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 functions for the corticogeniculate pathway, including alteration of LGN spatial frequency selectivity and facilitation of both binocular interactions and orientation selectivity. These changes may be due to either a tonic or a phasic cortical facilitation or both. The temporal differences between each of these inputs suggests that their impact on LGN cell temporal tuning should be unique. To test this hypothesis, we reversibly blocked the visual cortex (VI) and measured the effects on several indices of the temporal properties of LGN cells, including peak frequency, bandwidth, and response phase. Macaque monkeys were anesthetized and paralyzed during single cell recording from the LGN while area VI was cryogenically deactivated. Single-cell responses were visually evoked with drifting, luminance-modulated, sine-wave gratings and discrete-Fourier analyzed. Cortical cooling produced statistically significant increases or decreases in response amplitude in 64% of cells recorded. In most cases, alterations in response amplitude occurred for stimuli that varied in spatial as well as temporal frequency. For those cells 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 bandwidth. Response phase angles for all temporal frequencies tested were 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.