Increased sensitivity after repeated stimulation of residual spatial channels in blindsight

Abstract

Lesions of the occipital cortex result in areas of cortical blindness affecting the corresponding regions of the patient's visual field. The traditional view is that, aside from some spontaneous recovery in the first few months after the damage, when acute effects have subsided the areas of blindness are absolute and permanent. It has been found, however, that within such field defects some residual visual capacities may persist in the absence of acknowledged awareness by the subject (blindsight type 1) or impaired awareness (type 2). Neuronal pathways mediating blindsight have a specific and narrow spatial and temporal bandwidth. A group of cortically blind patients (n = 12) carried out a daily detection "training" task over a 3-month period, discriminating grating visual stimuli optimally configured for blindsight from homogeneous luminance-matched stimuli. No feedback was given during the training. Assessment of training was by psychophysical measurements carried out before and after training and included detection of a range of spatial frequencies (0.5-7 cycles per degree), contrast detection at 1 cycle per degree, clinical perimetry, and subjective estimates of visual field defect. The results show that repeated stimulation by appropriate visual stimuli can result in improvements in visual sensitivities in the very depths of the field defect.

Fig. 1.

Daily performance of one patient on a detection task. Results are shown for targets presented in the training area (Top), additional training area (Middle), and area straddling the sighted area (Bottom) in the blindfield.

Fig. 2.

Discrimination (Upper) and reported awareness (Lower) of cortically blind patients for stimuli presented within the blindfield, before and after taking part in the training task. Stimuli were circular spatial gratings (6° diameter) at a range of spatial frequencies and a fixed contrast of 50%.

Fig. 3.

Discrimination (Upper) and reported awareness (Lower) of cortically blind patients for stimuli presented within the blindfield, before (filled diamonds) and after (filled squares) taking part in the training task. Stimuli were circular spatial gratings of 1 cycle per degree (6° diameter) at a range of contrasts.

Fig. 4.

Discrimination (Upper) and reported awareness (Lower) of cortically blind patients before (filled diamonds) and after (filled squares) training, for stimuli presented at a control location in the blindfield that was not stimulated during training. Stimuli were circular spatial gratings (6° diameter) at a range of spatial frequencies and a fixed contrast of 50%.

Fig. 5.

Increase in visual field sensitivity in the central 30° and 10° of the affected hemifield after training, as measured by using a Humphrey visual field analyzer (Upper), and reduction in blindfield reported by using subjective fields (Lower).