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Melanie Palomares, Anthony Norcia; The effect of temporal frequency on the local and global structure of Glass patterns. Journal of Vision 2010;10(7):1206. doi: https://doi.org/10.1167/10.7.1206.
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Glass patterns are moirés created from a sparse random-dot field paired with it spatially shifted copy. Because discrimination of these patterns cannot be based on local features, they have been used extensively to study global integration processes. Using a multi-frequency tagging technique to record visual evoked potentials (VEPs), we can simultaneously measure neural sensitivity to local and global structure to Glass patterns. We have previously found that sensitivity to local and global structures of Glass patterns have different specificities: global responses were largely independent of luminance contrast while local responses were not (Palomares, et al, 2009, Journal of Cognitive Neuroscience), global responses were unaffected by directed attention while local responses were not and scalp topographies of global responses were localized more laterally than local responses (Palomares, et al, VSS 2009). Here, we evaluated the specificity of local and global responses to the local temporal frequency of Glass patterns. If sensitivity to global structure is independent from local structure, one strong expectation is that global responses to Glass patterns would remain unaffected by the local update of the dots. Random dot patterns were spatially offset to create concentric Glass patterns and alternated with randomized versions every 600 ms. Thus the global structure changed at 0.83 Hz. Different exemplars of concentric Glass patterns or randomly-oriented dipoles were sequentially presented at faster rates (every 66, 50 or 33 ms); the local structure changed at 15, 20 or 30 Hz. Our results show that sensitivity to local responses were highest at lower frequencies, while global responses were best at higher frequencies. VEP source-imaging on fMRI-based regions of interest suggest that this pattern is strongest in V4. Our data further demonstrate that sensitivity to local and global structure in dynamic Glass patterns is mediated by different, complementary mechanisms.
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