McCollough effect: what is this illusory phenomenon?

This strange phenomenon causes two images to merge together and we see an optical illusion.

Optical illusions can be very curious. Generally, they occur when we are exposed for a period of time to very specific patterns of shape and/or color; altering the conventional way in which we perceive lines, curves and even more complex stimuli (the human face, for example).

Illusions are, moreover, phenomena that are expressed in completely healthy people, and that generally revert in just a few seconds. This is precisely what distinguishes them from hallucinations, which usually persist and do not derive from objects within the perceptual frame.

In the present article we will deal with what is one of the most relevant optical illusions in the history of science, the McCollough effect, whose peculiarities have surprised the entire research community for decades. community for decades. In fact, we still do not have a theoretical model that can explain it in its entirety.

What is the McCollough effect?

The McCollough effect was discovered in 1965 by Celeste McCollough, a psychologist who was already studying other perceptual phenomena in the first decades of the last century, although it was defined more exhaustively over the following years.

It is an optical illusion included in the category posteffect, that is, that is, afterimages that require a prior period of exposure to a specific pattern of stimuli in order to unfold. to unfold. In these cases, very specific colors or shapes are usually used, which only affect the most superficial aspects of visual processing and last for a few seconds.

The effect in question, however, is somewhat more complex and has been considered as a potential explanatory mechanism for the way in which visual stimuli are usually integrated centrally. This is why for many years, since it was first described, it has prompted a variety of investigations in which the original methodology has been changed in order to explore the exact origin of the phenomenon. Nevertheless, there is still a lack of definitive knowledge on the subject, although there are some clues that point the way to continue searching.

In the following we will see how to carry out the procedure, what are the "effectsWe will see below how to carry out the procedure, what "effects" can be foreseen and what mechanisms are at the basis of it. However, it is essential to point out at this point that we are not dealing with a game, but with a methodology that promotes changes in brain structures and that can be maintained for a long time (depending on how it is carried out). Everything that is described below comes from the scientific body of knowledge on this issue and it is interesting to know it, but it should not be carried out without the corresponding information and always under the full responsibility of the person who decides to do so.

Procedure

The McCollough effect (like the rest of the illusions of its category) requires a previous induction stage, in which the person must be exposed to two color grids alternately. Specifically, it is a pattern of red horizontal lines (on a black background) and another of green vertical lines (with the same background). Both are shown to the subject for about three seconds, oscillating from one to the other. for a period of time that generally lasts three minutes (although it may vary according to the intention of the person conducting the study).

After this adaptation period, the person is shown a figure composed of black/white lines, arranged both horizontally and vertically. This is a complex grid that includes the patterns described above, but which at this stage lacks any chromaticity (only the direction of the lines is preserved). This new stimulus is presented to the subject in a contingent manner (at the end of the induction phase). (at the end of the induction phase), and the first evidence of perceptual alteration is manifested on it. Sometimes, this monochrome figure is also shown before the induction, so that it can be appreciated that it really lacks colors and the effect is more evident.

The illusory effect

After exposure to the colored gratings, the person will notice that the originally monochromatic grating will acquire different shades in the white color spaces. More specifically, it will be seen that the horizontals will acquire a greenish hue and the verticals will acquire a reddish/pinkish hue. That is, the "inverse" of those shown during the previous induction period (your negatives). So far it is nothing too striking or novel, since there is a great variety of "tricks" to reproduce the same effect, but in the one that concerns us there would be a particularity: it can last for many days, up to three and a half months in the case of very long induction periods.

The effect has also been reported with lines of different colors, such as blue and orange, with results whose duration is related to the duration of the induction.with results whose duration is directly related to the induction time. Thus, it has been tested with periods of just a few seconds and up to 150 minutes, being in the latter cases in which a greater persistence of the afterimage is reported. In any case, many people who used computer monitors (green phosphor) in the 60s and 80s were able to offer pioneering testimony on this issue, since they reported reading books with a pink or soft red background.

Furthermore, it has been proven that the intensity of the post-image colors is also linked to the total time of previous exposure, such that those who only observe a few minutes of the original green/red screens will only be able to distinguish pale negatives of both (light greenish and pinkish tones), but those exposed for ten minutes or more will appreciate them much more vividly and sharply.. Thus, both intensity and duration will depend closely on the previous induction period.

