Cognitive Psychology: Perception

Question 1

What is perception and sensation?

Answer 1

Perception – the process of acquiring knowledge about environmental objects or events via the senses and is broken down into two stages:
1. Sensation: the process of transforming physical stimuli to electrical signals
2. Perception: interpretating these signals for conscious awareness

Question 2

What is the perceptual process?

Answer 2

Distal stimuli – Proximal stimuli – (Sensation) conversion to neural signals which are sent to the brain – (Perception) signal processing and interpretation

Question 3

Evolutionary utility: What is perception for?

Answer 3

• Perception has evolved to aid the survival and reproduction of organisms
• Use our five senses (Somatosensory, visual, gustatory, auditory and olfactory)
• Different types of energy in the environment determines which senses has evolved, some species sense energies that humans cannot (Caribou can sense UV spectrum to detect camouflaged predators, Elephants are sensitive to very low frequency sounds so they can communicate over large distances)

Question 4

Is perception veridical?

Answer 4

• Senses would not evolve if they didn’t provide accurate information, however perception is not always a clear window onto reality

Question 5

What are illusions and ambiguous figures?

Answer 5

Illusions – situations in which perception differs from reality
Ambiguous figures – images that can give rise to two or more distinct perceptions (bistable images)

Question 6

Ambiguous sounds

Answer 6

Ambiguous sounds – can give rise to multiple bistable and stable perceptions (Laurel and Yannie)

• Sometimes sensory input is interpreted by the brain as representing objects or scenarios that are physically impossible
• Objects are not perceived directly
• Two sources of information (1. Current sensory input, 2. Existing knowledge about the environment)

Question 7

What are top-down and bottom-up processes?

Answer 7

Top-down – use knowledge about the structure of the world to influence perception
Bottom-up – take information from the senses and make judgements about the nature of the world

Question 8

Constructivist theories of perception

Answer 8

• Emphasise the importance of top-down processing
• Helmholtz argued that the inadequate information provided by the senses is argumented by unconscious inference
• Our brains are reluctant to interpret face images as convex

Question 9

Direct theories of perception

Answer 9

• Emphasise the importance of bottom-up processing
• James Gibson argued that the constructivist approach many underestimate the richness of the sensory evidence of receive
• There are a great variety of cues in the natural world that provide much information about the structure of the environment
• The perceiver is not a passive observer but interacts with the environment – this interaction is also the key to picking up useful information

Question 10

What is visible light?

Answer 10

is a band of energy within the electromagnetic spectrum
- Different wavelengths of lights (photons) are associated with different colour perceptions
- Light intensity is associated with brightness
- Light can be absorbed, reflected and transmitted

Question 11

Structure of the human eye

Answer 11

• Single-chambered eye uses convex cornea and lens to project an image onto the retina and enables directional sensitivity (represent the spatial structure)
• Photoreceptors transduce light into an electrical potential
• These signals than flow through a network of neurons to retinal ganglion cells and then out the back of the eye via the optic nerve

Question 12

Where are rods and cons located?

Answer 12

• Rods – located primarily in peripheral retina (low light levels)
• Cons – concentrated in fovea (require higher light levels)

Question 13

The journey light takes from retina to primary visual cortex

Answer 13

Retina – optic nerve – optic chiasm – LGN – primary visual cortex

Question 14

Brightness perception: Bottom-up and top-down processes

Answer 14

1. Bottom-up – The retina does not simply record light intensities and responses are shaped by processes occurring within the retina
2. Top-down – The brain also uses knowledge about how light interacts with objects when determining perceived brightness

Question 15

What is the light/dark adaptation theory

Answer 15

• The sensitivity of the retina is constantly adjusted to compensate for changes in mean luminance (sensitivity is reduced when mean intensity is high and increased when it is low)
• Negative afterimages – can produce illusions under some circumstances

Question 16

What is lateral inhibition?

