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Huseyin Boyaci, Fang Fang, Scott Murray, Gina Albanese, Daniel Kersten; Time course of cortical responses to illusory and real lightness changes. Journal of Vision 2008;8(6):960. doi: 10.1167/8.6.960.
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Using fMRI with short repetition times (TR = 227 ms), we measured the time course of BOLD signals in response to illusory and real lightness changes in human early visual cortex.
The illusory stimulus was composed of a background and foreground region. The background consisted of four horizontally arranged, contiguous, gray rectangular bands. The rightmost and leftmost bands (the “flanks”) had the same luminance. One of the central bands was lighter and the other darker than the flanks. The foreground consisted of two narrow vertical bars positioned to hide the two vertical luminance edges, one between each central band and its bordering flank. In this configuration the background is seen as a planar surface with the flank closer to the lighter central band appearing lighter, and the flank closer to the darker central band appearing darker. When the contrast of the two central bands is reversed, the apparent lightness of the flanks reverses. We constructed a “real stimulus” identical to the illusory stimulus except that the two flanks differed in luminance, psychophysically matched to the illusory difference. In the fMRI experiment attention was controlled through a demanding fixation task.
We analyzed the BOLD signal within independently determined ROIs corresponding to small regions within the flanks of the stimuli far from any edges (1.6 × 1.6 deg at ±8 deg from the central fixation mark). We found that the peak cortical activity to illusory lightness change in V1 was significantly delayed by as much as 1 second compared to that of a real change. We psychophysically compared the critical flicker rate at which the illusory and real flanks no longer appeared to change in lightness. The rate for the illusory stimulus was significantly longer than that for the real stimulus, consistent with the lag found in the V1 BOLD signal.
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