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(2) Tri A Visual Neuro > Simons Summaries > Flashcards

Simons Summaries Flashcards

1
Q

What is visual sensation, and visual perception?

A

Visual sensation - transformation of physical stimuli into electrical (neuronal) signal.
• Data driven; bottom-up processing (public, objective, observer-independent)

Visual perception - process of interpreting these signals for conscious awareness of action.
• Knowledge driven; top-down processing (private, subjective, observer-dependent)

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2
Q

What are the four challenges to visual perception?

A
  1. Mind-body problem
  2. Veridictality
  3. Measurement
  4. Inverse problem
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3
Q

What is the Mind-body problem
and Veridictality problem?

A
  1. Mind-body problem:
    > what we feel/believe/think are related to the physical reality of processes in our body.
    > The way we see this relationship will dictate our dealings with the world.
    > Monism (only one of them is real - materialism & idealism) vs dualism (both are real)
  2. Verdictality: degree to which perceptual experience accurately represents reality.
    - Perceptual experience is stable, but sometimes not a true representation of the physical world
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4
Q

What is the measurement and inverse problem?

A
  1. Measurement: we do not have direct access to our perceptual experience. ‘public vs ‘private’ phenomena - can we reliably and objectively make inferences about private event such as perception?
  2. Inverse problem: forward problem - given the cause determine the effect, inverse problem given the effect determine the cause.
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5
Q

What did the greeks say about optical theories?

A

• Greeks - offered a comprehensive and systematic analysis of optical theories (9th & 10th century). The Greeks acknowledged the crystalline lens as the receptive organ for vision
• Euclid - vision and geometry, defined visual size by visual angle
• Ptolemy - added distance, orientation and binocularity to visual angle for spatial perception

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6
Q

What did desecrates argue?

A

• Separated body and mind as different substances
• Argued that the mind and body are strongly connected.
• Argued that brain activity is especially correlated with the attributes of the mind - was not clear on the problem of mind-body interaction.

• Provided the first systematic account of the relationship between the observed and the observer
- enabled him to reformulate many issues in sensation and perception.

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7
Q

What did Johannes Kepler discover?

A

First to correctly recognise the image of the retina is inverted

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8
Q

What was Gesalts theories?

A

• Theory of mind from berlin school. Gestalt effect looks at organising principles of perception:

  1. Emergence
  2. Reification
  3. Multistability
  4. Invariance
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9
Q

What does Emergence and reification mean in Gestalt’s theory?

A
  1. Emergence - the process of complex pattern formation. We perceive a scene or object as a whole at one instead of identifying individual components first
  2. Reification - the consideration of an abstract thing as if were concrete, or an inanimate object as it were living.
    - Constructive or generative aspect of perception, by which the experienced perception of an object contains more information than the sensory stimulus that it’s based on
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10
Q

What does Multistability and Invariance mean in Gestalt’s theory?

A
  1. Multistability - spontaneous switching of perceptual experience between two or more alternative interpretations. When boundaries between scenes aren’t clear, perception is open for multiple interpretations
  2. Invariance - the property of perception whereby simple geometrical objects are recognised the same, independent of rotation, translation, lighting and scale
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11
Q

Which 8 factors affect grouping?

A

• Proximity
• Parallelism

• Symmetry
• Similarity
• Synchronicity

• Closure
• Continuity

• Good Gestalt (Pragnanz)

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12
Q

What is Proximity, similarity and closure?

A

• Proximity - grouping objects on how close they are to each other
• Similarity - grouping objects based on how similar they are to each other
• Closure - perceiving objects as being whole/complete even when some parts are missing or occluded by filling in.

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13
Q

What is Symmetry, parallelism and synchronicity?

A

• Symmetry - grouping objects based on symmetry
• Parallelism: parallel objects are perceived in groups
• Synchronicity) - we tend to see elements moving (changing) in the same direction (way) together than elements moving (changing) in different direction (way).

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14
Q

What is continuity and Good Gestalt?

