I was just experimenting in making visual illusions but my first efforts failed. I was looking at this cognitive science website set up by Dutch PhD student Sebastiaan Mathôt when I came across the curiously named ‘Boogie-Woogie‘ illusion. Basically a dot moves across the screen. While you focus on the dot the surrounding line appears to be moving. I reproduced the basic premise of the illusion with a moving dot and on the periphery I placed two chequered patterns anticipating that this set-up would stimulate a movement effect. There was nothing there (at least when I looked).
Returning to the original illusion there are several differences. There is a grey background. I’m sure this doesn’t make too much of a difference. The moving dot isn’t too significant either as I am able to reproduce the movement illusion by simply scanning my eyes back and for across the image. In the video i’ve created the speed of the moving circle is perhaps a little too slow. However even when I scan back and for across the image it doesn’t produce an illusion of movement or any other unusual effects.
My working hypothesis is that the movement effect results from differences between the peripheral and central aspects of the visual field. The basic aspects of the anatomy of the eye and physiology of the early part of the visual system are explained in the videos below.
The central part of the retina is the Macula and allows for a higher visual resolution of the environment than the peripheral areas. We focus on the moving circle with the Macula and in the original ‘Boogie-Woogie’ illusion it is in the lower resolution peripheral vision where the movement effect is taking place. When we pursue a moving object we use a special form of movement known as smooth pursuit eye movements (SPEM’s) to ensure that we track an object with our Macula. The video below discusses a study which uses tracking of SPEM’s for writing*.
Returning to the original illusion we can use the results above as well as the reasoning to hypothesise that the effect can be reproduced simply by producing a thin circular line which alternates between black and white on a white background combined with rapid scanning of the image. This is borne out with the following crude image which I created to test this. Rapid scanning of the circle produce a weak movement effect in the periphery – at least on my initial examination.
The circle above contains a large number of black segments and I suspect that on scanning across the circle there is too much information too deal with in the peripheral vision. This might impact on visual memory, processing of individual segments or a gestalt effect.
Finally the video that triggered this search of is a so-called anomorphic illusion. These are typically produced by street artists who will draw images which convey depth on the pavements. In order to achieve this effect the observer must see the image from a certain perspective. In the video below I couldn’t come to any conclusions. The cube is initially shown with a surface reflection on the table (which perhaps might have formed the basis for the cube image) and the accompanying image provided by brusspup is slightly different from that shown in the video. The premise of the illusion is that if the image remains constant and is rotated in the same perspective it shifts suddenly from a realistic cube sitting in the scene to a disproportionate paper image. I haven’t yet reproduced the experiment.
Nevertheless there is another illusion which I have covered previously showing a large optical illusion structure in a drive-by.
* This video illustrates a trend for scientists to communicate directly with the public
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