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Jane E. Aspell, Oliver J. Braddick, Janette Atkinson, John Wattam-Bell, Holly Bridge; Concentric and parallel textures differentially activate human visual cortex. Journal of Vision 2005;5(8):189. doi: 10.1167/5.8.189.
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© 2015 Association for Research in Vision and Ophthalmology.
Mid level visual processes integrating local orientation information to detect global structure can be investigated using global form stimuli of varying complexity. Several lines of evidence suggest that the identification of concentric and parallel organisations have different underlying neural substrates. The current study compared brain activation by concentric and parallel arrays of line segments.
Four subjects were scanned in a blocked design fMRI experiment. In ‘parallel’ blocks short line elements within a circular central region had parallel orientations; in ‘concentric’ blocks they were aligned tangential to concentric circles. In ‘random’ blocks the elements were randomly oriented. The early retinotopic regions - V1, V2, V3, V3A and V4 - were previously localised in each subject. We compared the percentage BOLD signal change (against the random blocks as baseline) for the parallel and concentric conditions within these regions of interest.
In general, there was more significant and more widespread activation of posterior brain regions by the parallel than by the concentric stimulus. In particular, for the retinotopic regions, the percentage BOLD signal change was greater for the parallel stimulus compared to concentric by ratios varying from 3.2:1 in V3 to 2.2:1 in V1. Areas that were better activated by concentric compared to parallel were found in only one of the subjects, in the right postcentral gyrus and left cingulate gyrus.
This unexpected result, of greater and more widespread activation produced by the parallel stimulus, is compatible with it being a less ‘specific’ stimulus than the concentric, and activating neurons across many cortical regions. Psychophysical studies (including our current work) suggest that there are specialized detectors involved in the global processing of concentric structure. The lower activation by the concentric stimulus suggests that these detectors are not very numerous or widespread.
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