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Cibu Thomas, Kestutis Kveraga, Moshe Bar; The basis of global and local visual perception revealed by psychophysical ‘lesions’. Journal of Vision 2010;10(7):972. doi: https://doi.org/10.1167/10.7.972.
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© ARVO (1962-2015); The Authors (2016-present)
We can extract the gist of scenes and objects very rapidly. This ability to “see the forest before the trees” is known as the Global Precedence Effect (GPE). The GPE is affected primarily by three factors: the spatial frequency content of the stimulus, the global and local grouping properties of the visual cortex, and the tuning curves of the magnocellular (M) and parvocellular (P) neurons in the lateral geniculate nucleus of the thalamus. Global information is conveyed primarily by low spatial frequencies (LSF) and local information, by high spatial frequencies (HSF). Moreover, the M-pathway is thought to be tuned primarily to LSF, while the P-pathway is thought to convey HSF. Therefore, the M-pathway is assumed to mediate global processing (and GPE) and the P-pathway, to local processing. To examine whether this mapping is true, we employed psychophysical techniques to selectively ‘lesion’ the M and P pathways and examined the relationship between spatial frequencies, M and P pathways and global/local processing. In Experiment 1 (N=10), we used hierarchical stimuli that were either M-biased (achromatic, low luminance contrast), P-biased (chromatic, isoluminant), or unbiased (black-on-white, resolved well by M and P cells). In Experiment 2 (N=10), we used hierarchical stimuli that were either ‘scrambled’ (phase of low and mid-range spatial frequencies randomly redistributed in the image), or unbiased. In both experiments subjects were tested for global/local processing using a focused attention paradigm. Contrary to the prevailing view, we found that both M and P pathways contribute significantly to global and local processing. Interestingly, we found that P-biased stimuli show a stronger GPE than M-biased stimuli. Our data also suggests that LSF is necessary for the GPE and HSF is sufficient for local processing. These findings describe for the first time the relationship between spatial frequencies, visual pathways, and global/local visual processing.
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