Project Summary For sensory systems, feedforward projections from thalamic relay cells provide the cortex with information about the external environment. The cortex, in turn, sends extensive feedback to the thalamus. The cortex thus functions both to process information supplied by the thalamus as well as to influence dynamically the transmission of thalamic input. The primary goal of the experiments presented in this proposal is to determine how the primary visual cortex (V1) and the lateral geniculate nucleus (LGN) of the thalamus interact to dynamically influence visual processing. The study involves four sets of experiments. The first major series of experiments (Specific Aim 1) will determine the functional topography of corticogeniculate feedback to the LGN and will test the hypothesis that there is a center/surround organization of feedback with a central region providing net excitation and a surround providing net suppression. The second series of experiments (Specific Aim 2) will determine the influence of corticogeniculate feedback in augmenting extraclassical surround suppression in the LGN and will reveal whether effects of feedback are specific for feedforward channels to cortex. The third series of experiments (Specific Aim 3) will test the hypothesis that corticogeniculate feedback serves to modulate retinogeniculate communication and to transform visual signals en route from retina to cortex. The final series of experiments (Specific Aim 4) will uncover the interactions between corticogeniculate feedback and visual attention and will determine whether attentional demand adjusts the dynamics of feedback effects on visual signals relayed from thalamus to cortex. Given the central importance of corticothalamic pathways for governing the excitability of thalamocortical networks, it is important that we understand the functional properties of the corticothalamic pathway, as disorders of the pathway likely underlie several illnesses affecting vision and visual processing.