Humans are fascinated by visual illusions, which occur when there is a mismatch between the pattern of light that falls on the retina, and what we perceive.
Before books, films, and the internet allowed illusions to be shared widely, people were captivated by illusions in nature.
Indeed, it is here that the long history of the study of illusions begins. Both Aristotle and Lucretius described motion illusions following observation of flowing water.
Aristotle observed pebbles beneath flowing water for some time and noticed that afterwards, pebbles beside the water appeared to be in motion.
Lucretius, meanwhile, looked at the stationary leg of his horse when in the middle of a fast flowing river and noted that it seemed to be moving in the opposite direction to the flow.
This is called induced motion and it has long been observed when clouds pass the moon – the moon can seem to move in the opposite direction.
But a more compelling account of such illusions was first provided by Robert Addams, a travelling natural philosophy lecturer, in 1834 following his observation of the Falls of Foyers in Scotland.
After watching the waterfall for a while, he observed that the adjacent rocks appeared to move upwards:
Having steadfastly looked for a few seconds at a particular part of the cascade, admiring the confluence and decussation of the currents forming the liquid drapery of waters, and then suddenly directed my eyes to the left, to observe the vertical face of the sombre age worn rocks immediately contiguous to the water fall, I saw the rocky face as if in motion upwards, and with an apparent velocity equal to that of the descending water, which the moment before had prepared my eyes to behold this singular deception.
A demonstration of the Waterfall Illusion using a video of the Falls of Foyers (Scotland) taken from where Robert Addams famously observed the effect in 1834. Video courtesy of Nick Wade.
This description of the phenomenon helped stimulate a torrent of research, with the effect becoming known as the “waterfall illusion”.
Basically, after looking at something moving in one direction for a while, something that is still will appear to move in the opposite direction.
Addams did not need a theory to know that this was an illusion: the rocks looked stationary before looking at the waterfall but appeared to move upwards after having stared at the waterfall.
All that was required was a belief that objects remain the same over time, but that the perception of them could change.
This illusory movement – one that we see in a still pattern following observation of motion – is known as the motion aftereffect.
Later descriptions of the motion aftereffect were based on moving images like rotating spirals or sectored discs that can be stopped after motion.
Once stopped, such shapes appear to move in the opposite direction.
Addams did provide a possible basis for the illusion.
He argued that the apparent motion of the rocks was a consequence of unconscious pursuit eye movements when viewing descending water.
That is, although he thought he was keeping his eyes still, he argued that, in fact, they were moving involuntarily in the direction of the descending water and then rapidly returning.
But this interpretation was completely wrong.
Eye movements cannot explain this aftereffect because they would result in the whole scene appearing to move, not an isolated part of it. T
his was pointed out in 1875 by the physicist Ernst Mach, who showed that motion aftereffects in opposite directions can be seen at the same time but the eyes cannot move in opposite directions simultaneously.
The brain and motion illusions
So what is going on in the brain in the case of this illusion?
This is fascinating to visual scientists because motion aftereffect illusions tap into an essential aspect of processing in the brain – how neurons respond to motion.
Many cells in our visual cortex are activated by movement in one particular direction. Explanations of these illusions are related to differences in the activity of these “motion detectors”.
Aristotle observed pebbles beneath flowing water for some time, and noticed that afterwards pebbles beside the water appeared to be in motion. The image is in the public domain.
When we look at something that is stationary, then the “up” and “down” detectors have nearly the same activity.
But if we watch water falling down, the “down” detectors will be more active than the “up” detectors, and we say we see downwards movement.
But this activation, after a while, will adapt or fatigue the “down” detectors, and they will not respond as much as before.
Say we then look at stationary rocks.
The activity of the “up” detectors will now be relatively high compared to the adapted “down” detectors, and we, therefore, perceive upward motion. (This is the simple explanation – in fact, it’s all a bit more complicated than this.)
Observing the waterfall illusion, we can notice another interesting effect – things can appear to move without seeming to change in position.
