The Effects of Changing Contrast and Colour in an Image: Continuing with a Visual Illusion Experiment – Part 6

Returning to the visual illusion experiment the video above shows the results of applying a change in contrast and colour to the original image. The illusion consists of two parts – the first is produced by the circumference. The alternating colours can produce an afterimage effect particularly when the image is static. The second effect is a ‘leaking’ of colour within the circle as it moves from one side of the screen. The colour gradient within the circle remains invariant during the movement.

In the first transformation demonstrated in the video above, the contrast is increased. This transformation effectively removes the subtle grading effect on the left side of the circle. This lessens the second effect. However the reader may observe that there is a sharp contrast between the grey and white areas within the circle on the left. This boundary may appear to change slightly during the movement and is a third type of subtle effect. As in previous images complex gradients within the image may require more processing than a circle which is the same colour throughout.

The argument for this would be based on an explanation at the level of the retina. In a previous post I looked at an explanation by Professor Meiker of two visual illusions (see below).

Applying the same explanation to the image above, the visual system needs to process the circle as it moves across the screen. Since the circle is filled with a gradient the cells in the retina will be picking up the contrast across the circle. If the circle was stationary there would be no need for the cells to change their activity as there is no new information being presented (the explanation is more complex as under experimental conditions without absolute stability of the fovea in relation to an image, that image will fade. Additionally the eyes undergo microsaccadic movements even during periods of visual fixation). However as the circle is moving across the screen there is a complex change of gradient within the image. This should trigger firing of contrast detection cells in the area of the retina receiving light from the image.

Compared to no change in the image there should be many more contrast cells firing. The signals from these cells would be conveyed to the visual cortex and the visual scene reassembled as the scene is perceived. If we say that the processing of these signals is equivalent to a calculation (the interested reader can find further details of theories about how the organisation of neuronal circuits might facilitate this type of operation here) then a rapidly changing scene with a lot of contrast would require more calculations by the retinal apparatus than a simple unchanging scene. An increase in the number of calculations may lead to an increase in the number of errors from deconstruction of the scene all the way through to reassembly of the scene/perception. This error rate may lead to reduced stability of the boundary in the image in the video above where there is increased contrast. This is one explanation at least. Nevertheless it can be tested by assessing the response to images of increasing complexity.


Part 5

Part 4

Part 3

Part 2

Part 1

Index: There are indices for the TAWOP site here and here Twitter: You can follow ‘The Amazing World of Psychiatry’ Twitter by clicking on this link. Podcast: You can listen to this post on Odiogo by clicking on this link (there may be a small delay between publishing of the blog article and the availability of the podcast). It is available for a limited period. TAWOP Channel: You can follow the TAWOP Channel on YouTube by clicking on this link. Responses: If you have any comments, you can leave them below or alternatively e-mail Disclaimer: The comments made here represent the opinions of the author and do not represent the profession or any body/organisation. The comments made here are not meant as a source of medical advice and those seeking medical advice are advised to consult with their own doctor. The author is not responsible for the contents of any external sites that are linked to in this blog.


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