Binocular Facilitation and Inhibition in the Lateral Geniculate Nucleus of the Cat

The lateral geniculate nucleus (LGN) relays visual information from the retinas to the cortex, segregating input from each eye into separate laminae. The LGN receives an equally large input back from the visual cortex, whose cells are driven from both eyes. Therefore, binocular interactions in the LGN were studied by systematically varying visual stimuli known to fire cortical neurons. Binocular to monocular responses were compared by interleaving them using computer driven shutters in order to eliminate errors due to LGN cell response variability. Full statistical analysis was used to identify significant binocular facilitation and inhibition. Significantly more and stronger binocular feedback (BF) was seen with this approach than in previous studies. The vast majority of LGN cells showed both binocular facilitation and inhibition; as many as half showed BF amplitudes exceeding 50% of their typical monocular firing rate. Importantly, BF was found to be well tuned to velocity, relative retinal disparity, and sweep direction, parameters known to profoundly affect cortical firing. Multiple regions of BF were found for most cells, with the majority located near the monocular receptive fields in visual space. Regions of facilitation required zero retinal disparity twice as often as inhibition. Further, most BF reached maximum amplitudes at 6°/sec to 12°/sec. These results are a strong indication that the BF is cortical in origin. It is likely that this BF has a role in highlighting visual features on the plane of fixation. Because BF is very sensitive to parameters of motion, it is also conceivable that it is involved in interpreting the visual signals generated by eyes constantly in motion. This and other possible roles of BF are discussed.