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Jennifer B. Swettenham, Stephen J. Anderson, Tian E. Holliday; Magnetoencephalographic investigation on the neural basis of global shape analysis. Journal of Vision 2004;4(8):671. doi: 10.1167/4.8.671.
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Magnetoencephalography was used to study neuronal activity associated with the perception of global structure from local image features. Glass patterns (Glass, 1969), comprised of 150 pairs of white dots (0.06 deg) on a black background, were arranged to form a horizontal, rotational or radial pattern. They were viewed monocularly at a distance of 2m using central fixation, and had an overall size of 8.0 deg square. The percentage of signal dot pairs in the Glass patterns was increased linearly from zero to 100% over 100 ms, remained at 100% for 300 ms, then decreased to zero over 100 ms. The interstimulus interval was 3000 ms, during which the dot pairs retained the same spatial separation but were uncorrelated. Both the signal and noise patterns appeared dynamic as the dot pairs were re-plotted every 10 ms. Dot density and mean luminance remained constant throughout the entire display period. Responses were recorded from eight normally sighted participants using a 151-channel whole-head neuromagnetometer. We used Synthetic Aperture Magnetometry to spatially map event-related power changes of cortical activity across a wide range of frequency bands, and the results were co-registered with each participant's MRI scan. Reductions in power were evident in the 10-30 Hz frequency band, beginning 300–400 ms after the signal pattern onset and lasting for approximately 500 ms. Power reductions were observed in extra-striate visual areas (BA 18/19), the lateral occipital cortex and at the parietal/occipital border. No power changes were evident in the primary visual area. We conclude that a network of extrastriate cortical areas are involved in the analysis of global form (shape), and in all areas the most prominent change in neural activity was a reduction in oscillatory power within the range 10–30 Hz. The dynamic nature of the stimulus may have contributed to the observed cortical responses at the parietal/occipital border.
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