A new visual illusion, and its mechanism

A new visual illusion is reported, in which a sine-wave grating appears to tilt when doubly sheared perpendicularly to the grating lines. It is shown that the illusory percept is related to the Münsterberg and Café Wall illusions. The probable mechanism at the root of all such illusions is postulated by reference to the neuroarchitecture of the retina and striate cortex.     

A new dynamic visual illusion: The bending hourglass

Attention, Perception, & Psychophysics - A white hourglass moving laterally across a black surface appears to bend in such a way that the narrow portion lags behind its true position, but only...     

A new optical-geometrical illusion

We describe and attempt to explain a new and unusual optical-geometrical illusion with three levels of distortion. The illusory figure is made up of three juxtaposed bands of the same width, which, when appropriately juxtaposed, appear to be of different widths. We hypothesised that the effect would depend on the combined action of various factors: (i) the band shapes and their reciprocal spatial position; (ii) the degree of coincidence of the sides of the juxtaposed bands; and (iii) the inability of the perceptual system to account for all the projective transformations. An experiment was conducted in which the shape of three stimuli was manipulated through affine transformation as well as variation of side lengths. The participants' task was to evaluate the width of the bands. The results revealed a robust and stable illusory effect; the factors that seem to influence the illusion most are band shape and conjoining side lengths.     

A new visual illusion: flickering fields are localized in a depth plane behind nonflickering fields

Flickering regions of the visual field are perceived to lie well behind regions which are not flickered. The depth segregation is not due to luminance differences since the average temporal luminance across all the regions was equal. This depth effect produced by flicker is not dependent on the texture of the visual field; nor does it depend on a specific configuration of the flickering and nonflickering areas. It is optimal at a temporal frequency around 6 Hz, which suggests that visual channels responding maximally to high temporal frequencies are involved in the segregation of perceptual regions in depth.     

A new visual illusion: neonlike color spreading and complementary color induction between subjective contours.

A new visual illusion is described as a neonlike spreading of color between subjective contours. Spreading of an actually present color as well as spreading of a complementary color appear to be possible. Spreading of brightness can be demonstrated also.Two related classes of illusions are mentioned and some indications of central factors involved in the effect are discussed.     

A new explanation for the Poggendorff illusion

Kanizsa (1972, 1974) has observed that the surface upon which a figure is amodally completed undergoes shrinkage. That observation is investigated here as a possible explanation of the Poggendorff illusion, on the assumption that the shrinkage of the surface behind the surface defined by the two vertical parallel lines results in displacement of the two visible segments of the oblique line. The first two experiments attempted to quantify the shrinkage of the amodal surface by measuring the enlargement of the vertical strip required to achieve perceived collinearity; the oblique lines intercepted the vertical strip at 45- and 30-deg angles. In both cases, the enlargement required to counterbalance the assumed amodal shrinkage was approximately 30%. In the third experiment, the oblique line was rotated to the horizontal, and again the perceived shrinkage of the amodal surface was approximately 30%. The application of this explanation to the Poggendorff illusion is discussed, as well as the relevance of this explanation to the common experimental finding that magnitude of the illusion is dependent upon the slope of the oblique line.     

A new "fat face" illusion

We report a novel fat face illusion that when two identical images of the same face are aligned vertically, the face at the bottom appears 'fatter'. This illusion emerged when the faces were shown upright, but not inverted, with the size of the illusion being 4%. When the faces were presented upside down, the illusion did not emerge. Also, when upright clocks were shown in the same vertically aligned fashion, we did not observe the illusion, indicating that the fat illusion does not generalize to every category of canonically upright objects with similar geometric shape as a face.     

A new set of illusions--the dynamic luminance-gradient illusion and the breathing light illusion

A novel set of illusions that break brightness constancy and size constancy at the same time is reported. The illusions occur when observers move towards or away from these patterns. Many variations of these phenomena and a possible explanation are discussed.     

