Rapid fading of visual sensations in a subject with a parietal-occipital tumour.

We present psychophysical data measured for a single subject, who had a tumour in the left parieto-occipital region, located by CT scan and post-mortem study. Her vision was characterised by rapid fading during voluntary fixation of images located in the right hemifield. We describe a number of simple tests which we developed in order to study this phenomenon. The results of these tests established that significant loss of function occurs within 1 s of fixation and that the effects associated with fixation are spatially localised. We compare our subject's response characteristics with those observed in other cases of visual disturbance and we examine the possible underlying mechanisms which give rise to the rapid fading of her visual images.