Optical illusions demonstrate how our brains filter sensory stimulii and fit it into a construct that ‘makes sense’ to us. Can we measure our differing perceptions?
In the early 1990s a group of children were asked to identify a drawn picture. Six months later, a similarly sized group of children were asked to identify the exact same picture (above). In April, most saw a rabbit. In October, most saw a duck. The picture had not changed, so what had? The first group had seen the image around Easter time, prompting their perception towards a furred rabbit over a feathered duck.
The rabbit-duck illusion is one of many illusions identified by Joseph Jastrow in 1899. In his essay The Mind’s Eye, published in Popular Science Monthly, Jastrow brings the reader’s attention to “a mind behind the eye and the ear and the finger tips which guides them in gathering information, and gives value and order to the exercise of the senses” Our thinking mind, he tells us, makes sense of the world. It chooses our perspective and fills in gaps, sometimes incorrectly.
The rabbit-duck illusion is known as an ambiguous figure. It can be seen as either animal. Other illusions play on our perception of relative shape or size. A circle might seem smaller or larger depending upon what is around it.
Two identical shapes can seem to be different sizes if the shorter line of one shape is placed adjacent to the longer line of the other.
Still others operate on the principle of gestalt, the human brain’s tendency to fill in spaces in order to create a organized whole, even when our source material is not organized. When Jastrow asked a class of his psychology students to copy the following picture exactly, for example, only three or four noticed and reproduced the actual lines observed. The rest wrote “EDITOR.”
And finally, there is the fading out of an entire image that we have simply seen too much of. Our nose for instance, which remains in our field of view but is never seen unless we consciously try to. Or try to stare at the picture below (called Troxler’s effect) for a minute and watch it simply vanish.
Optical illusions can be literal, physiological, or cognitive. A literal illusion generally uses small images to create a whole that is quite different from the image being used in its construction. A physiological illusion is more tricky, and often occurs after overstimulation of visual senses. Ambiguous figures fall under the cognitive illusion umbrella. These are illusions in which the viewers perception determines the form of illusion.
Illusion or reality?
It is tempting to dismiss these as specific extraordinary examples of how the mind is occasionally tricked. However, these are just simple demonstrations that provide a window into what the mind is doing all the time. Sensory stimulus of various kinds enters the brain all the time and are filtered at various stages to leave just a fraction of what impinges on the senses to be incorporated into an ongoing sea of activity. Some sensory elements are simply ignored or faded from view and some systematically misconstrued to be built into a construct that ‘makes sense’ within the larger sea of activity. As the perceptual object extends beyond a simple image to complex sensory landscapes of situation and circumstance, we begin to differ in how we ‘see’ things. Indeed, as well put by novelist Anais Nin, “We see things as we are, not as they are.” In the larger perceptual construct, is there a single truth?
Can we find evidence of how our brain processes the same stimulus differently to see it differently? A 2016 study makes an attempt. They used ambiguous figures and tracked changes in EEG readings as subjects switched between available modes of perception. The study by Kornmeier et al at the University of Freiburg included three distinct types of figures – geometry-based, motion-based, and semantics-based.
For geometry they used Necker’s cube (above left), a three dimensional cube drawn in two-dimensional space. Which panel of the cube is the front panel is up to the viewer. For motion, they used stroboscopic alternative motion stimulus (SAM). In SAM two screens are shown in quick succession. It appears that the dots on the screen are moving, but it is up to the viewer to determine whether they are moving horizontally, vertically, or in a circular motion. For their semantics-based ambiguous figure, the researchers used Boring’s old/young woman (above right). As the name suggests, the image can be seen as either an old or young woman.
The team looked at event-related potentials (ERP), time averages of the EEG signal locked to the event when shown images of various levels of ambiguity – for example, an equally likely old/young woman to a more definitive young or old woman. They found that across the board – in all three categories – the process of disambiguating the object was accompanied by larger positive ERPs (e.g. like the figure below). Moreover, the amplitudes of documented ERP readings were inversely related to the scale of object ambiguity. According to the researchers, the consistency across types of illusions suggests a cognitive process as the underlying reason for the ERP surges.
Making practical use of our mind’s misperceptions
As we learn more about how our brains respond to optical illusions, we are also learning more about how to use these illusions to our advantage. In Japan, researchers are looking into how lines on busy highways can help to maintain consistent traffic flow by sending the right signals to drivers. Energy-conscious designers are using illusion to create light bulbs that seem to glow constantly, but really take tiny breaks that we don’t notice, even though they total 13 percent of total time. Dieters are taking advantage of relative size illusions to eat from smaller plates in an attempt to consume less without feeling deprived.
The more we learn about the inner workings of our minds, the more easily we can reach our goals as a society, and optical illusions are just one underexploited window that can be used to peer into the human brain.