Answer 16

• Early form of information processing in retina
• Retinal ganglion cells receive both excitatory and inhibitory input from neighbouring photoreceptors
• Lateral inhibition makes the visual system sensitive to changes in luminance but can have dramatic effects on perceived brightness

Question 17

What are top-down influences?

Answer 17

• Our visual systems also uses knowledge of how light interacts with 3D objects in the world when determining brightness (result in ‘errors’ in 2D images portraying 3D scenes)

Question 18

What is colour perception?

Answer 18

• Only rod photoreceptors are sensitive enough to operate and contain a single type of photopigment (rhodopsin)
• Light of different wavelengths and intensities can elicit identical responses

Question 19

Trichromacy: the three different cones

Answer 19

1. S-cones: blue cones (short wavelengths)
2. M-cones: green cones (Middle wavelengths)
3. L-cones: red cones (Long wavelengths)

Question 20

What are the three types of trichromacy (colour blindness)

Answer 20

1. Monochromacy: individuals have either 0 or 1 functioning cone type
2. Dichromacy: only 2 functioning cone types (Protanopia is missing L-cones, Deuteranopia is missing M-cones and Tritanopia is missing S-cones)
3. Anomalous trichromacy: defect in one of the cone types (Protanomaly is defect in L-cone, Deuteranomaly is defect in M-cone and Tritanomaly is defect in S-cone)

Question 21

What is opponency?

Answer 21

• Retinal ganglion cells receive excitatory and inhibitory input from different cone types which results in distinct red/green and blue-yellow pathways
• Negative afterimages (adaptation to red causes a reduction in the sensitivity of long wavelength cones, creating an imbalance in the input to red/green opponent retinal ganglion cells

Question 22

Top-down influences

Answer 22

• Colour constancy – the tendency for the perceived colour of objects to remain the same, even if the lighting changes
• This can give rise to illusions in which the same wavelengths of light is perceived as different colours

Question 23

Why is it difficult to design a perceiving machine?

Answer 23

• Visual input provides ambiguous information about the 3D structure of the world e.g. The same 2D retinal image could be produced by an infinite number of 3D objects
• Image complexity makes it tricky for computers to organise the visual scene into distinct objects

Question 24

Perceiving depth: two types of cues and examples

Answer 24

Humans make use of a variety of sources of image information to infer depth in a visual scene
- Monocular cues (work with one eye) e.g. relative height, shading
- Binocular cues (require both eyes) e.g. disparity

Question 25

5 features of monocular cues

Answer 25

1. Relative height and size – objects that are below the horizon are typically perceived as being more distant. If two objects are equal size the more distant one will take up less of your field of view. We need prior knowledge about the relative sizes of objects when judging distance
2. Occlusion – closer objects will occlude further away ones
3. Linear perspective and texture gradient – parallel lines extending away from observer converge in the distance. Texture elements get smaller and more dense with distance
4. Motion parallax – as we move, more distant objects will glide past us more slowly than nearer objects
5. Shadows and shading – cast shadows can create a strong perception of depth. Brightness of a surface depends on its orientation with respect to the light source

Question 26

Binocular disparity

Answer 26

1. Stereoscopic vision – our two eyes receive a slightly different image of the world
2. Disparity – this creates a differences in image location of an object seen by left and right eyes. The size of the disparity depends on an objects depth
3. Horopter – set of points that project to corresponding positions in the two retinas. Includes the fixation point

• Objects closer than the horopter have crossed disparities
• Objects further than the horopter have uncrossed disparities

Question 27

How do we perceive objects?

Answer 27

• Structuralism (Wilhelm Wundt) – proposed that perceptions are simply the sum of ‘atoms’ of sensation
• The Gestalt school (Wertheimer, Kohler & Koffka) – argued that the whole form or configurations is greater then the sum of its parts

Question 28

What are illusory contours?

