Basic principles of visual and auditory perception

2. Basic principles of visual and auditory perception



When perceiving an object it needs to be differentiated from its background. Determinants of figure vs. ground differentiation include
  1. Contour – surroundedness
  2. Size
  3. Orientation
  4. Symmetry

This is also influenced by perceptual set (see below).
Reversal of figure-ground perception frequently occurs so that sometimes, the ground is perceived as figure and vice versa e.g. try googling for images of Rubin’s vase illusion. This indicates that same stimuli can produce more than one perception. Figure-ground differentiation is also crucial for perceiving auditory stimuli e.g. when we are at a crowded party we are still able to filter our friend’s voice and have a conversation amidst all noisy background (cocktail party phenomenon). Shadowing is an experimental extension of this effect where two different messages are given to the right and the left ear and the subject is asked to follow one and suppress the other. These experiments are called dichotic listening tests (see below for further information in ‘attention’ section).

The principle of Gestalt: Gestalt means shape or form; it also refers to the global whole of an object. Gestalt law of perceptual organisation includes

 Proximity

  • Objects close to each other are perceived as one figure

Closure

  • Incompletely closed figures are perceived as fully closed

Continuity

  • Continuous items are perceived as one object.

Similarity

  • Similar items are grouped together based on colour or shape etc.

Common fate

  • Things moving together are perceived as one object.

According to Gestalt laws, global processing occurs before local processing of components. The whole is different from the sum of its parts. This is true at least for 2 dimensional objects though ecological validity is lower for 3D objects.

Depth perception depends on pictorial and non-pictorial primary cues. The non-pictorial cues are generally binocular cues and include
  1. Retinal image disparity
  2. Stereopsis
  3. Accommodation (monocular)
  4. Convergence


Pictorial cues (secondary) include largely monocular elements such as
  1. Size
  2. Brightness
  3. Superimposition
  4. Texture
  5. Linear perspective (rails converge at distance, wide apart when closer)
  6. Aerial perspective (colour – blue mountains means a distant sight)
  7. Motion parallax (closer it is faster it seems)


Visual cliff is an apparatus used to test an infant’s perception of depth. A pane of thick glass covers a shallow drop and a deep drop. The underlying surfaces of both deep and shallow sides are covered with the same chequered pattern. Children of six months and older will not venture to the ‘deep side’ and this is taken as an indication that the child can perceive depth.

Perceptual constancy is defined as the ability to perceive objects to be the same and unchanging in character despite varied inputs. It consists of
  1. Size constancy
  2. Shape constancy e.g. a door is always a door no matter which angle it is showing to the viewer
  3. Location constancy – movement of the head gets nullified somehow so we do not perceive objects around us getting relocated as we move our head!
  4. Brightness, hue and colour constancy

Autokinesis refers to the phenomenon that if light is shown from a small, dim, and fixed light source for an extended period of time in a dark room, it will appear as if the light source is moving. This visual illusion can explain UFO sightings and can also affect pilots.

The phi phenomenon is a perceptual illusion described by Wertheimer. This refers to the phenomenon in which a false perception of motion is produced by a succession of still images shown with fixed time interval rapidly.

Theories of perception:


Bottom-up theory: Gestalt is an example of a bottom-up theory. According to bottom up theories, perception is purely data driven and directly starts with the optic array. Piecing together of basic elements of the data gives rise to more complex systems. This makes the original elements sub-systems of the ‘emergent system’. But perception is not just seeing, it is ‘seeing as’.

Top-down theory: Gregory’s constructivist theory is an example of a top-down theory. According to this theory, retinal images are sketchy and cannot explain the complex and fully formed perceptions that we experience. Perception is best defined as a process of using information known already to formulate and test a hypothesis. It is driven from the ‘top down’ – i.e. from higher cortical areas. Illusions such as Muller Lyer (i.e. when you compare >----< and ← → , despite the horizontal line being of same length in both instances, the first one may appear to be longer) support top-down processing.

A perceptual set is defined as the readiness to perceive selected features as an object. This is related to the level of motivation e.g. hunger, emotional state, values, beliefs, context and expectations (e.g. UFOs are sighted only by those who believes in them and ‘expects’ them).

Illusions and hallucinations

Illusion is defined as any perceptual situation in which a physical object is perceived but appears different from what it really is e.g. a white wall appears yellow if a yellow light is shone on it. A hallucination is an experience in which an object (e.g. sound or light) is perceived in the absence of any corresponding object in the real world. A hallucination is often indistinguishable from genuine perception.

Human visual perception

The development of human visual perception is an illustration of a constitutional-environmental interaction. Most of the time during development, complex visual stimuli such has human faces are preferred. Innate visual processes such as visual scanning, tracking, fixating, figure-ground discrimination are present from birth. Learnt visual processes include size constancy, shape constancy, depth perception, shape discrimination.

From birth we have the ability to discriminate brightness and carry out eye tracking, visual acuity is significantly impaired and focusing is fixed at 20cm. At 2-4 months – depth perception is apparent (as evidenced by visual cliff experiments). By 4 months – accommodation and colour vision seems to be present in most children. By 6 months – 6:6 acuity is achieved.

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