Pattern specificity of contrast adaptation.

Contrast adaptation is specific to precisely localised edges, so that adapting to a flickering photograph makes one less sensitive to that same photograph, but not to similar photographs. When two low-contrast photos, A and B, are transparently superimposed, then adapting to a flickering high-contrast B leaves no net afterimage, but it makes B disappear from the A+B picture, which now simply looks like A

Rotating Squares Look Like Pincushions.

Rotating squares appeared to be distorted into pincushions with concave sides. These illusory shape changes were caused by a perceived compression along the curved path of motion

Illusory drifting within a window that moves across a flickering background.

When a striped disk moves across a flickering background, the stripes paradoxically seem to move faster than the disk itself. We attribute this new illusion to reverse-phi motion, which slows down the disk rim but does not affect the stripes

Motion-Driven Transparency and Opacity.

When two adjacent surfaces move in step, this can generate a sensation of transparency, even in the absence of intersections. Stopping the motion of one surface makes it look suddenly opaque

induced movement the flying bluebottle illusion

Two small objects (flies) followed identical circular orbits. However, a large background that circled around behind them in different phases made one orbit look twice as large as the other (size illusion) or made the circles look like very thin horizontal or vertical ellipses with aspect ratios of 7.5:1 or more (shape illusion). The nature of the perceptual distortion depended upon the relative phase between the movements of the background and those of the flies. Brief snatches of the moving background that added up to a circular motion were also effective

Visual adaptation to a negative, brightness-reversed world: Some preliminary observations

Abstract There have been many studies of visual adaptation to spatial rearrangements, starting with Stratton's (1897) classic studies on adaptation to an upside-down world. These have been reviewed by Rock (1966) and In recognising faces, why is it so hard to recognise photographic negatives of famous faces? The Fourier power spectra are identical to those of positives, and the phase spectra nearly so. The difficulty arises only with 3-D lith (black/white) or half tone (gray scale) portraits, not with outline drawings. Probably, luminance reversal disrupts shape from shading which is a crucial step in recognising the 3-D shape of faces. In order to learn whether humans could perform these tasks when normal brightness relations were disturbed, we have studied visual adaptation to a world that was reversed not in position but in brightness. Adaptation procedure. We examined the effects of long exposure to a visual world in which brightnesses were reversed by means of a closed circuit video link. During passive adaptation the observer watched TV in negative. During active adaptation he walked about, or sat and viewed his own hands, conversed and interacted with others, while viewing a negative monitor fed from a TV camera. Perceptual phenomena studied before, during and after this adaptation included the perception of highlights and shadows and perception of depth in convex and concave face masks which were lit from above or from below. The observer was also confronted with negative TV images and asked to extract 3-D shape from shading, to recognise facial emotions such as anger, surprise or happiness, and to identify celebrities from their negative faces. It was observed that....

Perception of Fourier and non- Fourier motion by larval zebrafish

articles Zebrafish larvae innately begin responding to moving stimuli shortly after hatching. In their optomotor response, which is elicited by large moving stimuli presented from below or the side 1,2 , larvae swim in the direction of perceived motion. The distance they travel in a given time indicates the effectiveness of the stimulus. By observing the response of many larvae to computer-animated displays, we could determine which attributes of a moving stimulus the zebrafish visual system detects. If luminance-defined features drift smoothly or jump in space, they can produce strong sensations of motion. A number of proposed models explain how motion information can be extracted. In a simple model, a point-to-point comparison is made between the luminance pattern and a spatially displaced copy of the pattern that was seen a short time before 3 . The displacement that gives the best fit tells the brain the direction and speed of movement. A more complex strategy is to look at the Fourier motion energy in the visual scene Although there is evidence that humans can use both feature matching and motion energy to detect movement 7 , they may also sense motion when presented with stimuli in which only secondorder features such as contrast, texture or flicker are moving Here we find that the fish larvae detect moving features of visu- A moving grating elicits innate optomotor behavior in zebrafish larvae; they swim in the direction of perceived motion. We took advantage of this behavior, using computer-animated displays, to determine what attributes of motion are extracted by the fish visual system. As in humans, first-order (luminance-defined or Fourier) signals dominated motion perception in fish; edges or other features had little or no effect when presented with these signals. Humans can see complex movements that lack first-order cues, an ability that is usually ascribed to higher-level processing in the visual cortex. Here we show that second-order (non-Fourier) motion displays induced optomotor behavior in zebrafish larvae, which do not have a cortex. We suggest that second-order motion is extracted early in the lower vertebrate visual pathway. al stimuli in a way that is qualitatively similar to humans: both firstorder and second-order cues drive their behavioral response. Our demonstration of second-order motion detection in fish challenges the idea that higher-level, cortical mechanisms are necessary to explain this capacity of the visual system. RESULTS Optomotor responses to Fourier motion The assay used to measure optomotor responses is similar to the one described previously 2 (Methods). Movies showing drifting gratings evoke strong optomotor responses in almost all fish in a clutch. Fish do not respond to a moving grating with a stripe width narrower than approximately 9°, which is slightly less than the predicted resolution limit of the larval cone mosaic, 6°at this age In the following experiments, responses were normalized to the effect of a designated strong stimulus, a 100% contrast square wave subtending 100°of visual angle per cycle and moving at 1 Hz for 30 seconds Although the fish seemed to follow a motion signal in the movies, it was possible that they were tracking features such as light or dark regions or edges that were being displaced. We did an experiment to show that the optomotor response is truly a response to motion. A motion display was shown of a sine wave grating tha

Pattern specificity of contrast adaptation.

Contrast adaptation is specific to precisely localised edges, so that adapting to a flickering photograph makes one less sensitive to that same photograph, but not to similar photographs. When two low-contrast photos, A and B, are transparently superimposed, then adapting to a flickering high-contrast B leaves no net afterimage, but it makes B disappear from the A+B picture, which now simply looks like A