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.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Consciousness of the first order in blindsight

At suprathreshold levels, detection and awareness of visual stimuli are typically synonymous in nonclinical populations. But following postgeniculate lesions, some patients may perform above chance in forced-choice detection paradigms, while reporting not to see the visual events presented within their blind field. This phenomenon, termed “blindsight,” is intriguing because it demonstrates a dissociation between detection and perception. It is possible, however, for a blindsight patient to have some “feeling” of the occurrence of an event without seeing per se. This is termed blindsight type II to distinguish it from the type I, defined as discrimination capability in the total absence of any acknowledged awareness. Here we report on a well-studied patient, D.B., whose blindsight capabilities have been previously documented. We have found that D.B. is capable of detecting visual patterns defined by changes in luminance (first-order gratings) and those defined by contrast modulation of textured patterns (textured gratings; second-order stimuli) while being aware of the former but reporting no awareness of the latter. We have systematically investigated the parameters that could lead to visual awareness of the patterns and show that mechanisms underlying the subjective reports of visual awareness rely primarily on low spatial frequency, first-order spatial components of the image

Improved detection following neuro-eye therapy in patients with post-geniculate brain damage

Damage to the optic radiation or the occipital cortex results in loss of vision in the contralateral visual Weld, termed partial cortical blindness or hemianopia. Previously, we have demonstrated that stimulation in the Weld defect using visual stimuli with optimal properties for blindsight detection can lead to increases in visual sensitivity within the blind Weld of a group of patients. The present study was aimed to extend the previous work by investigating the eVect of positive feedback on recovery of visual sensitivity. Patients’ abilities for detection of a range of spatial frequencies within their Weld defect were determined using a temporal two-alternative forced-choice technique, before and after a period of visual training (n = 4). Patients underwent Neuro-Eye Therapy which involved detection of temporally modulated spatial grating patches at speciWc retinal locations within their Weld defect. Three patients showed improved detection ability following visual training. Based on our previous studies, we had hypothesised that should the occipital brain lesion extend anteriorly to the thalamus, little recovery would be expected. Here, we describe one such case who showed no improvements after extensive training. The present study provides further evidence that recovery (a) can be gradual and may require a large number of training sessions (b) can be accelerated using positive feedback and (c) may be less likely to take place if the occipital damage extends anteriorly to the thalamus