Sensory Compensation


    You have probably heard the opinion that blind people develop better senses of touch and hearing to compensate for their lack of vision. If you consult with knowledgeable persons you may be told that this is just a myth. For example, the class notes (PSYC 442Y5Y) of Professor Sandra Trehub at the University of Toronto recently included the following statement: "Despite widespread beliefs that blind people have better hearing (and more sensitive touch) than sighted people (the myth of sensory compensation), there is no evidence that this is the case. In fact, vision helps us integrate information across modalities (e.g., providing information about what we are hearing or touching). Thus, the absence of vision may lead to poor performance on many non-visual tasks. Nevertheless, experience and practice can allow blind children and adults to use their intact senses effectively so that they seem to have greater sensitivity in hearing and touch than sighted individuals" (http://www.erin.utoronto.ca/~w3psy/courses99w/442/blindfeb29.html). When I most recently tried to re-access this page, I found that this page and her home page returned 404 (File Not Found) error messages, but she is still listed as on the faculty. Please do pay special attention to the last sentence in this quote -- blind persons may learn to use their intact senses more effectively, even though those senses are no more sensitive than those of sighted persons.

    You can find similar statements at many places on the Internet. For example, at A New Perspective the following question was posted: "Do blind people have better hearing and touch than people who can see?" The posted answer was "No," with an explanation that blind people do not have super-sensitive touch or hearing but that since they use their senses of touch and hearing more than do sighted people they become more 'aware' of them. It was also suggested that sighted persons could achieve such greater awareness too, if they just practiced use of those senses.

    There is, however, recent evidence that persons blind from birth may, in fact, have different perceptual capabilities which enable them to compensate, somewhat, for their lack of vision. Note that I used the term "perceptual," not "sensory." When we study sensation, we study the means by which organisms become aware of those energies and substances their bodies and brains can detect. For example, when I take a sip of a good, single-malt Scotch, what is it that causes me to have that pleasant experience as I roll the Scotch around my mouth and then swallow it? Without doubt, there are several sensory systems involved in this experience, involving sensory organs in my mouth and in my nose. When we study perception, we study how sensory information is interpreted. It is generally assumed that this interpretation takes place in the brain rather than in the peripheral sensory organs, but, in fact, the sensory organs themselves already be extracting "information" from the raw sensory data before delivering it to the brain. That said, I should add that the distinction between "sensation" and "perception" is fuzzy at its boundary.

    N. Lessard, M. Paré, F. Lepore, and M. Lassonde (Early-Blind Human Subjects Localize Sound Sources Better Than Sighted Subjects, Nature, 1998, 395, 278-280, abstracted at PubMed) found that early-blind subjects were better able than sighted subjects to locate the source of sounds when using only one ear. The authors concluded that their results provided convincing evidence that blind individuals can compensate for their lack of vision by having a better than normal ability to localize sounds. More recent research suggests that Lessard et al. have made a stronger conclusion than is warranted by the available evidence.

    M. P. Zwiers, A. J. Van Opstal, and J. R. M. Cruysberg (A Spatial Hearing Deficit in Early-Blind Humans, The Journal of Neuroscience, 2001, 21, RC142) note that two opposing views have been advanced with regards how blindness affects the other senses. One view is that blindness may lead to a compensatory sharpening of the other senses. The other view is that early blindness may lead to deficient development of the perceptual and cognitive mechanisms involved in utilizing the other senses.

    The authors cite several studies done on nonhuman animals that show that the ability to localize sounds in space is compromised in animals that are blinded early in life. Apparently visual feedback is necessary properly to learn how to locate sounds in space, to produce an auditory mental map. Studies in cats and ferrets, however, have indicated that early loss of vision is associated with improved ability to localize sounds. The authors cited one study that indicated that blind humans do not differ from sighted humans in the ability to localize sounds, and three studies that indicated that blind humans are, under certain conditions (such as being able to use only one ear, not both, when trying to localize the sound), better able to localize sounds than are sighted humans.

    The research conducted by Zwiers et al. indicated that there is little difference in the ability of blind versus sighted persons in a simple auditory scene, that is, when there is no background noise, just the auditory signal that is to be located. However, when tested in a more realistic situation, where the target sound was embedded in a background of noise, sighted subjects were better able to locate the target than were blind subjects. The authors conclude that their results are consistent with the position that visual feedback is necessary for full development of the ability to localize sounds and not consistent with the position that compensatory mechanisms involving other senses can replace the role of visual feedback.

    Another interested read is Cross-Modal Plasticity: Where and How? (Daphne Bavelier and Helen J. Neville, Nature Reviews | Neuroscience, 2002, 3, 443 - 452). This article reviews the results of research comparing the brains of individuals who have a particular sense with those who do not have that sense, with emphasis on differences in brain areas known to be associated with the other senses -- for example, differences between sighted and blinded rats in the area of the brain that processes somatosensory information.

    With respect to differences in sensory sensitivity, Bavelier and Neville note that research has indicated no differences between blind and sighted persons with respect to hearing, but that on some tasks involving hearing or touch blind persons out-perform sighted persons. Likewise, on some tasks that involve vision or touch deaf persons outperform persons with normal hearing ability. So, what brain differences have been associated with the loss of one sense and what parts of the brain are involved? Bavelier and Neville note that the changes typically take place in areas of the brain that are normally involved in processing cross-modality input, that is, input from more than one sensory system. In the individual lacking one sense, these areas may become reorganized such that neurons that normally would serve the missing sense are recruited into serving a different sense. There is also some evidence that even neurons in the primary sensory areas, areas which typically serve only one particular sense, may also be recruited to serve a different sense. Please read the article for much more detail on the nature of the changes possibly involved in reorganization.

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This page most recently revised on the 22nd of November, 2013.