| |
Abstract:
The nature of the object recognition deficits in Alzheimer's
disease (AD) was examined by comparing AD patients' performance
across two sets of visual stimuli: 1) real objects with preexisting
semantic representations; and 2) novel objects without preexisting
semantic representations that were comparable to the real objects
in terms of featural and hierarchical complexity (i.e., Greebles).
A match-to-sample paradigm was used in which participants were
asked to judge whether each of a series of test stimuli matched an
earlier presented target. The degree of perceptual similarity
between targets and nontargets was systematically varied within
each series. Moreover, for the real objects, nontargets were either
exemplars from the same semantic category as the target, or were
exemplars from a different semantic category that shared similar
perceptual features with the target category (e.g. eyeglasses and
bike). In the real object case, impaired performance was
attributable primarily to perceptual rather than semantic
impairments. In the novel object case, impairment was most
pronounced at intermediate levels of perceptual organization, but
performance was less impaired at a higher level. Taken together
these findings suggest that AD patients display high level
perceptual impairments characterized by over-generalized perceptual
representations. Thus, the marked disruption in corticocortical
connections thought to produce degradation in semantic knowledge
representations in AD may also extend to the high-level visual
representations mediating object perception.
|
