fMRI adaptation for visual forms in the monkey brain

The aim of the study is to understand how local image features are integrated into configurations that may represent visual forms. We used fMRI in the anesthetized monkey and employed an adaptation paradigm (sufficiently prolonged presentation of a stimulus resulting in decreased fMRI responses over time) to test the role of various visual areas into such an integration process. The stimuli consisted of target-shapes embedded in a background of randomly oriented lines. The target-shapes were defined by collinearly arranged lines of the same height and width as the background lines. Following the presentation of an adapting stimulus, three conditions were tested:(A)presentation of a pattern identical to the adapting stimulus, (B) presentation of the same target-shape as the adapting stimulus but embedded in a different background (i.e. the background lines were rotated 90 deg), and (C) presentation of a different target-shape (orientation-changes of target rather than background lines). The selection of these conditions was motivated by the hypothesis that increased responses in the test phase for a new pattern are likely to indicate areas with neural populations that are selective for the global configuration of shapes, rather than local features. Initial experiments show that the time course of the fMRI signal varies in different visual areas. Not surprisingly, early visual areas failed to show shape selective adaptation, suggesting that the neural populations in these areas primarily encode local features. Differences in the time course of adaptation in higher visual areas are currently studied using a variety of visual patterns