Psychology Units 1&2 Chapter 8 & 9

Visual Perception & Illusions

Attention

The process of focusing on specific stimuli whilst ignoring and therefore excluding others

Sustained Attention

Involves maintenance of attention on a specific stimulus or task for a continuous period of time without being distracted

Divided Attention

The ability to distribute our attention so that two or more activities may be performed simultaneously

Selective Attention

Choosing and attending to a specific stimulus while at the same time excluding other stimulus

Perception

Is the process by which we interpret and give meaning to sensory information

Perception: Top-down processing

When we process information from past experience and the context it occurs

Perception: Bottom-up processing

When we process visual perception entirely on the sensory data and no previous knowledge

Depth Perception

The ability to accurately estimate the distance of objects and therefore perceive the world in three dimensions

Depth Cues

Our experience with the external world relating to the interpretation of an object’s distance in the visual field

Binocular

Requiring two eyes to operate

Monocular

Requiring one eye to operate

Binocular Depth Cues

Require the use of both eyes working together in order to provide information to the brain about depth and distance

Binocular depth cue: Convergence

Changes in tension in the eye muscles that occur when the two eyes turn inwards to focus on objects that are close. The closer an object gets, the greater tension in the eye muscles. The further away an object gets, the less tension in the eye muscles

Binocular depth cue: Retinal Disparity

Refers to the very slight difference in the location of the images on the retinas, which enables us to make judgements about the depth or distance of an object. Each retina receives a slightly different image because our eyes are 6 to 7 centimetres apart

Monocular Depth Cues

Require the use of only one eye to provide information to the brain about depth and distance, but they also operate with both eyes

Monocular depth cue: Accomodation

Involves the automatic adjustment of the shape of the lens to focus on an object in response to changes in how far away the object is. The brain monitors the movement of the ciliary muscles that control the shape of the lens. These muscles contract to focus on objects that is closer, and elongate to focus on distant objects

Monocular depth cues: Pictorial Depth Cues

Used by artists to create a 3D perception of something that exists on a 2D surface

Pictorial depth cues: Linear Perspective

Is the convergence of parallel lines as they recede into the distance

Pictorial depth cues: Interposition

Occurs when one object partially blocks or covers another, and the partially blocked object is perceived as further away than the object that obscures it

Pictorial depth cues: Texture Gradient

Refers to the gradual reduction of the detail that occurs in an object or surface as it recedes into the distance, compared with an object or surface that is close and perceived in fine detail

Pictorial depth cue: Relative Size

Refers to the tendency to visually perceive the object that produces the largest image on the retina as being closer, and the object that produces the smallest image on the retina as being further away. However the objects are still expected to be about the same size in real life

Pictorial depth cue: Height in visual field

Objects that are located closer to the horizon are perceived as being more distant than objects located further from the horizon.

Gestalt Principles

Refers to ways in which we organise the features of a visual stimulus by grouping them to perceive a whole, complete form

Gestalt Principles: Figure-ground

By dividing a visual stimulus into a ‘figure’, which stands out from the ‘ground’, which is its surroundings. The contour belongs to the figure, and defines it against the ground

Gestalt Principles: Closure

Is the tendency to mentally ‘close up’, fill in or ignore gaps in a visual image and to perceive incomplete objects as complete (‘whole’).

Gestalt Principles: Similarity

Involves the tendency to perceive parts of a visual stimulus that have similar features (such as size, shape, texture or colour) as belonging together in a unit, group or ‘whole’

Gestalt Principles: Proximity

To perceive parts of a visual image which are positioned close together as belonging together in a group

Perceptual Set

Is a temporary readiness to perceive something in accordance with what we expect it to be. We are more likely to interpret the object or event in the predetermined way

Perceptual Set: Context

Refers to the setting or environment in which a perception is made

Perceptual Set: Motivation

Refers to processes which activate and sustain goal directed behaviour

Perceptual Set: Past Experience

Refers to our personal experiences throughout our lives. Our visual perception relies to a very large extent on our past experience.

Perceptual Set: Memory

Memory is the process of storing and retrieving this perceived information for use when needed

Culture

Involve influences from the external social environment in which we interact with others

Perceptual Distortion

Involves an inconsistency between a perceptual experience and physical reality

Visual illusions

A misperception of external visual stimuli that occurs as a result of a distortion or mistake when interpreting the stimuli

Müller-Lyer illusion

A visual illusion in which one of two lines of equal length, each of which has opposite shaped ends, is incorrectly perceived as being longer than the other. The line with the ‘feather tail’ is perceived as being longer than the line with the ‘arrow head’

Ames Room illusion

A perceptual misinterpretation involving a trapezium shaped room that appears rectangular when viewed through a peephole only using one eye and people appear small or large depending on where they stand in the room

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

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

Spinning Dancer illusion

A visual illusion in which the silhouette of a female dancer can be interpreted as spinning clockwise or anticlockwise. This is due to the lack of depth cues

Agnosia

Characterised by loss or impairment of the ability to recognise and identify objects, people, sounds or other sensory stimuli when using one or more of the 5 senses despite otherwise using other normally functioning senses

Visual Agnosia

Loss or impairment of the ability to recognise visual stimuli, particularly familiar objects and faces. Related to the damage of the ventral stream, which is the pathway from the primary visual cortex in the occipital lobe to the temporal lobe

Apperceptive visual agnosia

An inability to accurately perceive visually presented stimuli. What is seen cannot be recognised

Associative visual agnosia

An inability to associate a visual stimulus with stored information about objects in memory despite having otherwise normal perceptual abilities

Prosopagnosia

An inability to recognise a familiar face, including their own when seen in a mirror or photograph

Simultanagnosia

An inability to recognise more than one object at a time in a scene that contains more than object.

Topographical agnosia

An inability to find one’s way around familiar environments

Colour agnosia

An inability to identify and distinguish between different colours, despite normal basic colour vision and colour discrimination mechanisms

Agnosic alexia

An inability to recognise or comprehend written or printed words

Synaesthesia

A perceptual experience in which stimulation of one sense produces additional sensations in another

grapheme-colour synaesthesia

In which a person sees colours when viewing letters, numbers or words

Spatial Neglect

A neurological disorder whereby individuals are unable to notice anything either on their left or right side even though there may be no sensory loss. They tend to behave as if that one side of their world does not exist