Contrast and assimilation in the Baldwin illusion

Four experiments were conducted to investigate the role of "large" squares on the occurrence of assimilation and contrast in the Baldwin illusion. In Exps. 1 and 2, in which subjects reproduced the length of the test line, large squares induced illusions of assimilation. In Exp. 3, reproduction of the entire length of the test line also resulted in illusions of assimilation but bisection of the test line and reproduction of one-half the length of the test line resulted in contrast. Exp. 4 replicated the findings involving judgments of one-half the length of the test line. Over-all, the results indicate that judgments of one-half the length of the test line result in contrast, whereas judgments of the entire extent result in assimilation.     

Lack of sex differences with the Baldwin illusion

Responses of college students (16 men and 16 women) to the Baldwin illusion showed a significant effect for size of square but not for sex. Findings are consistent with those reported previously by Porac, Coren, Girgus, and Verde for adults and for the oldest group of children tested by Pressey and Wilson.     

Auditory streaming and the continuity illusion

In the present experiment, auditory stream organization was investigated in the presence of perceptually restored continuity. It was found that auditory streaming processes tend to yield the same perceptual organization independently of the presence or absence of perceptual restoration. Other observations include the dominance of frequency proximity over trajectory as a perceptual organization principle, and the effect of harmonic enrichment on perceptual grouping.     

Spatial factors of brightness illusion in the Ehrenstein figure

The strength of brightness illusion in an Ehrenstein figure has been examined as a function of two variables--inducing line length and gap size--by a two-alternative forced-choice procedure. The results show an interaction between the two variables; the length of the inducers and gap size are both involved in the formation of the brightness illusion, with gap size as the stronger factor. A spatial limit (corresponding to a gap size of 2.4 deg) was found, below which the illusion is always present regardless of the length of the inducers. An area ratio, defined as the ratio of the area of the ring formed by the four inducing lines of an Ehrenstein figure to the area of the illusory surface, takes into account the different spatial factors studied in the experiment and the global size of the Ehrenstein figure. The strength of the illusion was found to increase linearly with increasing values of the area ratio.     

The effects of location,orientation, and cumulation of boxes in the Baldwin illusion

Three experiments were conducted on variants of the Baldwin illusion. Experiment 1 showed that placing a box on each side of the standard line produced a larger illusion than placing both boxes on the same side of the line. These results failed to support the assimilation theory proposed by Brigell, Uhlarik, and Goldhorn (1977). In Experiment 2, the side of a rectangular box was varied when that side was either parallel or perpendicular to the standard line. The parallel rectangle produced a function that was similar to the one found with the classical Baldwin figure, but the perpendicular boxes produced a monotonically decreasing function with a reversal of illusion evident at the large sizes. The latter function did not support any version of the assimilation theory. The findings of Experiment 3 replicated previous findings that showed that cumulating contours as box size increased had no effect on the illusion. These findings explain two longstanding puzzles about the Müller-Lyer illusion: why a multifinned form is not the sum of its single-finned parts and why the shrinkage form produces a smaller effect than does the expansion form.     

Structural alterations of an ambiguous musical figure: the scale illusion revisited.

The scale illusion (Deutsch, 1975) shows the importance of frequency range in the perceptual organization of a sequence of notes. This paper includes three experiments on the scale illusion. Experiments 1 and 2 demonstrated that if the structure of the pattern of notes used in the original scale illusion study is altered slightly, by adding or subtracting a pair of notes from the ends of the sequence, there is a significant decrease in the rate of frequency-based responses, suggesting a weaker illusion. Experiment 3 investigated two features of the note patterns that may have led to this change. Specifically, it asked whether the decrease in the strength of the illusion is due to (1) the nature of the notes at the extremes of the frequency range and/or (2) the nature of the notes at the crossing point of the two scales. While both sources were found to affect the strength of the scale illusion, the former had a greater influence.     

The influence of figure size and orientation on the magnitude of the horizontal-vertical illusion

Two experiments were performed to examine the effect of display size and figure orientation on the horizontal-vertical illusion. According to the visual field hypothesis of Künnapas (1957a, 1957b) if the relation of the figure components to the surrounding frame is held invariant neither experimental manipulation should exert an appreciable influence. However, both manipulations produced significant effects indicating that the visual field hypothesis is untenable as the primary determinant of the horizontal-vertical illusion. An alternative explanation was discussed.     

The role of figure orientation and apparent depth in the perception of the horizontal-vertical illusion.

An experiment was performed which examined the role of figural orientation directly, and the role of an inappropriately invoked size-constancy mechanism indirectly, in the actuation and magnitude of the horizontal-vertical illusion. When the vertical line of the stimulus figure was aligned above the horizontal line, the illusory effect was significant and positive; in contrast, when the vertical line was located below the horizontal line, the illusion was negative. Under the assumption that a vertical line can appear as a foreshortened line in depth, these findings support an explanation based on the operation of a misapplied size-constancy mechanism.     

Structural alterations of an ambiguous musical figure: The scale illusion revisited

The scale illusion (Deutsch, 1975) shows the importance of frequency range in the perceptual organization of a sequence of notes. This paper includes three experiments on the scale illusion. Experiments 1 and 2 demonstrated that if the structure of the pattern of notes used in the original scale illusion study is altered slightly, by adding or subtracting a pair of notes from the ends of the sequence, there is a significant decrease in the rate of frequency-based responses, suggesting a weaker illusion. Experiment 3 investigated two features of the note patterns that may have led to this change. Specifically, it asked whether the decrease in the strength of the illusion is due to (1) the nature of the notes at the extremes of the frequency range and/or (2) the nature of the notes at the crossing point of the two scales. While both sources were found to affect the strength of the scale illusion, the former had a greater influence.     