A

• Continuity - elements arranged on a straight or curved path are perceived together more than elements not on the same path.
• Good Gestalt (Prägnanz) - we tend to see and interpret ambiguous, irregular, or complex images as the simple, orderly, coherent form as possible - global regularity takes precedence over local relations.

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15
Q

What is the Marrs theory?

A

• Marr treated vision as hierarchy information processing system. Provided a unique framework for vision built upon interdisciplinary approach
1. Computation/theoretical levels identifies function and goal
2. Algorithm/representation level identifies input > algorithm > output
3. Hardware/implementation level identifies how it is physically implemented

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16
Q

What is meant by Pixels per degree?

A

• Pixel element - unit change of an image
• An image is a distribution intensity values across a 2D surface made up of pixels
• What matters is the extent of retinal image regardless of viewing distance
• Size of visual stimuli is measured in degrees or minutes of arc that falls on the retina

“Pixel resolutionalpha”

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17
Q

What is spatial frequency, amplitude and phase?

A

• Spatial frequency (f) - how many cycles are contained within a defined span (e.g. a degree of visual angle. Units - cpd
- Increase no. of cycles per degree - spatial frequency increases
• Amplitude - how high peaks and troughs are
• Phase - relative location of peaks and troughs

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18
Q

Define contrast

A

• Contrast - the difference in luminance or colour within an image or object or between an object and a scene (background). Human visual system is more sensitive to contrast than the absolute luminance

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19
Q

What is Weber contrast (Cw)

A

• Used to define the contrast of isolated features of an imagine against a large uniform background (e.g. black optotype on a white background)
• Can vary between -100% (white on black) and infinity (black on white)

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20
Q

What is Michaelson Contrast?

A

• Generally used for patterns or images in which the luminance profile is periodically fluctuating and repeating, such as in a sine wave grating
• Can range from 0 to 100%

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21
Q

What is root mean contrast?

A

• Used for more complex image and patterns such as natural images or random dot stereograms.
• Usually varies from 0 to 1, given no outliers in the image

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22
Q

What is absolute threshold and difference threshold?

A

• Absolute threshold - minimum intensity of any aspect of stimulus that an observer can barely detect against a null. Intensities over the threshold are detectable
• Difference threshold - minimum intensity difference between a pair of stimuli that is noticeable to the observer - just noticeable difference. Intensity difference above threshold is noticeable to the observer

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23
Q

During an experiment how is Psychophysics data collected?

A

• Refers to the way in which PP data is collected
• Forced choice - detection of spot
• Yes-no - do you see the spot of light
• 2 alternative forced choice - which side of the cross do you see the light - target is always presented
• N-alternative forced choice - two alternatives are most common however you can have as many as possible (e.g. VA chart)

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24
Q

What is the fechnerian- method of limits for absolute threshold?

A

• Absolute threshold
1. First trial, stimulus intensity well above or below threshold (ascending or descending series)
2. Observer reports if seen - yes/no
3. After each trial, intensity is changed until observer changes answer > end
4. Threshold estimate: average intensity between the last and second last intensity

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25
Q

What is the fechnerian- method of limits for difference threshold?

A

• Difference threshold
1. First trial, standard stimulus compared to test stimulus with different intensity
2. Observer reports which stimulus appeared stronger
3. Each trial, alter intensity of test stimulus until observer changes responds › end
4. Threshold estimate: average intensity between the last and second last intensity difference

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26
Q

What is the fechnerian- method of limits for VA limits threshold?

A

• Stimulus: letters
• Measure: letter size is threshold in VA measurement
Task: recognition of levels on the chart
• Procedure: alternative forced choice task
• Method: method of limits - descending series
• Analysis: best line read

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27
Q

What is the method of adjustment?

A

• Observed has control over stimulus to measure their own threshold, except from initial intensity
• Initial intensity is set to be far from expected threshold value
• Can be ascending or descending series
• Instructed to adjust intensity by a physical or software device until the stimulus is just detectable or just undetectable

28
Q

What are the advantages and disadvantages to method of limits and adjustment?

