Increasing Perspectival Obliqueness Increases the Leaning Tower Illusion

Abstract

The leaning tower illusion is a perceptual illusion in which two identical images of a tower photographed from below appear to diverge when juxtaposed. We manipulated the perceived obliqueness of the (upright) St Mark bell tower in Venice by modifying two parameters both related to the position of the camera with respect to the tower: (a) increasing the peripherality of the tower and (b) reducing the distance between the camera and the tower. The resulting images clearly show that the illusory leaning effect increases as a function of the obliqueness. Another crucial condition for the leaning tower effect must be that the twin images are perceived as parts of a unitary display: The illusion increases when the distance between the photos is progressively increased, but beyond a certain level of separation, the integration of the images should, of course, break down, and the illusion vanish.

The Misperception of Orientation in Depth When Processing Multiple Pictures With Linear Perspective Cues

“…Parovel and Costall (2018) also show that increasing the peripheral position (obliqueness) increases the LTI effect. Our results show a more a complicated interpretation of obliqueness may be necessary.…”

“…Parovel and Costall (2018) showed that once the images are too far apart in a LTI display, the images are perceived as two separate images and no LTI occurs. In Experiments 3 through 5, we showed other factors can influence LTI effects.…”

“…Alternating presentation of the images also reduced the LTI effect (Experiment 5). Taken together, there are several factors that influence the strength of the LTI effect based on how much the stimuli are visually integrated: attention (Experiments 3 and 4), alternating presentation of stimuli (Experiment 5), and distance between images when presented to the observer (Parovel & Costall, 2018).…”

“…These data can be used to rule out some explanations of how people integrate perspective across multiple pictures (e.g., a new observation point, convergence vectors) and could be used to inform models of perspective space (Erkelens, 2017). It may also be fruitful to investigate the relationship of the current findings to individual differences in perception of how parallel objects appear (Hiris, 2019) and the individual differences in the distance required for images to be no longer visually integrated (Parovel & Costall, 2018).…”

“…Kingdom et al (2017) noted that no LTI is experienced for pictures of towers taken at large distances, suggesting that an object receding in depth is necessary for an LTI. Parovel and Costall (2018) showed that increasing either perspective obliqueness of the image or distance between images (up to a point) increases the magnitude of the LTI. They attribute their results as being the result of (a) interpreting the images as part of one visual scene and (b) the visual system creating a new apparent point of observation between the two images which in turn results in new interpretations of the perspective cues in both images.…”