Interactions between simultaneous contrast and adaptation to gradual change of luminance

Diamond Patterns: Cumulative Cornsweet Effects and Motion-Induced Brightening

A Cornsweet edge creates the perception of a step in surface lightness between two adjacent regions of identical mean luminance due to a gradient on both sides. We might imagine that in a concatenated set of these gradients, the lightness steps would accumulate, but they do not. However, a diamond pattern, with each diamond filled with an identical luminance gradient does give a cumulative Cornsweet effect. Here, we offer an illumination explanation for why the cumulative effect is visible in the diamonds but not in the basic ramp grating and we demonstrate that when the diamonds drift, they produce a strong brightening effect (depending on the direction of the motion) and a dimming aftereffect. These effects are consistent with the local luminance gradients and not with the global lightness shift of the cumulative Cornsweet effect.

Afterimages and the breathing light illusion

Gori and Stubbs (2006 Perception 35 1573-1577) have published some visual illusions elicited in observers who fixate very blurred disks while they move their head towards and away from them. We interpret these illusions as afterimages. We support this with examples of eccentric, colored, and striped afterimages.

In honour of Lothar Spillmann - filling-in, wiggly lines, adaptation, and aftereffects

I have studied a number of visual phenomena that Lothar Spillmann has already elucidated. These include: Neon spreading: when a small red cross is superimposed on intersecting black lines, the red cross seems to spread out into an illusory disk. Unlike the Hermann grid, neon spreading is relatively unaffected when the black lines are curved or wiggly. This suggests that the Hermann grid, but not neon spreading, involves long-range interactions. Neon spreading can be shown in random-dot patterns, even without intersections. It is strongest when the red crosses are equiluminous with the gray background. Adaptation, aftereffects, and filling-in: direct and induced aftereffects of color, motion, and dimming. Artificial scotomata and filling-in: the "dam" theory is false. Staring at wiggly lines or irregularly scattered dots makes them gradually appear straighter, or more regularly spaced. I present evidence that irregularity is actually a visual dimension to which the visual system can adapt. Conjectures on the nature of peripheral fading and of motion-induced blindness. Some failed experiments on correlated visual inputs and cortical plasticity.

Visual adjustments to temporal blur

After observers have adapted to an edge that is spatially blurred or sharpened, a focused edge appears too sharp or blurred, respectively. These adjustments to blur may play an important role in calibrating spatial sensitivity. We examined whether similar adjustments influence the perception of temporal edges, by measuring the appearance of a step change in the luminance of a uniform field after adapting to blurred or sharpened transitions. Stimuli were square-wave alternations (at 1 to 8 Hz) filtered by changing the slope of the amplitude spectrum. A two-alternative-forced-choice task was used to adjust the slope until it appeared as a step change, or until it matched the perceived transitions in a reference stimulus. Observers could accurately set the waveform to a square wave, but only at the slower alternation rates. However, these settings were strongly biased by prior adaptation to filtered stimuli, or when the stimuli were viewed within temporally filtered surrounds. Control experiments suggest that the latter induction effects result directly from the temporal blur and are not simply a consequence of brightness induction in the fields. These results suggest that adaptation and induction adjust visual coding so that images are focused not only in space but also in time.

Experiments on the afterimages of stimulus change (Dvorák 1870): a translation with commentary

In 1870 Dvorák rejected Helmholtz's eye-movement account of motion aftereffects (MAEs) on the grounds that it was inconsistent with previous reports of nonuniform rotation in MAEs induced with Plateau spirals. Subsequent observations with spirals that were modified to induce both expanding and contracting MAEs simultaneously, together with the use of stationary negative afterimages during induction and test, were offered as further counter-examples to the eye-movement hypothesis. Dvorák's conjectures that perception (and misperception) of movement involves a unitary perceptual dimension of stimulus change also led him to investigate whether aftereffects comparable to MAEs could be induced along other stimulus dimensions in vision (luminance gradients), and in audition (gradients of pitch and intensity). It is suggested that Dvorák's observations, taken as a whole, may be interpreted as an attempt to provide evidence challenging the Helmholtzian traditions underpinning eye-movement accounts of MAEs. The nature and outcomes of these observations are provided in a translation of the original work, and are subsequently discussed in relation to some contemporary empirical counterparts.

Recovering motion information from luminance

We review evidence that visual transient channels responding to temporal change of luminance provide inputs to motion mechanisms, and also play a part in judgments of static brightness. These channels can be adapted to give after effects of apparent dimming or brightening. Nonlinearity in these channels causes a sawtooth grating to look dark (or light) while it is moving to the left (or right). The perceptual outcome in a competitive motion situation is governed by the larger temporal change in luminance: when a white bar and a black bar suddenly change places, on a dark (light) surround it is the white (black) bar that appears to move. The motion system responds to linear, not log luminance. If a black and white picture dissolves (fades) to its own photographic negative which if shifted a few min arc to the right "reversed apparent motion" is seen toward the left. These results constrain possible models of motion perception.