A review of the footsteps illusion

Studies on the footsteps illusion proposed by Anstis (2001) and its variants are reviewed in this article. The footsteps illusion has been explained as a difference in perceived speed depending on edge contrast (Thompson, 1982). In addition to this explanation, it is suggested that the footsteps illusion and its variants can also be attributed to the geometrical illusion presented by Gregory and Heard (1983), to the extinction effect similar to hidden images by Wade (1990), and to subsequent position or motion captures. Related illusions, for example, the kickback illusion (Howe, Thompson, Anstis, Sagreiya, & Livingstone, 2006), the kick-forward illusion, the driving-on-a-bumpy-road illusion, or the footsteps illusion based upon reverse phi motion, are discussed in this article.

Spatial Filtering and the Zöllner—Judd Geometrical Illusion: Further Studies

In a geometrical figure in which long vertical lines are each crossed by a series of short oblique lines, an illusory effect is obtained such that the orientations of the long lines are perceived as nonvertical and shifted away from the orientation of the oblique lines (the Zöllner illusion). In addition, the vertical separation between the crossing (oblique) lines is perceived as less than that if the crossing lines are horizontal (the Judd illusion). It has previously been shown that these two effects are closely related, and a single-process account has been proposed in which both effects are explained by a computational model involving band-pass spatial filtering of the figure by means of difference-of-Gaussians (DOG) filters. Two arguments are presented against the latter account. First, in an opposite-contrast-polarity figure with, for example, white vertical lines and black crossing lines on a mid-grey background, the peaks in the DOG filter output are such as to predict the reversal of the Zöllner—Judd effects. It is shown by demonstration that this prediction is disconfirmed, and that the normal effects are obtained. Second, it is shown that the normal Zöllner—Judd effects are obtained in the absence of the long vertical lines, and in the presence of anomalous contours. The latter effects are also in contradiction to the band-pass-filtering model. These findings are discussed in relation to a dual-process account of the Zöllner—Judd effects.

Obliquity Integration: Variations on a Theme of Delbœuf

An optical—geometrical illusion, described by Delbœuf and not familiar to specialists, is investigated. The results of two experiments show that the divergence between a bar filled with parallel slanting lines and a line drawn above it is clearly related to this angle of the lines which fill the bar. The illusion is already present when this angle is 10°, reaches its maximum at 20°, decreases at 30°, and almost disappears at 40°. These results are similar to those found for the tilt illusion, are slightly different from those found for the rod-and-frame illusion, and differ greatly from those found for the Zöllner illusion. The other variables considered—the distance between the slanting lines filling up the bar, the distance between the upper line and the bar, and the width of the bar—do not influence the illusion as much. Since either the line appears as diverging from the bar, or the bar seems inclined in relation to the line, the illusion should be considered a complex one. The small oblique lines inside the bar induce obliquity in the opposite sense in the display, but which of the elements is seen as diverging from the other depends on which of the two is established as the frame of reference.

The Optical-Geometrical Illusion “Corner Poggendorff” Re-Examined

In 1988 Greene noted, if a straight pair of obliques are drawn outside of two orthogonal lines, the segments appear to be angled slightly one relative to the other. This illusion, designated as “corner Poggendorff,” is different from the Poggendorff effect (the two obliques seem to lie on a parallel path). The results of the present experiments ( N = 76 students) suggest that the general conditions for the corner Poggendorff are similar to those for the Judd illusion.

Another Optical—Geometrical Illusion

A new optical—geometrical illusion is described. The parallelism of short rows of dots is affected by some unknown factor, so that the rows appear as pivoting on their middle point. Some explanations of the illusion are considered, but with no success.

The Contribution of Relational Factors to Line-Length Matches

Both Künnapas and Rock and Ebenholtz investigated the effects of surrounding-frame size upon line-length judgments. Whereas Künnapas obtained errors of the order of 10–15%, typical of those which occur in other geometrical illusion figures, Rock and Ebenholtz reported errors closer to 100%. Rock and Ebenholtz claimed that perceived size is largely relationally determined and that this fact was obscured in Künnapas' experiment by allowing observers to compare the test lines directly with each other and within a common framework provided by the wall of the room. On the contrary, the experiments reported here suggested that errors in the Rock and Ebenholtz study may have been inflated by their use of nondirective instructions and by the confounded effects of other variables. When the Rock and Ebenholtz experiments were repeated with adequate controls over these variables, the effect of surrounding frames on line-length matches were of the order of 15–19%, similar to those of Künnapas but much smaller than those reported by Rock and Ebenholtz. The complexity of the stimuli in the Rock and Ebenholtz type of experiment is such, however, that the effects of a number of variables and possible cue conflicts remain to be investigated.

Autokinetic Movement in Geometric Illusions: A Note

Four static geometrical illusions were presented in an autokinetic movement situation. 50 Ss saw each form four times. Of the total responses, 36% were no-movement responses while 37% of the time the total figure was seen as moving. The remaining 27% of the responses involved apparent movement in only a part of the figure. In general, the elements seen as moving are the parts misperceived in the static geometrical illusion.