MOTOR INTENTION ACTIVITY IN THE MACAQUES LATERAL INTRAPARIETAL AREA .2. CHANGES OF MOTOR PLAN

1. In the companion paper we reported that the predominant signal of t he population of neurons in the lateral intraparietal area (area LIP) of the monkey's posterior parietal cortex (PPC) encode the next intend ed saccadic eye movement during the delay period of a memory-saccade t ask. This result predicts that, should the monkey change his intention of what the next saccade will be, LIP activity should change accordin gly to reflect the new plan. We tested this prediction by training mon keys to change their saccadic plan on command and recording the activi ty of LIP neurons across plan changes. 2. We trained rhesus monkeys (M acaca mulatta) to maintain fixation on a light spot as long as this sp ot remained on. During this period we briefly presented one, two, or t hree peripheral visual stimuli in sequence, each followed by a delay ( memory period, M). After the final delay the fixation spot was extingu ished, and the monkey had to quickly make a saccade to the location of the last target to have appeared. The monkey could not predict which stimuli, nor how many, would appear on each trial. He thus had to plan a saccade to each stimulus as it appeared and change his saccade plan whenever a stimulus appeared at a different location. 3. We recorded the M period activity of 81 area LIP neurons (from 3 hemispheres of 2 monkeys) in this task. We predicted that, if a neuron's activity refle cted the monkey's planned saccade, its activity should be high while t he monkey planned a saccade in the neuron's motor field (MF), and low while the planned saccade was in the opposite direction. The activity of most of the neurons in our sample changed in accordance with our hy pothesis as the monkey's planned saccade changed. 4. In one condition the monkey was instructed by visual stimuli to change his plan from a saccade in the neuron's preferred direction to a saccade planned in th e opposite direction. In this condition activity decreased significant ly (P < 0.05) in 65 (80%) of 81 neurons tested. These neurons' activit y changed to reflect the new saccade plan even though the cue for this change was not presented in their RF. 5. As a control we randomly int erleaved, among trials requiring a plan change, trials in which the mo nkey had to formulate two consecutive plans to make a saccade in the n euron's preferred direction. The activity remained unchanged (P < 0.05 ) in 22 of 31 neurons tested (79%), indicating that the neurons contin ued to encode the same saccade plan. 6. In a variant of the task, the cue to the location of the required saccade was either a light spot or a noise burst from a loudspeaker. Of 22 neurons tested in this task, 16 (73%) showed activity changes consistent with plan changes cued by visual or auditory stimuli. 7. Alterations in the monkey's intentions, even in the absence of overt behavior, are manifested in altered LIP activity. These activity changes could be induced whether visual or au ditory cues were used to indicate the required plan changes. Most LIP neurons thus do not encode only the locations of visual stimuli, but a lso the intention to direct gaze to specific locations, independently of whether a gaze shift actually occurs.