Journal of Neuroscience, Vol 9, 1628-1640, Copyright © 1989 by Society for Neuroscience
The "motion-blind" patient: low-level spatial and temporal filters
RH Hess, CL Baker Jr and J Zihl
Physiological Laboratory, University of Cambridge, United Kingdom.
The "motion-blind" patient previously described by Zihl et al. (1983) was
investigated using standard psychophysical procedures with stimuli whose
spatial and temporal properties could be separately manipulated. Detection
experiments for sinewave grating stimuli of varying spatial and temporal
frequency showed sensitivity in this patient to be only slightly impaired.
Temporal integration for stimuli of varying spatiotemporal frequency
exhibited the expected space-time covariation seen in normal vision. An
examination of the suprathreshold discriminative capacity of this patient
was undertaken for spatial frequency, contrast, and temporal frequency.
Although all of these discriminative functions were impaired, those
concerning temporal frequency or velocity were dramatically reduced. No
similar loss was seen for spatial frequency discrimination for moving or
temporally varying stimuli. No measurable temporal frequency discrimination
was present above 6 Hz and no velocity discrimination above 6 degrees/sec.
Experiments involving the direction discrimination of suprathreshold
drifting gratings of arrays of random dots revealed an inability to
perceive direction of movement above a velocity of about 6 degrees/sec.
Contrast thresholds contingent on direction of motion of a drifting grating
also showed a much greater deficit than simple detection. Apparent motion
using 2-flash random dot kinematograms revealed that the residual motion
vision of this patient corresponded to the "short- range" motion process of
normal vision. This process originally defined by Braddick (1974) operates
over restricted space and time intervals. Apparent motion could only be
supported by a narrow range of intermediate spatial displacements. These
results suggest that this patient does exhibit some residual motion
perception, probably corresponding to a severely impaired "short-range"
mechanism. The patient's relatively intact ability to perform simple types
of discrimination but severe impairment of performance at making judgments
relevant to the nature of motion of the same stimuli suggests that while
the components necessary for the analysis of motion are intact their more
global associations have been disrupted. This implicates an extrastriate
locus of the brain damage. Alternative explanations for the nature of the
deficit are discussed.
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