The Leaning Tower of Pisa effect: an illusion mediated by colour, brightness, and motion

We report a novel, easily observed, and extraordinarily striking optical illusion mediated by interactions of colour, brightness, form, and motion perception--the Leaning Tower of Pisa (LTOP) illusion. Under some circumstances, the perception of orientation of coloured forms is radically altered by rotary movement. We demonstrate that this kinetic effect--easily reproduced with a common record turntable--is optimised by particular colour and brightness differences between foreground and background with an illusory tilt of 8 degrees and more. The described illusions can be easily studied at home by downloading the colour figures from www.perceptionweb.com/perc1000/ditzinger, printing them on a common colour printer and placing them on a rotating record turntable.     

The fluttering-heart illusion: a new hypothesis

The fluttering-heart illusion is a perceived lagging behind of a colour target on a background of a different colour when the two are oscillated together. It has been proposed that the illusion is caused by a differential in the perceptual latencies of different colours (Helmholtz 1867/1962), a differential in rod-cone latencies (von Kries 1896) and rod-cone interactions (von Grünau 1975, 1976 Vision Research 15 431-436, 437-440; 16 397-401; see list of references there). The purpose of this experiment was to assess the hypothesis that the fluttering-heart illusion is caused by a differential in the perceived velocities of chromatic and achromatic motion. To evaluate this hypothesis, we tested observers possessing normal colour vision and deuteranopes. The perceived delay of a chromatic target relative to an achromatic target was measured as a function of background cone contrast and target colour. For observers with normal colour vision, the perceived delay of the chromatic target is greater in the L-S than the L-M testing conditions. The reverse is observed in deuteranope observers. We suggest that this is caused by the absence of an L-M opponent mechanism contributing to chromatic motion in deuteranopes. Greater background cone contrasts tended to yield smaller perceived delays in both normal and deuteranope observers, indicating that greater chromatic modulation decreases the perceived delay of the colour target. These results support the hypothesis that the fluttering-heart illusion can be explained by a differential in the perceived velocities of chromatic and achromatic motion.     

On a changing perspective illusion within Vermeer's The Music Lesson

The effect of line of sight on the perception of spatial configuration has been investigated in a well-known painting (The Music Lesson by Vermeer) and in two control patterns. In experiment 1, subjects indicated the perceived inclination of two major contours which defined the sidewall-floor and backwall-floor joints in (i) a projected image of the painting, (ii) a three-line representation of the major spatial elements of the painting, and (iii) a three-dimensional wire model of these same contours, when standing in each of eighteen positions on a line running parallel to the surface of the screen. Results indicated a significant change in the perception of the sidewall-floor, but not of the backwall-floor contour, as viewing angle changed, in both the painting and the three-line representation. However, the angular setting in the latter case was significantly less than when the painting was used, ie subjects underestimated the depicted inclination. Settings for the wire model did not deviate with viewing angle and reflected geometrically correct adjustments. In experiment 2, the results of experiment 1 were confirmed using enantiomorphs. These findings are discussed in the light of other view-dependent illusions in paintings.     

Stability and change in a perspective reversal illusion

A phenomenologieal explanation is presented for the stability and instability of the appearance of the form of objects as a consequence of changes in (1) the spatial position of the observer with respect to the object, and (2) the amount of time the object is observed from a specific spatial position. In support of the explanation, an object was constructed so that it would have multiple appearances when observed continuously from the same viewpoint. The initial appearance of this object when observed from different spatial positions and the constancy over time of these appearances were demonstrated to be in agreement with the theoretical expectations.     

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...     

The solitaire illusion: An illusion of numerosity

A new illusion in which the apparent number of elements of two kinds is determined by their spatial arrangement is described. The illusion is such that one large cluster appears to contain more elements than several small clusters, clustering being determined by Gestalt principles. The illusion was found both in adults and in children of 8 years.     

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.     

Leaning to the left makes the Eiffel Tower seem smaller: posture-modulated estimation

In two experiments, we investigated whether body posture influences people's estimation of quantities. According to the mental-number-line theory, people mentally represent numbers along a line with smaller numbers on the left and larger numbers on the right. We hypothesized that surreptitiously making people lean to the right or to the left would affect their quantitative estimates. Participants answered estimation questions while standing on a Wii Balance Board. Posture was manipulated within subjects so that participants answered some questions while they leaned slightly to the left, some questions while they leaned slightly to the right, and some questions while they stood upright. Crucially, participants were not aware of this manipulation. Estimates were significantly smaller when participants leaned to the left than when they leaned to the right.     

Wheels: a new illusion in the perception of rolling objects

A new illusory effect is described which consists of a conspicuous perceptual overestimation of the speed at which a wheel rolling across an observer's visual field appears to be rotating. Wheels appear to revolve much faster than is compatible with their linear displacement. Experimental verification of the genuineness and magnitude of the effect is reported, along with a discussion of some of the variables upon which it may depend.     

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 illusion: The underestimation of distance during pursuit eye movements

A new visual illusion is reported. The apparent distance through which a displaced target appears to move is significantly shorter when pursuit tracked than when that same distance is observed by means of a saccadic eye movement. This misperception of distance seems to be related to the Aubert-Fleischl paradox, the underestimation of the velocity of a tracked target, and to reveal a consistent characteristic of the pursuit eye movement system.     

Illusory three-dimensional rotation of horizontal lines: a new motion-depth illusion.

A new motion--depth illusion is reported. When a curved aperture translates vertically and stationary horizontal lines can be seen through it, the line lengths on the retina change continuously because of the occlusion. Instead of seeing the aperture translate, subjects sometimes see the lines rotate in depth around a vertical axis. This is a rare kind of illusion: an ambiguous motion which can be seen as either stationary in two dimensions or rotating in three dimensions. Three-dimensional rotation was more often observed when the luminance difference between the horizontal lines and the background was larger than that between the aperture and the background. This illusion demonstrates that motion detection and the structure-from-motion process correlate with figure--ground segregation, depth stratification, and figural-completion processes based on luminance contrast.