Motor Intention Activity In The Macaque's Lateral Intraparietal Area. I.
Dissociation Of Motor Plan From Sensory Memory.
Mazzoni, Pietro, R. Martyn Bracewell, Shabtai Barash, and Richard A. Andersen.
Dept. of Brain and Cognitive Sciences, MIT, Cambridge, MA 02139, USA.
APStracts 3:0045N, 1996.
SUMMARY AND CONCLUSIONS
1. The lateral intraparietal area (area LIP) of the monkey's posterior
parietal cortex (PPC) contains neurons that are active during saccadic eye
movements. These neurons' activity includes visual and saccade-related
components (Mountcastle et al. 1975; Andersen et al. 1987). These responses
are spatially tuned and the location of a neuron's visual receptive field (RF)
relative to the fovea generally overlaps its preferred saccade amplitude and
direction (i.e. its motor field, MF) (Barash et al. 1991a). When a delay is
imposed between the presentation of a visual stimulus and a saccade made to
its location (memory saccade task), many LIP neurons maintain elevated
activity during the delay (memory activity, M), which appeared to encode the
metrics of the next intended saccadic eye movement (Gnadt and Andersen 1988).
Recent studies have alternatively suggested that LIP neurons encode the
locations of visual stimuli (Goldberg et al. 1990; Duhamel et al. 1992)
regardless of where the animal intends to look. We examined whether the M
activity of LIP neurons specifically encodes movement intention or the
locations of recent visual stimuli, or a combination of both. In the
accompanying study (Bracewell et al. 1996) we investigated whether the
intended-movement activity reflects changes in motor plan. 2. We trained
monkeys ( Macaca mulatta ) to memorize the locations of two visual stimuli and
plan a sequence of two saccades, one to each remembered target, as we recorded
the activity of single LIP neurons. Two targets were flashed briefly while the
monkey maintained fixation; after a delay the fixation point was extinguished
and the monkey made two saccades in sequence to each target's remembered
location, in the order in which the targets were presented. This "delayed
double saccade" (DDS) paradigm allowed us to dissociate the location of visual
stimulation from the direction of the planned saccade and thus distinguish
neuronal activity related to the target's location from activity related to
the saccade plan. By imposing a delay we eliminated the confounding effect of
any phasic responses coincident with the appearance of the stimulus and with
the saccade. 3. We arranged the two visual stimuli so that in one set of
conditions at least the first one was in the neuron's visual receptive field
(RF), and thus the first saccade was in the neuron's motor field (MF). M
activity should be high in these conditions according to both the sensory
memory and motor plan hypotheses. In another set of conditions the second
stimulus appeared in the RF but the first one was presented outside the RF,
instructing the monkey to plan the first saccade away from the neuron's MF. If
the M activity encodes the motor plan it should be low in these conditions,
reflecting the plan for the first saccade (away from the MF). If it is a
sensory trace of the stimulus' location it should be high, reflecting
stimulation of the RF by the second target. 4. We tested 49 LIP neurons (in 3
hemispheres of two monkeys) with M activity on the DDS task. Of these, 38
(77%) had M activity related to the next intended saccade. They were active in
the delay period, as expected, if the first saccade was in their preferred
direction. They were less active or silent if the next saccade was not in
their preferred direction, even when the second stimulus appeared in their RF.
5. The M activity of 8 (16%) of the remaining neurons specifically encoded the
location of the most recent visual stimulus. Their firing rate during the
delay reflected stimulation of the RF independently of the saccade being
planned. The remaining 3 neurons had M activity that did not consistently
encode either the next saccade or the stimulus' location. 6. We also recorded
the activity of a subset of neurons (n=38) in a condition in which no stimulus
appeared in a neuron's RF but the second saccade was in the neuron's MF. In
this case the majority of neurons tested (23/38, 60%) became active in the
period between the first and second saccade, even if neither stimulus had
appeared in their RF. Moreover, this activity appeared only after the first
saccade had started in all but two of these neurons. In general the neurons'
responses thus did not anticipate the saccades in the DDS task. 7. The
majority of LIP neurons have activity related to the next intended saccade.
Cells in LIP also carry a signal coding the memory of the location of the
sensory stimulus, although at the population level this signal is less
prominent than the intended movement signal in the DDS task. The intended
movement signal is not simply an attention signal for a spatial location
because it was reduced or absent when a location required attention but not a
saccade to it. The posterior parietal cortex is thus not only involved in
sensory and attentional processing, but also participates in the formulation
of movement plans.
Received 27 April 1995; accepted in final form 7 February 1996.
APS Manuscript Number J220-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 20 March 96