Simons Summaries

Question 1

What is visual sensation, and visual perception?

Answer 1

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)

Question 2

What are the four challenges to visual perception?

Answer 2

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

Question 3

What is the Mind-body problem
and Veridictality problem?

Answer 3

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

Question 4

What is the measurement and inverse problem?

Answer 4

3. 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?
4. Inverse problem: forward problem - given the cause determine the effect, inverse problem given the effect determine the cause.

Question 5

What did the greeks say about optical theories?

Answer 5

• 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

Question 6

What did desecrates argue?

Answer 6

• 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.

Question 7

What did Johannes Kepler discover?

Answer 7

First to correctly recognise the image of the retina is inverted

Question 8

What was Gesalts theories?

Answer 8

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

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

3. 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
4. Invariance - the property of perception whereby simple geometrical objects are recognised the same, independent of rotation, translation, lighting and scale

Question 11

Which 8 factors affect grouping?

Answer 11

• Proximity
• Parallelism

• Symmetry
• Similarity
• Synchronicity

• Closure
• Continuity

• Good Gestalt (Pragnanz)

Question 12

What is Proximity, similarity and closure?

Answer 12

• 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.

Question 13

What is Symmetry, parallelism and synchronicity?

Answer 13

• 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).

Question 14

What is continuity and Good Gestalt?

Answer 14

• 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.

Question 15

What is the Marrs theory?

Answer 15

• 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

Question 16

What is meant by Pixels per degree?

Answer 16

• 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”

Question 17

What is spatial frequency, amplitude and phase?

Answer 17

• 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

Question 18

Define contrast

Answer 18

• 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

Question 19

What is Weber contrast (Cw)

Answer 19

• 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)

Question 20

What is Michaelson Contrast?

Answer 20

• 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%

Question 21

What is root mean contrast?

Answer 21

• 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

Question 22

What is absolute threshold and difference threshold?

Answer 22

• 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

Question 23

During an experiment how is Psychophysics data collected?

Answer 23

• 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)

Question 24

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

Answer 24

• 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

Question 25

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

Answer 25

• 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

Question 26

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

Answer 26

• 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

Question 27

What is the method of adjustment?

Answer 27

• 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

Question 28

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

Answer 28

• 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

Question 29

What are the steps for Constant stimuli: Absolute threshold?

Answer 29

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

Question 30

What are the steps for Constant stimuli: Difference threshold?

Answer 30

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

Question 31

What is the Psychometric function?

Answer 31

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

Question 32

Modified fechner - adaptive methods - staircase steps:

Answer 32

• Stimulus placement • Termination • Threshold estimation

Question 33

Modified fechner - adaptive methods - staircase:

Answer 33

• 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 (

Question 34

Whats the equation in psychophysics for sensitivity?

Answer 34

Sensitivity = 1/threshold

Question 35

Adapting to light and duplex nature of retina:

Answer 35

• 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

Question 36

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

Answer 36

• 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

Question 37

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

Answer 37

• 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

Question 38

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

Answer 38

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

Question 39

What is Webers Law?

Answer 39

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

Question 40

What is spatial processing?

Answer 40

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.

Question 41

What is Fourier analysis?

Answer 41

• 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

Question 42

What is the contrast sensitivity function?

Answer 42

• 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

Question 43

How is the physiological determination if CSF done?

Answer 43

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

Question 44

Draw the chart from Page 7 of Simons summaries

Question 45

Letter acuity and cut off SF

Answer 45

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

Question 46

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

Answer 46

6/12=15cpd

Question 47

2. Expected VA of px with cut of 60 cpd?

Answer 47

6/3=60cpd

Question 48

What is a Peli-Robson chart?

Answer 48

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

Question 49

What is temporal vision and how is it measured?

Answer 49

• 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

Question 50

Temporal contrast and temporal frequency

Answer 50

• 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

Question 51

Temporal summation, and temporal resolution

Answer 51

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

Question 52

Critical duration

Answer 52

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

Question 53

Temporal CSF:

Answer 53

• Sensitivity to temporal changes in luminance

Question 54

5 steps in psychophysical determination of tCSP

Answer 54

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)

Question 55

Draw the chart on page 8 of simons notes

Question 56

What is critical fusion flicker?

Answer 56

• 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

Question 57

Visual motion:

Answer 57

• 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

Question 58

How we sense motion - Reichardt detector

Answer 58

• 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

Question 59

Functions of motion perception

Answer 59

• 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

Question 60

Corollary discharge theory - Eye movement on static image

Answer 60

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

Question 61

Corollary discharge theory - Real motion over fixation

Answer 61

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

Question 62

Corollary discharge theory - Smooth pursuit

Answer 62

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

Question 63

What are the 5 situations of motion perception?

Answer 63

1. Retinal motion 2. Smooth pursuit 3. Apparent motion 4. Relative motion 5. Motion aftereffect (SARRM)

Question 64

Retinal motion and smooth pursuit:

Answer 64

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

Question 65

Apparent motion, relative motion and motion aftereffect:

Answer 65

3. Apparent motion - stationary stimuli present in sequence in slightly different locations over a short period of time give the impression of motion 4. Relative motion - motion of an object with regard to another object 5. Motion aftereffect - visual illusion experience after viewing a moving stimulus for a period of time