Are visual illusions due to low-level, so-called "bottom-up" processing, or are they aberrations of high-level, more "cognitive", processing?

Authors Avatar

Cognitive psychology 2nd year.                Minal Jivraj

Are visual illusions due to low-level, so-called “bottom-up” processing, or are they aberrations of high-level, more “cognitive”, processing?

When a perception departs from the external world, to disagree with physical reality, we say we experience an illusion (Gregory, 2003). An illusion must, therefore be defined as a discrepancy between what is perceived and what is objectively present in the world. Ironically, most visual illusions can be attributed to perceptual constancies that ordinarily help us to perceive more accurately (Frisby, 1980). Perception in daily life is shot through with illusions. Sometimes we are aware of them but more often we are not-for the simple reason that we do not go around analysing and measuring objects we perceive (Rock, 1995). But how and why do we experience these visual illusions?

To what extent does experience influence perception of visual stimuli? Psychologists have traditionally taken two opposing stands. Perceptions influenced by the visual field itself, or changes in stimuli are referred to as “bottom-up” influences (sometimes referred to as direct perception). If a perception is influenced by what a person expects, or has experienced before, it is referred to as “top-down” influence (sometimes referred to as indirect perception).

Lower level, so called “bottom-up” or “data driven” processing, refers to processing that begins “at the bottom” with raw sensory data that feed “up” to the brain. A bottom-up explanation of visual perception argues that the brain forms perceptions by combining the responses of multiple feature detectors in the primary cortex, which themselves integrate input from neurons lower in the visual system. It involves the analysis of individual elements of the stimulus and works up to the brains integration of them into a unified perception. In contrast top down processing (also known as concept driven processing) is the perceptual processing in which existing knowledge, concepts, ideas or expectations are applied in order to make sense of incoming stimuli. Top down processing starts at the top, from the observer’s expectations and knowledge.

The question concerned asks whether visual illusions are due to bottom-up or top-down processing? This essay aims to describe the different processes involved (including examples) in perceiving visual illusions and conclude whether it is an either or debate OR maybe a case of both processes contributing to the perceiving of visual illusions.

Visual illusions and ambiguities allow us to separate effects of bottom-up signals from the eyes, from top-down knowledge and assumptions (Gregory, 2003). According to Gregory (1970, 1980), many classic visual illusions can be explained by assuming that previous knowledge derived from the perception of three-dimensional objects is applied inappropriately to the perception of two-dimensional figures. For example, people typically see a given object as having a constant size by taking account of its apparent distance. A good example of this is effect is ‘The Ames Room’. The Ames room consists of what seems to be a standard room with four walls all equal in height and size. Two people stand inside the room on opposite sides while the observer peers through a hole on the other side. From the viewers perspective the two people seem to be very different in size (i.e. one is much bigger than the other) when actually they are about the same size. This illusion stems from the unusual construction of the room. The floor-to-ceiling height as well as the sizes and shapes of the windows in the room are distorted to make the room appear rectangular and hence normal when viewed from a particular location. This illusion is due to the fact that people assume, via top-down expectations, that the room is ‘normal’ and therefore perceive the people in it to be different in size (Sekuler & Blake, 2002).

Join now!

Another example of how perspective can be influenced by the expectations of size is the Ponzo illusion. In the Ponzo illusion, the depth cues of linear perspective (the tracks converging) and height in the horizontal plane provide distance cues that make the upper bar appear farther away than the lower bar. Because it seems farther away, the perceptual system concludes that the bar in the background must be larger than the bar in the foreground, despite the fact that the two bars cast retinal images of the same size (Passer & Smith, 2001). This, known as the misapplied constancy ...

This is a preview of the whole essay