Agnosia for global patterns: When the cross-talk between grouping and visual selective attention failS

We present a single case study of a 72-year-old mild AD patient, MC, with a highly specific deficit in deriving the global pattern of visual stimuli, in the absence of visuospatial neglect. MC shows a specific difficulty in segregating overlapping figures, in object decision, and in all neuropsychological tasks requiring perception of a global structure from local cues, such as the Street Completion Test and the perception of illusory contours and of the global level of hierarchical stimuli. The detailed neuropsychological assessment prompted a psychophysical experiment aiming to quantify the limits of perceptual grouping in MC. We measured the thresholds of integration for a closed chain of Gabor Patches as a function of background noise using stimuli with different values of the distance between the local elements. When compared to normal controls, the patient displays a statistically significant drop of performance for stimuli with the larger interelement distance. The data are interpreted in the context of the "association field" theory (Field, Hayes, & Hess, 1993). As MC presents with a marked atrophy of the right temporoparietal junction, we interpret our data as providing further evidence of a neuromodulatory role of the right temporoparietal junction over the occipital cortices, in line with recent functional evidence (Fink et al., 1997a). The study also highlights the benefits of complementing classical neuropsychological investigations with more quantitative psychophysical procedures.