Eye position effects in monkey cortex. II. Pursuit and fixation related activity in posterior parietal areas
LIP and 7A
F. Bremmer, C. Distler & K.-P. Hoffmann
Journal of Neurophysiology, 77:962-977, 1997
- We studied the effect of eye position on pursuit-related discharges and activity during fixation in darkness
for neurons of monkey visual cortical areas (lateral intraparietal area) LIP and 7A. In a first step, neurons were
tested for direction-specific activity related to pursuit eye movements while the monkey tracked a moving target.
In consecutive trials the pursuit target moved in random order in one of four directions on a translucent screen.
For 39% of the neurons, located mostly in a dorsoposterior region of area LIP, as well as 42% of the neurons tested
in area 7A, a direction-specific pursuit-related activity could be found. To test whether responsiveness of these
neurons was modulated by eye position, we employed a pursuit paradigm. In this paradigm, the monkey had to track
a target that started to move in the preferred direction with constant speed from five different locations on the
screen in random order. For the majority of cells in both areas, pursuit-related activity was modulated by eye
position. Most of the neurons tested also revealed an influence of eye position on their spontaneous activity during
fixation in darkness (fixation paradigm). For the majority of cells (> 50%) recorded in both areas, two-dimensional
regression planes could be approximated significantly (P < 0.05) or nearly significantly (P < 0.1) to the
neuronal discharges observed on the fixation paradigm and pursuit paradigm. For 79% of the LIP neurons and 83%
of the 7A neurons tested in both experimental paradigms, the directions of the gradients of the regression planes
pointed into the same hemifield, suggesting a common neuronal mechanism mediating the eye position effect regardless
of the behavioral task the monkey was performing. The observed effects very much resemble the effects of eye position
on light-sensitive and saccade-related responses already described for areas LIP and 7A. Regarding also our results
observed for the middle temporal and medial superior temporal areas, it is suggested that the observed modulatory
effect of eye position on neuronal activity is a common phenomenon in the macaque visual cortical system subserving
an internal representation of the external space in a nonretinocentric frame of reference.
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