For example, in the video of the waterfall illusion, the water seems to be surging upwards but it does not get any closer to the top.
This suggests that movement and position might be processed independently in the brain. In fact, rare brain injuries can prevent people from seeing movement, while still perceiving changes in position.
We call this condition akinetopsia. One such patient, for example, described that flowing water looked like a glacier.
Humans have always been intrigued by illusions, but it’s only within the last century that they have been able to teach us about the workings of the brain.
With many ongoing advances in neuroscience, we still stand to learn much about awareness and cognition by studying these perceptual mismatches.
Visual illusions show us that we do not have direct access to reality.
They can also provide an inkling of the mental processing that delivers our experience of the viewable world.
Indeed, it is the processing happening inside our brains that is the basis for many illusions.
Rather than delivering information from our eyes in nearly raw form as a camera would, the brain tries to determine what is actually out there – what are the shapes and the objects in the scene?
When the information entering the eye is ambiguous, the brain must make educated guesses. The three displays below demonstrate this in rather delightful ways.
The Illusion of Sex
In this illusion by Richard Russell, the same face appears to be female when the skin tone is made lighter (left image) and male when the skin tone is made darker (right image).
The illusion works because changing the skin tone affects the face’s contrast – the difference between the darkest parts of the face (lips and eyes) and lightest parts (the skin).
Few would regard facial contrast as a defining feature of either sex, but in fact, contrast is on average higher in females than males.
Even without consciously knowing it, our brains are attuned to the difference in contrast between the sexes, and so contrast is one cue the brain uses to determine gender.
When other cues are removed, contrast can be the deciding factor.
Perhaps the most interesting thing about the illusion is that the contrast doesn’t simply help us work out the sex of the face – it provides an experience of ‘seeing’ a face that is male or female.
The use of the contrast cue is done by unconscious processes.
The image in our mind’s eye has incorporated information that we already hold, and uses this to resolve ambiguity in the image.
The Coffer Illusion
The Coffer Illusion may initially appear as a series of sunken rectangular door panels, but after a few seconds, your brain’s representation of the image may ‘flip’ to give you the experience of 16 circles.
People have been fascinated by such ambiguous figures since at least the time of the ancient Romans.
The Coffer Illusion plays on the fact that the visual brain is heavily geared towards identifying objects.
‘Pixels’ are grouped to form edges and contours, shapes, and finally objects.
Sometimes, as in the Coffer Illusion, there is no ‘right’ grouping because the image is inherently ambiguous.
Two different groupings make sense – a single set of horizontal lines can either form a circle, or be the intersection between two rectangles.
For most people, the grouping into rectangles initially dominates.
This may be because rectangles (including the ones we see in door panels) are often more common than circles in our daily environment, and so the brain favours the grouping that delivers rectangular shapes.
Mask of Love
In Gianni Sarcone’s Mask of Love, a Venetian mask can be seen to contain either a single face or the faces of two people kissing.
The illusion operates in a similar way to the Coffer illusion – the contours in the image can be grouped in two different ways, leaving the brain uncertain about which to choose.
The difference with this illusion is that, at least for some people, neither grouping tends to dominate.
The image appears to flip reasonably freely between the two plausible alternatives.
Flipping is an interesting way for the visual brain to deal with ambiguity.
Other parts of the brain have mechanisms that average ambiguous information, or simply choose the most likely representation and ignore all alternatives.
Video reveals how your brain gets fooled by optical illusions
Flipping has the advantage of providing coherent information about what the image could be, which might be useful to know in working out how to interact with the world.
Together, these three illusions demonstrate that visual processing is highly geared towards identifying what an object is.
The representation in our mind’s eye is designed to be functional, so rather than delivering a mess of pixels, we have elaborate visual experiences of circles, rectangles, faces, and even the gender of faces.
Nick Wade & Niia Nikolova – The Conversation
The image is in the public domain.