After providing all the funding for The Brain from Top to Bottom for over 10 years, the CIHR Institute of Neurosciences, Mental Health and Addiction informed us that because of budget cuts, they were going to be forced to stop sponsoring us as of March 31st, 2013.

We have approached a number of organizations, all of which have recognized the value of our work. But we have not managed to find the funding we need. We must therefore ask our readers for donations so that we can continue updating and adding new content to The Brain from Top to Bottom web site and blog.

Please, rest assured that we are doing our utmost to continue our mission of providing the general public with the best possible information about the brain and neuroscience in the original spirit of the Internet: the desire to share information free of charge and with no adverstising.

Whether your support is moral, financial, or both, thank you from the bottom of our hearts!

Bruno Dubuc, Patrick Robert, Denis Paquet, and Al Daigen

Thursday, 11 February 2021
Revisiting an optical illusion in terms of predictive processing

I recently came across a little experiment that I posted years ago on this website to show how the blind spot in each of your eyes works. The blind spot is a part of the retina where there are no photoreceptors, because it is where the axons of the retina’s ganglion cells converge and exit the eye, forming the optical nerve. As a result, there’s a corresponding area in your field of vision that doesn’t register on the retina. Hence, in theory, you shouldn’t see anything there. But in reality, you don’t see any such blank spot in your field of vision.

To find out why not, let’s revisit this optical illusion from the standpoint of predictive-processing theory, which has become more and more accepted in cognitive science over the past 10 years or so. In the traditional view, the brain passively waits to receive inputs, then processes them and ultimately produces outputs, as if it were nothing more than a biologically based computer. In contrast, according to predictive-processing theory, the brain is a proactive organ that is constantly making predictions about its environment and what may be about to happen there, so as to operate within it more effectively.

According to predictive-processing theory, the reason that you don’t notice your blind spot is that your brain makes you blind, so to speak, to the blindness in that part of your field of vision. It does so by filling this area in with the visual information that is most likely to occur there, according to the experience of the world stored in your memory.

The two graphics above give you two ways of experiencing your blind spot.

For the top graphic, close your right eye, look at the + sign with your left eye, then move your head toward or away from the screen slowly while continuing to watch the + sign. The big black dot will disappear as it passes through the blind spot of the retina of your left eye, because your brain projects the surrounding white background onto the corresponding spot in your field of vision.

For the bottom graphic, do the same thing. This time, when the image is at the right distance from your eye, the two lines will look like one solid line. This seems like pure magic, until you apply the theory of predictive processing to explain it: the brain predicts what is most likely to appear in the space between the two lines—a continuation of them—and inserts it there!

You can readily imagine the evolutionary advantages of your brain’s operating in this way. Most of the signals that it receives from the outside world involve a certain degree of ambiguity. If your brain couldn’t project its predictions about the world based on your past experiences, you might, for example, have to wait until the catlike form jumped out of the high grass on the trail ahead of you before you recognized it as a tiger. By then it would probably be too late to run. If your humanoid ancestors’ brains had worked like that, you might not be around here now to worry about it.

The Senses | Comments Closed

Monday, 4 February 2013
Adelson’s Checkerboard

Optical illusions are always a humbling experience for people who think that they see the world “the way it really is”. Often, when such people are confronted with optical illusions induced by context, the context has to be removed and then restored several times before these people can be convinced that, for example, two lines that appear to be different lengths are actually the same.

The world that we see is often ambiguous, and our visual system tries to give it a meaning on the basis of certain recurrent clues. Scientists today are familiar with some of these clues and can combine them to produce some truly astounding optical illusions. One famous example is Adelson’s checkerboard, in which one square is perceived as black and another as white, even though both are actually the same shade of grey! (more…)

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Monday, 11 June 2012
The Phi Effect Is Not the Beta Effect!

illusionScience is not immune to historic errors that can be passed along for decades. One such error was the confusion of the phi effect with the beta effect, which persisted until Robert M. Steinman and his colleagues published their clarification in 2000.

The phi effect was first described in Experimental Studies on the Seeing of Motion, a book published in 1912 by Max Wertheimer, one of the fathers of Gestalt psychology. The problem was that in his book, Wertheimer did not describe the conditions for the appearance of the phi effect precisely. He said that this phenomenon occurs when two lines are projected on a screen in very close chronological succession, thus creating the impression (under certain observation conditions that he left undefined) that a fuzzily defined area the same colour as the background is moving between these two lines. (more…)

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