Perceptual Illusions and Constancies

Perceptual illusion occurs when a stimulus is perceived inaccurately. It can equally be explained as a sensory distortion in scientific parlance.

How to Describe Illusions?

Illusions provide us important clues on the information processing capabilities of our brain. Even scientific investigations by way of empirical researches may have limitations because the information perceived by way of direct observation can also be quite misleading. Whatever we perceive from the environmental factors around us need not always be true. In fact there may be errors in perception. The image falls directly onto to the retina which is later processed and interpreted by the brain. The interpretations may not necessarily be always correct and this misinterpretation of information which is perceived by our sensory receptors is called as illusion. For an example, in darkness we may perceive a rope to be a snake.

Illusions can take place with any of our senses. Given below is a description on the different types of illusions:

  1. Optical Illusion: An image may be constructed in such a way that misleading information is relayed to the brain. Optical illusions can make use of various colours, patterns or light for creating images, which can be misleading or deceptive for the brain. Also known as Visual Illusions, optical illusions involve visual deception. This can be very well explained with the help of Moon Illusion example. By spending sometime gazing at the night sky, one will be able to understand the phenomenon of optical illusion, in which the moon in the horizon looks far bigger than when it is high up in the sky. In this case, visual illusion can be distinguished from hallucinations or delusion. Illusions are normal phenomenon, whereas hallucinations and delusions are the pathological states of the mind.
  2. Auditory Illusion: When an individual hear those sounds which do not exist in reality or are not created or is being able to hear different sounds which is the distorted version of the actual sound, this phenomenon can be regarded as auditory illusion. Shepard tone can be considered as one best example for explaining the phenomenon of auditory illusion which seems to continuously rise or fall in pitch but in reality nothing occurs.
  3. Tactile Illusions: It’s the sensation of touch stimuli which is perceived by the brain, though it is actually not present or at least not in the way in which the brain perceives it to be present. It’s example is Phantom Limb syndrome or the feeling which one undergoes or experiences in case of an amputated limb.
  4. Smell and Taste Illusions: Certain people may perceive smells differently from what others perceive and the same may be applied to taste as well.

Causes of Illusions

Cognitive illusions are good examples for proving how our expectations can influence our perceptions. Illusions can be experienced by anybody and these are not necessarily a symptom of some psychiatric issue like hallucinations, as hallucinations can take place in the absence of external stimuli. One can experience illusions because of many reasons lighting effect on an object, lack of availability of sensory information related to an object or could also be possible because of errors in the processing of sensory details by an individual. Certain conditions called pseudo-hallucinations can be a sign of a psychiatric condition, which individuals experience under situations of anxiety or fear. Illusions can be caused by diverse factors like stimulus factors, cultural factors and psychological factors. It demonstrates whatever we experience largely depends upon several processes which goes far beyond any sensory inputs (Morgan et al, 1986). Studies reveal that in several cases when perceptual constancies fail, illusion takes place. In case of perceptual constancies, the brain processes information as a computer by taking into consideration all the possible parameters or cues for taking a decision pertaining to the probable size as well as object distance.

Perceptual Constancies

The phenomenon of perceptual constancy is expected to have taken place when one perceives the object as the same, despite the sensory changes. In the absence of perceptual constancy there will be chaos and a lot of confusion around us, because the objects will appear to be different, whenever we look at them. Perceptual constancy can be explained as stability in our perception of the environment around us, even when the object is perceived in wide variations or circumstances. Let’s understand the two important types of Perceptual Constancy:

Shape Constancy: For example, when we look at an object from different angles, different images in the retina appear. However, the shape which is perceived by the perceiver remains the same. It is because those objects which are familiar help in attaining shape constancy. It is because familiar objects help in perceiving the distance and size of objects quite accurately. Shape constancy largely depends upon our constant interaction with the outside world and determines how we perceive the objects.

Size Constancy: By moving away from an object, the size of that object appears to be smaller and smaller with further increase in the distance. On the contrary, with the decrease in distance, the size of the object will appear to be bigger and bigger. But, in both these experiences the size of the object remains the same. When the perception about the size does not change it is called as Size Constancy. It is because we do have this tendency to perceive the changes in the retinal image in conjunction with the perceived distance. The distance is perceived with the help of different cues around us.

Colour Constancy: It is the tendency of perceiving objects having same colour even under different lighting arrangements (Rathus, 1990). For example a green colour car will be perceived as the same green colour whether it is seen in dim light, bright sunlight or in different set ups. On the other hand, if the actual colour of the car is not known, our perception to a great extent will depend upon the wavelength of the light.


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