Figure 217

What brain systems underlie blindsight? Kehler and Moscovitch (1997) discussed findings from several patients who had had an entire cerebral hemisphere removed. These patients showed evidence of blindsight for stimulus detection, stimulus localisation, form discrimination, and motion detection. These findings led Kehler and Moscovitch (1997, p. 322) to conclude: "The results. suggest that subcortical rather than cortical regions may mediate blindsight on tasks that involve these visual functions".

Fendrich, Wessinger, and Gazzaniga (1992) favoured an alternative position. According to conventional assessment, their patient had no conscious awareness of visual stimuli within a large area. However, when they used a more sensitive method, they discovered that the patient could report visual stimuli presented to certain small regions of the visual field. They concluded that their patient had preserved "islands" of function within the cortex that permitted him to show the phenomena of blindsight. However, it is unlikely that this is true of most other blindsight patients.

Another possibility is that there is a "fast" pathway that proceeds directly to V5 without passing through V1 (primary visual cortex). Evidence supporting this view was reported by ffytche, Guy, and Zeki (1995). They obtained visual event-related potentials for moving stimuli, and found that V5 became active before, or at the same time as, V1. Blindsight patients may use this pathway even if V1 is totally destroyed.

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Business Correspondence

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