Neural basis of a perceptual decision in the parietal cortex (area LIP) ofthe rhesus monkey

We recorded the activity of single neurons in the posterior parietal cortex (area LIP) of two rhesus monkeys while they discriminated the direction of motion in random-dot visual stimuli. The visual task was similar to a moti on discrimination task that has been used in previous investigations of mot ion-sensitive regions of the extrastriate cortex. The monkeys were trained to decide whether the direction of motion was toward one of two choice targ ets that appeared on either side of the random-dot stimulus. At the end of the trial, the monkeys reported their direction judgment by making an eye m ovement to the appropriate target. We studied neurons in LIP that exhibited spatially selective persistent activity during delayed saccadic eye moveme nt tasks. These neurons are thought to carry high-level signals appropriate for identifying salient visual targets and for guiding saccadic eye moveme nts. We arranged the motion discrimination task so that one of the choice t argets was in the LIP neuron's response field (RF) while the other target w as positioned well away from the RE During motion viewing, neurons in LIP a ltered their firing rate in a manner that predicted the saccadic eye moveme nt that the monkey would make at the end of the trial. The activity thus pr edicted the monkey's judgment of motion direction. This predictive activity began early in the motion-viewing period and became increasingly reliable as the monkey viewed the random-dot motion. The neural activity predicted t he monkey's direction judgment on both easy and difficult trials (strong an d weak motion), whether or not the judgment was correct. In addition, the t iming and magnitude of the response was affected by the strength of the mot ion signal in the stimulus. When the direction of motion was toward the RF, stronger motion led to larger neural responses earlier in the motion-viewi ng period. When motion was away from the RF, stronger motion led to greater suppression of ongoing activity. Thus the activity of single neurons in ar ea LIP reflects both the direction of an impending gaze shift and the quali ty of the sensory information that instructs such a response. The time cour se of the neural response suggests that LIP accumulates sensory signals rel evant to the selection of a target for an eye movement.