There are various visual illusions
that exhibit different perceptual processes. In scientific psychology, the
question that always draws attention is to what extent perceptual processes are
driven by the top-down perception theory as opposed to bottom-up information.
Eysenck (1998) stated that bottom-up processing is reliant on external stimuli,
whereas top-down one is influenced by prior knowledge, expectations, context,
and so on. Carlson, Miller, Heth, Donahoe and Martin (2010) stated that the
process of perception, in the case of bottom-up processing, starts with the
features of the image that the retina captures. This information is then
processed in a hierarchical manner by the successively higher levels of the
visual system until the top of the system receives the information, and the
perception of the object occurs. In contrast, top-down processing entails
contextual information extracted from memory, and here the information is
passed on from higher to lower neural areas (Carlson et al., 2010). Top-down
effects are voluntary, whereas bottom-up is more of physiological in nature (Watanabe,
Nanez and Sasaki, 2001). Top-down theory of perception is supported by
constructivists like Bruner and Neisser, whereas
bottom-up theory is upheld by direct-realist like J.J. Gibson (Shea, 2013). The
conflict between these two groups of psychologists arises while interpreting
optical illusions, which are images ambiguous in nature, and therefore, can be
interpreted in dual ways. This paper will argue that illusions are the product
of top-down perceptual rather than bottom-up sensory errors.
The top-down processing theory was
proposed by psychologist Richard Gregory who argued that stimulus information
gathered from our environment is ambiguous in nature, and hence in order to
interpret it correctly, we need higher cognitive information based on our past
experiences or knowledge stored in our brain to deduce a perception (Gregory,
1974). He believed that a lot of information (about 90%) our eyes see are lost
by the time this information reaches the brain and hence, our brain needs to
rely on the prior knowledge to make a perception of reality (Gregory, 1970).
Our perceptions are hypotheses influenced by our past and therefore, if we form
incorrect ones, then it will lead to errors in perception, such as forming
visual illusions like the Necker cube. The optical illusion of Necker cube was
first observed by Louis Albert Necker in 1982. The cube is ambiguous, because
it can be differently interpreted by different people. At the intersection
point, where two lines meet, it is unclear which is at the front, and
therefore, it can be perceived in two different ways (Woodson, 1979).
The question that arises is if
Gregory's theory of top-down perceptions involves interpreting an image based
on a hypothesis, what kinds of hypotheses are they. Hypotheses vary from one
person to another, because they are situational and are based on memory and
past experiences. Therefore, the same optic
illusion can be interpreted in two ways in two different situations. For
instance, the picture below showcases a box.
When viewed alone, the picture engages your brain in bottom-up processing, because apparently, it does not involve any optical illusion. The three thin horizontal lines and two thick vertical lines seen in the picture do not give any special context to derive a specific meaning. Hence, it does not involve any top-down processing (Carbon, 2014). But if the picture is positioned in two different situations, the brain engages in making a perception based on hypotheses. For example, when the image is placed in between two letters A and B, the brain perceives the image as letter 'B', whereas when the same image is put between two numbers 11 and 12, the brain hypothesizes it as number '13'. This shows that hypotheses vary in different situations.
On the other hand, Gibson, who
proposed the bottom-up perception theory, argued against Gregory's concept of
top-down visual illusions on the grounds that Gregory's perceptions of reality
are based on artificial images and not real ones found in normal visual
environments. Gibson (1966) believed that perceptions are survival mechanisms
influenced by evolution, and it does not involve learning. He claims that perceptions
are direct and not the result of hypotheses as Gregory proposed. Gibson argued
that when we receive information from the visual environment about an object's
size, shape, or color, our brain perceives it as what we see (Gibson, 1972).
The perception does not involve interpretation because the whole perception is
based on sensory stimuli. According to Gibson (1972), perception is a bottom-up
process because it follows a pattern of how the information is received. The
whole process begins with an analysis of the pattern of light, known as optic
array, that reaches the eyes, with all the visual information required for
further processing. The information received is passed on to the retina where
the process of transduction into electrical impulses starts, and these impulses
are then relayed to the brain which triggers further responses along the visual
trajectories until they reach the visual cortex for the final processing of
information (Gibson, 1972).
Though Gibson's theory provided an
explanation for the direct perception of the environment, he, however, could not
provide any logical reasoning for the optical illusions. He reasoned that the
images used for optical illusions are highly artificial in nature, and the
chances for such images to be encountered in real word are very rare (Gibson,
1972). However, it is not true that optical illusions cannot be experienced in
natural environmental conditions. For instance, a stick looks straight if you
hold it in front of your eyes, but if you put the stick in a glass of water, it
looks bent. The stick looking bent is not an artificial image that we do not
encounter in real life. The difference between the two perceptions is that in
the case of the stick looking straight, the stimulus energy reflecting from the
stick to the retina sent information to the brain that is the direct mental
representation, which is supported by Gibson’s theory of direct perception
(Favela & Chemero, 2016). However, in the case of the stick looking bent in
water, the stimulus energy reflecting from the stick to the retina sent
information to the brain that results in an illusion (Favela & Chemero,
2016). The stick looks bent, however, it is not really bent. This example of a
bent stick suggests that Gibson's theory explains perceptions based on ideal
viewing conditions, whereas Gregory's theory involves viewing in conditions
that are not ideal.
