The role of apparent distance in the Poggendorff illusion

This study was designed to test the hypothesis that apparent shrinkage of the distance between the oblique lines is responsible for the Poggendorff illusion. The results from one experiment, which provided an indirect test by increasing the length of the oblique arm, supported the shrinkage hypothesis. However, a second experiment, in which apparent distance was measured directly, did not support the hypothesis. Instead, the distance between the oblique lines appeared longer than a control distance. It was concluded that the argument, made by assimilation theory, that the Poggendorff illusion is caused by changes in the apparent distance between oblique lines must be reassessed.     

The moon tilt illusion.

Besides the familiar moon illusion [e.g. Hershenson, 1989 The Moon Illusion (Hillsdale, NJ: Lawrence Erlbaum Associates)], wherein the moon appears bigger when it is close to the horizon, there is a less known illusion which causes the moon's illuminated side to appear turned away from the direction of the sun. An experiment documenting the effect is described, and a possible explanation is put forward.     

Terrestrial passage theory of the moon illusion

Theories of the celestial, or moon, illusion have neglected geometric characteristics of movement along and above the surface of the earth. The illusion occurs because the characteristics of terrestrial passage are attributed to celestial passage. In terrestrial passage, the visual angle subtended by an object changes discriminably as an essentially invariant function of elevation above the horizon. In celestial passage, by contrast, change in visual angle is indiscriminable at all elevations. If a terrestrial object gains altitude, its angular subtense fails to follow the expansion projected for an orbital course: Angular diminution or constancy is equivalent to distancing. On the basis of terrestrial projections, a similar failure of celestial objects in successive elevations is also equivalent to distancing. The illusion occurs because of retinal image constancy, not--as traditionally stated--despite it.     

The role of frame size on vertical and horizontal observers in the rod-and-frame illusion.

The rod-and-frame illusion was used to examine a proposed distinction between the mechanism responsible for frame effects on rod-adjustment errors with large displays and the mechanism responsible for errors with small displays. It was suggested that visual-vestibular mechanisms are involved only when the rod is surrounded by a large tilted frame. Errors in the perceived vertical with small frame would instead be due to purely visual mechanisms. To examine this dual process model, we compared errors at small and large frame when the body was vertical or horizontal. There is evidence to suggest that tilting the body affects visual-vestibular interactions, but there is no reason to expect that body tilt would affect intravisual interactions. Hence, we hypothesized that body tilt would increase errors for large frame, but not for small frame. Eight subjects were tested in four different conditions, corresponding to the combination of two body orientations (vertical versus horizontal) and two frame sizes (47.5 versus 10.5 deg of visual angle). Fourier analysis of data was performed. Repeated measures ANOVA tested the hypothesis about frame size and body orientation. The hypothesis was not confirmed. More specifically, we found that tilting the body increased errors for the small frame as well as for the large frame. The interaction between frame size and body orientation was not significant. Results are discussed in relation to the proposed dual-process model.     

The role of distance from the body and distance from the real hand in ownership and disownership during the rubber hand illusion

A key tool for investigating body ownership is the rubber hand illusion, in which synchronous multisensory feedback can induce feelings of ownership over a fake hand. Much research in the field aims to tease apart the mechanisms that underlie this phenomenon. Currently there is conflicting evidence as to whether increasing the distance between the real and fake hands (within reaching space) can reduce the illusion. The current study examines this further by modulating, not only the absolute distance between the real and fake hands but also their relative distance from body midline. It is found that the strength of the illusion is reduced only when the fake hand is both far from the real hand and far from the trunk; illusion scores over a fake hand in the same position can then be increased by moving the real hand nearer. This is related to peripersonal space surrounding the trunk and the hand. Subjective disownership of the real hand, and proprioceptive drift measures were also taken and may be driven by different mechanisms.     

A simple but powerful theory of the moon illusion

Modification of Restle's theory (1970) explains the moon illusion and related phenomena on the basis of three principles: (1) The apparent sizes of objects are their perceived visual angles. (2) The apparent size of the moon is determined by the ratio of the angular extent of the moon relative to the extents subtended by objects composing the surrounding context, such as the sky and things on the ground. (3) The visual extents subtended by common objects of a constant physical size decrease systematically with increasing distance from the observer. Further development of this theory requires specification of both the components of the surrounding context and their relative importance in determining the apparent size and distance of the moon.     

A test of size-scaling and relative-size hypotheses for the moon illusion

In two experiments participants reproduced the size of the moon in pictorial scenes under two conditions: when the scene element was normally oriented, producing a depth gradient like a floor, or when the scene element was inverted, producing a depth gradient like a ceiling. Target moons were located near to or far from the scene element. Consistent with size constancy scaling, the illusion reversed when the "floor" of a pictorial scene was inverted to represent a "ceiling." Relative size contrast predicted a reduction or increase in the illusion with no change in direction. The relation between pictorial and natural moon illusions is discussed.     

The role of converging lines on the perception of vertical lines: A Ponzo illusion variant

An experiment was performed to examine the effect of the angle of converging contours on the subjective midpoints of vertical lines. According to the inappropriate size-constancy scaling theory of Gregory (1963) increasing the angle of converging contours should increase the apparent depth of the figure and therefore exert a measureable influence on the subjective midpoints of vertical lines enclosed by the contours. However, this manipulation was not found to affect the subjective midpoints of test lines indicating that an explanation of the Ponzo illusion based on the operation of inappropriate size-constancy scaling must be restricted to certain test configurations.     

