Perception
According to the ecological view of perception, most of what we perceive is already present in the rich array of stimuli in the environment
The primary goal of perception is to support actions such as walking or driving. The constructionists argue that the perceptual system must construct a representation of reality from fragments of sensory information· The computational view explains how complex computations within the nervous system might turn raw sensory stimulation into a representation of the world.
Psychophysics
Absolute thresholds
The minimum detectable amount of physical energy (light, sound, pressure, etc.) is called the absolute threshold. Because of variability, psychophysicists have redefined the absolute threshold as the minimum amount of energy that can be detected 50 % of the time· Variability arises from:
1.internal noise is the spontaneous, random firing of cells in the nervous system that occurs in varying degrees whether or not we are stimulated by physical energy
2.the response criterion (a.k.a. bias) reflects a person’s willingness or reluctance to respond to a stimulus and is affected by: ·motivation ·expectancies
Difference threshold
The minimum difference that has to exist between two sources for them to be perceived separately
Signal detection theory
Signal detection theory is a mathematical model of how people’s sensitivity and response criterion combine to determine decisions about whether they say that a near-threshold stimulus has occurred. Sensitivity refers to one’s ability to discriminate a stimulus from its background. The theory explains: ·detecting cancer cells in a pap smear ·spotting a weapon in an airport x-ray.
Weber’s Law (Ernst Weber)
States that the smallest detectable difference in stimulus energy is a constant fraction of the intensity of the stimulus. The smallest detectable difference in the stimulus is called the difference threshold or just-noticeable difference (JND). e.g. noticing a 50 p difference in change from a £1 bus fare, but not noticing a 50 p difference in the monthly rent (since it is below the JND). It does not hold when stimuli are very intense or very weak.
Fechner’s Law (1860)
As stimulus magnitude increases, larger and larger changes in physical energy are necessary to obtain equal changes in perceived magnitude – sensory perception is a logarithmic function of stimulus intensity. i.e. constant increases in physical energy will produce progressively smaller increases in perceived magnitude. Applies to most, but not all, stimuli - e.g. does not apply to an electric shock, which takes less and less energy to increase the perceived intensity.
Perceptual organization
Figure and ground
When you look at a complex scene or listen to a noisy environment, some stimuli are emphasized and stand out clearly (figure) and others are perceived to be less relevant background (ground). e.g. you see a person standing in front of building, not a building with a person-shaped whole in it. The effect is known as figure-ground differentiation. e.g. vases/ faces picture demonstrates reversal of figure and ground. Camouflage demonstrates blurring of figure and ground.
Grouping
Certain inherent properties of the stimulus environment lead people to group them together, more or less automatically. The Gestalt psychologists argued that people perceive sights and sounds as organized wholes - the whole is greater than the sum of its parts. Most perceptual phenomena demonstrate Gestalt effects. They proposed a number of principles that describe how the perceptual system organizes stimuli:
1.proximity - the closer objects or events are to one another, the more likely they are to be perceived as belonging together
2.similarity - similar elements are perceived to be part of a group, even if separated
3.continuity - sensations that appear to create a continuous form are perceived as belonging together
4.closure - people tend to fill in missing contours to form a complete object
5.texture - when basic features of stimuli have the same texture, those stimuli are grouped together
6.simplicity - people tend to group features of a stimulus in a way that provides the simplest interpretation of the world
7.common fate - sets of objects that are moving in the same direction at the same speed are perceived together (e.g. flock of birds)
8.common region - elements located within the same boundary tend to be grouped together
Auditory scene analysis
Through closure, we hear a tone as continuous even if it is repeatedly interrupted by bursts of static. Auditory scene analysis is the perceptual process of mentally representing and interpreting sounds. Sounds with similar characteristics are grouped into separate auditory streams which are sounds perceived as coming from the same source
Perception of depth, distance, and motion
Depth & distance perception
Made possible by stimulus cues - characteristics of visual stimuli and therefore illustrate the ecological view of perception:
1.principle of relative size - the object producing the larger image on the retina is perceived to be closer
2.height/ elevation - more distant objects are usually higher in the visual field
