Effects of test line position and age on the magnitude of the Ponzo illusion

An earlier study by Quina and Pollack (1971) suggested that the two test lines of the classical Ponzo figure underwent simultaneous distortion in opposite directions. In the present study, test lines were shown individually with comparison lines outside the influence of the wedge to 70 school children in Grades 2–8 and to 10 adults. The apical test line was overestimated by all age groups, and the distal line was consistently underestimated. Dissimilar age trends were found, supporting the concept of the Ponzo illusion as the product of two distinct processes.     

Depth separation and the Ponzo illusion

The Ponzo illusion refers to an apparent change in length of objectively equal parallel lines induced by enclosure within an acute angle. The present study investigated this illusory change in stimulus extent as a function of the relative depth positions of the parallel lines and the inducing angle. To permit facile and unconfounded manipulation of apparent depth, the stimuli comprising the Ponzo configuration were stereoscopic contours formed from dynamic random-element stereograms. The main results were: (1) apparent depth separation exerted a strong influence on illusion magnitude; (2) this influence was asymmetrical in that illusion magnitude decreased when the inducing angle appeared in depth behind the parallel lines and increased when the inducing angle appeared in depth in front of the lines. These data are consistent with a general theory of space perception that assumes that information about depth position is processed prior to information about stimulus characteristics.     

Response confidence and the Ponzo illusion.

The number of longer line judgments and the ratings of confidence in these judgements were significantly higher when the comparison line was located near the apex of the Ponzo wedge but only confidence ratings were affected by the orientation of the wedge (ns = 24 men, 24 women). Differences between the two measures suggest that perceptual and nonperceptual processes may be involved in the illusion.     

Effects of contextual and local factors on Ponzo illusion magnitude

Ponzo illusion has been explained by considering either just the inducing elements present in a restricted area of the visual field, the same area in which the test elements are located, or the stimulus configuration as a whole in which even the most distal figural elements – i.e., the external converging lines, here called “Ponzo wedge”– play a crucial role. The two studies reported here aimed at showing that both global configurational characteristics and inducing elements locally interacting with the test stimuli can independently affect the illusory effect. This hypothesis was tested using stimuli in which graphic-inducing elements giving rise to a herringbone pattern (Coren & Girgus, 1978) were drawn in the same area of the test segments. Results of Exp. 1 confirmed the effect of the two factors. In particular, both factors proved to determine the illusion, since they induced illusory effects either in isolation or in the same/opposite direction. In Exp. 2 the relative weight of these two factors was evaluated in relation to the width of the angle of the inducing elements and to the distance of the test segments from the vertex. Results showed no linear relationships between the distance of the test segments from the external inducing elements and the weight of the Ponzo wedge factor. Received: 25 June 1996 / Accepted: 1 October 1997     

The Ponzo illusion and the perception of orientation

A new theory, called the tilt constancy theory, claims that the Ponzo illusion is caused by the misperception of orientation induced by local visual cues. The theory relates the Ponzo illusion-along with the Z?llner, Poggendorff, Wündt-Hering, and cafe wall illusions-to the mechanisms that enable us to perceive stable orientations despite changes in retinal orientation or body orientation. In Experiment 1, the magnitude of the misperception of orientation was compared with the magnitude of the Ponzo illusion. In Experiment 2, predictions of the tilt constancy theory were compared with accounts based on (1) low spatial frequencies in the image, (2) memory comparisons (pool-and-store model), and (3) relative sizejudgments. In Experiment 3, predictions of the tilt constancy theory were tested against predictions of the assimilation theory of Pressey and his colleagues. In the final experiment, the orientation account was compared with theories based on linear perspective and inappropriate size constancy. The results support the tilt constancy theory.     

Temporal processing characteristics of the Ponzo illusion

Many visual illusions result from assumptions of our visual system that are based on its long-term adaptation to our visual environment. Thus, visual illusions provide the opportunity to identify and learn about these fundamental assumptions. In this paper, we investigate the Ponzo illusion. Although many previous studies researched visual processing of the Ponzo illusion, only very few considered temporal processing aspects. However, it is well known that our visual percept is modulated by temporal factors. First, we used the Ponzo illusion as prime in a response priming task to test whether it modulates subsequent responses to the longer (or shorter) of two target bars. Second, we used the same stimuli in a perceptual task to test whether the Ponzo illusion is effective for very short presentation times (12 ms). We observed considerable priming effects that were of similar magnitude as those of a control condition. Moreover, the variations in the priming effects as a function of prime-target stimulus-onset asynchrony were very similar to that of the control condition. However, when analyzing priming effects as a function of participants’ response speed, effects for the Ponzo illusion increased in slower responses. We conclude that although the illusion is established rapidly within the visual system, the full integration of context information is based on more time-consuming and later visual processing.     

Cross-cultural and educational aspects of the Ponzo perspective illusion

The magnitude of the Ponzo illusion was determined for two groups of Ugandans, university students and villagers. For the university population, increase in the depth cues in which the illusion was embedded produced an increase in illusion magnitude similar to that observed among students in an American university. For the villagers, however, no illusion was observed for any of the stimulus conditions. Since the environmental ecology of the two groups was similar, the lack of illusion among the villagers cannot be attributed to differential past history of exposure to depth cues. Rather it is hypothesized that responses to two-dimensional stimuli are determined by both depth and flatness cues, and that the responses of the uneducated observers were determined primarily by flatness cues.     

Measuring the Ponzo illusion with the method of production

The Ponzo illusion was varied as a function of angle of oblique arms and was measured by the method of production. The results showed that the method of production produced very similar results to those obtained by the method of limits. The data also indicated that relatively stable individual scores could be obtained if about six judgments were required and if the targets elicited a large illusion. It was concluded that the method of production should be given serious consideration when choosing a psychophysical method.     