A

• Advantages: simple, quick, efficient & clinical advantage (e.g. VA testing)
• Disadvantages: habituation (tendency to repeat same answer even after sensation has changed), expectation (premature change in response prior to change in sensation, due to anticipation. Choice of initial intensity & step size can be tricky.
• Recommendations: repeat both series multiple times and take average for final estimation because each series may yield different results & results can be variable even within the same series

29
Q

What are the steps for Constant stimuli: Absolute threshold?

A
  1. Select range of stimulus intensities for visible to invisible
  2. Pick a few (4-9) in uniform steps; this will make up the constant stimuli set tested
  3. Present on intensity as a trial, ask observer to report if seen or not
  4. Test each intensity (20-100 times) in random order
  5. Calculate proportion of seen and not seen for each intensity
  6. Plot % seen against stimulus intensity
  7. Absolute threshold: stimulus intensity at its detected 50% of the time
30
Q

What are the steps for Constant stimuli: Difference threshold?

A
  1. Select standard range of stimulus intensities
  2. Pick a few (4-9) of which difference from standard to notable to indistinguishable
  3. Present pairs of standard and comparison many times (20-100)
  4. Each trial - observed reports which of the pair looks brighter/weaker
  5. For each difference level, % of brighter vs weaker responses calculated and plotted as a PF

• Advantages: PF possible, unbiased/reliable threshold information
• Disadvantages: insufficient stimulus intensity selection, time consuming

31
Q

What is the Psychometric function?

A

• Shows probabilistic relationship between one nominal measurement as one interval or ratio measurement
• Typically S (sigmoidal) shape

32
Q

Modified fechner - adaptive methods - staircase steps:

A

• Stimulus placement
• Termination
• Threshold estimation

33
Q

Modified fechner - adaptive methods - staircase:

A

• Stimulus placement
1. Initial stimulus intensity for the first trial: well below or above threshold depending on the initial direction of charge intensity
2. Stimulus intensity for subsequent trials: changes upon the preceding respond of the observer. Reversal: stimulus intensity at which the direction of stimulus intensity change reverses.
3. Step size: some % from the last intensity or an octave scale is used

• Termination: after n number of reversals OR n number of trials OR whatever is first
• Threshold estimation: how the calculate final threshold - average of stimulus at the last m (<n) reversals

• Advantages: simple & economical, efficient as stimuli intensity quickly approaches to
threshold and non-parametric so no assumption is required for underlying PF
• Disadvantages: selection of initial value, if selection size is too small it takes more time, if its too coarse the final threshold estimate will be more variable, expectation and thresholds only.

34
Q

Whats the equation in psychophysics for sensitivity?

A

Sensitivity = 1/threshold

35
Q

Adapting to light and duplex nature of retina:

A

• Vision functions under wide range of light intensities across 10 orders of magnitude
• However, at any given time, eyes only sensitive to 3-4 orders by restricting the dynamic range of its response to match the current ‘ambient’ light levels and maintain relative sensitivity
• To reach under a wide range of light levels, visual system needs adapting time to switch between two modes of operation - duplicity of visual system

36
Q

Define for the Scotopic system: Photoreceptor, colour, light sensitivity, spatial resolution, spatial summation, temporal resolution - CFF, temporal summation, Weber’s fraction

A

• Photoreceptor: Rod
• Colour: Colour blind
• light sensitivity: Dim light sensitive
• spatial resolution: Poor (6/60)
• spatial summation: Excellent
• temporal resolution - CFF: Poor (20Hz)
• temporal summation: Excellent
• Weber’s fraction: 0.14

37
Q

Define for the Photopic system: Photoreceptor, colour, light sensitivity, spatial resolution, spatial summation, temporal resolution - CFF, temporal summation, Weber’s fraction

A

• Photoreceptor: Cone
• Colour: Colour sensitive
• light sensitivity: Bright lights sensitive
• spatial resolution: Excellent (6/6)
• spatial summation: Poor
• temporal resolution - CFF: Excellent (70Hz)
• temporal summation: Poor
• Weber’s fraction: 0.015

38
Q

What are the 8 properties for comparing scotopic and photopic systems?

