Figure 418

Face recognition for normal and inverted faces in normals and in a prosopagnosic patient (LH). Data from Farah (1994a).

left hemisphere, particularly lesions encompassing the angular gyrus in the posterior region of the parietal lobe" (Gazzaniga et al., 1998, pp. 202-203).

Third, it is assumed within the theory that reading and object recognition both involve analytic processing. Thus, it is predicted that patients with alexia (who have problems with analytic processing) should be impaired in their object recognition. This contrasts with the conventional view that patients with "pure" alexia have impairments only to reading abilities. This issue was studied by Behrmann, Nelson, and Sekuler (1998) in six patients who seemd to have "pure" alexia. Their key finding was that five out of six patients with this condition were significantly slower than normal participants to name visually complex pictures. These findings are in line with the prediction from Farah's theory.

Evaluation

There is reasonable evidence from the research of Farah (1990, 1994) and elsewhere to suggest that the processes typically involved in face recognition differ somewhat from those involved in object recognition and reading. The two-process model describes some of the major similarities and differences in processing across these three types of stimuli.

On the negative side, Farah's approach is at a very general level that incorporates various oversimplifications. For example, Farah argued that faces are processed holistically, but there is evidence of a left-hemisphere system that is involved in processing faces more analytically in terms of their features (Parkin & Williamson, 1986). As Humphreys and Riddoch (1993) pointed out, the case of HJA (discussed earlier) provides evidence against Farah's theory. HJA can read common words well, but is extremely poor at recognising faces, suggesting that he has problems with holistic processing. However, when asked on the object decision test to decide whether objects are real or artificial, he performed better when they were presented as silhouettes rather than as line drawings. This last finding suggests that HJA has reasonably good ability to process holistically, which is difficult to handle within Farah's theoretical approach.

Finally, Farah's failure to distinguish between apperceptive and associative agnosia seems ill advised. For example, consider the case of HO, who had had herpes simplex encephalitis. His performance was essentially perfect on the unusual views test (see earlier) and he performed well on the object decision test (Steward, Parkin, & Hunkin, 1992). However, he could name only 50% of objects on a naming task, and he did not know the functions of most objects. HO's problems are clearly related to associative agnosia rather than to apperceptive agnosia.

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