sentences (NX + PX > XX) yielded a bilateral network of hemodynamic responses in posterior to anterior STG and STS. Similarly, we identified brain regions associated with the processing of emotional sentence meaning relative to neutral sentences in the left superior/middle temporal cortex (XN + XP > XX).

Table 1

Brain regions associated with significant BOLD signal increases for our main contrasts regarding incongruence and valence effects

Regions Right/left Brodmann's area Z score local maximum Cluster size (Voxels) MNI coordinates
 
X Y Z 
Incongruent > congruent prosody [(NP + PN) > (NN + PP)] 
    ACC 32 3.99 56 16 44 
    STG 22 3.53 92 48 −24 −8 
    Middle temporal gyrus 21 3.30 12 58 −12 −10 
[PN > (NP + NN)] 
    Middle temporal gyrus 22 3.88 51 −54 −40 
    IFG 47 3.52 43 −54 20 −4 
    IFG 47 3.52 42 −40 26 −16 
[NP > (PN + PP)] 
    Caudate nucleus NA 4.42 174 −20 10 22 
    Thalamus NA 4.28 322 −2 −18 16 
[(NX + PX) > XX] 
    STG 22 4.69 764 −56 −46 
    STG 21/22 3.82 157 60 −8 −4 
[(XN + XP) > XX] 
    Middle temporal gyrus 22 4.68 189 −50 −44 
Regions Right/left Brodmann's area Z score local maximum Cluster size (Voxels) MNI coordinates
 
X Y Z 
Incongruent > congruent prosody [(NP + PN) > (NN + PP)] 
    ACC 32 3.99 56 16 44 
    STG 22 3.53 92 48 −24 −8 
    Middle temporal gyrus 21 3.30 12 58 −12 −10 
[PN > (NP + NN)] 
    Middle temporal gyrus 22 3.88 51 −54 −40 
    IFG 47 3.52 43 −54 20 −4 
    IFG 47 3.52 42 −40 26 −16 
[NP > (PN + PP)] 
    Caudate nucleus NA 4.42 174 −20 10 22 
    Thalamus NA 4.28 322 −2 −18 16 
[(NX + PX) > XX] 
    STG 22 4.69 764 −56 −46 
    STG 21/22 3.82 157 60 −8 −4 
[(XN + XP) > XX] 
    Middle temporal gyrus 22 4.68 189 −50 −44 

In order to detect voxels that show elevated responses to incongruent emotional prosody compared with congruent emotional prosody, we created the contrast (NP + PN) > (NN + PP) (Fig. 1). We report activity in the right anterior cingulate cortex and in the right middle STS. In order to obtain valence-specific brain activity irrespective of conflict-related effects, we contrasted 2 PN > (NP + NN) and 2 NP > (PN + PP), respectively. These contrasts revealed left inferior frontal (BA 47) and left mid STG/STS clusters related to the processing of positive emotional prosody. Activation related to the processing of angry emotional prosody was found in the thalamus and the left caudate nucleus. The time courses of the ROIs can be seen in Figure 2.

Figure 2.

Time courses for the selected ROIs. Conflict-related effects are most prominent in the right ACC ROI. Contrast estimates in arbitrary units (y-axis); TR after sentence offset (x-axis).

Figure 2.

Time courses for the selected ROIs. Conflict-related effects are most prominent in the right ACC ROI. Contrast estimates in arbitrary units (y-axis); TR after sentence offset (x-axis).

Compared with sentences, which were correctly identified as spoken with happy, angry, or neutral prosody, error trials elicited the largest clusters of brain responses in the medial prefrontal cortex (BA 6/8) as well as in bilateral insular and IFG (BA 47) (Fig. 3, Table 2).

Table 2

Brain regions specifically activated during error processing

Regions Right/left Brodmann's area Z score local maximum Cluster size (Voxels) MNI coordinates
 
X Y Z 
Error-related activity (errors > correct trials) 
    Medial prefrontal cortex 8/6 4.80 1,828 26 48 
    Insula/IFG 13/47 4.76 948 −30 26 
    IFG 47 4.75 1,001 42 16 −10 
    Inferior parietal lobule 40 4.38 509 42 −38 46 
    Posterior cingulate cortex/precuneus 31/7 4.27 676 −10 −28 44 
    Inferior parietal lobule 40 4.26 388 −46 −34 42 
Regions Right/left Brodmann's area Z score local maximum Cluster size (Voxels) MNI coordinates
 
X Y Z 
Error-related activity (errors > correct trials) 
    Medial prefrontal cortex 8/6 4.80 1,828 26 48 
    Insula/IFG 13/47 4.76 948 −30 26 
    IFG 47 4.75 1,001 42 16 −10 
    Inferior parietal lobule 40 4.38 509 42 −38 46 
    Posterior cingulate cortex/precuneus 31/7 4.27 676 −10 −28 44 
    Inferior parietal lobule 40 4.26 388 −46 −34 42 
Figure 3.