The peripheral drift illusion: a motion illusion in the visual periphery

Circularly repeating patches containing sawtooth luminance gradients produce a sensation of motion when viewed in the periphery. Illusory motion is perceived in a dark-to-light direction, but only when one's gaze is directed to different locations around the stimulus, a point outside the display is fixated and the observer blinks, or when the stimulus is sequentially displayed at different locations whilst the observer fixates one point. We propose that the illusion is produced by the interaction of three factors: (i) introducing transients as a result of eye movements or blinks; (ii) differing latencies in the processing of luminance; and (iii) spatiotemporal integration of the differing luminance signals in the periphery.     

Handgrip maximum force and the visual horizontal-vertical illusion

The visual horizontal-vertical illusion (HVI) refers to the tendency to overestimate vertical distances relative to horizontals in both 2-D and 3-D presentations. Although the HVI is evident across a wide range of different stimuli, no general theoretical account fully explains the illusion. Some recent authors have proposed the 'effort' account of HVI, contending that vertical overestimation is mediated by effort assessment of gravitational challenges offered by the stimulus. The theory has been supported by a set of studies showing that the height overestimation of large-scale 3-D objects is inversely related to perceivers' fitness and strength. We explored if the large-scale HVI/strength dependence extends to the evaluation of small-scale 2-D line stimuli, traditionally used in HVI studies. We measured the maximum handgrip strength, and assessed the HVI with a computerised line-adjustment task in thirty-two individuals. Compatible with earlier findings in the context of large-scale 3-D stimuli, a significant negative correlation was found between the strength of the dominant hand and amount of HVI. In addition, the variability of HVI was negatively correlated with maximum grip strength of both hands. The results are discussed with reference to the 'effort' account of HVI.     

The vista paradox: a natural visual illusion

Suppose an observer views a distant object through a window in the far wall of a room or corridor--a visual scene constituting a vista. If the observer moves toward the window, then the distant object will shrink in apparent size and appear farther away. These effects are paradoxical, because the distant object appears smaller as its visual angle increases. The vista paradox occurs under many other real-world conditions, such as viewing a distant object while moving out of the mouth of a valley, or driving across a topographic crest. In the present study, framing effects and the equidistance tendency are considered as possible factors. However, an explanation based on the dynamic relationship between the visual angle of the framing portion of a vista and the visual angle of a distant object appears more promising.     

A variant of the anomalous motion illusion based upon contrast and visual latency

We examined a variant of the anomalous motion illusion. In a series of experiments, we ascertained luminance contrast to be the critical factor. Low-contrast random dots showed longer latency than high-contrast ones, irrespective of whether they were dark or light (experiments 1 -3). We conjecture that this illusion may share the same mechanism with the Hess effect, which is characterised by visual delay of a low-contrast, dark stimulus in a moving situation. Since the Hess effect is known as the monocular version of the Pulfrich effect, we examined whether illusory motion in depth could be observed if a high-contrast pattern was projected to one eye and the same pattern of low-contrast was presented to the other eye, and they were binocularly fused and swayed horizontally. Observers then reported illusory motion in depth when the low-contrast pattern was dark, but they did not when it was bright (experiment 4). Possible explanations of this inconsistency are discussed.     

A new variant of the Ouchi illusion reveals Fourier-component-based processing

We report that anomalous motion illusion in a new variant of the Ouchi figure is well predicted by the strength of its Fourier fundamentals and harmonics. The original Ouchi figure consists of a rectangular checkerboard pattern surrounded by an orthogonal rectangular checkerboard pattern, in which illusory relative motion between the two regions is perceived. Although this illusion has been explained in terms of biases in integrating one-dimensional motion signals to determine the two-dimensional motion direction, the physiological mechanism has not been clarified. With our new stimuli, which consisted of thin lines instead of rectangles, we found that the perceived illusion is drastically reduced when the position of each line element is randomly shifted. This is not predicted by simple models of local motion integration along the visible edges. We demonstrate that the relative amplitude of the relevant Fourier fundamentals and harmonics leads to a quantitative prediction. Our analysis was successfully applied to other variants of the Ouchi figure (Khang and Essock 1997 Perception 26 585-597), closely predicting the reported rating. The results indicate that the underlying physiological mechanism is sensitive to the Fourier components of the stimuli rather than the visible edges.