Velocity invariance of receptive field structure in somatosensory corticalarea 3b of the alert monkey

This is the second in a series of studies of the neural representation of t actile spatial form in cortical area 3b of the alert monkey. We previously studied the spatial structure of 330 area 3b neuronal receptive fields (RFs ) on the fingerpad with random dot patterns scanned at one velocity (40 mm/ sec; DiCarlo et al., 1998). Here, we analyze the temporal structure of 84 n euronal RFs by studying their spatial structure at three scanning velocitie s (20, 40, and 80 mm/sec). As in the previous study, most RFs contained a s ingle, central, excitatory region and one or more surrounding or flanking i nhibitory regions. The mean time delay between skin stimulation and its exc itatory effect was 15.5 msec, Except for differences in mean rate, each neu ron's response and the spatial structure of its RF were essentially unaffec ted by scanning velocity. This is the expected outcome when excitatory and inhibitory effects are brief and synchronous. However, that interpretation is consistent neither with the reported timing of excitation and inhibition in somatosensory cortex nor with the third study in this series, which inv estigates the effect of scanning direction and shows that one component of inhibition lags behind excitation. We reconcile these observations by showi ng that overlapping (in-field) inhibition delayed relative to excitation ca n produce RF spatial structure that is unaffected by changes in scanning ve locity. Regardless of the mechanisms? the velocity invariance of area 3b RF structure is consistent with the velocity invariance of tactile spatial pe rception (e.g., roughness estimation and form recognition).