Activation map for error-related effects.

Figure 3.

Activation map for error-related effects.

Discussion

In the current study, we used fMRI to examine the hypothesis that the dACC plays a prominent role during the monitoring of emotional conflict. Young healthy participants listened to spoken emotional or neutral sentences that were either incongruent or congruent. This should allow disentangling brain areas associated with monitoring conflict between emotional prosody and emotional semantic content. A variety of fMRI studies has demonstrated that voice-sensitive regions in the associative auditory cortex in the middle part of the STG (mid STG) adjacent to the STS revealed a BOLD signal increase in response to happy and angry prosody as compared with neutral intonations (Kotz et al. 2003; Mitchell et al. 2003; Grandjean et al. 2005; Belin 2006; Ethofer et al. 2006; Kotz et al. 2006; Beaucousin et al. 2007; Wiethoff et al. 2008). Activations related to emotional prosody seem to be independent of whether the participants were instructed to attend to it (Ethofer et al. 2006; Beaucousin et al. 2007) or to any other stimulus dimension (Grandjean et al. 2005). These data speak for a bottom-up mechanism relying mainly on stimulus features. Our data are in line with these assumptions, revealing bilateral clusters in the posterior STG/STS for the comparison of sentences with emotional intonation and neutral (not interfering) semantic content compared with sentences, in which both features were neutral.

In accordance with our hypothesis and with the existing literature on cognitive and emotional conflict paradigms, we found dorsal ACC activity related to the detection of interfering information between happy intonation and negative semantic content or angry intonation and positive semantic content, respectively (see also Schirmer et al. 2004). However, in contrast to Mitchell (2006), our data suggest that the brain handles conflict monitoring in the context of a particular emotional processing type in a similar way compared with other types of emotional or nonemotional conflicts (Botvinick et al. 2004; Ridderinkhof et al. 2004; Ullsperger and von Cramon 2004; Etkin et al. 2006; Egner et al. 2008). This view is supported by recent results by Egner et al. (2008). They found that the dACC increases its BOLD response in the visual domain whenever a conflict between a relevant and an irrelevant stimulus feature is detected. Most importantly, this occurred irrespective of whether the irrelevant information was emotional or nonemotional.

Furthermore, it is arguable whether the prosody-only condition can serve as a neutral baseline condition. During these events, participants may automatically try to comprehend the semantic meaning of a sentence, despite filtering of the sentences. Previous evidence indicates that even if there is no meaning in a stimulus, participants attempt to generate meaning and screen for putative intentions (Brüne 2005; Kotz et al. 2006). It is difficult to see how these processes should be experimentally controlled, and for this reason may be confounded in the study of Mitchell (2006). In a similar vein, in a classical color-word Stroop task (Stroop 1935), a “neutral” condition would be if one would turn letters upside down or filter them in a way that they cannot be read. Unlike the analysis procedure applied in Mitchell (2006), most studies on cognitive conflict have contrasted incongruent versus congruent conditions (e.g., Ullsperger and von Cramon 2001; Fan et al. 2003), unless one is interested in finding activation patterns associated with monitoring of stimulus feature–related processing regardless of whether a conflict is induced or not (Milham and Banich 2005).

Observed right hemispheric clusters in the superior and middle temporal cortex related to conflict processing indicate modulation when incongruent prosody and semantics are processed. These areas have been associated with early processing of suprasegmental cues of language intonations (Schirmer and Kotz 2006). Although one may interpret the finding that the detection of conflicting stimulus dimensions leads to an amplification of emotional prosody processing in these regions, future studies have to clarify whether control mechanisms modulate in an upward or downward fashion. Lateralization of activation clusters to the right side support the evidence of a right-sided network of prosody processing (Ross and Monnot 2008); although it has also been proposed that both hemispheres participate equally in interference resolution (Chiarello and Maxfiled 1996; Grimshaw 1998). One may assume that this lateralization effect indeed reflects a characteristic of emotional prosody processing in the human brain.

