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Sensation and Perception > Midterm > Flashcards

Midterm Flashcards

1
Q

Perception is a result of:

A

available physical energy
sensitivities of our sense organs
information processing in our brain

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2
Q

What is the “input” and output” of human vision?

A

Distal stimulus (outside image, 3D) -> proximal stimulus (retinal image, 2D) -> Visual percept, 3D

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3
Q

What are qualities the eye looks for in an image?

A

Angle, Shape, Size, Lightness and brightness

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4
Q

Fundamental problem of perception:

A

Every proximal stimulus is consistent with many different distal stimuli.

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5
Q

Optics

A

The mapping of the 3D scene to the projected image

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6
Q

Inverse optics:

A

mapping of the projected image to the 3D scene

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7
Q

levels of analysis of perception

A
  1. What problem is it solving? (computational analysis)
  2. What strategy is it adopting? (algorithm)
  3. How is it implemented in hardware? (brain circuits)
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8
Q

multiple approaches to sensation and perception

A
  1. Theoretical (computational)
  2. Psychological (behavioral)
  3. Biological (neuroscience)
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9
Q

Psychophysics

A

Study of relationship between physical world and “psyche” (Gustav Fechner)

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10
Q

Absolute threshold

A

Minimum intensity needed to evoke a sensation - Boundary between undetectable and detectable

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11
Q

Difference threshold

A

Minimum change in intensity that leads to a noticeably different stimulus. boundary between “look the same” and “look different”

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12
Q

Weber’s law

A

Difference threshold is proportional to stimulus intensity ^I = K . I
K = “weber fraction”

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13
Q

Each difference threshold corresponds to a

A

just noticeable difference (JND)

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14
Q

Method of constant stimuli

A

fixed set of stimuli
undetectable to easily detectable
Presented multiple time in random order
Respond: YES or NO
Plot “percentage of detections”
Ideal case: 100% detections at and post the absolute threshold
What actually happens: More of a ramp, take 50% as absolute threshold
Plot graph from intensity and proportion of “yes” responses

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15
Q

Method of limits

A 
Fixed set of stimuli
Start with weak (undetectable) stimulus
Gradually increase intensity
Mark "crossover point"
Threshold = mean of crossovers
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16
Q

Method of adjustment

A

Intensities not fixed in advance
Interactively adjusted by observer
Some concerns: No “right answer”, differences in individual criterion/motivation level

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17
Q

Forced-choice methods

A

Set up task so there’s always a right answer
Example: Dim light flashes either on left or right of screen
If invisible, observer has to guess
If clearly visible -> Accuracy ~100%
75% point is threshold, scale starts at 50

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18
Q

Doctrine of specific nerve energies

A

What matters is which nerves are stimulated, not how they are stimulated (Johannes Muller)

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19
Q

Lesion studies

A

Locus of lesion loss in performance
Example: Damage to area MT and motion-blindness
Difficulty in interpretation: correlation does not imply causation
ex. 1: economy of san francisco / golden gate bridge

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20
Q

Single-cell recording

A

Measure electrical activity from a single neuron, using a microelectrode

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21
Q

Neurons

A

Cells that integrate and transmit signals

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22
Q

Dendrites

A

Collect chemical signals

Convert into electrical activity

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23
Q

cell body

A

integrates electrical activity

Generates nerve impulses

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24
Q

axon

A

Transmits nerve impulses

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25
Q

terminals

A

Convert impulse to chemical signals

Pass on other neurons

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26
Q

action potential

A

“firing” of a nerve impulse
All-or-none
Travels from cell body to terminals
1. Resting potential = -70mV
2. Given sufficient +ve charge (“depolarization”), a sudden upsurge is generated.
3. Spike travels along axon
4. Dies down but overshoots RP before returning

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27
Q

Synapse

A

Small gap between pre-synaptic and post-synaptic neurons
Neurotransmitters sent across synapse
Modify likelihood of post-synaptic neuron firing

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28
Q

Two kinds of synapses:

A

Firing of the pre-synaptic neuron either…
increases chances of post-synaptic neuron firing (excitatory synapse) +ve charge (depolarizing)
decreases chances of post-synaptic neuron firing (inhibitory synapse) -ve charge (hyperpolarizing)

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29
Q

The Rate Law

A

One impulse is not the basic element of information
Continuous information is encoded by rate of firing
(Hertz = # per second)

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30
Q

What counts as “no response”?