3.interposition (a.k.a. occlusion) - closer objects block the view of things further away
4.the apparent convergence of parallel lines is linear perspective - the closer together two converging lines are, the greater the perceived distance
5.since greater distances usually produce less clarity, reduced clarity is interpreted as a cue for greater distance
6.light and shadow - brighter objects are perceived as being nearer
7.a textural gradient is a graduated change in the texture of a visual field - texture appears less detailed as distance increases
8.movement gradient (a.k.a. motion parallax) is the difference in the apparent movement of objects - objects closest to you move faster than distant objects
Depth and distance perception are abnormal in:
·schizophrenia ·temporal lobe epilepsy ·derealization ·acute brain syndromes
Cues based on properties of the visual system
·accommodation produces information about muscle activity, which helps to create the perception of distance
·convergence - the closer the object, the more the eyes must converge, and the greater the proprioceptive information going to the brain
·the difference between the two retinal images of an object is called binocular disparity - the disparity decreases with increasing distance
·a similar principle underlies location of an auditory stimulus
Perception of motion
Not due to movement of retinal image (which has to move or habituation renders it invisible). Brain differentiates between pursuit and searching eye movements (? Action at superior colliculus). Looming is the rapid expansion in the size of an image so that it fills the retina - when an image looms, there is an automatic tendency to perceive it as an approaching stimulus. If the expansion is as fast to the right as to the left, and as fast as above as below, this signals that the object is approaching the eyes and we duck! Relatively large objects are assumed stationary (e.g. moon and clouds). Motion aftereffects show spatial frequency specificity. Perception of apparent motion depends on interstimulus interval. The magnitude and pattern of texture moving across the retina provide a cue to your speed and to whether you are accelerating.
Perceptual constancy
Experience of a consistent world in spite of incomplete, ambiguous, confusing sensory information; often lost in schizophrenia
Size constancy: approaching object produces increasingly large retinal image but does not appear to grow - objects retain size regardless of distance (constancy scaling). People are better at judging the true size and distance of familiar objects. May explain why small cars have more accidents than large ones in countries where cars vary greatly in size.
Shape constancy: e.g. window perceived as rectangle in spite of non-rectangular and varying retinal image. In Western countries, most corners are at right angles, and most curved surfaces are circular or spherical.
Brightness constancy: white paper in dim light and grey paper in bright light retain colours in spite of overlap in amount of light reflected
Colour constancy: similar retention of colour in differing coloured illumination.
Location constancy: spatial position is constant regardless of viewer’s movement.
Perceptual illusions
Ponzo Illusion - can be explained by a misapplication of size constancy and the depth cue of linear perspective
Müller-Lyer Illusion - represents a misapplication of the depth cue of linear perspective
Ebbinghaus illusion - similar to the misjudgment of brightness
visual illusions suggest an active striving after meaning. Reproduce completely in drawings what is present visually but not experienced by touch (e.g. front parts of bus omitted but access platform at the rear is included). Maintain perceptual constancy for size and shape.
Mechanisms of pattern recognition
Matching of stimulus to template acquired by learning/ experience does not allow for generalization. Possibility of abstractions (prototypes) generated by comparisons and categories of stimuli but context not accounted for. Feature recognition followed by feature combination and ultimately pattern recognition.
Top-down processing
Knowledge of the world and experience in perceiving allow people to make inferences about the identity of stimuli, even when the quality of raw sensory information is low. · for example, a small, dark, moving shape at the end of a leash is perceived as a dog, because the stimulus occurs at a location where we would expect a dog to be· Motivation can also affect perception.
Bottom-up processing
Raw sensory information is analyzed into basic features, such as colour or movement. These features are then recombined at higher brain centres, where they are compared to stored information about objects or sounds - for example, you recognize a dog because it has 4 legs, a tail, and is barking, and it matches your perceptual category for ‘dog’. Network, or PDP (parallel distributed processing). Recognition depends on communication among feature-analysis systems operating simultaneously and enlightened by past experience. for example, a dog standing behind a picket fence will look like a dog, even though each ‘slice’ may not look like a dog.
