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Abstract

The integration of individual features is a fundamental process for the intact perception of a global scene. Disturbance of this process has been found in patients with Bálint's syndrome following bilateral damage of the temporo-parietal cortex. However, the cortical mechanisms underlying grouping processes remain largely unknown.

In an event-related fMRI study, we investigated the neural correlates of spatial integration of visual features in healthy subjects. We compared the observed neural activity with the psychophysical performance and tested for attentional modulation of the fMRI responses. The stimuli consisted of images of circles or squares (global level) rendered by smaller images of stimuli from these categories (local level) resulting in four combinations: a) global circle, local circle; b) global circle, local square; c) global square, local circle and d) global square, local square. Furthermore, we parametrically degraded the objects at the global level by exchanging the smaller images of objects at the local level with each other resulting in the following conditions: 1) Intact Global Perception at 20%- Scrambled, 2) Reduced Global Perception at 40%- Scrambled, 3) Reduced Global Perception at 60%- Scrambled and 4) Disturbed Global Perception at 80%- Scrambled. Subjects were engaged in a 2AFC-task on the category at the global (Experiment 1) or local (Experiment 2) level.

The fMRI responses of Experiment 1 revealed a network of cortical areas to be involved in the spatial integration of visual features including the precuneus (PC), anterior cingulate cortex (ACC) and the temporo-parieto-occipital junction (TPJ). Strong relation between neural activity and psychophysical performance was found for TPJ but not PC and ACC. In contrast, no significant differences across conditions were observed in the fMRI responses of Experiment 2.

Our results suggest a critical involvement of the TPJ in the spatial integration of visual features.