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Sensation and Perception > Midterm > Flashcards

Midterm Flashcards

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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
terminals
Convert impulse to chemical signals | Pass on other neurons
26
action potential
"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
27
Synapse
Small gap between pre-synaptic and post-synaptic neurons Neurotransmitters sent across synapse Modify likelihood of post-synaptic neuron firing
28
Two kinds of synapses:
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)
29
The Rate Law
One impulse is not the basic element of information Continuous information is encoded by rate of firing (Hertz = # per second)
30
What counts as "no response"?
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
31
EEG
Record brain activity using electrodes on scalp Difficulties: (a) Hard to pinpoint precisely (b) Many signals too weak
32
Neuroimaging
Highly active regions will have greater blood flow - PET, fMRI
33
To understand visual perception, we must study
1. Light and its interaction with objects 2. Structure and function of the eye 3. Information processing in the eye and brain
34
Light
Dual nature: Light is a particle and a wave
35
Light as particles
Travels in straight lines "rays" | Smallest 'packet': Photon
36
Light as wave
Has a wavelength | Refraction: bends when it encounters a new medium
37
What is an eye?
Def 2: a structure/organ that can compare light from different directions
38
convex lens
Convex lenses converge light rays
39
Focal length
Distance at which parallel rays converge Diopters = 1 / focal length (in m) Example: 5 diopters: f = 20cm (=1/5 m)
40
Main functions of the human eye
Main functions: Form a sharp image Transduction Initiate image processing
41
Optical power is made up of
cornea (2/3) + lens (1/3) | Optical power of the lens in adjustable (Ciliary muscles, known as accomodation)
42
Hyperopia (Farsightedness)
Eyeball too short or lens too weak nearby objects are blurred (rays do not converge enough) Correction: Convex lens
43
Myopia (Nearsightedness)
Eyeball too long or lens too "strong" Distant objects are blurred (rays converge too much) Correction: concave lens
44
Presbyopia ("old sight")
Lens becomes inflexible Cannot focus on nearby objects Near point: closest distance at which an object can be focused.
45
The retina
Light -> Ganglion ->Bipolar -> horizontal -> rod in physical direction Ganglion cell axons bond to form the optic nerve
46
Fovea
Small central "pit where vision is most acute"
47
Optic disk
Where the ganglion fibers leave the eye
48
rods
Higher sensitivity to light Lower resolution Scotopic vision Color-blind
49
cone
Lower sensitivity to light Higher resolution Photopic (color) vision
50
Distribution of receptors
Fovea: Cones only -> high-resolution vision Periphery: rods and cones
51
How does light at different locations affect a ganglion cell?
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
Receptive field
that region on the retina which, when stimulated, influences the baseline firing rate of a neuron. Combination of disk and ring = receptive field
53
Center-surround antagonism or lateral inhibition
when neuronal activity antagonizes (turns off) surrounding activity (center-surround antagonism)
54
What do ganglion cells respond to?
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
P-cells
parvocellular (small) Comprise 80% of cells Smaller receptive fields -> higher spatial resolution Lower sensitivity Thinner axons -> worse temporal resolution color sensitive
56
M-cells
Comprise 10% of cells Larger receptive fields -> lower spatial resolution higher sensitivity thicker axons -> better temporal resolution Color blind
57
Why respond to changes in brightness?
That's where there is the most information in a scene.
58
Perceptual consequences of ganglion-cell processing
1. Neural signal depends on local intensity and surrounding intensity 2. Signal emphasizes contrast borders; de-emphasizes homogeneous regions
59
Optic chiasm
Temporal half of retina -> Ipsilateral visual cortex | Nasal half of retina -> Contralateral visual cortex
60
Why does the optic chiasm split the visual field as it does?
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
Retinotopic map in V1
Each hemisphere represents contralateral visual field
62
Cortical magnification
80% of cells devoted to central 10 degrees
63
Main new features of V1 cells
orientation selectivity Selectivity for direction of motion Binocularity
64
Simple V1 cells
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
Complex V1 cells
also orientation selective No separate ON / OFF regions Exact positioning of edge / bar not required
66
Binocularity
First site of binocular cells | Note: these cells have two receptive fields!
67
Ocular dominance
slightly stronger responses to one eye Example: pattern, looks like a fingerprint Black stripes: right-eye dominant White stripes: left-eye dominant
68
Where does the signal go from V1?
Dorsal stream goes towards parietal (M pathway) | Ventral stream goes towards temporal (P pathway)
69
Spatial vision
ability to visually detect spatial patterns | example: seashells at multiple scales, zooming out looks like mona lisa
70
How good is our vision at different scales?
Relation of RF size? Large RF's -> coarse scale Small RF's -> fine scale
71
Multi-channel model
Campbell & Robson (1968) The visual system analyzes information through multiple channels Each channel is responsible for a particular spatial scale
72
Fourier's theorem
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
Fechner’s law
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
spatial frequency
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
Why use sine gratings?
``` 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
Contrast-sensitivity function (CSF)
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
Contrast threshold
Minimum amount of contrast (on a sine grating) that is visible How low can you go?
78
Measuring the CSF
``` 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
Selective adaptation experiment
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
Adaptation
decrease in the strength of a neuron's response after prolonged firing
81
Selective adaptation
Only those neurons sensitive to the adapting frequency get fatigued Different channels respond to different frequency ranges
82
Compare vision across different conditions:
Scotopic (low), Mesopic (medium), Photopic (high)
83
Perception
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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