Spatial and temporal structure of receptive fields in primate somatosensory area 3b: Effects of stimulus scanning direction and orientation

This is the third in a series of studies of the neural representation of ta ctile spatial form in somatosensory cortical area 3b of the alert monkey. W e previously studied the spatial structure of >350 fingerpad receptive fiel ds (RFs) with random-dot patterns scanned in one direction (DiCarlo et al., 1998) and at varying velocities (DiCarlo and Johnson, 1999). Those studies showed that area 3b RFs have a wide range of spatial structures that are v irtually unaffected by changes in scanning velocity. In this study, 62 area 3b neurons were studied with three to eight scanning directions (58 with f our or more directions). The data from all three studies are described accu rately by an RF model with three components: (1) a single, central excitato ry region of short duration, (2) one or more inhibitory regions, also of sh ort duration, that are adjacent to and nearly synchronous with the excitati on, and (3) a region of inhibition that overlaps the excitation partially o r totally and is temporally delayed with respect to the first two component s. The mean correlation between the observed RFs and the RFs predicted by t his three-component model was 0.81. The three-component RFs also predicted orientation sensitivity and preferred orientation to a scanned bar accurate ly. The orientation sensitivity was determined most strongly by the intensi ty of the coincident RF inhibition in relation to the excitation. Both orie ntation sensitivity and this ratio were stronger in the supragranular and i nfragranular layers than in layer IV.