Answer 28

some images evoke the perception of edges in locations where there is no change in luminance or colour. Difficult to explain via the structuralist approach

Question 29

The 6 Gestalt principles

Answer 29

Having rejected structuralism, the Gestalt psychologists proposed a number of principles by which elements in an image are grouped to created larger objects
1. Proximity – things that are close together group together
2. Similarity – things that are similar group together
3. Common fate – things that move together group together
4. Good continuation – group elements to form smoothly continuing lines rather than abrupt or sharp angles. Helps preserve grouping of occluded objects
5. Closure – group elements to form complete figures, even if incomplete (requires top-down knowledge)
6. Symmetry – elements more likely to be formed into groups that are balanced or symmetrical

Question 30

Assessment of Gestalt principles

Answer 30

• Seem correct about many things
• Generally hold across wide range of images
• Some of the principles seem vague and imprecise
• No coherent workable account of the underlying neural mechanisms
• No empirical evidence

Question 31

What is sound? perceptual and physical definition

Answer 31

Perceptual definition – sound is the experience we have when we hear
Physical definition – sound is pressure changes in the air or other medium caused by the vibration of an object

Question 32

Characteristics of sound: Pure Sounds

Answer 32

1. Amplitude size of the vibration in air pressure (related to perception of loudness) Related to perception of loudness
2. Frequency number of cycles per second (1 Hertz = 1 cycle) Related to perception of pitch

Question 33

Characteristics of sound: Complex Sounds

Answer 33

• All sound waves can be described as some combination of sine waves
• Natural sounds often consist of a fundamental frequency superimposed by additional waveforms with higher frequencies (the harmonics)

Question 34

Overview of the ear: Outer ear

Answer 34

(The human ear is divided into 3 sub-divisions: outer, middle and inner)
1. Outer ear:
- Pinnae – visible external parts of ear
- Auditory canal – 3cm tube-like structure and protects middle ear
- Tympanic membrane (eardrum) – cone-shaped membrane separting the outer and middle ear. Sound waves induce a difference in pressure either side of tympanic membrane, causing it to vibrate (larger amplitude = larger vibrations, higher frequency = faster vibrations)

Question 35

Overview of the ear: Middle ear

Answer 35

• Contains the three smallest bones in the human body (ossicles) – Malleus, Incus and Stapes
• The bones amplify the vibrations

Question 36

Overview of the ear: Inner ear

Answer 36

• The main structure of the inner ear is the cochlea, a snail-like liquid-filled organ
• Vibrations of the oval window displaces fluid in cochlea, resulting in a change in pressure which propagates up and down the spiral structure
• The cochlea consists of three parallel canals (vestibular, middle & tympanic)
• Auditory transduction is triggered by the motion of the basilar membrane which is translated into neural signals by structures in the Organ of Corti. A voltage is generated when specialised hair cells are bent which sends impulses in auditory nerve cells to the brain

Question 37

Loudness: How’s it measured?

Answer 37

• Our perception of loudness is related to the amplitude of sound waves (extremely large range)
• Measured in decibels (dB), a change of 2odB corresponds to a ten-fold increase in amplitude
• Rate code: sound amplitude is coded in the firing rate of auditory nerve fibres (responses increase with increasing sound intensity)
• Some fibres have high spontaneous rates and saturate rapidly and others have low rates

Question 38

How does loudness depend on frequency

Answer 38

• Our auditory systems are not equally sensitive to all sound frequencies
• The red curve here indicate the number of decibels requires to create the some perception of loudness at different frequencies
• Pitch: the highest frequency humans can hear is around 20,000Hz

Question 39

Pitch: difference between place code and timing code

Answer 39

• Place code – sound frequencies cause vibration in specific areas along the basilar membrane in the cochlea (low frequencies = near apex, high frequencies = near base)
• Timing code – auditory nerve responses are synchronised to changes in pressure and this property is called phase-locking and occurs up to frequencies of about 4000Hz

Question 40

Why does the same noise not sound different when played on different instruments?

