Why the Conventional Concept of “Visual Illusions” is Misleading

A common conception is that we see the world in accord with physical reality, but are sometimes fooled in special circumstances that give rise to discrepancies between reality and perception (“visual illusions”). Evidence accumulated over the last decade, however, indicates that all visual perceptions are at odds with the parameters revealed by physical measurements. Because retinal images cannot specify stimulus sources due to a loss of information, visual animals have evolved a strategy of vision based on the value stimuli have for reproductive success. In contrast to received views, this criterion demonstrates that all visual perceptions are equally “illusory”.

Purves, D., Wojtach, W.T., & Lotto, R.B. (2017).  The Oxford Compendium of Visual Illusions. Oxford University Press.

 

Perception and Reality: Why a Wholly Empirical Paradigm is Needed to Understand Vision

A central puzzle in vision science is how perceptions that are routinely at odds with physical measurements of real world properties can arise from neural responses that nonetheless lead to effective behaviors. Here we argue that the solution depends on: (1) rejecting the assumption that the goal of vision is to recover, however imperfectly, properties of the world; and (2) replacing it with a paradigm in which perceptions reflect biological utility based on past experience rather than objective features of the environment. Present evidence is consistent with the conclusion that conceiving vision in wholly empirical terms provides a plausible way to understand what we see and why.

Purves, D., Morgenstern, Y. & Wojtach, W.T. (2015).  Frontiers in Systems Neuroscience, 9:156. doi: 10.3389/fnsys.2015.00156.

 

Will Understanding Vision Require a Wholly Empirical Paradigm?

Based on electrophysiological and anatomical studies, a prevalent conception is that the visual system recovers features of the world from retinal images to generate perceptions and guide behavior. This paradigm, however, is unable to explain why visual perceptions differ from physical measurements, or how behavior could routinely succeed on this basis. An alternative is that vision does not recover features of the world, but assigns perceptual qualities empirically by associating frequently occurring stimulus patterns with useful responses on the basis of survival and reproductive success. The purpose of the present article is to briefly describe this strategy of vision and the evidence for it.

Purves, D., Morgenstern, Y. & Wojtach, W.T. (2015).  Frontiers in Psychology, 6:1072. doi: 10.3389/fpsyg.2015.01072.

 

How Biological Vision Succeeds in the Physical World

Biological visual systems cannot measure the properties that define the physical world. Nonetheless, visually guided behaviors of humans and other animals are routinely successful. The purpose of this article is to consider how this feat is accomplished. Most concepts of vision propose, explicitly or implicitly, that visual behavior depends on recovering the sources of stimulus features either directly or by a process of statistical inference. Here we argue that, given the inability of the visual system to access the properties of the world, these conceptual frameworks cannot account for the behavioral success of biological vision. The alternative we present is that the visual system links the frequency of occurrence of biologically determined stimuli to useful perceptual and behavioral responses without recovering real-world properties. The evidence for this interpretation of vision is that the frequency of occurrence of stimulus patterns predicts many basic aspects of what we actually see. This strategy provides a different way of conceiving the relationship between objective reality and subjective experience, and offers a way to understand the operating principles of visual circuitry without invoking feature detection, representation, or probabilistic inference.

Purves, D., Monson, B.B., Sundararajan, J., & Wojtach, W.T. (2014). Proceedings of the National Academy of Sciences, USA, 111: 4750-4755.

 

Understanding Vision in Wholly Empirical Terms

This article considers visual perception, the nature of the information on which perceptions seem to be based, and the implications of a wholly empirical concept of perception and sensory processing for vision science. Evidence from studies of lightness, brightness, color, form and motion all indicate that, because the visual system cannot access the physical world by means of retinal light patterns as such, what we see cannot and does not represent the actual properties of objects or images. The phenomenology of visual perceptions can be explained, however, in terms of empirical associations that link images whose meanings are inherently undetermined to their behavioral significance. Vision in these terms requires fundamentally different concepts of what we see, why, and how the visual system operates.

Purves, D., Wojtach, W.T., & Lotto, R.B. (2011). Proceedings of the National Academy of Sciences, USA, 108 (3): 15588-15595.