Another very curious fact about the McCollough effect is that it produces what is known as an interocular transfer: although the test is carried out using only one eye, its consequences extend to both eyes. Many of our readers may be able to conjure up the experience of exposing their eyes (unwillingly) to different levels of light, with one eye perceiving the darker tones and the other the lighter. In such an assumption there would be no interocular transfer, since the effect is in the retinal cells related to light perception (rods/cones), but then.... what happens in the present case? Let's continue to investigate.

Why does it happen?

Many different theories have been postulated over the years to explain why the McCollough effect occurs, but we still have only a partial understanding of the issue. The first hypotheses were based on the possibility that it was a phenomenon based on the principles of classical or Pavlovian learning (remodeling of the nervous system based on the continuous presentation of the stimulus), but this was dismissed because it only occurred with linear figures, and not with curves or other more complex shapes.

The second hypothesis was related to the activity of the retinal cells associated with color: the cones.The second hypothesis was related to the activity of retinal cells associated with color: the cones, since they possess photopigments for blue (cyanolabe), red (erythrolabe) and green (chlorolabe), which tend to decay due to chromatic antagonism resulting from simple overexposure. This is what usually happens during a conventional post-image illusion. However, in these cases the change is only maintained for a few seconds (a few minutes at most), and there is never a transmission of this change to both eyes, so it is a line that was discarded for the McCollough effect.

On the other hand, it is evident that in a natural environment the linear green/red pattern that forms the stimulus that triggers this illusion can rarely be seen, so it is possible that the brain interprets this pattern as a line. the brain may interpret it as a kind of sensory aberration and and propitiates mechanisms to "compensate" it during the processing of visual information. In this case, for its explanation, we should resort to the substrate of the central nervous system, obviating the learning and sensory thesis. Is it possible that the answer lies precisely in this mysterious organ?

The brain and the McCollough effect

When we are exposed to a stimulus for a long time, the brain stops making an effort to pay attention to it and simply "understands" that it is so, "ignoring" it and leaving its resources available to perceive the rest of the things in the environment. That may be happening to you right now if you are reading this article from a cell phone: even though you are holding it with one hand, your brain is isolating from the experience everything accessory (its sense of weight, e.g.), and is only working on understanding the text. Well, a similar phenomenon happens with the illusion at hand.

When the eyes are exposed to the green/red lines constantly, the brain comes to understand that this pattern (very rare in nature) will always be so in any possible situation. For this reason, will anticipate it in the presence of stimuli that have a similar relationship to it, such as horizontal and/or vertical monochrome lines.. This will occur, moreover, throughout the first phases that occur as part of visual processing, but beyond what happens before reaching the retina (due to the aforementioned interocular transfer effect).

Thus, the primary visual cortex, located in the posterior region of the brain parenchyma (occipital lobe), has been the focus of attention in recent years. This area (V1) is specialized in the perception of static and moving objects, but especially in the recognition of patterns (such as those that occur during the induction phase of the McCollough effect). It is also the point at which images from both eyes merge, forming integrated and coherent scenes.forming integrated and coherent (binocular) scenes.

The hypothesis currently under consideration involves alterations in this area, which is basic to understand how we represent colors and shapes at a cortical level. Despite this, they are still uncorroborated models, heuristics that serve to guide research activity (based on neuroimaging techniques and comparative studies that include subjects with very different brain lesions).

Although the aforementioned effect tends to dilute as time goes by, there is also a supposed method to tackle it. In such a case, new gratings would be presented (but with their color reduced) to help the brain to relearn that the previous pattern is no longer valid (and to recover a "normalized" perception). The McCollough effect is considered a method of "modifying" brain structure through exposure to an image, and although its effect is not permanent, it should not be undertaken without precise knowledge of what it is and its extent.

Bibliographical references:

• Ans, B., Marendaz, C., Herault, J. and Séré, B. (2010). McCollough Effect: a Neural Network Model based on Source Separation. Visual Cognition, 1(6), 823-841.
• Ramachandran, V. and Zeve, M. (2017) Synesthesia and McCollough Effect. i-Perception, 8(3), 201-211.