Mechanisms of Shape Processing in Macaque Area V4
Speaker: Dr. Winrich Freiwald , University of Bremen and Harvard Medical SchoolContact:
Date: September 14 2005
Time: 4:30PM to 5:30PM
Host: Prof. Tomaso Poggio, Dept. of Brain & Cognitive Sciences and McGovern I
Mary Pat Fitzgerald, 253-0551, email@example.comRelevant URL:
Shape processing along the ventral pathway critically depends on cortical area V4. Understanding shape processing in V4, therefore, has been an active area of research over the last twenty years. The goal of our study has been to identify some of the intermediate level processes and to relate them to underlying receptive field mechanisms. To address this question, we used single unit recordings in the fixating macaque monkey. Receptive field properties and shape selectivity have been studied with a set of experiments, all employing rapid stimulus presentation updates and reverse correlation analysis, but differing in stimulus content and complexity. In the first experiment, cells were stimulated with a sparse noise stimulus consisting of pairs of small squares of same or opposite contrast dots. These experiments revealed complex and often dynamic receptive field organization beyond what has been reported for early cortical areas. Shape selectivity was probed with experiments using gratings of Cartesian and non-Cartesian categories. All neurons exhibited marked within- and across-category tuning properties, with an overall population preference for circular shape - a preference which could be predicted from their second-order receptive field maps. Thus, we begin to understand shape selectivity in V4 neurons in terms of their receptive field mechanisms.
Dr. Winrich Freiwald is head of a Junior Research Group at the University of Bremenís Center for Advanced Imaging (Germany) and a Research Scholar at Harvard Medical School in the laboratory of Marge Livingstone. He received his PhD from Tuebingen University in 1998 for work on the binding problem, performed in Wolf Singerís group at the Max Planck Institute for Brain Research. He has since worked on mechanisms of object representation, attention, and executive control at various institutions, including BCS as a postdoc with Nancy Kanwisher, using electrophysiology and fMRI in macaque monkeys and humans.
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