CHAPTER 15 Flashcards
(30 cards)
Perceptual Constancies
Perceptual constancies enable us to maintain a stable perception of a stimulus, although the image on the retina may change (for example, in size or shape). The more familiar we are with the observed object, the more likely it is that we will maintain perceptual constancy
of it. Perceptual constancies are usually learned early in childhood.
Size constancy
Size constancy refers to the fact that we maintain a constant perception of an object’s size, even though the size of the image on the retina alters as the object moves
nearer to or further from us. For example, when we look at a friend from a distance of 5 m, he produces a retinal image twice the size of the one he produces when he is 10 m away. Because we are familiar with his height we don’t change our perception of it, but we use the size of the image as a cue to provide information about how far away he is.
Shape constancy
Shape constancy refers to the fact that an object is perceived to maintain its known shape despite the changing perspective from which it is observed. This is a learned skill – a toddler may have di culty perceiving a familiar toy if it is viewed from an unusual angle. Objects that are familiar to us can be accurately interpreted when viewed from any direction. For example, as a door opens, the retinal image of it changes shape from a rectangle to a trapezium, but we easily maintain our perception of a rectangular door.
What is gestalt psychology based on
Gestalt psychology developed early in the twentieth century and is based on the principle that ‘the whole is greater than the sum of its parts’. In terms of sight, it deals with the tendency for our visual system to perceive what we see as a meaningful whole.
Figure ground organisation and camouflage
The rst Gestalt principle of visual perception that develops in infants is gure– ground organisation. The part of the visual eld being attended to and focused on is the gure, and its surroundings are the ground. Figure and ground are separated by an imagined contour (line). The contour is ‘owned’ by the gure. In children’s drawings, we often see a black line around a gure, emphasising the separation of gure and ground.
Camou age is where the Gestalt principle of gure–ground is used to break up the contour of the gure, meaning that gure and ground will blend, making the gure more di cult to see. We see camou age in nature, for example with leopards, tigers and gira es, and the army uses camou age uniforms to enable soldiers to blend in to their surroundings and avoid detection.
Closure
Closure occurs when we perceive an object as being whole, despite it actually being incomplete. Many company logos use such incomplete gures in their designs.
Similarity
Similarity is when the individual parts of a stimulus pattern are similar (for example, in size, shape or colour). We tend to group them together as a meaningful ‘whole’ – a single unit. In the examples below, we tend to perceive rows of ‘X’ and ‘O’ in Group A, and columns of ‘X’ and ‘O’ in Group B.
Proximity
Proximity is when the individual parts of a stimulus pattern are close to each other. We tend to group them together as a meaningful ‘whole’ – a single unit. We can choose how to perceive a group of shapes such as those below (as rows or columns), but we tend to perceive the ones in Group A as rows and Group B as columns.
AB
Depth perception and depth cues
Depth perception is the ability to accurately judge 3D space and distance, using cues in the environment. Depth cues are vital to us. This is because we exist in a 3D world but have only 2D images on our two retinas from which to judge depth and distance.
Binocular and monocular depth cues
Depth cues may be binocular (using both eyes) or monocular (using one eye alone or both eyes together).
Binocular:retinal disparity, convergence
Monocular: accommodation and pictorial depth cues,
Retinal disparity
Retinal disparity is a cue that comes about because our eyes are set about 6–7 cm apart. When an object is within about 20 m of a viewer (with the strongest depth cue occurring within 7 m), each eye receives a slightly di erent image on the retina, as a result of the di erent angles of view from each eye to the object being observed. The brain fuses these two images together in stereoscopic vision. The more di erent the two images, the closer the object will be to the viewer.
Convergence
Convergence is also a binocular depth cue. As an object comes closer to us, our eyes turn inwards to keep the object centred on the retina. Again, this cue operates for objects within about 7 cm. The brain reads the amount of turning from the tension of the muscles that move the eyes and uses this to make judgements of distance. The more the turning, the closer the object is to the viewer.
Accommodation
Accommodation involves the lens of the eye (located behind the iris) changing shape so that it can focus light rays onto the retina. Small muscles called ciliary muscles control whether the lens bulges (for closer objects) or attens (for more distant objects). At the same time, the tension in the ciliary muscles is received by the brain to con rm the location of the object being viewed; the greater the tension, the closer the object.
Pictorial depth cues
Pictorial depth cues are so called because they are used by artists to create a 3D perception of something that exists on a 2D surface.
Linear perspective
one of the most basic
skills an artist uses to create apparent depth. Parallel lines are made to converge as they extend along the page to an imaginary point (where in theory they meet) at the horizon, as shown in Figure 15.6.
Interposition
overlap) is based on the partial blocking or obscuring of one object by another. The obscured object appears to be further away than the object obscuring (overlapping) it. This is an e ective cue for determining which objects are closer than others, but it is not as e ective for actually judging distance.
Texture gradient
is used to make surfaces in a picture appear to recede into the distance. Artists draw less and less detail as a surface is more and more distant, the same way we see it in real life. This is illustrated by the photo of the Rhyll boardwalk, Phillip Island, as shown in Figure 15.7. In the foreground we can see every detail of the wood and the mesh, but as the boardwalk gets further away it becomes much less detailed.
Relative size
is based on our tendency to perceive the object producing the largest retinal image as being the nearest, and the object producing the smallest retinal image as being the farthest. For this cue, it is necessary to know the
real size of the objects so that accurate comparisons can be made. Think about watching a game of football from behind your team’s goal – you realise that the players at the opposite goal are far away; you don’t think they are tiny!
Height in the visual field
shows depth by portraying objects further away as being closer to the horizon. In a picture, objects in the sky – such as aeroplanes, clouds and birds – will be perceived as further away as they become lower in the visual eld (closer to the horizon). On the other hand, objects on the ground – such as trees, houses and people – will be perceived as further away as they become higher in the visual eld (closer to the horizon).
What can predispose us to perceive the world in a certain Wayne
Expectations that are created by our previous experiences, the environment in which a stimulus is seen, our motivation and even our emotional state can combine to predispose us to perceive the world in a certain way
Perceptual set
> a predisposition to perceive stimuli in a speci c way; that is, interpreting what we
see according to certain preconceptions (interpretation)
a predisposition to attend to only certain features or aspects of our eld of view
(selection).
Several factors can in uence perceptual set
Previous experience
> Previous experience: Previous experience can affect visual perception, especially if the experience holds signi cant personal meaning. The same stimulus can be
interpreted di erently by di erent people.
Context
Context refers to the environment in which a perceived stimulus is
observed. Context sometimes has an immediate e ect on our expectations.
Motivation
> Motivation: Very often we see what we want to see. On a long drive, running
low on petrol, a sign reading ‘FOOD AHEAD’ could be interpreted as ‘FUEL
AHEAD’.