A

Baseline firing rate: ~1-5 hz
Excitation increases firing rate (100-500 hz)
Inhibition decreases firing rate (< 1 hz)
Firing rate is always measured relative to baseline

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31
Q

EEG

A

Record brain activity using electrodes on scalp
Difficulties:
(a) Hard to pinpoint precisely
(b) Many signals too weak

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32
Q

Neuroimaging

A

Highly active regions will have greater blood flow - PET, fMRI

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33
Q

To understand visual perception, we must study

A
  1. Light and its interaction with objects
  2. Structure and function of the eye
  3. Information processing in the eye and brain
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34
Q

Light

A

Dual nature: Light is a particle and a wave

35
Q

Light as particles

A

Travels in straight lines “rays”

Smallest ‘packet’: Photon

36
Q

Light as wave

A

Has a wavelength

Refraction: bends when it encounters a new medium

37
Q

What is an eye?

A

Def 2: a structure/organ that can compare light from different directions

38
Q

convex lens

A

Convex lenses converge light rays

39
Q

Focal length

A

Distance at which parallel rays converge
Diopters = 1 / focal length (in m)
Example: 5 diopters: f = 20cm (=1/5 m)

40
Q

Main functions of the human eye

A

Main functions:
Form a sharp image
Transduction
Initiate image processing

41
Q

Optical power is made up of

A

cornea (2/3) + lens (1/3)

Optical power of the lens in adjustable (Ciliary muscles, known as accomodation)

42
Q

Hyperopia (Farsightedness)

A

Eyeball too short or lens too weak
nearby objects are blurred (rays do not converge enough)
Correction: Convex lens

43
Q

Myopia (Nearsightedness)

A

Eyeball too long or lens too “strong”
Distant objects are blurred (rays converge too much)
Correction: concave lens

44
Q

Presbyopia (“old sight”)

A

Lens becomes inflexible
Cannot focus on nearby objects
Near point: closest distance at which an object can be focused.

45
Q

The retina

A

Light -> Ganglion ->Bipolar -> horizontal -> rod in physical direction
Ganglion cell axons bond to form the optic nerve

46
Q

Fovea

A

Small central “pit where vision is most acute”

47
Q

Optic disk

A

Where the ganglion fibers leave the eye

48
Q

rods

A

Higher sensitivity to light
Lower resolution
Scotopic vision
Color-blind

49
Q

cone

A

Lower sensitivity to light
Higher resolution
Photopic (color) vision

50
Q

Distribution of receptors

A

Fovea: Cones only -> high-resolution vision
Periphery: rods and cones

51
Q

How does light at different locations affect a ganglion cell?

A
  1. Most of the retina - “no response”
  2. Small circular region where light excites the cell - ON response
  3. Donut-shaped region where light inhibits the cell - OFF response
52
Q

Receptive field

A

that region on the retina which, when stimulated, influences the baseline firing rate of a neuron. Combination of disk and ring = receptive field

53
Q

Center-surround antagonism or lateral inhibition

A

when neuronal activity antagonizes (turns off) surrounding activity (center-surround antagonism)

54
Q

What do ganglion cells respond to?

A
  1. Uniform illumination: + and - responses cancel
    do not respond well to overall light level.
  2. Dark-light border: strong response
    Respond to changes in light level
  3. Orientation change: no influence
    not sensitive to edge orientation
55
Q

P-cells

A

parvocellular (small)
Comprise 80% of cells
Smaller receptive fields -> higher spatial resolution
Lower sensitivity
Thinner axons -> worse temporal resolution
color sensitive

56
Q

M-cells

A

Comprise 10% of cells
Larger receptive fields -> lower spatial resolution
higher sensitivity
thicker axons -> better temporal resolution
Color blind

57
Q

Why respond to changes in brightness?

A

That’s where there is the most information in a scene.