Perceptual set
A perceptual set is the readiness or predisposition to perceive a stimulus in a certain way
·demonstrates top-down processing
·expectancy may also be shaped by the context in which the stimulus occurs
·emotional connotations e.g. perceptual defence
·individual values e.g. poor children exaggerate size of coins
·personality characteristics may also predict the kinds of top-down processing that people engage in (field dependence)
Information theory
·can extract information from sensory stimuli and form hypotheses about the world e.g. reading using only the tops of letters, or some rather than all words in a text
·sensory input contains redundant information
·chunking of information facilitates processing
·grouping information into aggregations according to attributed meaning
Perception and Human Development
Habituation - infants stop looking when they repeatedly see stimuli that are perceived to be the same. if a stimulus appears that is perceived to be different, looking resumes - this is dishabituation
Learning vs. innate in perception
Neonates capable of levels of brightness discrimination, can track, have slightly impaired visual acuity (6:50) and fixed focus (20 cm)
2/12: depth perception (as tested with the visual cliff)
4/12: colour vision, accommodation
6/12: acuity is 6:6
Preference for complex stimuli
in blindness from birth, which has been corrected surgically, the subject can:
·fixate, track, scan ·distinguish figure from ground ·but not able immediately to: ·identify visually objects previously known by touch ·the following functions are believed to be innate: ·visual scanning ·tracking ·fixating ·figure ground discrimination
Attention
Attention is the process of directing and focusing certain psychological resources to enhance perception, performance, and mental experience
Three important characteristics:
1.improves mental processing - 2.attention takes effort 3.attentional resources are limited
·shifting attention involves:
1.overt orienting - pointing sensory systems at a particular stimulus e.g. looking behind you
2.covert orienting - e.g. able to shift attention to an image of a friend’s face without moving a muscle
Types of attention
1.Selective/ Focused
2.Divided: a)two sources are attended to simultaneously
3.Sustained: a)the environment is monitored constantly
4.Controlled: a)effort is required
5.Automatic: a)little conscious effort is required. b)Stroop effect: the automatic process is so ingrained that it interferes with controlled processing
Dual-task interference: refers to the loss of performance occurring during divided attention
According to the ecological view of perception, most of what we perceive is already present in the rich array of stimuli in the environment
The primary goal of perception is to support actions such as walking or driving. The constructionists argue that the perceptual system must construct a representation of reality from fragments of sensory information· The computational view explains how complex computations within the nervous system might turn raw sensory stimulation into a representation of the world.
Psychophysics
Absolute thresholds
The minimum detectable amount of physical energy (light, sound, pressure, etc.) is called the absolute threshold. Because of variability, psychophysicists have redefined the absolute threshold as the minimum amount of energy that can be detected 50 % of the time· Variability arises from:
1.internal noise is the spontaneous, random firing of cells in the nervous system that occurs in varying degrees whether or not we are stimulated by physical energy
2.the response criterion (a.k.a. bias) reflects a person’s willingness or reluctance to respond to a stimulus and is affected by: ·motivation ·expectancies
Difference threshold
The minimum difference that has to exist between two sources for them to be perceived separately
Signal detection theory
Signal detection theory is a mathematical model of how people’s sensitivity and response criterion combine to determine decisions about whether they say that a near-threshold stimulus has occurred. Sensitivity refers to one’s ability to discriminate a stimulus from its background. The theory explains: ·detecting cancer cells in a pap smear ·spotting a weapon in an airport x-ray.
Weber’s Law (Ernst Weber)
States that the smallest detectable difference in stimulus energy is a constant fraction of the intensity of the stimulus. The smallest detectable difference in the stimulus is called the difference threshold or just-noticeable difference (JND). e.g. noticing a 50 p difference in change from a £1 bus fare, but not noticing a 50 p difference in the monthly rent (since it is below the JND). It does not hold when stimuli are very intense or very weak.
Fechner’s Law (1860)
As stimulus magnitude increases, larger and larger changes in physical energy are necessary to obtain equal changes in perceived magnitude – sensory perception is a logarithmic function of stimulus intensity. i.e. constant increases in physical energy will produce progressively smaller increases in perceived magnitude. Applies to most, but not all, stimuli - e.g. does not apply to an electric shock, which takes less and less energy to increase the perceived intensity.
