final exam Flashcards
What causes sensory illusions?
Caused by a limitation in how information is encoded
What causes perceptual illusions?
Brain misinterprets the sensory representation, often due to perceptual constancy or global interpretation
Define a bistable percept
An image that can be perceived in two ways, depending on what is interpreted as the figure and the background
How can results be skewed when using bistable percepts in experiments?
Some people have emotional responses to bistable percepts, which are linked to other cognitive processes, such as cognitive empathy. These responses can influence the target effect
Describe the perceptual shape constancy illusion
The brain’s tendency to assume the perceptual constancy in the physical attributes of an object causes a misinterpretation of a retinal image
What are the underlying processes that cause perceptual constancy illusions?
When producing a percept, the brain only has access to the retinal image during visual processing.
The retinal image is constantly changing when we move, therefore the brain actively interprets input in order to construct a stable percept
What can sensory illusions tell us about how the brain works?
- What brain cells are tuned to and what information cortical areas process
- How brain cells/areas interact
- How information is transformed across brain areas
What can perceptual illusions tell us about how the brain works?
- How the brain interprets sensory representations
- What assumptions the brain makes about the outside world
What does perceived size depend on?
The size of the retinal image and the (estimate of) viewing distance
The brain scales the size of the retinal image by an estimate of viewing distance to generate a percept size
What subconscious inference does the brain make when representing size constancy?
The distance to the object, which is then used to scale the retinal image accordingly at a subconscious level
Define the visual angle in regard to size perception
A measurement of how big a retinal image produced by an object is on the eye
Describe Holway and Boring’s experiment and their results
Task: test disks of various sizes at various distances were compared by participant to a comparison disk at a fixed distance
Results: the size of the retinal image changes with viewing distance. Removing distance cues creates a dysfunction in perceptual size constancy as the brain can only use the retinal image
What is Emmert’s Law?
The size of a retinal after-image is fixed
Perceived size of the after-image depends on viewing distance
What are two factors that affect perceived size of a viewed object?
Relative size: comparison to the size of surrounding objects
Shape/object constancy: perceived shape of the object doesn’t change, compared to retinal image
What is constructivism?
A form of empiricism proposed by Ames. Notion that the aim of visual processing is construct an internal model of the outside world, using assumptions to form a percept
What assumption does the brain prioritise when viewing the Ames room illusion?
The brain prioritises the assumption that rooms are typically rectangular over that of constantly sized people, regardless of conscious awareness
Define an absolute threshold
Detecting the presence of a stimulus
Minimum intensity at which the stimulus is ‘just’ detectable
Define a relative threshold
Telling two stimuli apart
What does a psychometric curve plot?
Detectability vs stimulus intensity
Shape of the curve is influenced by noise
What are the two kinds of noise that can influence a psychometric curve?
External noise: variation in the stimulus or the environment that affects performance
Internal: variation in the observer
What are the two kinds of noise that can influence threshold values?
Random noise: the direction of the error varies randomly from trial to trial
Systematic: the direction of the error is constant
What are the outcomes of noise in threshold measurement on the shape of psychometric curve?
Random: curve is more shallow, more variability and less robust data
Systematic: offsets the curve producing the wrong curve
What are the desired properties of a threshold?
Validity: the observer can actually perform the task at threshold level
Reliability: you would get close to the same value if measured again
What are the issues with subjective threshold measures?
Individual variation in threshold detection
No measure of if they can actually see it when they say they can
Describe the method of adjustments/limits for psychophysical techniques
The experimenter adjusts the stimulus intensity until the it is said to be detectable by the participant, in both directions.
Threshold = value at which the response changes from detectable/undetectable
Habituation: tendency to zone out and keep saying same response
Expectation: falsely anticipate a change before it occurs
Describe the method of constant stimuli for psychophysical techniques
Present a fixed set of stimulus intensities a fixed number of times in a random order
Plot frequency of responses as a function of stimulus intensity
Describe the staircase method for psychophysical techniques
Start at an intensity far from threshold value, change stimulus depending on observer’s response (+ = down, - = up) stopping after a specified number of reversals
Reversal point = where the direction of change reverses
What are the advantages/disadvantages of the method of limits?
Advantages: quick and minimal pre-testing
Disadvantages: inprecise
What are the advantages/disadvantages of the method constant stimuli?
Advantages: produces full psychometric curve, avoids habituation and anticipation effects
Disadvantages: inefficient, need to pilot test
What are the advantages/disadvantages of the staircase method?
