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Abstract:
The aim of the study is to understand the perception of global shapes from local image features. Specifically, we tested the role of various visual areas that are characterized by neural populations with different receptive
field size in the integration of local features into global shapes at different spatial scales. To this end, we used fMRI in the anesthetized monkey and employed an adaptation paradigm. The paradigm entails prolonged presentation of a stimulus, resulting in decreased fMRI response, after which a change in a stimulus dimension elicits rebound of activity. The magnitude of the rebound correlates with the selectivity of an area to the changed dimension. The adapting stimulus was a rectangular area filled with randomly oriented line segments, followed by one of three test stimuli: a pattern identical to the adapting stimulus; a pattern where 1/3 of the line segments changed orientation randomly; a pattern in which change of line segment orientation resulted in a colinear shape. Spatial scale was manipulated by changing the size and the distance between the line segments. Differential responses to colinear shapes and random patterns indicated areas (V1,V2/V3) with neural populations that are selective for the global configuration of shapes, rather than local features. Rebound was observed in peripheral and central V1 for collinear shapes at large and small scales respectively. These findings suggest, that in the processing of global shapes from local features different visual areas are involved at different spatial scales.
