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Haptic perceptual illusions

Edouard Gentaz and Yvette Hatwell

Perceptual illusions refer to systematically oriented errors in the perception of figures or scenes, and these errors are observed in almost all people. For centuries, they have been called ‘opto-geometric illusions’ because it was thought that they concerned only visual perception. Although visual errors are relatively rare, comments and questions about visual illusions are found as early as in the Greek and Roman literature. In scientific psychology, the theoretical and practical problems raised by these deformations have been intensively studied since the end of the 19th Century, and the elements of the figure inducing each error are now identified. However, there is no general theory explaining all the visual illusions. Instead, each figure must be analysed in order to determine the specific processes leading to the error.
The question as to whether analogous illusions occur in the haptic modality was asked only since the 1930s by the gestalt psychologists. According to these researchers, systematic errors result from the general functioning of the nervous system and particularly from the interactions (called ‘field effects’) between different parts of the figure. Therefore, because the same processing rules are at work in all perceptual modalities, illusions analogous to visual ones must be present in touch. Indeed, this was observed by Revesz [1] and Bean [2] who found haptically almost all visual illusions. However, further studies with better methodological controls produced some contradictory results, which will be discussed here.
The theoretical interest of the studies of haptic illusions is two-fold [3]. First, they confront purely visual explanations of visual illusions with non modality-specific theories. They can also answer the question as to whether the perceptual processes implemented in tactual perception are similar or not to those implemented in visual perception (for reviews, see [4–7]). Thus, not finding a visual illusion in touch is an argument in favour of specific haptic perceptual processes. However, observing the same illusion in vision and touch does not indicate whether the error is a result of similar and/or specific perceptual processes. To do that, it is necessary to know if the factors responsible for the presence of the tactual illusion are identical to those affecting the same phenomenon in vision. An affirmative answer brings arguments in favour of similar visual and haptic processes, whereas a negative answer favours specific visual and haptic functioning. In this latter case, the problem would be to identify these modality-specific processes.
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