The influence of the angle of gaze upon the excitability of the light-sensitive neurons of the posterior parietal cortex.

The responses of parietal visual neurons are markedly increased during attentive fixation, as compared to those evoked in relaxed wakefulness, an effect specific for directed attention and unrelated to putative differences in the general level of arousal. Those responses are also strongly influenced by the angle of gaze, an effect observed only during directed visual attention. The change in response is smoothly graded along a meridian for about one-half the neuron population; the average spatial gradient from maximum to minimum is 78% response for a 20 degrees shift in eye position. No lateral preference was observed. For the remaining half, responses were either maximal or minimal for fixations dead ahead, and changes occurred with deviations in any direction. Angle of gaze effects were observed for neurons with foveal as well as eccentrically located receptive fields, all of which were organized in retinotopic not spatial coordinates. Control experiments showed that the effect was not produced by changes in visual background with changes in the angle of gaze, nor to changes in fixation distance, nor to variations in the intensity of stimuli viewed from different angles. The effect depends upon the position of the eye in the orbit, but is unlikely due to a direct central action of changes in nonretinal orbital afferent activity at different angles of gaze, for the effect was rarely observed with changes in the angle of gaze during relaxed wakefulness without directed visual attention. The evidence supports the interpretation that the effect is produced by a central influence of the systems controlling directed visual attention and the angle of gaze upon those linking the retinae to the parietal lobe.