Brain, Vol. 121, No. 8, 1565-1588,
August 1, 1998
© 1998 Oxford University Press
Blindsight and shape perception: deficit of visual consciousness or of visual function?
AbstractTwo people with homonymous right hemianopias were tested on a number of measures of non-conscious and conscious perception of shape in the blind field. Experiment 1 examined preparatory manual adjustments in grasping objects. Both subjects performed well above chance not only in three-dimensional location but also in preforming the hand to the shape, size and orientation of objects. In Experiment 2 single upper- case letters were briefly exposed in the blind field, and subjects made forced choices among 6 alternatives in the sighted field. Performance improved over blocks of trials and was above chance, but not dramatically. In Experiment 3 single upper-case words were briefly presented in the blind field, and subjects chose which of two words exposed after in the intact field was semantically closer. In Experiment 4 subjects had to give the meaning of single ambiguous words (e.g. BANK) presented both visually in the intact field and auditorily. Each ambiguous word was preceded by a single upper-case word briefly presented in the blind field, biasing each meaning on different blocks of trials (e.g. MONEY and RIVER). In Experiment 3, although results were in the appropriate direction, they were not consistently well above chance. By contrast, in Experiment 4 both subjects were consistently semantically biased to a high degree by words in the blind field. Experiments 2, 3 and 4 taken together suggest that indirect techniques (priming) are more sensitive to showing effects of non- conscious perception than direct ones (forced-choice). More importantly the experiments indicate that not only orientation but curvature, structural descriptions of component strokes and spatial ordering of letters are registered non-consciously in the blind field. Experiment 5 examined after-images in the blind and sighted fields, showing veridical conscious perception of shape in the blind field provided it was accompanied by a shape in the sighted field which together formed a good Gestalt. Experiment 6 showed conscious perception of illusory contours spanning the hemifields induced by Kanizsa figures. The experiments suggest that aspects of shape are much better perceived in blindsight than previously thought, that this is independent of their use in motor control, that the main deficit in blindsight is one of consciousness, and that the loss of conscious vision in the blind field is far from total. The effects and their relationship to those in other neuropsychological deficits suggest an intimate link between perceptual consciousness, attention and object perception.
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