The "motion-blind" patient: low-level spatial and temporal filters

Abstract

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.