A

• Photoreceptor
• Colour
• light sensitivity
• spatial resolution
• spatial summation
• temporal resolution - CFF
• temporal summation
• Weber’s fraction

39
Q

What is Webers Law?

A

The noticeable difference between two stimuli is reliant on the magnitude of the two stimuli, I.e if a stimulus is brighter, the same Change of a lower strength stimulus won’t be as noticeable
k= ΔI/Ib where ΔI is the JND (noticeable difference) and Ib is baseline

40
Q

What is spatial processing?

A

Cognitive and neurological mechanisms that allow individuals to perceive, analyze, and understand spatial information in the environment.
_ It involves the ability to interpret spatial relationships, positions, and orientations of objects and events, enabling functions such as spatial awareness, perception, reasoning, and navigation.

41
Q

What is Fourier analysis?

A

• Any 2D image can be created from or decomposed into a set of sinusoidal gratings
• To construct an image, all you need to do is find the right combination of SF, contrast, orientation and phase of each sinusoidal component and add them together

42
Q

What is the contrast sensitivity function?

A

• Function relating contrast sensitivity of a range of spatial frequencies
(SF)
• Evaluate performance of entire visual system (optical and processing)
• Since wave gratings used to measure CSF
• CS is computed by measuring the contrast threshold for a given and taking the reciprocal

43
Q

How is the physiological determination if CSF done?

A
  1. Select spatial frequency of grating to be tested
  2. Determine the contrast method using a PP method (e.g. staircase)
  3. CS = 1/CT
  4. Repeat for other spatial frequencies
  5. Plot sensitivity on y-axis as a function of spatial frequency (x-axis) in a loglog scale
44
Q

Draw the chart from Page 7 of Simons summaries

A
45
Q

Letter acuity and cut off SF

A

Letter acuity and cut of SF
• Letter acuity Is closely related to high-spatial cut-off frequency on CSF
• Minimum angle of resolution (MAR)
- The resolution acuity limit
- The angular separation of acuity limit
- 1 arc min = 6/6 = 30 cpd
- 6/3 = 60 cpd
- 6/12 = 15 cpd

46
Q

Whats the cut off frequency with Px VA 6/12?

A

6/12=15cpd

47
Q
  1. Expected VA of px with cut of 60 cpd?
A

6/3=60cpd

48
Q

What is a Peli-Robson chart?

A

Peli-robson chart
• Optotypes with carrying contrast, 8x6 letters
10 Sloan letters of same size, 0.5 degrees at 3m (about 6/18)
Depending on SF, component on each letter, the chart tests about 3-5 cd

49
Q

What is temporal vision and how is it measured?

A

• Changes in luminance over Time not Space
• Temporal sinusoids/temporal equivalent of spatial sinewave grating used to test temporal vision, they are defined by temporal contrast and temporal frequency

50
Q

Temporal contrast and temporal frequency

A

• Temporal contrast - visibility of a temporally modulated stimulus.
• Temporal frequency - low temporal frequency = slow flicker, high temporal
frequency = fast flicker, measured in ps or Hz

51
Q

Temporal summation, and temporal resolution

A

• Temporal summation - ability to sum light over time
• Temporal resolution - ability to resolve two events of light occurring in temporal succession

52
Q

Critical duration

A

• Critical duration - time period over which the light is summed (photopic system = 10
- 50ms, and scotopic system 100-200ms)

53
Q

Temporal CSF:

A

• Sensitivity to temporal changes in luminance

54
Q

5 steps in psychophysical determination of tCSP

A
  1. Select temporal frequency of grating to be tested, e.g. 10Hz (ps)
  2. Determine contrast threshold using PP method (e.g. staircase)
  3. Contrast sensitivity = 1/threshold
  4. Repeat for other temporal frequencies
  5. Plot sensitivity on y-axis as a function of temporal frequency (loglog scale)
55
Q

Draw the chart on page 8 of simons notes

A
56
Q

What is critical fusion flicker?