Advantages: quick and accurate
Disadvantages: spend most of the time around threshold, sensitive to false errors
Define ‘Just Noticeable Difference’ (Weber’s Law)
Difference between intensities required to tell them apart - discrimination threshold
Why is spatial frequency analysis useful?
- Any image can be broken down and represented as a series of sinewaves
- Can be used to model visual system processes
- Can be used to determine what any observer should see
- Can account for and explain different percepts
What happens to a sinewave when spatial frequency increases and contrast decreases?
The peaks of the sinewaves become smaller and closer together
Define contrast in relation to spatial frequency analysis
The difference in the strength or intensity of a sinewave
Define spatial frequency in relation to sinewaves
The scale of the sinewave, cycles per degree (peak to peak)
Define phase in relation to spatial frequency analysis
The starting location of the sinewave in degrees
What does a spatial frequency spectrum show?
The energy in the stimulus at each spatial frequency -> how much of each sinewave is needed to form the stimulus
Define the frequency of a sinewave
The number of cycles within a certain time or space
Define the amplitude of a sinewave
The height of the wave
What are square waves and how are they constructed
A square wave is a periodic wave form that alternates between two fixed amplitudes.
Constructed by summing the fundamental and the odd harmonic components of the sinewave
What is the fundamental in spatial frequency analysis?
The square wave that has the same peak-to-peak distance as the sinewave
It is the lowest spatial frequency sinewave within an image
What are harmonics in spatial frequency analysis?
Sinewaves that have a higher frequency than the fundamental
(3rd harmonic is 3x the spatial frequency of the fundamental)
What are the two spectrums required to define an image in full?
Power spectrum: contrast of the different spatial frequencies
Phase spectrum: phase of the different spatial frequencies
What is the contrast level needed to discriminate a sinewave from a square wave?
A squarewave and sinewave can only be differentiated at the point where the third harmonic becomes detectable, otherwise they are both at the fundamental level
What is a modulation transfer function?
A plot of the sensitivity of the a non-human system (camera lens) to different spatial frequencies
Outputs the spectra of the input image, multiplied by the MTF
Describe the human contrast sensitivity function
Plot of sinewaves at each spatial frequency that represents where an stimulus is visible to the human visual system
What causes a loss of sensitivity at low spatial frequencies?
Low spatial frequencies are essentially uniform luminance. Simple cells in V1 are not tuned to uniform luminance, they are tuned to sinewave modulations (variation in luminance). Due to the spatial opponency of V1 simple cells, they have no excitation or inhibition responses.
What causes a loss of sensitivity at high spatial frequencies?
The ability of the visual system to extract high frequencies is limited by:
- the number of cells
- how tightly packed they are
- how many retinal ganglion cells they feed into
This causes undersampling (aliasing) which the optics of the eye handle by blurring out high frequencies
What information do high spatial frequencies convey?
- Rapid alternation between light and dark
- Short wave-length
- Fine detail
What information do low spatial frequencies convey?
- Slow alternation between light and dark
- Long wavelength
- Coarse detail
What are the upper frequency cutoffs for humans, cats and wedge-tail eagles?
Maximum spatial frequency that the observer is sensitive to
Humans: ~30cpd
Cats: ~12cpd
Wedge-tail eagle: ~140cpd
What information is required to predict the visibility of a stimulus?
The object’s Fourier spectrum and the observer’s contrast sensitivity function
Perceived image = stimulus FS*CSF
What are SF channels?
Cells/pathways tuned to specific stimulus features, such as orientation, SF, disparity
Describe adaptation to spatial frequencies
When prolonged viewing of a stimulus reduces the sensitivity of cells that are tuned to that stimulus
What pattern of results would suggest that the human CSF is due to a single channel sensitive to all spatial frequencies?
After adaption, sensitivity to all spatial frequencies will be effected (global change in adaptation)
What pattern of results would indicate that the CSF is due to multiple channels sensitive to different spatial frequencies?
After adaptation, only channels sensitive to the adapted SF would exhibit reduced sensitivity
How does the visual system process different spatial frequencies in the retinal image?
There are debated to be 4 to 7 channels that break images down into different spatial frequency bands, each channel is tuned to a different SF.
High SFs are processed by small receptive fields.
Low SFs are processed by large receptive fields
Describe the coarse-to-fine process analysis
The process by which the magnocellular system rapidly transmits the low spatial frequency information to the cortex, which maps out the rough outline of the image. Then the parvocellular system information, which has slower axon conduction speed, reaches the cortex and fills in the detail
What happens to the spatial frequency content of an image with increasing viewing distance?