Anisotropic perception of visual angle: Implications for the horizontal-vertical illusion,overconstancy of size,and the moon illusion

Three experiments investigated anisotropic perception of visual angle outdoors. In Experiment 1, scales for vertical and horizontal visual angles ranging from 20° to 80° were constructed with the method of angle production (in which the subject reproduced a visual angle with a protractor) and the method of distance production (in which the subject produced a visual angle by adjusting viewing distance). In Experiment 2, scales for vertical and horizontal visual angles of 5°–30° were constructed with the method of angle production and were compared with scales for orientation in the frontal plane. In Experiment 3, vertical and horizontal visual angles of 3°-80° were judged with the method of verbal estimation. The main results of the experiments were as follows: (1) The obtained angles for visual angle are described by a quadratic equation, θ′=a+bθ+cθ² (where θ is the visual angle; θ′, the obtained angle;a, b, andc, constants). (2) The linear coefficientb is larger than unity and is steeper for vertical direction than for horizontal direction. (3) The quadratic coefficientc is generally smaller than zero and is negatively larger for vertical direction than for horizontal direction. And (4) the obtained angle for visual angle is larger than that for orientation. From these results, it was possible to predict the horizontal-vertical illusion, over-constancy of size, and the moon illusion.     

The role of structural components in the Mueller-Lyer illusion

In order to assess the role of the structural components of the Mueller-Lyer illusion, subjects reproduced the central extent of standard Mueller-Lyer figures and configural variations. Illusory magnitude of the underestimated wings-in and overestimated wings-out figures was examined with selective amputations of the oblique wings and central line segment (shaft). Variations were presented at 0, 45, 90, and 135 deg from vertical. Orientation had no reliable effect on illusory magnitude. Elimination of the shaft effected a decrease in the apparent extent for all variations, presumably due to the addition of the filled-unfilled space illusory effect to the standard Mueller-Lyer effect. A second study corroborated this finding The decrease in apparent extent consequent to shaft removal occurred independently of any response factor. Selective wing removal differentially decreased the illusory magnitude of the standard Mueller-Lyer figures; this was discussed with regard to a dual-illusion hypothesis. Finally, variations that contained no intersecting lines produced a significant illusion in the direction of the standard Mueller-Lyer figures, suggesting the involvement of higher level, nonperipheral distortion mechanisms.     

Anisotropic perception of visual angle: implications for the horizontal-vertical illusion, overconstancy of size, and the moon illusion.

Three experiments investigated anisotropic perception of visual angle outdoors. In Experiment 1, scales for vertical and horizontal visual angles ranging from 20 degrees to 80 degrees were constructed with the method of angle production (in which the subject reproduced a visual angle with a protractor) and the method of distance production (in which the subject produced a visual angle by adjusting viewing distance). In Experiment 2, scales for vertical and horizontal visual angles of 5 degrees-30 degrees were constructed with the method of angle production and were compared with scales for orientation in the frontal plane. In Experiment 3, vertical and horizontal visual angles of 3 degrees-80 degrees were judged with the method of verbal estimation. The main results of the experiments were as follows: (1) The obtained angles for visual angle are described by a quadratic equation, theta' = a + b theta + c theta 2 (where theta is the visual angle; theta', the obtained angle; a, b, and c, constants). (2) The linear coefficient b is larger than unity and is steeper for vertical direction than for horizontal direction. (3) The quadratic coefficient c is generally smaller than zero and is negatively larger for vertical direction than for horizontal direction. And (4) the obtained angle for visual angle is larger than that for orientation. From these results, it was possible to predict the horizontal-vertical illusion, over-constancy of size, and the moon illusion.     

The role of bottom-up confirmation in the phonemic restoration illusion

Phonemic restoration is an illusion in which listeners hear spoken words as intact, even though parts of them have been replaced by an extraneous sound. An improved methodology was used to investigate how much the illusion depends upon the bottom-up confirmation of expectations generated at higher levels. A powerful bottom-up factor was phone class of the sound to be restored, and its acoustic similarity to the sound that replaced it. When white noise was the replacement sound, fricatives were better restored than vowels, whereas the pattern reversed with a pure tone replacement. Including a short silence period increased restoration of stop consonants. The data indicate that phonemic restoration depends upon the interplay between the listener's expectations and the acoustic signal.     

The role of hand size in the fake-hand illusion paradigm

When a hand (either real or fake) is stimulated in synchrony with our own hand concealed from view, the felt position of our own hand can be biased toward the location of the seen hand. This intriguing phenomenon relies on the brain's ability to detect statistical correlations in the multisensory inputs (ie visual, tactile, and proprioceptive), but it is also modulated by the pre-existing representation of one's own body. Nonetheless, researchers appear to have accepted the assumption that the size of the seen hand does not matter for this illusion to occur. Here we used a real-time video image of the participant's own hand to elicit the illusion, but we varied the hand size in the video image so that the seen hand was either reduced, veridical, or enlarged in comparison to the participant's own hand. The results showed that visible-hand size modulated the illusion, which was present for veridical and enlarged images of the hand, but absent when the visible hand was reduced. These findings indicate that very specific aspects of our own body image (ie hand size) can constrain the multisensory modulation of the body schema highlighted by the fake-hand illusion paradigm. In addition, they suggest an asymmetric tendency to acknowledge enlarged (but not reduced) images of body parts within our body representation.     

The role of preparatory muscular tension in the size-weight illusion

Muscle action potentials were recorded from lifting muscles during the foreperiod before lifts of large and small objects when the size-weight illusion occurred. There were increases in tension throughout the foreperiod of both lifts, culminating in a large increase following the instruction to lift. The increases were greater before lifts of the large can, supporting the peripheral theory of comparative weight judgment. The “set to lift” effect was shown to exist in the action potentials before the foreperiod of the second lift when compared to those just prior to the lift of the first can.