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Abstract:
A dissociation between the ability to recognize misoriented
objects and to determine their orientation has been reported in a
small number of patients with vascular lesions. We describe a 57
year old man with probable Alzheimer's disease, who shows the same
dissociation. Neuroimaging findings indicated marked hypometabolism
in the postero-superior parietal lobes. Clinically, the patient had
good object recognition accompanied by severely impaired spatial
abilities. Experimental investigations comprised tasks in which he
identified misoriented objects, evaluated the orientation of single
objects, or discriminated the orientation of simultaneously
presented items. Results revealed that his object recognition was
independent of orientation and was largely mediated by salient
features. With respect to orientation judgements, the patient
displayed a profound inability to judge the orientation of
non-upright objects, but remarkably intact (though largely
implicit) knowledge of the upright orientation. Strikingly, his
orientation judgements were also more accurate for upside-down
objects than for other orientations (i.e. 90°). We interpret
these results as evidence that judgements about object orientation
are facilitated when the orientation of the principal axis of the
object matches that of an internal representation. We propose that
the inability to determine other orientations may be due to the
failure of an "axis-finding" mechanism implemented in the posterior
parietal lobes, that translates between object-centered and
eye-centered coordinates appropriate for guiding visual
scanning.
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