Perceptual organization
Figure and ground
When you look at a complex scene or listen to a noisy environment, some stimuli are emphasized and stand out clearly (figure) and others are perceived to be less relevant background (ground). e.g. you see a person standing in front of building, not a building with a person-shaped whole in it. The effect is known as figure-ground differentiation. e.g. vases/ faces picture demonstrates reversal of figure and ground. Camouflage demonstrates blurring of figure and ground.
Grouping
Certain inherent properties of the stimulus environment lead people to group them together, more or less automatically. The Gestalt psychologists argued that people perceive sights and sounds as organized wholes - the whole is greater than the sum of its parts. Most perceptual phenomena demonstrate Gestalt effects. They proposed a number of principles that describe how the perceptual system organizes stimuli:
1.proximity - the closer objects or events are to one another, the more likely they are to be perceived as belonging together
2.similarity - similar elements are perceived to be part of a group, even if separated
3.continuity - sensations that appear to create a continuous form are perceived as belonging together
4.closure - people tend to fill in missing contours to form a complete object
5.texture - when basic features of stimuli have the same texture, those stimuli are grouped together
6.simplicity - people tend to group features of a stimulus in a way that provides the simplest interpretation of the world
7.common fate - sets of objects that are moving in the same direction at the same speed are perceived together (e.g. flock of birds)
8.common region - elements located within the same boundary tend to be grouped together
Auditory scene analysis
Through closure, we hear a tone as continuous even if it is repeatedly interrupted by bursts of static. Auditory scene analysis is the perceptual process of mentally representing and interpreting sounds. Sounds with similar characteristics are grouped into separate auditory streams which are sounds perceived as coming from the same source
Perception of depth, distance, and motion
Depth & distance perception
Made possible by stimulus cues - characteristics of visual stimuli and therefore illustrate the ecological view of perception:
1.principle of relative size - the object producing the larger image on the retina is perceived to be closer
2.height/ elevation - more distant objects are usually higher in the visual field
3.interposition (a.k.a. occlusion) - closer objects block the view of things further away
4.the apparent convergence of parallel lines is linear perspective - the closer together two converging lines are, the greater the perceived distance
5.since greater distances usually produce less clarity, reduced clarity is interpreted as a cue for greater distance
6.light and shadow - brighter objects are perceived as being nearer
7.a textural gradient is a graduated change in the texture of a visual field - texture appears less detailed as distance increases
8.movement gradient (a.k.a. motion parallax) is the difference in the apparent movement of objects - objects closest to you move faster than distant objects
Depth and distance perception are abnormal in:
·schizophrenia ·temporal lobe epilepsy ·derealization ·acute brain syndromes
Cues based on properties of the visual system
·accommodation produces information about muscle activity, which helps to create the perception of distance
·convergence - the closer the object, the more the eyes must converge, and the greater the proprioceptive information going to the brain
·the difference between the two retinal images of an object is called binocular disparity - the disparity decreases with increasing distance
·a similar principle underlies location of an auditory stimulus
Perception of motion
Not due to movement of retinal image (which has to move or habituation renders it invisible). Brain differentiates between pursuit and searching eye movements (? Action at superior colliculus). Looming is the rapid expansion in the size of an image so that it fills the retina - when an image looms, there is an automatic tendency to perceive it as an approaching stimulus. If the expansion is as fast to the right as to the left, and as fast as above as below, this signals that the object is approaching the eyes and we duck! Relatively large objects are assumed stationary (e.g. moon and clouds). Motion aftereffects show spatial frequency specificity. Perception of apparent motion depends on interstimulus interval. The magnitude and pattern of texture moving across the retina provide a cue to your speed and to whether you are accelerating.
Perceptual constancy
Experience of a consistent world in spite of incomplete, ambiguous, confusing sensory information; often lost in schizophrenia
Size constancy: approaching object produces increasingly large retinal image but does not appear to grow - objects retain size regardless of distance (constancy scaling). People are better at judging the true size and distance of familiar objects. May explain why small cars have more accidents than large ones in countries where cars vary greatly in size.
