The role of vision in "visual imagery" experiments: evidence from the congenitally blind.

Abstract

SUMMARY Responses of congenitally blind subjects were compared with those of sighted subjects to test whether performance on three tasks is dependent on specifically visual processing. In Experiment 1, subjects memorized the locations of several figures on a board and then were asked to form an image of the board and mentally to scan from one figure to another. There was a strong relationship between distance and scanning time both for blind and for sighted subjects, although the response times were significantly longer for blind than for sighted subjects. Thus the images of congenitally blind subjects, like those of sighted subjects, preserve metric spatial information. Experiment 2 varied the subjective size of imaged objects by requiring subjects to form images of a object, such as a radio, alongside a object that was either very large (a car) or very small (a paper clip). Subjects then were asked to verify whether a named physical attribute such as drawer or dial was a part of the imaged target object (radio). For both blind and sighted subjects, the time taken to verify whether a physical feature of a target object was included in the image was greater when the context item was large so that the target object was subjectively small. Thus for blind as well as for sighted subjects the features of a subjectively large image are noticed or accessed faster than those of a subjectively small image. Experiment 3 tested the mnemonic consequences of forming images of objects with differing spatial relationships to each other. Subjects heard descriptions and were instructed to form images of scenes in which a target object was described in one of three relationships to the rest of the scene: spatially separated, contiguous but visually hidden or concealed, or contiguous and clearly visible. The pattern of results on an incidental cued-recall test was similar for blind and sighted subjects, with objects imaged as spatially contiguous recalled better than those that were spatially separated. Visual picturability did not affect recall. Overall recall scores did not differ for blind and sighted subjects, but sighted subjects reported forming the images significantly faster than did the blind. Thus the images formed by blind subjects were as mnemonically effective as those created by sighted subjects and the memorability of imaged scenes was equivalently affected by the spatial relationships of their components. Taken together, the three experiments demonstrate that congenitally blind adults are capable of preserving and processing spatial images in a manner very similar to that used by sighted subjects, although such processing may require slightly less time with visually mediated than with nonvisually mediated imagery. The research raises questions about definitions of imagery that are tied specifically to the visual processing system and suggests that spatial imagery processing ability need not depend on visual perceptual experience or, in fact, on any specific sensory processing modality.