There is evidence suggesting that the functional network dedicated to the processing of prosodic information may consist of both cortical and subcortical structures (e.g., Kotz et al. 2003). With regard to the processing of angry prosodic information while ignoring positive semantic content, our results reveal activation clusters in the thalamus and the left caudate nucleus. We suggest distinct functional neuroanatomical networks for the processing of anger in the voice. There have been attempts to examine possible interactions of emotion and attention, especially for anger (Sander et al. 2005) that emphasize the importance of these subcortical structures. Anatomically, the caudate is strongly connected to the prefrontal cortex through a series of parallel loops projecting from the cortex to both input and output nuclei of the basal ganglia. Further pathways comprise the thalamus, which project back to the cortex (Alexander et al. 1986; Utter and Basso 2008). It could be demonstrated that the detection of emotional signals in the environment, which are potentially survival related, are processed independently from voluntary attention (Wambacq et al. 2004; Grandjean et al. 2005; Vuilleumier 2005). This seems to be crucial as the behavioral dimension of anger, most frequently referred to as aggression, may introduce a harmful situation (Weiner 1995; Cox and Harrison 2008). The processing of negative prosodic information (anger) may indicate a possible threat, which requires different neuronal routes. The basal ganglia seem to play an important part in this functional process. A proposed mechanism for this is provided by models of emotion recognition, in which an emotional state is inferred by mental simulation, that is, by generating similar states in oneself (Goldman and Sripada 2005). Thus, functional impairment of the basal ganglia leads to a decrease of emotion recognition abilities. This hypothesis is corroborated by recent findings in Parkinson's disease (Lawrence et al. 2007). In a recent patient study (Rymarczyk and Grabowska 2007), a close connection between valence type and the location of the brain lesion has been claimed. Patients with frontal damage are impaired in the recognition of happy prosody, whereas patients with subcortical lesions show impaired comprehension of angry prosody. Moreover, Dara et al. (2008) found in an emotional classification task that patients with Parkinson's disease and known impairment of basal ganglia have difficulties categorizing emotional prosody compared with healthy subjects. Interestingly; this impairment is mainly restricted to the recognition of negative displays of emotion highlighting a critical involvement of this region for negative stimuli. An important functional role of the basal ganglia has been suggested in various contexts regarding aprosodic syndromes in neurological disorders (Ross et al. 1988; Blonder et al. 1989), lesion studies (Cancelliere and Kertesz 1990; Starkstein et al. 1994; Ross et al. 1997; Breitenstein et al. 1998; Karow et al. 2001; Sidtis et al. 2006; Van Lancker Sidtis et al. 2006; Paulmann et al. 2008) or in an fMRI study, in which lateralization effects were the focus of the investigation (Kotz et al. 2003). However, until now, the role of the basal ganglia during prosodic processing has been seen mainly independent of emotional valence (Cancelliere and Kertesz 1990; Kotz et al. 2003). An involvement of subcortical structures is reasonable as even in lower animals, communication systems are represented similarly in nonneocortical areas (Jürgens 1998; Lieberman 2002). The incongruence of both stimulus features may be another factor increasing the significance of emotional processing. In accordance with our results, an involvement of thalamus and caudate nucleus in ambiguity resolution during language processing has recently been suggested (Ketteler et al. 2008).

Our data suggest distinct neural pathways for happy and angry prosody when presented in incongruent emotional semantic context. This evidence possibly reflects functionally different ways to handle positive and negative information with regard to potential “threats” for the participant. Concerning valence specificity, we found that the processing of happy prosody while ignoring negative semantic content engaged a left-sided neural network of middle temporal and inferior frontal areas, which have been previously associated with the processing of prosodic information per se (Kotz et al. 2003; Ethofer et al. 2006; Wildgruber et al. 2006). Interestingly, peak activation coordinates of the left IFG found in our study (−40, 26, −16) and IFG activation maxima found in the study of Mitchell (2006) (−45, 26, −14) lie close together. Research suggests an involvement of IFG in attitudinal ambivalence during coactivation of positive and negative information (Cunningham et al. 2003; Cunningham and Zelazo 2007). Against the background of studies investigating the functional role of the IFG, our findings may reflect the increasing effort to process incongruence between prosodic and semantic information, as this region has been associated with various paradigms that required participants to discriminate aspects of language (Burton et al. 2000; Giraud et al. 2001; Poldrack et al. 2001; Kotz et al. 2003), or music (Maess et al. 2001).

