THE INFLUENCE OF THE VISUAL-CORTEX ON THE SPATIOTEMPORAL RESPONSE PROPERTIES OF LATERAL GENICULATE-NUCLEUS CELLS

Previous studies of the cortical input to the mammalian dorsal lateral geniculate nucleus (LGN) have identified a number of possible functio ns for the corticogeniculate pathway, including alteration of LGN spat ial frequency selectivity and facilitation of both binocular interacti ons and orientation selectivity. These changes may be due to either a tonic or a phasic cortical facilitation or both. The temporal differen ces between each of these inputs suggests that their impact on LGN cel l temporal tuning should be unique. To test this hypothesis, we revers ibly blocked the visual cortex (V1) and measured the effects on severa l indices of the temporal properties of LGN cells, including peak freq uency, bandwidth, and response phase. Macaque monkeys were anesthetize d and paralyzed during single eel recording from the LGN while area V1 was cryogenically deactivated. Single-cell responses were visually ev oked with drifting, luminance-modulated, sine-wave gratings and discre te-Fourier analyzed. Cortical cooling produced statistically significa nt increases or decreases in response amplitude in 64% of cells record ed. In most cases, alterations in response amplitude occurred for stim uli that varied in spatial as well as temporal frequency. For those ce lls influenced by changes in stimulus temporal frequency, the majority showed changes over a broad range of frequencies. A minority of cells showed changes in either peak temporal tuning or temporal frequency b andwidth. Response phase angles for all temporal frequencies tested we re unaffected by cortical cooling. Overall, these results suggest that the cortical input may alter the temporal response properties of LGN cells, perhaps by tonic, but not exclusively excitatory, corticofugal influences.