When Gibson argued that the
ambiguous images used for top-down theory of perceptions are unreal, he failed
to take a note of a few factors. Firstly, it is true that some pictures with
ambiguous features or two-dimensional or three-dimensional drawings used for
experimentation purpose are unnatural, but we encounter a lot of ambiguous
images in real-life situations that are not the result of unnatural lab
experiments. The straight stick looking bent in water is one such example. The
illusion of the stick looking bent can be experienced by two individuals at the
same time. Secondly, perceptual apparatuses of different animals are different,
and therefore, a natural image may produce optical illusion in a natural
environmental condition because of how the brain perceives information (Favela
& Chemero, 2016). For example, though the illusion of a bent stick in water
can appear same to the eyes of two people who have the same perceptual
mechanisms, the stick may look straight in water to the eyes of a gannet that
has the capacity of accommodating light refraction at the boundary of air and
water. Gibson failed to consider the visual capacity of the perceiver while
dismissing optical illusions.
Direct realists like Gibson argue
that since the top-down processing of perception depends on hypotheses while
interpreting information, if the hypotheses are misleading, then it will result
in a wrong perception of an image or a situation (Carbon, 2014). For example, making
typos during texting or typing is common among people and the readers while
reading through a typo use the filling-in phenomena by filling in missing
details. In such a case, the brain simply extracts prior information from
memory and rearranges or fills in the missing letters within the words. If the
prior knowledge registered in the brain is faulty, then there are chances for
the brain to make wrong hypotheses and erroneous processing of typos.
What direct realists overlook is
that the fundamental principle behind top-down processing theory lies in making
a connection between sensory inputs and our semantic networks. The
knowledge base or the semantic network determines what kind of hypotheses an
individual is likely to make (Costall, 1980). The hypotheses made by one
individual could be different from that of another one, depending on their
previous experience (Kohn, 2007). Since no two people have similar past
experiences, their perceptual system varies. For example, if we read an old
book where coffee stains have obscured partial information, some of the letters
have become blurred, and the decay processes have made the white paper turn
into a yellowish substance, we can read only the fragments of the text and then
reconstruct the general meaning of what we read by making hypotheses (Carbon,
2008). In other words, we fill the gaps of missing texts and passages depending
on our prior experience. The meaning gathered from the text could vary from one
individual to another, as both have different past experiences.
Another good example is the famous
man-rat illusion where the sketching is ambiguous in nature and not easily
decipherable. So, one can interpret that the image is of a man while another
can interpret it as an image of a rat (Carbon, 2014). For most people, the
perception of the images switches from man to rat. This is a fascinating
example of an illusion that demonstrates the mental capacity of a human being
to switch from one meaning to another (Carbon, 2014). It also reveals the
intriguing process of our brain that makes the initial perception through the
activation of our semantic network. If one has the experience of seeing a
picture of a man before, or if one knows what a man looks like or has heard the
word "man", then one is more likely to interpret the ambiguous
picture as an image of a man. If one's previous experiences were more
associated with a rat, or another animal of such kind, then one is more likely
to interpret the image as a rat (Carbon, 2014).
Another factor that counteracts
Gibson's claim that if the knowledge base is erroneous, then it will lead to
perceptual errors is how our perception is based on the expectation we keep
from a scene. For example, while entering a classroom, we expect to see a black
or whiteboard and desks. If these objects are present in the classroom, we
rapidly interpret them. If these elements that we are expecting to see in a
classroom are not present in the scene, it leads to perceptual errors. Biederman
(1981) showed in his experiment that there was an increased error in
identifying fire hydrants when they were placed in kitchens. The same error
happened in identifying sofas when they were placed in city streets. It happens
because our brain is used to seeing an object in its designated place. The
brain will have difficulty in processing information when the object is not
seen in its usual context.
In conclusion, it is worth mentioning that there are two groups of
perceptual theorists: top-down perceptual theorists and bottom-up theorists. The
first ones, known as constructivists, believe that perception is influenced by
contexts, prior experience, and expectations. Bottom-up theorists, also called
realists, believe that perception is sensation and is the direct result of
external stimuli. Constructivists interpret optical illusions on the ground of
prior knowledge, contextual information, and expectations, whereas direct
realists dismiss the existence of optical illusion, calling it an unnatural
phenomenon that does not take place in natural environment condition. However,
optical illusions are not only two-dimensional or three-dimensional distorted
images. They can be encountered in the natural environment too, as seen in the
example of a straight stick looking bent in water. Direct realists also argue
that the interpretation of illusions is based on hypotheses and hypotheses
influenced by prior knowledge are prone to errors if the knowledge base is
misleading. However, the determination of hypotheses to be erroneous in nature
is not an easy process, because hypotheses are influenced by prior experience,
expectations, and contextual information. All these three variables could be
subjective in nature, and therefore, it cannot be stated with conviction that
hypotheses could be misleading.
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