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Jess Rowland, Alex Wade; Decoding foveal stimulus chromaticity using the peripheral V1 BOLD response. Journal of Vision 2010;10(7):386. doi: https://doi.org/10.1167/10.7.386.
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Introduction: Although much is known about chromatic properties of classical receptive fields, the effect of chromatic stimuli on extraclassical suppressive surrounds is less well-understood. If extraclassical receptive fields are chromatically-tuned, neurons outside directly-stimulated regions might carry information about stimulus chromaticity. Here we show that fMRI BOLD signals in peripheral regions of V1 carry significant information about the color of a foveal stimulus.
Methods: Subjects fixated 5 degree diameter contrast-reversing gratings defined by chromatic contrast along the three different color directions in MacLeod-Boynton space in a block-design fMRI experiment. Data from directly stimulated cortical regions and from a peripheral ‘surround’ in retinotopically-defined V1 were analyzed both by univariate analysis and by a multivariate pattern classification algorithm.
Results: The univariate analysis showed little difference in central response amplitudes but distinct differences in the surround for all three color directions: Luminance stimuli created a large negative BOLD response while S-cone and (L-M)-cone isolating isoluminant stimuli did not. Although isoluminant stimuli did not generate a mean activity change in the surround, it was possible they caused changes in small-scale patterns of voxel responses in this region. By removing periphery mean responses, we used the classification routine to ask whether we could predict the color of foveal targets based on responses of peripheral cortical neurons. We found that we could discriminate all three color directions using a multivariate pattern classifier operating on peripheral voxels as well as central ROIs.
Conclusions: The spatially-extended negative BOLD effect is largely generated by luminance stimuli. This is consistent with single studies showing the largest suppression from extraclassical receptive fields in magnocellular neurons of the early visual system. However, our ability to classify isoluminant stimuli based on peripheral population responses confirms that some V1 neurons must have large, chromatically-tuned suppressive surrounds.
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