Target detection and movement discrimination in the blind field of hemispherectomized patients

Four hemispherectomized patients were tested with a variety of tasks designed to investigate the extent of residual vision in the blind field. The first set of studies was aimed at evaluating the ability of these subjects to detect and localize at various eccentricities three types of targets which differed in their spatiotemporal properties (stationary, flashing and moving). The subjects could detect effectively the presence of any of these stimuli in their blind field and they experienced little difficulty in discerning blank from target trials. When manual pointing was used to measure localization quality, the hemianopes made more errors in their blind field but their accuracy, when detection was correct, was comparable with that observed in their intact visual field and in that of the control subjects. In the second set of experiments, the capacity of 3 of the subjects to detect in their blind field a moving grating, as well as to discriminate between relative grating velocities and directions in their blind field and in both fields simultaneously, was assessed. Two subjects could detect the movement in their blind field, although this was in part affected by stimulus velocity. When discrimination of relative velocities in the blind field and in both fields simultaneously was evaluated, individual differences were observed. However, all subjects showed some capacity to carry out the task. Finally, none of the experimental subjects was able to discriminate the relative directions of the moving gratings. The results are discussed in terms of collicular involvement to account for the residual vision observed in the blind field of hemispherectomized subjects.