THE CHARACTERISTICS OF RESIDUAL MOTION PERCEPTION IN THE HEMIFIELD CONTRALATERAL TO LATERAL OCCIPITAL LESIONS IN HUMANS

Unilateral damage to the lateral occipital region in humans can give rise to impaired motion perception in the contralateral visual field [Plant et al. (1993), Brain, 116, 1303-1335]. We report the following characteristics of the residual vision. (i) Spatial acuity and spatial frequency discrimination are not affected. (ii) Contrast thresholds for direction-of-motion (DOM) discrimination of luminance modulated (LMod) sine-wave gratings is unaffected regardless of drift temporal frequency and the effect of spatial and temporal frequency on drifting/counterphase sensitivity ratios is normal (providing further evidence that cortical directionally selective mechanisms are intact). (iii) Contrast thresholds for DOM discrimination of contrast modulated (CMod) gratings are elevated by a log unit across a range of drift velocities. (iv) The residual motion perception shows neither a directional nor a naso-temporal asymmetry. (v) Weber fractions for velocity discrimination are shown in one patient in whom this measurement was carried out, to be elevated by around a factor of three but the functions relating velocity discrimination to stimulus contrast and to the velocity of the standard are parallel in the affected and unaffected hemifields. (vi) Weber fractions for temporal frequency discrimination using counterphase modulated gratings are also elevated. We conclude that the degraded motion perception is mediated by mechanisms which have similar contrast and temporal properties to those subserving normal motion perception. Mechanisms subserving DOM discrimination of LMod gratings may be spared because they are more widely distributed in extra-striate cortex than mechanisms subserving non-Fourier (second-order) motion perception or velocity discrimination. The anomaly resembles that described in some recent animal studies of impaired motion perception after extra-striate cortical damage.