Abstract
Recent research suggests a role of top-down modulatory signals on perceptual processing, particularly for the integration of local elementary information to form a global holistic percept (e.g., Piccini et al., 2003). In this study we investigate whether prefrontal cortex may be instrumental in this top-down modulation in humans. We recruited 6 patients with prefrontal lesions. Four out of the 6 patients showed impaired performance in standard neuropsychological tests requiring perception of a global
structure from local cues (e.g., Street Completion Test, Overlapping Figures subtest of the BORB). We then conducted a psychophysical experiment aiming to quantify the limits of perceptual grouping in our patients. A control group of 4 patients with temporal lesions and a control group of 17 age-matched healthy individuals were also tested. We measured detection thresholds for perceiving a circle defined by a closed chain of Gabor patches over a noisy background (e.g., Field et al., 1993). Crucially, the number of patches defining the circle was manipulated across experimental conditions, hence varying the inter-element distance values. Performance of patients with prefrontal lesions was far worse than that of patients with temporal lesions and normal controls when the patterns
were sparse, requiring integration across relatively extensive regions of space. In contrast, for denser patterns performance of prefrontal patients was similar to that of the control groups. These results implicate the prefrontal cortex in the process of integrating elementary features into a holistic global percept, when the elements do not form a “pop-out” display.