Memories of Morocco: The influence of age,schooling, and environment on memory

To what extent are models of memory general, in that they may be applied to children or to other cultural groups? In an attempt to answer this question, two experiments were undertaken in Morocco to investigate various cultural and experiential antecedents to memory development. A total of 384 children and young adults, ranging in age from 6 to 22 years, were tested in a design that contrasted schooled and nonschooled children in urban and rural environments. Three additional groups of subjects—Koranic students, Moroccan rug sellers, and University of Michigan students—were also studied because it was hypothesized that each might have particular “culture-specific” memory skills as a function of previous experience.A serial short-term recall task was used in Experiment I. Results showed that the recency effect or short-term store was generally invariant with age or experience. Control processes appeared to be a function of age, but only when coupled with schooling, and, to a lesser extent, urban environment. In Experiment II, a continuous recognition memory task was given with black and white photographs of Oriental rugs as stimuli. Forgetting rates were generally invariant with age and experience, while the acquisition parameter seemed to vary as a function of specific cultural experiences. Data from the three additional groups were useful in supporting the hypothesis of culture-specific memory skills.From Experiments I and II, and previous research, it was hypothesized that structural features of memory (e.g., short-term store and invariant forgetting rates) may be universal, while control processes or mnemonics in memory are probably culture-specific, or a function of a variety of experiential and cultural factors that surround the growing child.     

A comparison of the Ponzo illusion with a textural analogue

The inappropriate constancy scaling notion of geometric illusions was explored by employing a textural analogue of the Ponzo figure. Ten Ss estimated the length of a horizontal line by equating it with varying companion lines in the context of the Ponzo figure, a textural analogue, and a baseline control in which the lines appeared with no surrounding contours. The textural analogue had the added feature of imposing no contours at the ends of the horizontal lines. It was found that length estimates were significantly different between the horizontals of the Ponzo figure and control stimuli, but not between the texture figure and a context-free control. The results suggest that inappropriate constancy scaling plays a minor role at best in the perception of geometric illusions.     

Factors affecting the magnitude of the Ponzo perspective illusion among the Baganda

The magnitude of the Ponzo perspective illusion, utilizing stimuli with varying amounts of context, was determined for a population of Ugandan villagers. The subjects were classified as two-dimensional, three-dimensional, or mixed perceivers based upon their verbal responses to photographs portraying symbolic depth cues. The illusion magnitude among those subjects classified as three-dimensional was similar to that of college observers who demonstrate an increase in illusion magnitude with increasing background context. The two-dimensional observers’ responses were similar to those of villagers tested previously, showing no significant increase for the same conditions. The role of two-dimensional flatness cues was eliminated on the basis of a control experiment. The data are interpreted as reflecting the operation of a. cognitive factor determining responsiveness to symbolic depth cues in two-dimensional reproductions.     

The role of apparent depth and context in the perception of the Ponzo illusion

The role of apparent depth features and the proximity of the test lines to the adjacent contours in the actuation of the Ponzo illusion was examined. Six versions of the Ponzo figure were employed: a standard Ponzo figure and five modified figures in which the test lines varied in orientation (horizontal or vertical) and in location (inside or outside the converging contours). Both manipulations resulted in a significant decrease in the magnitude of the illusion in comparison to the standard Ponzo figure. The results suggest that the Ponzo illusion is significantly affected by contextual factors.     

Gradients of distortion seen in the context of the Ponzo illusion and other contours

It is suggested that the spatial distortion evident in the Ponzo figure is a special case of a more general illusion in which a gradient of attenuation appears within areas bounded by angular brackets. The magnitude of this gradient is measured in five lines seen against a number of angular contexts. A similar gradient appears also in the presence of single oblique lines. Accordingly, it is suggested that the distortions seen in the figures usually referred to as “the angle illusions” depend upon the presence of contours which do not necessarily define angles. The implications of these findings for certain existing theories which suggest that some illusions depend upon apparent-distortion of angular size and that they contain features usually associated with two-dimensional perspective projections of typical three-dimensional scenes are discussed.     

Perception of the Ponzo illusion by rhesus monkeys, chimpanzees, and humans: Similarity and difference in the three primate species

In Experiment 1, 3 rhesus monkeys and 1 chimpanzee were tested for their susceptibility to the Ponzo illusion. The subjects were first trained to report the length of the target bar presented at the center of the computer display by touching either of the two choice locations designated as “long” or “short.” When inverted-V context lines were superimposed on the target bar, the subjects tended to report “long” more often as the apex of these upward-converging lines approached the target bar. The perception of the Ponzo illusion was thus demonstrated. In Experiment 2, the same 3 rhesus monkeys and 2 new chimpanzees were tested using two types of context lines that provided different strengths of linear perspective. The subjects showed a bias similar to that found in Experiment 1, but there was no difference in the magnitude of the bias between the two types of context in either species. This failed to support the classic account for the Ponzo illusion, the perspective theory, raised by Gregory (1963). In Experiment 3, the magnitude of the illusion was compared between the inverted-V context and the context consisting of short vertical lines having the same gap as the former in the same 3 rhesus monkeys and 2 of the chimpanzees from the preceding experiments. While the chimpanzees showed the illusion for both types of stimuli, the monkeys showed no illusion for the latter. In Experiment 4, 6 humans were tested in a comparable procedure. As in the nonhuman primates, the illusion was unaffected by the strength of linear perspective. On the other hand, the humans showed considerably larger illusion for the context consisting of vertical lines than for contexts consisting of converging lines. Thus, there was a great species difference in the effect of the gap itself on the magnitude of the Ponzo illusion. Similarity found at first turned out to be no more than superficial. Possible sources of this species difference are discussed.     

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.