The detection and evaluation of residual vision in cortical blindness
Some cortically blind patients have the ability to detect and discriminate certain visual stimuli presented within their field defect; a phenomenon known as ‘Blindsight’ (Weiskrantz 1986). Although a wide-range of residual abilities has been investigated in patients with striate cortex damage, to date no systematic method has been used to characterise the residual capabilities of blindsight cases, which allows comparison and generalisation across cases. In the current thesis, a common metric was introduced to evaluate the fundamental characteristics of residual vision in a group of patients. Psychophysical investigation of the effect of spatial frequency, stimulus size, stimulus contrast, stimulus duration and temporal frequency revealed the importance of specific stimulus parameters in eliciting above chance discrimination within the field defect. Residual vision was characterised by sensitivity to a narrow range o flow spatial frequencies (<4c/°) and temporal frequencies between 5 and 20Hz. Stimulus size and contrast were also important for successful detection. Pupil grating responses (PGRs) to sinusoidal gratings presented at a range of spatial frequencies also demonstrated low spatial frequency sensitivity (<3.5c/°). Two patients with no significant performance in the psychophysical studies also had no significant blind field PGRs. The two negative cases sustained occipital brain damage extending more anteriorly compared to the positive cases of blindsight. Finally, in a series of investigations, detection and form discrimination were investigated in DB, the first documented blindsight patient, and revealed an improvement in residual visual abilities.