Thursday, October 13, 2005


Google Image Result for "Like in other primates, human visual areas are clustered along two "streams" diverging from the occipital pole: the ventro-temporal "what or perception" stream and the dorsal "where or action" stream. While the areas in the dorsal stream are tuned for visual stimuli and tasks related to stimulus location and/or action, the ventral stream consists of a web of exquisitely category selective areas. For example, a region in the lateral occipital cortex (LOC) extending anteriorly into the temporal cortex responds strongly to a variety of complex shaped objects such as polygonal figures, chairs, and gloves, etc. Furthermore, in the so called fusiform face area (FFA; located within the fusiform gyrus, cells are tuned to faces and facial stimuli (e.g., front-view photographs of faces and line drawings of faces, etc.) in a way comparable to the receptive field properties of face-selective neurons in primate inferotemporal cortex (IT). Further down the temporal cortex, in the so called parahippocampal place area (PPA), maximum functional response can be obtained using scenic or place type of stimuli. The description of highly specialized areas such as FFA and PPA raises the question how many category-selective regions of cortex exist in the human visual system, and, more generally, how the ventral temporal cortex is organized. Hypotheses range from the assumption that there are a few specialized processing modules, i.e., for faces, places, letters and human body parts up to the proposal of widely distributed and overlapping cortical object representations. Effects of category-related expertise and, more recently, different category-related resolution needs have also been proposed to explain the topology of the human what-pathway. Further insights into the question how objects are represented in ventral visual cortex might come from functional imaging studies investigating within-category responses, for example, by comparing responses to single object images, such as two different faces or two different houses (Kriegeskorte et al., personal communication). It has also been proposed that the eccentricity gradient observed in early visual areas continues into ventral visual cortex (Malach et al., 2002). For example, regions selective to faces (FFA) overlap with the representation of the fovea, while regions that are selective to houses (PPA) overlap with a peripheral visual representation located in the collateral sulcus."
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