Answer 40

• Pitch is typically determined by the fundamental frequency of a sound
• The number, frequency ratios and relative amplitudes of the harmonics dictates the quality or timbre of the sound

Question 41

The missing fundamental illusion

Answer 41

• We perceive a pitch consistent with the fundamental frequency, suggest that pitch isn’t simply determined in the cochlea, the brain infers the missing fundamental from the harmonics

Question 42

Where is visual information contained?

Answer 42

• Visual information for the relative location of objects is contained within the retinal image, but the place activated by a sound on the cochlea does not indicate its location

Question 43

How do we localise sound? Two types of cues

Answer 43

1. Binaural cues – require comparison of signals in left and right ears and are vital for signalling location of a sound in azimuth (Interaural time differences & interaural level differences)
2. Monaural cues – work with one ear can help localise the elevation and distance of a sound (Filter properties of the pinnae, intensity & reverberation)

Question 44

Binaural cues: What are interaural time differences (ITDs)?

Answer 44

• The relative time at which a sound arrives at the two ears depends on its location in the azimuth (left-right)
• If the source is straight ahead, the distance to each ear is the same and there is no difference in time
• The range of ITDs encountered depends on the; speed of sound (330m/s) & distance between the two ears (larger heads create bogger range of ITDs)

Question 45

Binaural cues: What are interaural level differences (ILDs)?

Answer 45

• The relative sound pressure level reaching the two ears also depends on the location of the source in the azimuth
• A reduction in sound level occurs for the far ear, due to the acoustic shadow created by the head (occurs for high-frequency sounds but not low frequency sounds)

Question 46

What is the physiology of binaural processing?

Answer 46

• Processing of ITDs and ILDs starts within the brainstem in the superior olivary complex (superior olive)
• Binaural localisation cues processed by different types of neurons, located in different parts of the superior olive (Lateral superior olive contains neurons that are sensitive to ILDs, Medial superior olive contains neurons that are sensitive to ITDs)

Question 47

Pro’s and Con’s of binaural processing

Answer 47

• Pros: ITDs work well for low-frequency sounds, ILDs provide information about high-frequency sounds
• Cons: provide ambiguous information about elevation and tell us nothing about distance (cone of confusion)

Question 48

Two factors in monaural localisation cues

Answer 48

1. Elevation – when sound reflects off your external ear, the relative intensity of different frequencies sound wave changes. This changes with sound source elevation and everyone will filter sounds in a different way
2. Distance – sound intensity decreases with distance, so closer objects will tend to have greater amplitudes than further ones (relative intensity) Multiple reflections combine to produce a persistence of sound called reverberation and the distance of a sound alters the relative intensity and timing of direct and reverberant sounds (reverberation)

Question 49

How do sounds localise wishing rooms?

Answer 49

• Reflected sound poses a potential problem for localisation (similar sounds arriving in quick succession from different locations are localised according to the direction of the first sound = Precedence effect)

Question 50

What is the auditory scene analysis?

Answer 50

(Your auditory system needs to be able to segregate the components of the sound that come from different sources and group the components of the sound that come from the same sound source)
1. Spectral grouping – combining different frequency sounds components that occur at the same time
2. Sequential grouping – combining sequences of sounds over time

Question 51

what is spectral grouping?

Answer 51

Frequency components of a sound are more likely to be grouped into a single sound if it is likely that they have been cause by the same sound-producing event
- Harmonicity – if a component is mistuned to other components, it will be heard as a separate sound
- Common frequency change – frequency components that change together tend to group together

Question 52

What is sequential grouping?

Answer 52

(The process of organising sounds over time into separate perceptual events is known as auditory stream segregation)
1. Similarity of pitch – sounds with similar pitch are often produced by the same source so increasing frequency difference promotes stream segregation
2. Temporal proximity – sounds that occur in rapid progression tend to be produced by the same source and increasing presentation rate also promotes stream segregation
3. Similarity of timbre – sound sources often have distinct timbre providing a good cue for stream segregation
4. Continuity – sounds that stay constant or change smoothly are often produced by the same source and are perceived as continuous even when interrupted by noise (Phonemic restoration: this effect is not restricted to tones, but can also occur with speech)