58
Q

Perceptual consequences of ganglion-cell processing

A
  1. Neural signal depends on local intensity and surrounding intensity
  2. Signal emphasizes contrast borders; de-emphasizes homogeneous regions
59
Q

Optic chiasm

A

Temporal half of retina -> Ipsilateral visual cortex

Nasal half of retina -> Contralateral visual cortex

60
Q

Why does the optic chiasm split the visual field as it does?

A

Because the controlateral brain areas correspond to the eyes’ visual field - the hemispherical set-up helps establish 3D vision both binocularly, and monocularly.
Left visual fields (both eyes) -> right visual cortex
Right visual fields (both eyes) -> Left visual cortex

61
Q

Retinotopic map in V1

A

Each hemisphere represents contralateral visual field

62
Q

Cortical magnification

A

80% of cells devoted to central 10 degrees

63
Q

Main new features of V1 cells

A

orientation selectivity
Selectivity for direction of motion
Binocularity

64
Q

Simple V1 cells

A

respond to edges and bars of specific orientations
Elongated RFs with clearly-demarcated ON and OFF regions
The edge or bar much be positioned exactly within RF

65
Q

Complex V1 cells

A

also orientation selective
No separate ON / OFF regions
Exact positioning of edge / bar not required

66
Q

Binocularity

A

First site of binocular cells

Note: these cells have two receptive fields!

67
Q

Ocular dominance

A

slightly stronger responses to one eye
Example: pattern, looks like a fingerprint
Black stripes: right-eye dominant
White stripes: left-eye dominant

68
Q

Where does the signal go from V1?

A

Dorsal stream goes towards parietal (M pathway)

Ventral stream goes towards temporal (P pathway)

69
Q

Spatial vision

A

ability to visually detect spatial patterns

example: seashells at multiple scales, zooming out looks like mona lisa

70
Q

How good is our vision at different scales?

A

Relation of RF size?
Large RF’s -> coarse scale
Small RF’s -> fine scale

71
Q

Multi-channel model

A

Campbell & Robson (1968)
The visual system analyzes information through multiple channels
Each channel is responsible for a particular spatial scale

72
Q

Fourier’s theorem

A

A mathematical procedure by which any signal can be separated into component sine waves at different frequencies. Combining these sine waves will reproduce the original signal.

73
Q

Fechner’s law

A

A principle describing the relationship between stimulus and resulting sensation that says the magnitude of subjective sensation increases proportionally to the logarithm of the stimulus intensity.

74
Q

spatial frequency

A

The number of cycles of a grating (e.g., dark and bright bars) per unit of visual angle (usually specified in cycles per degree).

75
Q

Why use sine gratings?

A 
1. Spatial frequency
How many cycles per unit distance?
2. Amplitude / contrast
low: dim, gets darker as you go from low to high
3. Orientation
0, +45, -45, 90
4. Phase
Position relative to a fixed landmark
76
Q

Contrast-sensitivity function (CSF)

A

A function describing how the sensitivity to contrast (defined as the reciprocal of the contrast threshold) depends on the spatial frequency (size) of the stimulus.

77
Q

Contrast threshold

A

Minimum amount of contrast (on a sine grating) that is visible
How low can you go?

78
Q

Measuring the CSF

A 
Pick a frequency f.
Measure contrast threshold for f.
Sensitivity = 1/threshold
repeat for different frequencies
CSF charts = x = spatial frequency, y=contrast sensitivity
CSF= window of visibility
79
Q

Selective adaptation experiment

A
  1. Measure an observer’s CSF
  2. Adapt the observer to a high-contrast grating with some frequency f
  3. Measure the CSF again
  4. compare pre- and post-adaptation CSFs.
80
Q

Adaptation

A

decrease in the strength of a neuron’s response after prolonged firing

81
Q

Selective adaptation

A

Only those neurons sensitive to the adapting frequency get fatigued
Different channels respond to different frequency ranges

82
Q

Compare vision across different conditions:

A

Scotopic (low), Mesopic (medium), Photopic (high)

83
Q

Perception

A

Our link and access to our world
Construction of our reality
sense of 3D space/distance
sounds/voices
tactile sensations
Perception informs an organism about: what is in its environment and where it is
Evolutionary significant actions: flee from predators, hunt/gather food, find mates, navigate

Sensation and Perception (4 decks)

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