The basic neurobiology behind the recently famous 12-dot illusion.

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At this point, you might already be familiar with the visual illusion which, according to Time Magazine, is here to break the internet. Exactly 12 small black circles are scattered around this image – but the tantalising bit is that you can only see a few of them at any one time. As you move your eyes across the picture, it might strike you that circles which you were just looking at suddenly vanish from sight, replaced by a few new ones that were previously invisible. This so-called extinction illusion, which was in fact published in a research journal already in 2000, gained immense popularity last Sunday when the Japanese psychologist Akiyoshi Kitaoka shared it on his Facebook site.

While visual illusions are fascinating to look at, what I find most interesting about them is that they only exist in the first place because they are an artefact of the way our visual system is organised. Thus, behind every great visual illusion is an even greater neurobiological reason why we see it the way we do. So what is it that prevents us from seeing more than a few dots in the extinction illusion? To begin understanding the basics, let’s take a look at how information is transmitted from the eye to the brain.

Lining the back of our eye is the retina – a thin sheet of tightly packed cells, called photoreceptors. Whenever these cells capture photons of light that enter the eye, they send continuous chemical signals to receiving cells in the retina in order to inform them that they have detected something in the tiny area of the visual field that is under their surveillance.

Retina1.pngWhile it might be intuitive to assume that the brain receives constant pixel-by-pixel updates of the light patterns that hit the eye as we scan our environments, this is not the case. If it were, then the eye would just be creating an exact replica of the external world and transmitting a photograph to the brain. We would then need to ask ourselves – is there a little man inside our visual brain, keeping an eye on the constant flux of images that is being projected from the retina and deciding how to interpret the images? This absurd idea is something that the American philosopher Daniel Dennett refers to as the problem of the Cartesian Theatre (discussed briefly in this YouTube video).

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On a fundamental level, the visual system cannot possibly be of any use to us if it merely reconstructs the light patterns that fall on the eye, for the perusal of the brain. It needs to use these patterns to extract information about the external world, which requires the initial light-detection signals from the photoreceptors to undergo some transformations before reaching the brain.  The illustration below explains one of the primary principles of retinal processing that holds the key to the extinction illusion.

 

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So what is the purpose of lateral inhibition in the retina? Let’s consider what kind of stimuli are optimal for activating this bipolar cell. If a diffuse light uniformly activates photoreceptors both in the centre and the surround of this bipolar cell’s receptive field, then the outcome will be a weak signal (since the positive effect of the central inputs will be largely cancelled out by the negative effect of inputs from the surround). On the other hand, the bipolar neuron is more likely to become excited and send a robust signal if it receives a strong input from photoreceptors in the centre of its receptive field and a weak signal from the surrounding area. And there you have it… a dot on a contrasting background is just perfect. Lateral inhibition means that retinal neurons like bipolar cells prefer receiving inputs that contain contrast, which is why it’s thought to be one of the most fundamental mechanisms that enables our visual system to be sensitive to dots and edges (which usually sit at points of change between light and dark regions). While the dots found at the intersection points of the extinction illusion are technically the ideal stimulus for our retinas, one other property of the visual system prevents all the dots scattered across the image from being simultaneously visible.

That property is convergence of information. A bipolar cell collects its signals from multiple photoreceptors, and researchers know that how widely the cell extends its network to collect information differs massively between different parts of the retina. As an example, if we were to examine the centre of the human retina, we would find that a bipolar cell often receives its central input from just one photoreceptor, and its surrounding input from a couple more. In contrast, a bipolar cell in the more peripheral regions of the retina might collect signals from 25 photoreceptors before joining signals with 5000 other bipolar cells when communicating with the retinal ganglion cell (which, in turn, transmits information to the brain). This means that a single retinal ganglion cell in the periphery of our retina collects information from up to 75,000 photoreceptors! This allows it to keep watch over a much larger area of the visual world than cells that pool information from a much smaller number of receptors (hence, cells in the periphery are said to have large receptive fields). A further perk of such great convergence is that these cells are exquisitely sensitive to light, since they can add up weak signals from thousands of receptors. This is primarily why astronomers have historically preferred to look for faint stars by directing the telescope just slightly off-centre of their retinas – a technique known as averted vision.

The downside of the vast convergence of photoreceptor signals that takes place in the periphery of the retina is exactly why the extinction illusion works the way it does. As neurons collect information from more photoreceptors and grow larger receptive fields, they become less capable of resolving small details. Thus, the brain is unlikely to be informed of the presence of small objects in the visual periphery – something that is responsible for the fact that the dots you see in the extinction illusion are never far away from the centre of your gaze.

To better understand this, take a look at the illustrations below and imagine that you are the visual brain whose task is to read and interpret the signals being transmitted to you from the retina as the eye is scanning the extinction illusion image. First, let’s examine the signals produced by a patch of retina close to the centre of the eye, where bipolar and retinal ganglion cells pool information from very small numbers of receptors and thus have small receptive fields (the centres and surrounds of their receptive fields are outlined by solid circles).

Here, you can see that when a small black dot on a lighter background happens to fall on the receptive field centre of one of these cells, it provokes a robust excitatory signal.

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If this dot happens to be located elsewhere (below), then it will likely stimulate the receptive field of another cell that keeps a slightly different area of the visual world under surveillance. As the brain reads the signals arriving from this patch of the retina in these two cases, it has reason to believe in the existence of two small stimuli in different locations. Thus, it provides you with a conscious experience of these dots as distinct entities.

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On the other hand, let’s briefly consider the types of signals the visual brain would receive from a more peripheral region of the retina, where retinal neurons collect their inputs from vast numbers of photoreceptors. Here, the centre of one such cell’s receptive field is outlined with a red circle (the surround extends far beyond the image). When a small dot falls within the centre of this cell’s large receptive field, it will likely provoke a signal.

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However, the vast size of this receptive field means that if another dot were to occur in the vicinity of the first one (below), there is a high probability that it would also stimulate the same receptive field’s centre.

Retina7.pngIn the case of these two dots, the visual brain is receiving the same signal from the same retinal ganglion cell. This means that the information it has access to does not allow it to reasonably infer where something might be happening in a particular area of the visual world. Thus, as receptive fields of neurons in the retina grow larger, their signals give the brain less and less certainty as to what exactly is happening and where. When such certainty about visual events is lacking, there is no reason for them to arise in our conscious experience. Instead, the brain appears to provide us with an experiential ‘filler’ (you don’t exactly go around seeing ‘uncertainty’) and some level of ignorance regarding just how poor our spatial vision is in the eye’s periphery.

Thus, as you move your eyes across the extinction illusion, small black dots dip in and out of your conscious awareness as the periphery of your retina quickly becomes ‘blind’ to the existence of those dots that might have been visible to you when you were gazing directly at them.

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