Concerning the comprehension of the semantic content of the sentences, one may raise the objection that the incongruity of the 2 stimulus dimensions could be absent in some trials. Although all sentences were carefully evaluated beforehand (see commentary in Materials and Methods section), participants may interpret emotional content according to their own experience. Hence, we cannot be sure that all expressions were judged according to the experimental condition they belonged to. If this were the case, no conflict effect would have been induced, and the activation should have been comparable with sentences where semantic content was interpreted according to the experimental instruction. Because we did not administer postscan stimulus ratings, we cannot rule out possible effects of this nature, and thus recommend the incorporation of such a measure in future studies of emotional denotation.

Further examinations are under way to test whether this valence specificity is influenced by attentional demands of the task (Kotz SA, Schröder C, Szymanowski F, Heinze H-J, Dengler R, Wittfoth M, in preparation). In other words, are different brain areas engaged if emotional semantic content has to be ignored or if emotional prosody has to be ignored?

Our starting point for the present study was the assumption that the mechanism related to the processing of conflicting emotional dimensions are similar to those involved in cognitive conflict. Recently, there has been increasing interest in understanding the effect of error trials in interference tasks. Thus, we conducted a posthoc analysis, in which we contrasted brain activation patterns of all incorrect responses to correct responses. We did this to find differences or similarities of error processing in the context of emotional or cognitive conflict, respectively. Our findings of an error-related network comprising the posterior medial prefrontal cortex extending into adjacent anterior cingulate areas together with bilateral inferior frontal and insula as well as posterior cingulate and inferior parietal lobule activity are in line with data of the existing error-monitoring literature (Kiehl et al. 2000; Braver et al. 2001; Menon et al. 2001; Ullsperger and von Cramon 2001, 2004; Garavan et al 2003; Hester et al. 2004). The present results demonstrate that error processing of emotional stimuli recruits brain regions that are largely comparable with the error network for nonemotional stimuli suggested in previous work (for a review, see Taylor et al. 2007).

Different theoretical attempts have been made to account for the hemodynamic responses during error processing (Botvinick et al. 2001; Holroyd and Coles 2002; Taylor et al. 2007). According to the reinforcement learning theory (Holroyd et al. 2005), the role of medial prefrontal regions is hypothesized to act as an error-detection system reflecting activity in the midbrain dopamine neurons, which indicate the absence of an anticipated reward. Concerning anterior insular activity, it has recently been suggested that this region is specifically associated with the awareness of error commission (Klein et al. 2007); in the broadest sense, the anterior insulae together with the adjacent inferior frontal gyri are thought to belong to a network responsible for implementation of goal-directed task sets (Dosenbach et al. 2006).

Given the similar set of regions recruited during error processing in an emotional and nonemotional context, it is reasonable to assume that the underlying mechanisms are also comparable and independent of stimulus type.

One potentially informative approach for future research is to determine whether interindividual differences modulate error processing following emotional stimuli because temperament and personality style have been identified to contribute to variance in error processing in studies of cognitive conflict (Luu et al. 2000; Hajcak et al. 2003, 2004; Hester et al. 2004).

In conclusion, our data propose a conflict-monitoring mechanism during emotional prosody and emotional semantic processing, which is similar to conflict monitoring during cognitive interference tasks. This is the first study reporting valence-related effects in the context of emotional conflict processing, as we found specific conflict-related networks for the processing of happy and angry intonations. Although there have been a number of reports of valence effects for facial affect (Jansari et al. 2000; Van Strien and van Beek 2000; Rodway et al. 2003), our results speak against the view that these effects are restricted to facial affect but need to be extended to vocal emotional processing. Based on our results, which provide evidence that the basal ganglia play an important role in the processing of angry prosody, we would encourage patient research on emotional conflict processing including patient groups that have known functional or structural alterations in these brain regions (e.g., Parkinson's disease, Huntington's disease). Furthermore, both the increase of negative attitude found in depressed patients and a greater bias toward threat-related stimuli observed in patients with anxiety disorders are thought to modulate the processing of emotional conflict (Haas et al. 2007; Fales et al. 2008; Mitterschiffthaler et al. 2008; Reinholdt-Dunne et al. 2009). As our stimulus set comprised of spoken sentences that are socially and communicatively valid, such emotional stimuli are also well suited for the investigation of patients with mood disorders.

Funding

The Deutsche Forschungsgemeinschaft (FOR 499 to M.W., C.S., R.D., and S.A.K.).

The authors wish to thank Denise Göttert for recruiting participants and conducting the measurements, as well as all the participants of this study for their motivation and patience. Conflict of Interest: None declared.

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