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Velitchko Manahilov, Julie Calvert, William A. Simpson, Denis M. Parker; Visual evoked responses of human cortex to contrast modulations of noise. Journal of Vision 2004;4(8):533. doi: 10.1167/4.8.533.
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Vision is sensitive to first-order luminance patterns and second-order modulations of carrier contrast. Research has suggested that these two types of visual information are processed by different pathways. The first-order pathway consists of linear filters that are sensitive to luminance modulations. The second-order pathway contains early linear filters, followed by a non-linear stage and second-stage linear filters which extract the modulating signal. The existence of additional stages in the second-order pathway may result in slower responses than the responses to luminance modulations. We recorded onset visual evoked potentials (VEPs) to first- and second-order modulations of dynamic binary noise. The electroencephalograph signals were recorded from three electrodes attached to the observer's scalp at Oz, O3 and O4. The Laplacian responses to 0.5-c/deg luminance gratings were positive with a latency of about 130 ms. For luminance modulations of 4 c/deg, the Laplacian responses were negative with a latency of about 100 ms. The early component of the Laplacian responses to contrast modulated noise of both low and higher spatial frequencies, however, had a negative polarity with a latency of about 200 ms. We carried out also multiple channel recording of VEPs to second-order patterns which were presented in the upper, lower, left and right parts of the visual field. The Laplacian responses to contrast modulations of 0.5 and 4 c/deg showed a reversal in amplitude between stimulation of lower and upper as well as between left and right parts of the visual field. Bearing the cruciform model of the primary visual cortex in mind, these findings suggest that the generators of the responses to second-order modulations are likely to be localised in the striate cortex. The longer latency of these responses may be due to the presence of additional stages in the second-order pathway which could be associated with neural structures located within the primary visual cortex.
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