FEATURE-LINKED SYNCHRONIZATION OF THALAMIC RELAY CELL FIRING INDUCED BY FEEDBACK FROM THE VISUAL-CORTEX

THE function of the massive feedback projection from visual cortex to its thalamic relay nucleus(1,2) has so far eluded any clear overview. This feedback exerts a range of effects(3-6), including an increase in the inhibition elicited by moving contours(7,8), but the functional l ogic of the direct connections to the thalamic cells that relay the re tinal input to the cortex(9-11) remains largely unknown. In contrast t o its thalamic nucleus, the visual cortex is characterized by cells th at are strongly sensitive to the orientation of moving contours. Here we report that when driven by moving oriented visual stimuli the corti cal feedback induces correlated firing in relay cells. This cortically induced correlation of relay cell activity produces coherent firing i n those groups of relay cells with receptive field alignments appropri ate to signalling the particular orientation of the moving contour to the cortex. Synchronization of relay cell firing means that they will elicit temporally overlapping excitatory postsynaptic potentials in th eir cortical target cells, thus increasing the chance that the cortica l cells will fire. Effectively this increases the gain of the input fo r feature-linked events detected by the cortex. We propose that this f eedback loop serves to lock or focus the appropriate circuitry onto th e stimulus feature.