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Frank E. Visco, Jr., Scott B. Stevenson; Lateral interactions modify the Pulfrich effect. Journal of Vision 2002;2(7):316. doi: 10.1167/2.7.316.
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When a neutral density filter is placed in front of one eye, a swinging pendulum appears to take on an elliptical orbit (the Pulfrich Effect), due to transmission delays caused by the drop in retinal illuminance. The temporal disparity between the fused moving targets is interpreted as depth with motion. This study investigated whether the Pulfrich Effect is driven locally, or if the delay is affected by light in the surround.
Two counter-phased, computer generated pendulum bobs (8.3 arcmin) on a black background were presented to each eye in a haploscope. The bobs presented to one eye had a luminance of 5cd/m2; the fellow eye saw two 50cd/m2 bobs. The bobs in each eye were flanked above and below by 1.7arcmin wide horizontal strips of the same luminance. The bobs “swung” back and forth at 0.95 Hz with an amplitude of 3.26 degrees. By adjusting the interocular phase delay of the two swinging bobs the subject nulled the rotational effect. This difference in phase, quantified in milliseconds, corresponds to the interocular difference in signal timings. To assess the effect of the flanks on the perceived delay we compared a no flank condition to flanks at various gap distances from the bobs (zero deg. — 1 deg.)
With no surround on a black background, the relative delay was 9ms. The addition of an abutting 1.7arcmin flank increases the interocular delay to 12ms (gap =0), and returns to 9ms with a gap size of 1 degree.
These findings suggest that the transmission delays responsible for the Pulfrich Effect are not strictly dependent on local illuminance. Bright objects within about 1 degrees of a foveal target speed up signal transmission, and the magnitude of the effect is much too large to be due simply to light scatter. When the flanks were abutting and several degrees wide, the relative interocular delay was about 16ms, in agreement with previous findings in the classic Pulfrich paradigm.
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