 

An Empirical Explanation of the Speed-Distance Effect

Understanding motion perception continues to be the subject of much debate, a central challenge being to account for why the speeds and directions seen accord with neither the physical movements of objects nor their projected movements on the retina. Here we investigate the varied perceptions of speed that occur when stimuli moving across the retina traverse different projected distances (the speed-distance effect).  By analyzing a database of moving objects projected onto an image plane we show that this phenomenology can be quantitatively accounted for by the frequency of occurrence of image speeds generated by perspective transformation. These results indicate that speed-distance effects are determined empirically from accumulated past experience with the relationship between image speeds and moving objects.

Wojtach, W.T., Sung, K., & Purves, D. (2009). Public Library of Science ONE, 4(8): e6771.

 

Reconsidering Perceptual Content

An important class of teleological theories cannot explain the representational content of visual states because they fail to address the relationship between the world, projected retinal stimuli, and perception. A different approach for achieving a naturalized theory of visual content is offered that rejects the traditional internalism/externalism debate in favor of what is termed “empirical externalism”. This position maintains that, while teleological considerations can underwrite a broad understanding of representation, the content of visual representation can only be determined empirically according to accumulated past experience. A corollary is that a longstanding problem concerning the indeterminacy of visual content is dissolved.

Wojtach, W.T. (2009). Philosophy of Science, 76: 22-43.

 

An Empirical Explanation of Aperture Effects

The perceived direction of a moving line changes, often markedly, when viewed through an aperture. Although several explanations of this remarkable phenomenology have been proposed, these accounts typically focus on the percepts elicited by a particular type of aperture and offer no biological rationale for these puzzling effects. Here we test the hypothesis that to contend with the inherently ambiguous nature of motion stimuli, the perceived direction of objects moving behind apertures of different shapes is determined by a wholly empirical strategy of visual processing. An analysis of moving line stimuli generated by objects projected through apertures shows that the directions of motion subjects report in psychophysical testing is accounted for by the frequency of occurrence of the 2-D directions of stimuli generated by simulated 3-D sources. The accuracy of these predictions supports the conclusion that the direction of perceived motion is determined entirely by accumulated behavioral experience with sources whose physical motions cannot be conveyed by image sequences as such.

Sung, K., Wojtach, W.T., & Purves, D. (2009). Proceedings of the National Academy of  Sciences, USA, 106: 298-303.

 

An Empirical Explanation of the Flash-Lag Effect

When a flash of light is presented in physical alignment with a moving object, the flash is perceived to lag behind the position of the object. This phenomenon, known as the flash-lag effect, has been of particular interest to vision scientists because of the challenge it presents to understanding how the visual system generates perceptions of objects in motion. Although various explanations have been offered, the significance of this effect remains a matter of debate. Here, we show that: (i) contrary to previous reports based on limited data, the flash-lag effect is an increasing nonlinear function of image speed; and (ii) this function is accurately predicted by the frequency of occurrence of image speeds generated by the perspective transformation of moving objects. These results support the conclusion that perceptions of the relative position of a moving object are determined by accumulated experience with image speeds, in this way allowing for visual behavior in response to real-world sources whose speeds and positions cannot be perceived directly.

Wojtach, W.T., Sung, K., Truong, S., & Purves, D. (2008). Proceedings of the National Academy of  Sciences, USA, 105 (42): 16338-16343.

 

Visual Illusions: An Empirical Explanation

The evolution of biological systems that generate behaviorally useful visual perceptions has inevitably been guided by many demands. Among these are: (1) the limited resolution of photoreceptor mosaics (thus the input signal is inherently noisy); (2) the limited number of neurons available at higher processing levels (thus the information in retinal images must be abstracted in some way); and (3) the demands of metabolic efficiency (thus both wiring and signaling strategies are sharply constrained). The overarching obstacle in the evolution of vision, however, was recognized several centuries ago by George Berkeley (1709), who pointed out that the information in images cannot be mapped unambiguously back onto real-world sources. In contemporary terms, information about the size, distance and orientation of objects in space are inevitably conflated in the retinal image; the same problem affects light itself, since the source of illumination, surface reflectances, and atmospheric transmittance also conflate information.  In consequence, the patterns of light in retinal stimuli cannot be related to their generative sources in the world by any logical operation on images as such; nevertheless, to be successful visually guided behavior must deal appropriately with the physical sources of light stimuli.  As explained here, visual illusions appear to arise primarily from the way the visual system contends with this problem.

Purves, D., Wojtach, W.T., & Howe, C. (2007). Scholarpedia, 3(6): 3706.