Dynamics in primary somatosensory cortex: A role for SI in the processing of tactile information

Traditional views of primary somatosensory cortex (SI) and its role in the processing of tactile information have limited its function as a dynamic component in the somatosensory pathway. Here I present evidence that the response in SI to stimuli at a given skin site is systematically modified with changes in the stimulus parameters and displays considerable dynamics. Optical intrinsic signal (OIS) imaging was used to study the responses (in vivo) evoked by 25 Hz (flutter) vertical skin displacement stimuli to the forelimb of squirrel monkey and cat. Responses to electrical stimulation were also measured in rat sensorimotor cortical slices using OIS imaging and local field potential (LFP) recordings. Results indicate that, contrary to traditional views, the intensive but not spatial attributes of the SI response are modified by increases in stimulus amplitude. Increasing the duration of flutter stimulation evokes increases in response magnitude in cortical regions near to the maximally responding center and simultaneous decreases in surrounding cortical regions; the net effect of this is the spatial sharpening of the SI response during prolonged stimulation. The distribution of decreased absorbance in surrounding cortex was non-uniform, indicating the possibility of stronger intracortical inhibition along the proximodistal axis of the body representation. Cortical slices in the sagital and coronal planes of the rat somatosensory cortex demonstrated a similar anisotropy in the distribution and impact of GABAergic inhibition on the horizontal spread of activity, and lend support to the idea that the non-uniformity observed in vivo may contain functional relevance. Bilateral stimulation of both forelimbs demonstrated that, although input to SI has been traditionally regarded as exclusively contralateral, not only can the response to an ipsilateral stimulus be measured in SI, but when stimulation is applied bilaterally the spatiotemporal characteristics of the evoked response cannot be accounted for by the responses of either stimulus alone or by the linear summation of the pair. All of these results taken together present a strong case for the necessity of strong dynamics in SI and the role of SI as an important site of cortical information processing in the somatosensory pathway.