Behavioral Modulation of Tactile Responses in the Rat Somatosensory System

We investigated the influence of four different behavioral states on tactile responses recorded simultaneously via arrays of microwires chronically implanted in the vibrissal representations of the rat ventral posterior medial nucleus (VPM) of the thalamus and the primary somatosensory cortex (SI). Brief (100 μsec) electrical stimuli delivered via a cuff electrode to the infraorbital nerve yielded robust sensory responses in VPM and SI during states of quiet immobility. However, significant reductions in tactile response magnitude and latency were observed in VPM and SI during large-amplitude, exploratory movements of the whiskers (at ∼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 were observed as well. When pairs of stimuli with interstimulus intervals <100 msec were delivered during quiet immobility, the response to the second stimulus in the pair was reduced and occurred at a longer latency compared with the response to the first stimulus. In contrast, during large-amplitude whisker movements and general motor activity, paired stimuli yielded similar sensory responses at interstimulus intervals >25 msec. These response patterns were correlated with the amount and duration of postexcitatory firing suppression observed in VPM and SI during each of these behaviors. On the basis of these results, we propose that sensory responses are dynamically modulated during active tactile exploration to optimize detection of different types of stimuli. During quiet immobility, the somatosensory system seems to be optimally tuned to detect the presence of single stimuli. In contrast, during whisker movements and other exploratory behaviors, the system is primed to detect the occurrence of rapid sequences of tactile stimuli, which are likely to be generated by multiple whisker contacts with objects during exploratory activity.