Beyond the dress — the science of illusion
A neuroscientist/psychologist shows how the brain can trick us into seeing movement and color that’s not there
By Pascal Wallisch, PhD Posted on 27 February 2015
This article originally appeared on Pascal's Pensées, the blog of NYU psychology professor and Elsevier author Dr. Pascal Wallisch. Here is an excerpt followed by links to the full post.
The brain lives in a bony shell. The completely light-tight nature of the skull renders this home a place of complete darkness. So the brain relies on the eyes to supply an image of the outside world, but there are many processing steps between the translation of light energy into electrical impulses that happens in the eye and the neural activity that corresponds to a conscious percept of the outside world.
In other words, the brain is playing a game of telephone and – contrary to popular belief – our perception corresponds to the brains best guess of what is going on in the outside world, not necessarily to the way things actually are.
This has been recognized for at least 150 years, since the time of Hermann von Helmholtz. As there are many parts of the brain that contribute to any given perception, it should not be surprising that different people can reconstruct the outside world in different ways.
This is true for many perceptual qualities, including form and motion. While this guessing game is going on all the time, it is possible to generate impoverished stimulus displays that are consistent with different mutually exclusive interpretations, so in practice the brain will not commit to one interpretation, but switch back and forth.
For example, Rubin's vase (at right) is a classical example of figure/ground segmentation. The image is fundamentally ambiguous. People perceive a vase or faces, but not both at the same time.
These are known as ambiguous or bistable stimuli, and they illustrate the point that the brain is ultimately only guessing when perceiving the world. It usually just has more information to go by and disambiguate the interpretation.
The image below shows a bistable motion stimulus. Do you see the dots moving from left to right or up and down?
This is also true for color vision. The fundamental challenge in the perception of color is to identify an object despite changing illumination conditions. The mixture of wavelengths that reaches our eye will be interpreted by the brain as color, but which part is due to the reflectance of the object and which part is due to the illumination?
This is a inherently ambiguous situation, so the brain has to make a decision whether to take the appearance of an object at face value or whether it should try to discount the part of the information that stems from the illumination. As the organism is not primarily interested in the correct representation of hues, but rather the identification of objects in light of dramatically varying conditions (e.g. a predominance of long wavelengths in the early morning and late afternoon vs. more short wavelengths at noon), it is commonly accepted that the brain strives for "color constancy" and is doing a pretty good job at that. But in this tradeoff towards discounting, something has to give, and that is that we are bad at estimating the apparent absolute hue of objects. For instance, a white surface illuminated by red light will objectively look reddish. The same white surface illuminated by blue light will objectively look blueish. In order to recognize both as the same white surface, the subjective percept needs to discount the color of the light source.
So it should not be surprising that inference of hue can be dramatically influenced by context. ...
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Elsevier Connect Contributor
Dr. Pascal Wallisch (@Pascallisch) received his PhD from the University of Chicago, did postdoctoral research at the Center for Neural Science at New York University, and currently serves as Clinical Assistant Professor of Psychology at NYU. His research interests are at the intersection of psychology and neuroscience, specifically cognitive and computational neuroscience. His work focuses on motion perception, autism and the appraisal of film. His blog is Pascal's Pensées.
Dr. Wallisch is the author of MATLAB for Neuroscientists, 2nd Edition, published by Elsevier. The text serves as the only comprehensive study manual and teaching resource for MATLAB, the globally accepted standard for scientific computing, in the neurosciences and psychology.