The distribution of stimulus-evoked 14C-2-deoxyglucose (2DG) labeling in primary somatosensory cortex (SI) of monkey (Macaca fasciculans) and cat was investigated. Reconstructions of the global pattern of labeling reveal that discrete skin stimuli evoke activity within an extensive region of SI, and that the activation pattern typically consists of multiple, elongated regions of above-background labeling (“modules,” typically 0.5–1.0 mm wide, and 1–4 mm long). Evidence obtained using recently developed methods (Tommerdahl, 1989) for quantitative analysis of 2DG activity patterns is shown to be consistent with the idea (Whitsel et al., 1991) that SI modules typically are bounded by zones dominated by stimulus-evoked inhibition. The labeling pat tern within individual 2DG modules In SI of both cats and monkeys is analyzed quantitatively (in the frequency domain). Within-module spatial activation patterns are demonstrated to he periodic, consisting of radially on ent.d profiles of above-background labeling separated from each other by less strongly labeled radial profiles. The spectral characteristics of within-module 2DG la beling change systematically with location along the module's long axis: spatial frequencies htween 18 and 35 cycles/mm are prominent in the labeling that oc cupies both the middle and upper layers at central locations in the module, but are a less obvious component of the labeling in both the middle and upper layers at locations remote to the module center. Since the radially oriented periodic variation both (1) in 2DG labeling in regions of SI outside modules and (2) in optical density in images of Nissl-stained sections of SI consists pre dominantly of spatial frequencies in the range of 18–35 cycles/mm, it is concluded that the radial profiles of labeling within individual 2DG modules correspond to groupings of minicolumna distinguishable from their neighbors on the basis of labeling intensity. The findings raise the possibility that highly structured, within-mod ule spatial patterns of SI minicolumnar activation en code information about the physical properties of tactile stimuli.

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