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
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.
•Objects close to each other are perceived as one figure
Proximity
•Incompletely closed figures are perceived as fully closed
Closure
•Continuous items are perceived as one object.
Continuity
•Similar items are grouped together based on colour or shape etc.
Similarity
•Things moving together are perceived as one object.
Common fate
© SPMM Course 9
Depth perception depends on pictorial and non-pictorial primary cues. The non-pictorial cues are
generally binocular cues and include
a. Retinal image disparity
b. Stereopsis
c. Accommodation (monocular)
d. Convergence
Pictorial cues (secondary) include largely monocular elements such as
a. Size
b. Brightness
c. Superimposition
d. Texture
e. Linear perspective (rails converge at distance, wide apart when closer)
f. Aerial perspective (colour – blue mountains means a distant sight)
g. 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’.
© SPMM Course 10
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