PERIPHERAL SHIFT REDUCES VISUAL SENSITIVITY IN CAT GENICULATE NEURONS

The sudden displacement of the retinal image during a saccade raises t he visual threshold of human observers to foveal stimuli. The fall in visual sensitivity observed during this phenomenon, known as saccadic suppression, seems to occur very early in the visual processing chain. The lateral geniculate nucleus (LGN) is a likely locus for the multip le retinal and extraretinal interactions occurring during saccadic eye movements, therefore we used the responses of relay cells of adult ca ts to simulate a psychophysical experiment. We first measured the resp onses of X and Y relay cells (27 X and 13 Y) to central spots of optim al size and different contrasts. The spots were presented either alone or time locked with the rapid movement of a large, high-contrast peri pheral pattern, referred to as shift. We measured the percentage of tr ials on which the relay cell fired more spikes when the spot (contrast : 0.03-1.0) was present than when it was absent. In experiments with h uman observers the task was to indicate, by a keypress, which of two o therwise identical temporal intervals contained the spot. The shift re duces the sensitivity (raises the contrast threshold) of neurones in t he cat relay cells to brief, stationary targets presented to the recep tive-field center. The suppression of visual sensitivity is significan tly greater in Y cells than in X cells (average sensitivity ratios 5.6 +/- 5.4 in Y cells, 1.59 +/- 0.9 in X cells: P < 0.001, U test). The shift also reduces the sensitivity of human observers to the same targ et. This suggests that the LGN is a potential locus for the modulation of visual responses that leads to saccadic suppression.