What this optical illusion tells us about the nature of consciousness

Focus on the black cross-hairs at the center of this image. As your eyes relax, you’ll start to notice that the purple dots disappear…

…and are replaced by a single green dot which revolves around the target. This is the phi phenomenon, a well-known optical illusion first discovered in 1912. Remarkably, our visual processing system is able to ‘fill in the space’ between the purple dots to create the illusion of smooth, continuous motion.

The scientist who discovered this effect was one of the founding members of Gestalt psychology, which studies the brain’s ability to impose top-down rules when constructing our perception. According to their motto, “the whole is something other than the sum of its parts”, we stitch together sensory inputs using automatic, pre-existing abstractions.

In the case of the phi phenomenon, the mind is processing discrete flashes of light and imposing the illusion of filled space in between them. Our minds are somehow capable of ‘editing’ the sensory data in order to give the appearance of smooth motion when none is present.

That the mind is capable of structuring our experience in this way has serious philosophical consequences. According to neuroscientist Daniel Dennett, this is because it contradicts the philosophical notion of the ‘Cartesian ego’: the idea that we are disembodied subjects of conscious thought. Put another way:

The phi phenomenon reveals a certain ‘gap’ or inconsistency with our most intuitive conception of the self.

Now we ask the question: what if the dots are different colors? In this variation, known as the phi color effect, we get a pretty surprising result:

Under these conditions, subjects consistently report that the dot changes color halfway through its course. This is a bizarre finding, because it implies that we experience a change in color before the dot reaches its next position.

Dennett provides two ways of understanding how this could be happening. Both ways give a possible explanation as to how our minds are ‘filling in’ the sight of the dot changing color.

The first way he calls the Orwellian revision. An Orwellian revision occurs post-experientially: there was a time T when you observed X, but milliseconds afterwards, the memory of X is replaced with Y, leaving you with the illusion of seeing Y at time T. As in George Orwell’s dystopian novel 1984, an event actually occurs but is later tampered with by the state’s censors.

As it relates to the phi color effect, an Orwellian revision would mean that you had the experience of seeing the gap, but your brain rewrote the memory right afterward to give you the experience of motion.

The second way he calls the Stalinesque revision. A Stalinesque revision occurs during the experience itself: your observation of X was contaminated with Y at time T, leaving you with the illusion of seeing Y at time T. This is similar to Stalin’s show trials, in which ostensibly legitimate proceedings were being fabricated in real-time.

As it relates to the phi color effect, a Stalinesque revision would mean that you never actually saw the gap: your experience of it was ‘tainted’ by the illusion of motion as it was happening.

The question that comes to anyone’s mind is: which one actually happens? That is to say:

Is our experience of seeing a color change mid-way along the path due to an ‘Orwellian’ or a ‘Stalinesque’ revision?

This may appear to be a mistake of imprecise measurement not not enough cleverness, but actually Dennett uses this fact to conclude that our everyday experience of the self is wrong. Eventually, he will introduce his own competing notion: a theory of consciousness he believes is more in line with findings of psychology and cognitive science.

Philosophically speaking, our common understanding of the self is known as the Cartesian ego. Rene Descartes arrived at this notion by committing himself to methodological skepticism: a process of questioning one’s own beliefs. Wanting to achieve absolute certitude, Descartes was eventually driven to more and more rigorous forms of doubt—until finally he started to question his own existence.

The insight which led him to certitude was summarized by his famous formula “I think, therefore I am”. Descartes believed that his capacity to use reason made the truth of his own existence an evident fact. In other words, Descartes affirmed his self-existence in the face of radical doubt by identifying the self in the thing doing the doubting in the first place.

This association between the self and the thinking subject is what the Cartesian ego signifies. It is among the most firmly-embedded intuitions we have about ourselves. Simply put, it is the ‘I’ of common use: the person ‘in there’ who thinks, self-reflects, and speaks.

This is perceptible in our everyday language. When we’re caught up in emotions and do something irrational, we might say in apology that “I wasn’t feeling like myself”. The implication is that the lapses in your reasoning process made us unable to tap into our true selves. The ‘true self’ we are referring to here is none other than the Cartesian ego: the subject of pure reason.

How does the phi color effect point to an inconsistency in this concept of the self? Let’s return to the Orwellian and Stalinesque revisions mentioned previously. We can sketch a rough visualization of the mental processes involved in each:

The loop indicates the revision that is made after the experience of seeing the dot, and the conjoining arrow indicates the revision being made during the experience.

What’s at stake in the argument between Descartes and Dennett’s conception of the self is the status of this supposed ‘loop’. We can trace it a bit differently to illustrate this point:

m1 demarcates the period when you first see the dot: an experience which can only be presupposed, since it is always ‘recorded over’ by m2, the illusory perception of motion.

The disagreement between Descartes and Dennett is over the nature of this mythical first experience, m1. According to the Cartesian, we can say that you, the thinking subject, ‘underwent’ m1 at some point in time and then a revision process took place to overwrite the memory of it.

But Dennett’s view of consciousness is more radical than this. It claims that what is ‘conscious’ is solely the final product of any revisions and mental processing that the mind performs. Only once a mental event is conscious, i.e. present in a global network of ‘edited content’, it is capable of being accessed for verbal report.

Therefore, m2 is not simply a retrograde revision of a previously-conscious moment. It is the only conscious content that exists: your consciousness can be said to have ‘skipped’ forward from the perspective of absolute time.

Dennett’s perspective on consciousness gives primary importance to what is ‘written down’ by your mental processes: there is no singular Cartesian observer moving through absolute time

But in his typical fashion, Dennett provides his critique in a clear and memorable way. The phi phenomenon is just one example of how philosophical ideas can be found in the most surprising forms. We just have to wait for the giants to find them.

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