Behavioral modulation of tactile responses in the rat somatosensory system

We investigated the influence of four different behavioral states on tactil e responses recorded simultaneously via arrays of microwires chronically im planted in the vibrissal representations of the rat ventral posterior media l nucleus (VPM) of the thalamus and the primary somatosensory cortex (SI). Brief (100 msec) electrical stimuli delivered via a cuff electrode to the i nfraorbital nerve yielded robust sensory responses in VPM and SI during sta tes of quiet immobility. However, significant reductions in tactile respons e magnitude and latency were observed in VPM and SI during large-amplitude, exploratory movements of the whiskers (at similar to 4-6 Hz). During small -amplitude, 7-12 Hz whisker-twitching movements, a significant reduction in SI response magnitude and an increase in VPM and SI response latencies wer e observed as well. When pairs of stimuli with interstimulus intervals,100 msec were delivered during quiet immobility, the response to the second sti mulus in the pair was reduced and occurred at a longer latency compared wit h the response to the first stimulus. In contrast, during large-amplitude w hisker movements and general motor activity, paired stimuli yielded similar sensory responses at interstimulus intervals >25 msec. These response patt erns were correlated with the amount and duration of postexcitatory firing suppression observed in VPM and SI during each of these behaviors. On the b asis of these results, we propose that sensory responses are dynamically mo dulated during active tactile exploration to optimize detection of differen t types of stimuli. During quiet immobility, the somatosensory system seems to be optimally tuned to detect the presence of single stimuli. In contras t, during whisker movements and other exploratory behaviors, the system is primed to detect the occurrence of rapid sequences of tactile stimuli, whic h are likely to be generated by multiple whisker contacts with objects duri ng exploratory activity.