An illusion of movement complementary to the horizontal-vertical illusion

Blindfolded subjects moved a stylus held in the hand over a standard distance of 4.5 ins. in a given direction. They then attempted to move the same distance in a direction at right angles to the first. Eight combinations of movements were investigated. The results reveal an illusion such that the extent of movements to left or right across the body is underestimated, while the extent of movements towards or away from the body in the mid-line is overestimated. The illusion applies to speed as well as extent of movement. Movement up or down in a vertical plane is equivalent to movement towards or away from the body in a horizontal plane.

The interaction of this illusion with the well-known horizontal-vertical illusion of visual perception explains a failure to find any net illusory effect where lines visually displayed in different orientations were matched for length by unseen movements in similar orientations.

Whether the visual and movement illusions simply co-exist or whether they are functionally related is not yet clear.     

The role of vision in the on-line correction of illusion effects on action.

In this study, participants reached out and picked up a bar placed on a background grating that induced an illusion in the perceived orientation of the bar. The illusion had a large effect on the orientation of the hand early in the reaches, but this effect decreased continuously as the hand approached the target. This pattern occurred whether or not participants were allowed vision of the hand and target while reaching. These results are consistent with a "planning/control" model of action, in which actions are planned using a context-dependent visual representation but monitored and corrected on-line using a context-independent visual representation. The hypothesized neural bases of these representations are discussed.     

A static geometrical illusion contributes largely to the footsteps illusion

When a black and a white rectangle drifts across a stationary striped background with constant velocity, the rectangles appear to alternately speed up and slow down. Anstis (2001, Perception 30 785-794; 2004, Vision Research 44 2171-2178) suggested that this 'footsteps' illusion is due to confusion between contrast and velocity signaling in the motion detectors of the human visual system. To test this explanation, three experiments were carried out. In experiment 1, the magnitudes of the footsteps illusion in dynamic and static conditions was compared. If motion detectors play an important role in causing the illusion, it should be reduced in the static condition. Remarkably, however, we found that the illusory misalignment between the black and the white rectangle was even more prominent in the static condition than in the dynamic condition. In experiment 2, we measured the temporal-frequency properties of the footsteps illusion. The results showed that the footsteps illusion was tuned to low temporal frequencies. This suggests that the static illusory misalignment can contribute sufficiently to the dynamic illusory misalignment. In experiment 3, the magnitude of the illusion was measured with the rectangles drifting on a temporally modulated background instead of a spatially modulated background. If contrast affects the apparent velocity of the rectangles, temporal modulation of a uniform background should also cause the footsteps illusion. However, the results showed that the magnitude of the illusion was much reduced in this condition. Taken together, the results indicate that the footsteps illusion can be regarded as a static geometrical illusion induced by the striped background and that motion detectors play a minor role at best.     

Auditory motion segregation: a limited analogy with vision

Interaural time delay (ITD) is the dominant cue to sound-source location. When one component of a complex sound changes rapidly but smoothly in ITD, it perceptually segregates from the complex. When different components were changed in ITD in succession, a recognizable melody was heard. Each note was more detectable from the transition than from a distinct ITD with respect to the complex. However, a transition relative to a coherently changing complex produced no segregation, whereas cyclic modulation of ITD, too rapid to be perceived as movement, did produce segregation. These two results suggest that the relevant cue is not movement per se but rather the lack of a well-defined ITD during the transition. Segregation largely disappeared when the transition in ITD was replaced by a temporal gap in the complex of the order of 100 ms. The effect seems similar to visual motion segregation but is best explained by the mechanism of binaural unmasking.     

The Ponzo illusion with auditory substitution of vision in sighted and early-blind subjects

We tested the effects of using a prosthesis for substitution of vision with audition (PSVA) on sensitivity to the Ponzo illusion. The effects of visual experience on the susceptibility to this illusion were also assessed. In one experiment, both early-blind and blindfolded sighted volunteers used the PSVA to explore several variants of the Ponzo illusion as well as control stimuli. No effects of the illusion were observed. The results indicate that subjects focused their attention on the two central horizontal bars of the stimuli, without processing the contextual cues that convey perspective in the Ponzo figure. In a second experiment, we required subjects to use the PSVA to consider the two converging oblique lines of the stimuli before comparing the length of the two horizontal bars. Here we were able to observe susceptibility to the Ponzo illusion in the sighted group, but to a lesser extent than in a sighted non-PSVA control group. No clear effect of the ilusion was obtained in early-blind subjects. These results suggest that, at least in sighted subjects, perception obtained with the PSVA shares perceptual processes with vision. Visual experience appears mandatory for a Ponzo illusion to occur with the PSVA.     

Sex differences in susceptibility to the Poggendorff illusion

This study presents experimental results indicating that there are sex differences in the susceptibility to a geometric optical illusion. Participants (57 male and 39 female undergraduate students) performed 3 trials on a test involving the Poggendorff illusion. Analysis indicated that the magnitude of the illusion diminished significantly with each trial and that the percent perceived error was significantly larger for women than for men. This finding is consistent with the numerous studies which have indicated better visuospatial abilities for men than for women.