A

• Limit of temporal vision - statistical
• Temporal frequencies of flicker beyond this point, humans will perceive as steady
• When not specified, CFF almost always refers to high frequency CFF

• Scotopic vision = 15-20Hz
• Photopic vision = 60-70Hz

• Factors affecting CFF = modulation depth, background illumination, wavelength, eccentricity (different distribution of photoceptors), degree of light/dark adaptation and physiological factors such as age or fatigue

• Grant-harper law - the CFF increases linearly with the log of the stimulus area - easier to detect a large flicker
• Ferry-porter law - the CFF (100% modulation, high frequency cut off) increases nearly linearly with the log of retinal illumination
• Blochs Law- Large targets fall onto more peripheral retina that is more sensitive to flicker of low contrast
- duration of time must be doubled, if luminance is halved

57
Q

Visual motion:

A

• Change in the position of a luminance pattern over time. Motion perception requires correlation of brain activity across space and time - individual photo receptors cannot detect motion
• Velocity = degree of visual angle/second
• Smallest detectable shift in motion is about 10arc sec

58
Q

How we sense motion - Reichardt detector

A

• Two spatially separate inputs to signal changes across space
• Temporal filters to signal changes across time
• Comparator to evaluate these changes - when a signal triggered from one point in space and time is multiplied with a delayed signal from a neighbouring detector, an event of motion and its direction is signalled

59
Q

Functions of motion perception

A

• Detection and identification - moving object draws immediate attention, more detailed info about the object can than be extracted
• Navigation - compute distance/direction to various object in the scene through optic flow
• Biological motion - movement of living organisms provides us with unique information that can be distinguished from motion of inanimate objects

60
Q

Corollary discharge theory - Eye movement on static image

A
  1. An MS (motor signal) is initiated when eye moves from point A to B
  2. Copy of MS (CDS - corollary discharge signal) is sent to comparator
  3. Rightward retinal motion occurs - IMS (image movement signal) sent to comparator
  4. Comparator = CDS - IMS, tells sensory system there was no real motion

Helps provide stable perception of world despite own movements

61
Q

Corollary discharge theory - Real motion over fixation

A
  1. No MS as eyes are fixated
  2. No copy of CDS sent to comparator
  3. IMS sent to comparator as retinal motion occurred
  4. Comparator only receives IMS and tells sensory system real motion occurred
62
Q

Corollary discharge theory - Smooth pursuit

A

Smooth pursuit
1. MS initiated due to eye movement
2. Copy of MS (CDS) is sent to comparator
3. No retinal motion occurs during smooth pursuit (imagine stationary at back of the eye) there no IMS
4. Comparator receives the CDS only, telling sensory system that real motion occurred

63
Q

What are the 5 situations of motion perception?

A
  1. Retinal motion
  2. Smooth pursuit
  3. Apparent motion
  4. Relative motion
  5. Motion aftereffect

(SARRM)

64
Q

Retinal motion and smooth pursuit:

A
  1. Retinal motion - fixating on centre and watch object move from left to right.
    Oculocentric motion. The image is physically moving on the retina, perception is from left to right
  2. Smooth pursuit - egocentric (viewer centred) motion, eye follows a target & no retinal motion. Perception is left to right
65
Q

Apparent motion, relative motion and motion aftereffect:

A
  1. Apparent motion - stationary stimuli present in sequence in slightly different locations over a short period of time give the impression of motion
  2. Relative motion - motion of an object with regard to another object
  3. Motion aftereffect - visual illusion experience after viewing a moving stimulus for a period of time

(2) Tri A Visual Neuro (2 decks)

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