The dependence of grating induction magnitude on retinal illuminance was examined in two subjects. Grating induction magnitude, as determined using the cancellation technique of McCourt, declines monotonically with decreasing retinal illuminance, effectively disappearing at a value of 0.3-0.5 phot td. In a second experiment, sensitivity differences for test lights of 500 and 600 nm were measured as a function of background illuminance in order to gauge the luminance operating range for grating induction with respect to duplex photoreceptor function. Cancelling contrast (and hence grating induction magnitude) fell below detection threshold contrast at retinal illuminances coinciding with the transition from photopic to scotopic visual function. In a third experiment, spatial contrast sensitivity was measured using both spatially extended (10 degrees) and truncated (2 degrees) sinewave gratings at frequencies below 2 c/deg, at three values of retinal illuminance. Illuminance values corresponded to those where grating induction magnitude was, as determined from the first experiment, either maximal, intermediate or negligible. Similar to grating induction, the strength of lateral inhibition, as indexed by the slope of the low-frequency decline in contrast sensitivity, is progressively reduced with decreasing retinal illuminance, particularly for the 2 degree field. There was, however, using the same criteria, evidence of lateral inhibition at a value of retinal illuminance which did not support grating induction. The implications of these results are discussed with respect to classical brightness contrast phenomena, recent neuroanatomical and neurophysiological evidence of segregated parvo- and magnocellular mediated contrast processing systems, and with results from previous studies of the grating induction effect.
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"Stationary phantom visibility is maximal when the IA luminance is close to the maximum or to the minimum level of the inducing grating , and between these levels, especially at mean luminance , the grating induction effect is dominantly perceived (for a review of similar findings for moving and flickering phantoms see Maguire & Brown, 1987). Moreover , Gyoba (1994a) found that stationary phantoms are perceived with only a low luminance (less than 80 cd/m 2 ) inducing grating, whereas the magnitude of GI increases as the luminance of the inducing grating increases into photopic levels (McCourt, 1990). Another important property in discussing the mechanisms underlying phantoms and GI is the differences in the f igural nature of their appearance. "
[Show abstract][Hide abstract] ABSTRACT: The visibility of stationary visual phantoms and the grating induction (GI) effect were concurrently analyzed with both black and gray inspection areas (IA) using the same subjects with counterbalanced orders of measurements. Oblique inducing gratings were employed in order to compare the visibility of obliquely aligned and vertically misaligned appearances between the two phenomena. Aligned and misaligned phantom responses with a black IA were similar, whereas overall phantom visibility was severely suppressed when the IA was gray. In contrast, misaligned GI dominated with a gray IA, whereas aligned and misaligned GI responses were similar with a black IA. Phantoms appear to be related to visual mechanisms' selectively utilizing relative luminance information between the inducing grating and IA in a manner consistent with more global figural characteristics of the display (e.g., modal and amodal completion). On the other hand, GI may be predominantly due to locally operating brightness/contrast mechanisms.
"The lowest luminance used in most of the experiments was − 0.8 log photopic td (0.05 cd/m 2 ). We assume that only the scotopic system is active under the lowest adapting level (− 1.5 log photopic td) (Hecht & Schlaer, 1936; Stabell & Stabell, 1981; Hood & Finkelstein, 1986; McCourt, 1990; Makous, 1997). Though the next lowest luminance used, − 0.8 log photopic td, is also assumed to be in the scotopic range (Stabell & Stabell, 1981), since the cone threshold depends not only on the stimulus parameters but also on individual variations (Makous, 1997), some cones could be active at this luminance level. "
[Show abstract][Hide abstract] ABSTRACT: To characterize scotopic motion mechanisms, we examined how variation in average luminance affects the ability to discriminate velocity. Stimuli were drifting horizontal sine-wave gratings (0.25, 1.0 and 2.0 c/deg) viewed through a 2 mm artificial pupil and neutral density filters to produce mean adapting levels from 2.5 to -1.5 log photopic trolands. Drift temporal frequency varied from 0.5 to 36.0 Hz. Grating contrasts were either three or five times direction discrimination threshold contrasts at each adaptation level. Following 30 min adaptation, two drifting gratings were presented sequentially at the fovea. Subjects were asked to indicate which interval contained the faster moving stimulus. The Weber fraction for each base temporal frequency was determined using a staircase method. As previously reported, velocity discrimination performance was most acute at temporal frequencies of about 8.0 Hz and greater than 20.0 Hz (though there are individual differences), and fell off at both higher and lower temporal frequencies under photopic conditions. As adaptation level decreased, discrimination of high temporal frequencies in the central retina became increasingly worse, while discrimination of low temporal frequencies remained largely unaltered. The overall scotopic discrimination performance was best at about 3.0 Hz. These results can be explained by a motion mechanism comprising both low-pass and band-pass temporal filters whose peak and temporal cut-off shifts to lower temporal frequencies under scotopic conditions.
"We also used adapting levels of 7.85 td (0.9 log photopic td), 0.785 td (–0.1 log photopic td), and 0.0785 td (–1.1 log photopic td), respectively. We assume that only the scotopicsystem is active under the lowest adapting level (Hecht & Shlaer, 1936; Hood & Finkelstein, 1986; McCourt, 1990). Subjects initially dark adapted for 25 min prior to the task, and the experiment always started at the lowest adapting level. "
[Show abstract][Hide abstract] ABSTRACT: We studied scotopic motion mechanisms, using a two-frame sinusoidal grating separated by various ISIs equated for mean luminance level. Perceived direction of displacement varied with both ISI and luminance. As luminance decreased, apparent motion reversal disappeared. This is predicted by a first-order motion model if the underlying temporal impulse response function varies from biphasic under photopic conditions to monophasic under scotopic conditions. Performance at long (but not short) ISIs depends upon stimulus contrast, suggesting there is also a scotopic feature-tracking mechanism. With isoluminant and high spatial frequency gratings, where the temporal impulse response function is monophasic, no motion reversal was observed.