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Journal of Neuroscience, Vol 7, 154-176, Copyright © 1987 by Society for Neuroscience
Functional properties of parietal visual neurons: mechanisms of directionality along a single axis
BC Motter, MA Steinmetz, CJ Duffy and VB Mountcastle
The directional properties of parietal visual neurons (PVNs) were examined
using the method of single-neuron analysis in waking monkeys. PVN
properties were determined with passive visual stimuli as the animal
executed a simple detection task. Parietal area PG was studied in 10
hemispheres of 6 male Macaca mulatta. Each class of parietal neurons was
identified in PG: the fixation, projection, visual, and oculomotor neurons;
613 PVNs were identified, 323 were studied quantitatively, and 188 were
studied with one or more of the protocols described. The receptive fields
of PVNs are commonly large and bilateral, and at the limit some may fill
the visual field; for many, the central zone of the visual field is spared
when the fields are determined by stimuli that enter from the periphery and
transit meridians. The receptive fields vary with the behavioral state, the
angle of gaze, and the parameters of the stimuli used to determine them.
PVNs are sensitive to stimulus movement but relatively insensitive to
stimulus speed; many respond over a speed range of 5 degrees-500
degrees/sec. Stimulus-response relations may be incremental or decremental
with increasing speed or show maxima or minima in the midrange of speed,
but the response variation over the full range is rarely greater than 2:1.
The directional preferences of PVNs with bilateral receptive fields are
opponently organized; the preferred directions point either inward toward
or outward away from the central line of gaze along the 4 meridians tested,
which were equally spaced in the circular dimension of the visual field.
The mechanism of the axis directionality of PVNs was studied using
conditioning-test paradigms. They revealed a feed-forward inhibition
preceding a stimulus, an effect that extends from the leading edge of the
stimulus for 10 degrees-20 degrees in front of the moving stimulus and
lasts for several hundred milliseconds. A double-Gaussian model of
superimposed but unequal excitatory and inhibitory effects suffices to
explain the present observations. It places demand upon the projection of
functional properties from the contralateral hemisphere or from the
ipsilateral prestriate areas that project upon PG over multistaged pathways
and minimal demands upon intracortical processing mechanisms.
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