Motor Intention Activity In The Macaque's Lateral Intraparietal Area. II.
Changes Of Motor Plan.
Bracewell, R. Martyn, Pietro Mazzoni, Shabtai Barash, and Richard A. Andersen.
Dept. of Brain and Cognitive Sciences, MIT, Cambridge, MA 02139, USA.
APStracts 3:0044N, 1996.
SUMMARY AND CONCLUSIONS
1. In the companion paper (Mazzoni et al. 1996a) we reported that the
predominant signal of the population of neurons in the lateral intraparietal
area (area LIP) of the monkey's posterior parietal cortex (PPC) encode the
next intended saccadic eye movement during the delay period of a memory-
saccade task. This result predicts that, should the monkey change his
intention of what the next saccade will be, LIP activity should change
accordingly to reflect the new plan. We tested this prediction by training
monkeys to change their saccadic plan on command and recording the activity of
LIP neurons across plan changes. 2. We trained rhesus monkeys ( Macaca mulatta
) to maintain fixation on a light spot as long as this spot remained on.
During this period we briefly presented one, two, or three peripheral visual
stimuli in sequence, each followed by a delay (memory period, M). After the
final delay the fixation spot was extinguished 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 reflected the
monkey's planned saccade, its activity should be high while the 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 hypothesis 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 the opposite direction. In this condition activity
decreased significantly (p<0.05) in 65 (80%) out of 81 neurons tested. These
neurons' activity 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
interleaved, among trials requiring a plan change, trials in which the monkey
had to formulate two consecutive plans to make a saccade in the neuron's
preferred direction. The activity remained unchanged (p<0.05) in 22 out of 31
neurons tested (79%), indicating that the neurons continued 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 auditory cues were used to indicate the required plan changes. Most LIP
neurons thus do not encode only the locations of visual stimuli, but also the
intention to direct gaze to specific locations, independently of whether a
gaze shift actually occurs.
Received 27 April 1995; accepted in final form 7 February 1996.
APS Manuscript Number J219-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96