Shape constancy: e.g. window perceived as rectangle in spite of non-rectangular and varying retinal image. In Western countries, most corners are at right angles, and most curved surfaces are circular or spherical.
Brightness constancy: white paper in dim light and grey paper in bright light retain colours in spite of overlap in amount of light reflected
Colour constancy: similar retention of colour in differing coloured illumination.
Location constancy: spatial position is constant regardless of viewer’s movement.
Perceptual illusions
Ponzo Illusion - can be explained by a misapplication of size constancy and the depth cue of linear perspective
Müller-Lyer Illusion - represents a misapplication of the depth cue of linear perspective
Ebbinghaus illusion - similar to the misjudgment of brightness
visual illusions suggest an active striving after meaning. Reproduce completely in drawings what is present visually but not experienced by touch (e.g. front parts of bus omitted but access platform at the rear is included). Maintain perceptual constancy for size and shape.
Mechanisms of pattern recognition
Matching of stimulus to template acquired by learning/ experience does not allow for generalization. Possibility of abstractions (prototypes) generated by comparisons and categories of stimuli but context not accounted for. Feature recognition followed by feature combination and ultimately pattern recognition.
Top-down processing
Knowledge of the world and experience in perceiving allow people to make inferences about the identity of stimuli, even when the quality of raw sensory information is low. · for example, a small, dark, moving shape at the end of a leash is perceived as a dog, because the stimulus occurs at a location where we would expect a dog to be· Motivation can also affect perception.
Bottom-up processing
Raw sensory information is analyzed into basic features, such as colour or movement. These features are then recombined at higher brain centres, where they are compared to stored information about objects or sounds - for example, you recognize a dog because it has 4 legs, a tail, and is barking, and it matches your perceptual category for ‘dog’. Network, or PDP (parallel distributed processing). Recognition depends on communication among feature-analysis systems operating simultaneously and enlightened by past experience. for example, a dog standing behind a picket fence will look like a dog, even though each ‘slice’ may not look like a dog.
Perceptual set
A perceptual set is the readiness or predisposition to perceive a stimulus in a certain way
·demonstrates top-down processing
·expectancy may also be shaped by the context in which the stimulus occurs
·emotional connotations e.g. perceptual defence
·individual values e.g. poor children exaggerate size of coins
·personality characteristics may also predict the kinds of top-down processing that people engage in (field dependence)
Information theory
·can extract information from sensory stimuli and form hypotheses about the world e.g. reading using only the tops of letters, or some rather than all words in a text
·sensory input contains redundant information
·chunking of information facilitates processing
·grouping information into aggregations according to attributed meaning
Perception and Human Development
Habituation - infants stop looking when they repeatedly see stimuli that are perceived to be the same. if a stimulus appears that is perceived to be different, looking resumes - this is dishabituation
Learning vs. innate in perception
Neonates capable of levels of brightness discrimination, can track, have slightly impaired visual acuity (6:50) and fixed focus (20 cm)
2/12: depth perception (as tested with the visual cliff)
4/12: colour vision, accommodation
6/12: acuity is 6:6
Preference for complex stimuli
in blindness from birth, which has been corrected surgically, the subject can:
·fixate, track, scan ·distinguish figure from ground ·but not able immediately to: ·identify visually objects previously known by touch ·the following functions are believed to be innate: ·visual scanning ·tracking ·fixating ·figure ground discrimination
Attention
Attention is the process of directing and focusing certain psychological resources to enhance perception, performance, and mental experience
Three important characteristics:
1.improves mental processing - 2.attention takes effort 3.attentional resources are limited
·shifting attention involves:
1.overt orienting - pointing sensory systems at a particular stimulus e.g. looking behind you
2.covert orienting - e.g. able to shift attention to an image of a friend’s face without moving a muscle
Types of attention
1.Selective/ Focused
2.Divided: a)two sources are attended to simultaneously
3.Sustained: a)the environment is monitored constantly
4.Controlled: a)effort is required
5.Automatic: a)little conscious effort is required. b)Stroop effect: the automatic process is so ingrained that it interferes with controlled processing
Dual-task interference: refers to the loss of performance occurring during divided attention
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