Object orientation agnosia: A failure to find the axis?

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. In this article, we describe a 57-year-old man with probable Alzheimer's disease who shows the same dissociation. Neuroimaging findings indicated marked hypometabolism in the posterior cortical regions , particularly the postero-superior parietal lobes. Clinically, the patient had good object recognition accompanied by severely impaired spatial abili ties. The experimental investigations comprised a variety of tasks in which he identified misoriented objects, evaluated the orientation of single obj ects, or discriminated the orientation of Simultaneously presented items. R esults revealed that his object recognition was independent of orientation and was largely mediated by salient features. With respect to orientation j udgements, the patient displayed a profound inability to judge the orientat ion of nonupright objects, but remarkably intact (though largely implicit) knowledge of the upright orientation. Strikingly, his orientation judgement s were also more accurate for upside-down objects than for other orientatio ns (i.e., 90 degrees). We interpret these results as evidence that judgemen ts about object orientation are facilitated when the orientation of the pri ncipal axis of the object matches that of an internal representation. We pr opose that the inability to determine other orientations may be due to the failure of an "axis-finding" mechanism implemented in the posterior parieta l lobes, that translates between object-centered and eye-centered coordinat es appropriate for guiding visual scanning.