What do illusions tell us

The problem with the idea of veridical perception of the world is further intensified when taking additional perceptual phenomena, which demonstrate highly constructive qualities of our perceptual system, into account.

A very prominent example of this kind is the perceptual effect which arises when any visual information which we want to process falls on the area of the retina where the so-called blind spot is located see Figure 1. Figure 1.

Demonstration of the blind spot, the area on the retina where visual information cannot be processed due to a lack of photoreceptors. The demonstration works as follows: Fixate at a distance of approx. Interestingly, visual information that is mapped on the blind spot is not just dropped—this would be the easiest solution for the visual apparatus. It is also not rigidly interpolated, for instance, by just doubling neighbor information, but intelligently complemented by analysing the meaning and Gestalt of the context.

Beside this prominent example which has become common knowledge up to now, a series of further phenomena exist where we can speak of full perceptual constructions of the world outside without any direct link to the physical realities.

A very intriguing example of this kind will be described in more detail in the following: When we make fast eye movements so-called saccades our perceptual system is suppressed, with the result that we are functionally blind during such saccades.

Actually, we do not perceive these blind moments of life although they are highly frequent and relatively long as such—actually, Rayner et al. In accordance with other filling-in phenomena, missing data is filled up with the most plausible information: Such a process needs hypotheses about what is going on in the current situation and how the situation will evolve Gregory, , If the hypotheses are misleading because the underlying mental model of the situation and its further genesis is incorrect, we face an essential problem: what we then perceive or fail to perceive is incompatible with the current situation, and so will mislead our upcoming action.

In most extreme cases, this could lead to fatal decisions: for instance: if the model does not construct a specific interfering object in our movement axis, we might miss information essential to changing our current trajectory resulting in a collision course. In such a constellation, we would be totally startled by the crash, as we would not have perceived the target object at all—this is not about missing an object but about entirely overlooking it due to a non-existing trace of perception.

Despite the knowledge about these characteristics of the visual system, we might doubt such processes as the mechanisms are working to so great an extent in most everyday life situations that it provides the perfect illusion of continuous, correct and super-detailed visual input.

We can, however, illustrate this mechanism very easily by just observing our eye movements in a mirror: when executing fast eye movements, we cannot observe them by directly inspecting our face in the mirror—we can only perceive our fixations and the slow movements of the eyes.

If we, however, film the same scene with a video camera, the whole procedure looks totally different: Now we clearly also see the fast movements; so we can directly experience the specific operation of the visual system in this respect by comparing the same scene captured by two differently working visual systems: our own, very cognitively operating, visual system and the rigidly filming video system which just catches the scene frame by frame without further processing, interpreting and tuning it.

We can utilize this phenomena for testing interesting hypotheses on the mental representation of the visual environment: if we change details of a visual display during such functional blind phases of saccadic movements, people usually do not become aware of such changes, even if very important details, e.

Gregory proposed that perception shows the quality of hypothesis testing and that illusions make us clear how these hypotheses are formulated and on which data they are based Gregory, One of the key assumptions for hypothesis testing is that perception is a constructive process depending on top-down processing.

Such top-down processes can be guided through knowledge gained over the years, but perception can also be guided by pre-formed capabilities of binding and interpreting specific forms as certain Gestalts.

The strong reliance of perception on top-down processing is the essential key for assuring reliable perceptual abilities in a world full of ambiguity and incompleteness. If we read a text from an old facsimile where some of the letters have vanished or bleached out over the years, where coffee stains have covered partial information and where decay processes have turned the originally white paper into a yellowish crumbly substance, we might be very successful in reading the fragments of the text, because our perceptual system interpolates and re- constructs see Figure 2.

If we know or understand the general meaning of the target text, we will even read over some passages that do not exist at all: we fill the gaps through our knowledge—we change the meaning towards what we expect.

Figure 2. A famous example which is often cited and shown in this realm is the so-called man-rat-illusion where an ambiguous sketch drawing is presented whose content is not clearly decipherable, but switches from showing a man to showing a rat—another popular example of this kind is the bistable picture where the interpretation flips from an old woman to a young woman an v.

Figure 3. The young-old-woman illusion also known as the My Wife and My Mother-In-Law illusion already popular in Germany in the 19th century when having been frequently depicted on postcards. So, we can literally say that we perceive what we know—if we have no prior knowledge of certain things we can even overlook important details in a pattern because we have no strong association with something meaningful.

The intimate processing between sensory inputs and our semantic networks enables us to recognize familiar objects within a few milliseconds, even if they show the complexity of human faces Locher et al. Top-down processes, however, are also susceptible to characteristic fallacies or illusions due to their guided, model-based nature: When we have only a brief time slot for a snapshot of a complex scene, the scene is if we have associations with the general meaning of the inspected scene at all so simplified that specific details get lost in favor of the processing and interpretation of the general meaning of the whole scene.

Biederman impressively demonstrated this by exposing participants to a sketch drawing of a typical street scene where typical objects are placed in a prototypical setting, with the exception that a visible hydrant in the foreground was not positioned on the pavement besides a car but unusually directly on the car.

