Sensory Intigration

CASE STUDIES

JW is a relatively young man, in his early forties, who, despite many preserved cognitive abilities, fails to recognize many common objects. In August 1992, JW suffered a severe cardiac event while exercising and was subsequently anoxic. A computed tomography (CT) scan revealed multiple hypodensities in both occipital lobes with minor hypodensities in his right parietal lobe. Although JW has normal visual acuity as well as intact color and motion perception, Behrmann and colleagues have shown that he recognizes approximately 20% of black and white line drawings and a slightly higher percentage of color pictures. He is almost totally unable to recognize photographs of famous people. He is poor at copying simple line drawings presented to him, at matching rectangles and squares of various dimensions, at simple shape detection (e.g., deciding that an "X" is present among a background of visual noise), and even at detecting symmetry in a visual image. Despite these impairments, he is able to recognize objects well from tactile/haptic input and from definitions that are read to him. These findings suggest that his long term knowledge of objects is preserved. This is further confirmed by his ability to generate visual images in his "mind's eye" and to describe those in detail. Needless to say, this impairment significantly limits his ability to interact with objects and his world. Whereas JW was the owner of a hardware computer company (and had a master's degree in computer science), currently he works as a volunteer and provides instruction on computer use to people who are blind.

CK, like JW, is impaired at recognizing objects and has been studied extensively by Behrmann, Moscovitch, and Winocur. CK sustained brain damage in a motor vehicle accident in 1988; he was struck on the head by the side mirror of a truck while he was jogging. Except for a hint of bilateral thinning in the occipito-temporal region, no obvious circumscribed lesion is revealed on magnetic resonance imaging (MRI) or CT scan. This may not be surprising given that his lesion was sustained via a closed head injury which often results in shearing of axons or more microscopic neuronal damage. Despite his deficits, CK functions well in his life; he has a responsible managerial job and makes use of sophisticated technology that allows him to translate written text into auditory output.

When asked to identify line drawings, CK misre-cognized a candle as a salt shaker, a tennis racquet as a fencer's mask, and a dart as a feather duster, presumably because of the feathers on the end. As Figure Patient JW's copies of simple line drawings. illustrated by these examples, CK, like JW, is clearly not blind. However, despite his intact visual acuity, he fails to recognize even familiar and common visually presented objects. This deficit holds irrespective of whether the objects are drawn in black and white on a piece of paper or whether they are shown in slides or even as real three dimensional objects, although the addition of information such as color and surface texture does assist recognition to some extent.

CK, like JW, can also use tactile/haptic information to recognize objects; he was perfectly able to recognize a padlock and a paper clip by touch alone. CK can also provide detailed definitions for an object whose name is presented to him verbally; for example, he defined a pipe as "a long cylindrical hollow object to convey liquid or gas" and a card of matches as "a cardboard container containing long sticks or matches which are struck against cordite." These definitions clearly demonstrate that his deficit is not attributable to a failure to name objects nor a loss of semantic knowledge.

CK is unable to read and, although he writes flawlessly, he cannot read his own writing presented to him at a later point in time. CK's hobbies have also been affected; he is no longer able to design complex configurations of his large plastic soldier collection or visually differentiate airplanes, a domain in which he had rather extensive knowledge premorbidly.

BACKGROUND

Despite the behavioral differences between JW and CK, they have a dramatic deficit: They are unable to recognize even common, familiar objects a disorder termed "agnosia" by Sigmund Freud (coined from the Greek "without knowledge"). Visual agnosia is a disorder of recognition, in which a person cannot arrive at the meaning of some or all categories of previously known visual stimuli, despite normal or near-normal visual perception and intact alertness, intelligence, and language. Despite the visual recognition problems associated with agnosia, there is normal recognition of objects through modalities other than vision (touch, auditory, and verbal definitions or description of their function), which suggests that the deficit is not simply a difficulty in retrieving names or in accessing the necessary semantic information. Visual recognition has been more extensively studied than recognition in other modalities, although similar deficits have been observed in patients with auditory (auditory agnosia) or tactile (tactile agnosia) deficits.

The traditional view of agnosia as a specific disorder of recognition has undergone considerable challenge in the past, with critics contending that all visual agnosias can be explained by a subtle alteration in perceptual functions likely accompanied by a generalized intellectual deterioration. Despite this early skepticism, there is now widespread acceptance of this disorder as a legitimate entity and detailed case studies have been concerned with characterizing both the underlying mechanisms that give rise to this disorder and the overt behaviors.

Lissauer was the first to classify visual object agnosia into two broad categories: apperceptive "mindblind-ness" and associative mindblindness. These impairments were evaluated by requiring patients to describe the formal features of a pattern, reproduce it by drawing, and recognize it among similar alternatives. Using Lissauer's classifications, a person with apperceptive agnosia is assumed to be impaired at constructing a perceptual representation from vision and subsequently is unable to copy, match, or identify a drawing. In contrast, a person with associative agnosia is one who cannot use the derived perceptual representation to access stored knowledge of the object's functions and associations but is able to copy and match the drawing even though he or she is unable to identify it.

Recent neuropsychological accounts by Humphreys and Riddoch as well as by Warrington and colleagues and computational accounts such as that of Marr