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BlindsightA Case Study and Implications$
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L. Weiskrantz

Print publication date: 1990

Print ISBN-13: 9780198521921

Published to Oxford Scholarship Online: January 2008

DOI: 10.1093/acprof:oso/9780198521921.001.0001

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The natural blind-spot (optic disc) within the scotoma

The natural blind-spot (optic disc) within the scotoma

(p.90) 10 The natural blind-spot (optic disc) within the scotoma

L. Weiskrantz

Oxford University Press

The optic disc provides ready-made control for determining the limits of intra-ocular diffusion. As the size of the natural and absolutely blind area is fixed and known (5° × 7°), a target light that cannot be detected on the disc sets the measurable limits to diffusion. The rationale of the experience was to compare detection of a small target (33′ diameter) when it fell on the objectively blind disc with detection at a number of neighbouring locations within the scotoma. The result was that he was at chance when the stimulus fell on the optic disc, but well above chance at other loci. With one exception, he always consistently and insistently reported that he was ‘just guessing’. The one exceptional region was relatively small; his report was that ‘I feel something coming in’ similar to his reports with some other abrupt-onset stimuli. It is interesting that he did not distinguish at all between his lack of experience when the target fell on the optic disc or when it fell in the scotoma in the ‘dead’ field. In all cases he felt or saw nothing and said he was ‘just guessing’. The test is a very stringent one for demonstrating the insufficiency of stray light as an explanation of D. B.'s ability to detect stimuli in his scotoma.

Keywords:   optic disc, blind spot, intra-ocular diffusion, chance performance, scotoma, stray light

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