Thalamic signal processing and the effects of cortical feedback

Thalamocortical circuits are critically involved in the processing and transfer of afferent sensory information from the periphery to higher levels of sensory processing in cerebral cortex. The present work focused on two inter-related aspects of signal processing in thalamocortical (TC) circuits, the nature of the thalamic signal to microcircuits in cortical layer 1V and the effects of corticothalamic (CT) feedback on it. We investigated stimulus-dependent synchronous activity in the thalamic ventral posteromedial nucleus (VPm) using local field potential (LFP) recording and simultaneous single-unit recording. We found that populations of neighboring thalamic barreloid neurons generate larger magnitude LFP responses to principal (PW) vs. adjacent (AW) whisker stimulation, to preferred vs. non-preferred movement directions, and to high vs. low velocity/acceleration deflections. Reliable and robust stimulus/response relationships were found only for the initial 1.2–7.5 ms of the thalamic LFP response, reflecting arrival of afferent information from the brainstem. Similar results were obtained using simultaneous recordings of TC cells located in the same electrophysiologically identified barreloid during on-going whisker deflections varying in the frequency domain. Stronger thalamic firing synchrony appears to reflect decreased response latency variability. Together with previous results, the findings underscore a critical role for thalamic firing synchrony in the encoding of small but rapidly changing, on-going perturbations of specific whiskers in particular directions.; Effects of CT feedback were examined by pharmacologically enhancing the activity of infragranular neurons within a region comprising a single barrel-related cortical column. We found that CT projections enhance responses in the topographically-aligned barreloids and suppress those in the immediately adjacent, non-aligned barreloids. PW deflections evoked larger cortical responses and correspondingly stronger CT-mediated facilitation of thalamic responses in topographically aligned barreloid than AW deflections. CT projections, via direct excitation and indirect, Rt-mediated inhibition, may thus selectively enhance thalamic responses to preferred cortical stimuli, sharpening thalamic receptive field tuning within an activated thalamic-cortical-thalamic loop. Preliminary data suggest further that CT feedback modulates thalamic firing synchrony and thus the saliency of the thalamic signal for the cortical barrels. The present work provides a firm basis for pursuing this issue in the future.