In this specific case, people have indeed been deceived, because they report a scene which was in accordance with their knowledge but not with the assessment of the presented scene—but for everyday actions this seems unproblematic.

Although you might indeed lose the link to the fine-detailed structure of a specific entity when strongly relying on top-down processes, such an endeavor works quite brilliantly in most cases as it is a best guess estimation or approximation—it works particularly well when we are running out of resources, e.

Actually, such a mode is the standard mode in everyday life. However, even if we had the time and no other processes needed to be executed, we would not be able to adequately process the big data of the sensory input. The whole idea of this top-down processing with schematized perception stems from F. There is clearly an enormous gap between the big data provided by the external world and our strictly limited capacity to process them.

The gap widens even further when taking into account that we not only have to process the data but ultimately have to make clear sense of the core of the given situation. The goal is to make one and only one decision based on the unambiguous interpretation of this situation in order to execute an appropriate action.

This very teleological way of processing needs inhibitory capabilities for competing interpretations to strictly favor one single interpretation which enables fast action without quarrelling about alternatives. In order to realize such a clear interpretation of a situation, we need a mental model of the external world which is very clear and without ambiguities and indeterminacies. Ideally, such a model is a kind of caricature of physical reality: If there is an object to be quickly detected, the figure-ground contrast, e.

If we need to identify the borders of an object under unfavorable viewing conditions, it is helpful to enhance the transitions from one border to another, for instance. If we want to easily diagnose the ripeness of a fruit desired for eating, it is most helpful when color saturation is amplified for familiar kinds of fruits. Our perceptual system has exactly such capabilities of intensifying, enhancing and amplifying—the result is the generation of schematic, prototypical, sketch-like perceptions and representations.

Any metaphor for perception as a kind of tool which makes photos is fully misleading because perception is much more than blueprinting: it is a cognitive process aiming at reconstructing any scene at its core.

The illusion is induced by the distribution of the peripheral gray values which indeed show a continuous shift of gray levels, although in a reverse direction. The phenomenon of simultaneous contrast helps us to make the contrast clearer; helping us to identify figure-ground relations more easily, more quickly and more securely.

Figure 4. Demonstration of the simultaneous contrast, an optical illusion already described as phenomenon years ago by Johan Wolfgang von Goethe and provided in high quality and with an intense effect by McCourt : the inner horizontal bar is physically filled with the same gray value all over, nevertheless, the periphery with its continuous change of gray from darker to lighter values from left to right induce the perception of a reverse continuous change of gray values.

The first one who showed the effect in a staircase of grades of gray was probably Ewald Hering see Hering, ; pp.

Teil, XII. Tafel II , who also proposed the theory of opponent color processing. Via the process of lateral inhibition, luminance changes from one bar to another are exaggerated, specifically at the edges of the bars. This helps to differentiate between the different areas and to trigger edge-detection of the bars.

Figure 5. Chevreul-Mach bands. Demonstration of contrast exaggeration by lateral inhibition: although every bar is filled with one solid level of gray, we perceive narrow bands at the edges with increased contrast which does not reflect the physical reality of solid gray bars. This reconstructive capability is impressive and helps us to get rid of ambiguous or indeterminate percepts.

However, the power of perception is even more intriguing when we look at a related phenomenon. Illusions can offer scientists new insights on how vision and the brain work -- and are more than intriguing parlor tricks. Visual illusions are not just some nice puzzle, like a crossword, or an entertainment feature, said Martinez-Conde. Beyond their amusement value, one researcher speculated that illusions may also serve an evolutionary purpose.

Some of these constructions can be fiction," said Mark Changizi, a neurobiologist and assistant professor of cognitive science at Rensselaer Polytechnic Institute in Troy, N.

Changizi came up with a theory to help explain why we see illusions. He argued that illusions are due to the brain's attempt to "see" the future. They occur during the slight time lag after light reaches the retina in your eye, but before your brain translates it into a visual perception.

According to Changizi, author of " The Vision Revolution ," when the brain attempts to generate a perception, it basically is taking a guess at the near future by trying to fast-forward a tenth of a second.

As a result of this "neural delay," you might not be perceiving an image as it actually is, but as you expect it might soon be. Although there is no single reason illusions take place, Martinez-Conde offered another possible explanation. The brain is a limited structure with limited resources, including its number of neurons, wires, and neuronal connections, she suggested. Color, motion, shape and the amount of light that hits your eye are just a few of the factors that might cause you to see an illusion.

Some people like to design new illusions and, in fact, there is an annual international contest to recognize the best -- and most novel -- visual illusion of the year. First place this year went to an entry called " The Break of the Curve Ball ," which depicted the path of a spinning disk and helped illustrate why the abrupt shift of this pitch is so good at fooling baseball hitters.

No doubt, the lessons learned from illusions extend beyond the baseball diamond -- and deep into your mind. We'll notify you here with news about. Turn on desktop notifications for breaking stories about interest? Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event. When we experience a visual illusion, we may see something that is not there or fail to see something that is there.

Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. By studying these failings, we can learn about the computational methods used by the brain to construct visual experience. Macknik and Sandra Blakeslee.

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