Dissociations of visual integration deficits in early DAT

Subject DB suffers from an early dementia of the Alzheimer type (DAT). MRI scans of her brain reveal a very mild generalised atrophy while an asymmetric hypoperfusion of the right occipito-parietal cortex is evident at cerebral SPECT imaging. Her neuropsychological profile is characterised by a selective and profound impairment in all visual tasks requiring the integration of local elements into a unitary percept: she is unable to report the global level of hierarchical stimuli, she does not perceive illusory contours, and cannot identify degraded and overlapping figures. We measured her thresholds for the detection of a closed chain of Gabor patches as a function of the background noise for different values of local interelement distance in the target. DB showed a consistent reduction of the integration thresholds for all the measured conditions when compared to normal subjects. However, her performance also differs from the one of a clinically very similar DAT patient, MC, at shorter interelement distances. Interestingly, MC and DB also differ in their ability to perceive biological motion. We discuss the two patients profiles in the light of a neurophysiological hypothesis assuming interactions between temporo-parietal neuromodulatory top-down signals and occipital bottom-up integration processing.