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.     

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     

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 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.     

Bisecting the Ponzo illusion.

A modification of the Ponzo illusion, in which the judged lines are centered in different sized obliques, was presented to matched normal and mentally retarded males (ns = 8) under conditions of equal and unequal retinal sizes. Magnitude of illusion was affected by IQ (WISC) and by physical proximity rather than retinal contour. The results seem to imply a central rather than peripheral explanation for the illusion.     

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.     

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.     

Ontogeny of the Ponzo illusion: Effects of age,schooling, and environment(1)

Subjects were 384 Moroccan males (age range 6–22 yrs.), divided into 16 equal groups, according to the factorial design: age (4) × schooling (2) × environment (2). Subjects were tested on four Ponzo configurations (differing in contextual information) from Leibowitz et al. (1969), the Ponzo perspective stimulus from Segall et al. (1966), the CEFT from Witkin et al. (1971), and a measure of pictorial depth perception. Individual measures of contact with mass-media and urban life were collected on each subject. Analyses indicated that all main factors of age, schooling, and environment played important, and differing, roles in inducing illusion susceptibility. Piaget's (1969) theory of primary and secondary illusions was found useful in understanding the results of the Ponzo configurations used in the study. Primary illusion configurations were found to be relatively insensitive to experiential variables, and illusion susceptibility decreased with chronological age. In contrast, secondary illusion configurations were affected by many experiential factors, and illusion susceptibility was mediated through perceptual development and pictorial depth perception rather than chronological age. It was concluded that single-factor theories of ontogenetic change in illusion susceptibility were inadequate to explain the complex interactions found in this study.     

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.     

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.