The processing of kinetically defined boundaries in the cortical
motion area MT of the macaque monkey.
Marcar, V.L., D. K. Xiao, S.E. Raiguel, H. Maes and G.A. Orban, K.U. Leuven.
Laboratorium voor Neuro- en Psychofysiologie, Campus Gasthuisberg, B-3000
Leuven, Belgium.
APStracts 2:0150N, 1995.
SUMMARY AND CONCLUSIONS
1) Electrophysiological recordings of 68 cells in the middle temporal area MT
were made in paralysed and anaesethetised macaque monkeys. 2) Testing with our
kinetic boundary stimuli always occurred under optimised conditions. To this
end the preferred direction, speed, stimulus position and stimulus size of
each cell was determined by quantitative tests. 3) The orientation selectivity
to stationary, luminance contrast edges served as a reference by which a
response to kinetic boundaries could be compared. We found cells in area MT to
be less selective to the orientation of luminance contrast stimuli than to the
direction of motion. We confirmed the presence of neurons with preferred
orientation aligned with their preferred direction (Albright, 1984). 4) The
responses to kinetic edges defined by motion vectors moving in opposite
directions, kinetic gratings with motion vectors in opposite directions,
kinetic edges containing coherent motion and a stationary complementary field
or coherent motion and a complementary field containing visual dynamic noise
were compared. Kinetic boundaries were generated so that the motion vectors
moved either parallel or orthogonal to the orientation of the discontinuity.
For a cell to be considered as responding to the orientation a kinetic
boundary it must exhibit the same preferred orientation when the local motion
vectors changed from parallel to orthogonal to the orientation of the kinetic
boundary. 5) All cells in area MT changed their preferred orientation by 90O
when the coherent motion vectors changed from moving parallel to moving
orthogonal to the boundary. This was the case independent of the types of
kinetic boundary tested. We concluded that cells in area MT appear to respond
to the motion vector over their CRF only and were unable to code the
orientation of the kinetic boundary. 6) In those cells exhibiting an
antagonistic surround we examined the ability of the cell to code the position
of a kinetic boundary. None of the cells tested signalled the position of a
kinetic boundary. The side preference of the stimulus of the cells changed
from left to right as the motion vectors in the stimulus reversed. This
indicated that the cells were only selective for the motion vectors present
over their CRF. 7) We found that the directional sensitivity of cells in area
MT remained unaltered by the presence of additional motion vectors within the
CRF. This suggests that cells in area MT extract a specific motion vector from
a spatial configuration of vectors. We further found that the response level
of cells in area MT could be related directly to the amount of motion in the
non-preferred direction present in the stimulus.
Received 8 May 1994; accepted in final form 8 May 1995.
APS Manuscript Number J596-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 18 May 1995.