The influence of attentive fixation upon the excitability of the light-sensitive neurons of the posterior parietal cortex.

We describe the effect of behavioral state upon the excitability of light-sensitive (Ls) neurons of the inferior parietal lobule, area 7a, studied in waking monkeys. The responses of parietal LS neurons to visual stimuli are facilitated during the state of attentive fixation of a target light as compared to their responses to physically and retinotopically identical test stimuli delivered during the eye pauses of alert wakefulness. Seventy percent of the neurons tested (n = 55) showed significant increments in responses in the state of attentive fixation; the median value of the increments was 3.5 times. Only 4 of the 55 cells examined completely showed the reverse relation. Three sets of control experiments were done. The facilitation occurred when the responses evoked during the trials of a reaction task with attentive fixation of a target were compared with those evoked by identical stimuli delivered to the same retinotopic locations at the end of each intertrial interval: the facilitation of attentive fixation is not due to a shift in the general level of arousal. The facilitation occurred when the animal maintained attentive fixation of a spot of the tangent screen without a target light or when an additional light mimicking the target light was presented along with testing stimuli in the state of alert wakefulness without attentive fixation: the facilitation is not produced by a sensory-sensory interaction between target and testing lights. Finally, the facilitation was observed whether or not the test stimuli were behaviorally relevant. We conclude that the act of attentive fixation exerts a specific and powerful effect upon the excitability of the neural systems linking the retinae and the inferior parietal lobule and that the facilitation plays an important role in visually guided behavior.