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Tobias Otte, Lothar A. Spillmann; The Effect of Surround Luminance Modulation on a Foveal Afterimage: Long-Range Interaction in Human Vision.. Journal of Vision 2004;4(8):484. doi: 10.1167/4.8.484.
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There is neurophysiological evidence that visual stimuli are not only locally processed, but that they are also subject to surround modulation from beyond the classical receptive field. We aimed at finding a psychophysical correlate by using a foveal afterimage as a probe. A flash-elicited afterimage appears bright on a dark background and dark on a bright background. Accordingly, modulation of the background luminance makes the afterimage pulsate between dark and bright in counter-phase to the background. A foveal afterimage on a disk-shaped background requires at least 4% contrast modulation for pulsation, depending on the time after the flash. We found that this threshold contrast is altered when the disk is surrounded by an annulus with an inner radius of 8 deg that is sinusoidally modulated in luminance at 0.75 Hz. When the annulus is modulated in phase to the disk, the afterimage continues to pulsate even when the background modulation approaches zero. In comparison, when the annulus is modulated in counter-phase, the contrast threshold for afterimage pulsation is significantly increased. Modulated stray light from the annulus falling onto the fovea was taken into account. Also, changes of pupil size were ruled out by the use of an artificial pupil (4 mm in diameter). We therefore attribute the observed threshold changes to a long-range neuronal process from the peripheral annulus onto the foveal afterimage. The processes underlying the afterimage pulsation may be akin to the periphery effect (McIlwain, 1964, J. Neurophys. 27, 1154ff) and shift effect (Krüger & Fischer, 1973, Exp. Brain Res. 18, 316ff) in cat retinal ganglion cells.
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