- An increase in SF when the image is further away and consequently smaller
- An increase in cycles per degree as distance increases
What happens to humans’ contrast sensitivity function with increasing viewing distance?
It does not change, sensitive to the same spatial frequencies
What does squinting your eyes do to change how we process spatial frequencies?
Squinting essentially creates a low-parse filter, which filters out high spatial frequencies
How does spatial acuity differ in the central and peripheral regions of the eye?
Our best spatial acuity is in the fovea. As eccentricity increases, spatial acuity decrease
How does the difference between acuity in the central and peripheral affect the contrast sensitivity function?
The CSF prioritises high SF information from the fovea, but when an object is viewed in the periphery, low SF information dominates
What causes the checker shadow illusion where squares A and B have the same physical luminance?
The visual system processes how information is represented across all SFs when interpreting a scene
Real edges have the edge information defined at all SFs, whereas shadows are only defined at low SFs, causing square B to be interpreted as in shadow
What are the two types of depth information?
Absolute: distance (depth) from the observer to an object
Relative: difference in depth between objects
What is the relative size monocular cue?
If two objects are similar in size, we perceive the one that casts a smaller retinal image to be farther away, either consciously or unconsciously
What is the occlusion/interposition cue to depth processing?
If two opaque objects are in the same line of sight, the near one occludes the other providing relative depth information
What is the lighting and shadow cue to depth processing?
The brain uses light and shadow to generate a percept of depth because 3D objects typically cast shadows.
Greater depth is indicated by deeper shadows, so less depth is perceived when light is directed straight onto a surface
What is the texture gradient cue to depth processing?
The SF information of visual detail on a surface varies with distance.
Increasing distance shifts the spectrum toward high SFs
What is convergence?
Simultaneous inward movement of both eyes to focus on something that is closer than the current depth of fixation
What is divergence?
Simultaneous outward movement of both eyes to focus on something that is further than current depth of fixation
Define decussation
The crossing over of nerve fibres from one side of the brain to the other
Define hemidecussation
Partial cross over of optical nerve fibres that results in cells in V1 receiving input from RFs that corresponds that correspond to the same region of space
Cells on the right VF of each eye project to the right hemisphere
Cells on the left VF of each eye project to the left hemisphere
What are the characteristics of a large monocular visual field?
Larger overall VF
Better visual coverage
Worse depth perception
What are the characteristics of a large binocular visual field?
Smaller VF overall
Worse visual coverage of the world
Better depth perception
Define binocular disparity
The difference in the location of an object’s image in the two eyes due to horizontal offset. Images off the horopter fall on non-corresponding points and therefore generate binocular disparity
What are the two factors that cue an object’s depth relative to a fixation point?
Disparity direction: direction left or right of the fixation point
Disparity magnitude: distance from fixation point, far or near
What is the horopter?
The multiple points in the visual space that lead to single, corresponding vision. The horopter’s shape changes as a function of viewing distance
What is the horopter’s shape, before, at and past the abathic distance?
Before: concave parabola
At: straight line
Past: convex parabola
What is Panum’s fusional area?
The region around the horopter where images can be fused despite non-correspondence
What is an anaglyph?
Stereoscope in which spatially overlapping red and green images are seen through filters in each eye eyes
What is a polaroid stereoscope?
Stereoscope in which different images are projected onto a screen via 2 projectors with oppositely oriented polaroid filters
Viewed through cross-polarised glasses
What is a shutter stereoscope?
Stereoscope in which different images are presented in rapid alteration on a computer monitor
Viewed via goggles that. alternately occlude each eye
What is the correspondence problem?
The issue of how the visual system needs to determine which images in each eye correspond to the same object
Must be done on an object-by-object basis because different objects have different disparities
What are the two theories of how the brain solves the correspondence problem?
A. Binocular combination prior to stereopsis and object recognition
B. Monocular object recognition prior to binocular combination and stereopsis
Which theory of how the brain solves the correspondence problem won out?
Julesz used random dot stereograms, which are images composed of light and dark dots in which depth can only occur through binocular matching
Demonstrated that monocular form recognition is not a prerequisite for stereopsis
What is the false-matching problem?
The fact that there are more false matches of objects that correct ones.
How does the visual system solve the false matching problem?
False matches along the same line of sight are solved through inhibitory connections between cells. Due to the assumption that objects are opaque. If there is one correct match in a line of sight, there cannot be more (1 cell active)
False matches along the same line of depth are solved via excitation and the assumption of object continuity. Objects tend to be bigger than the RF of single V1 cell.
What are the ecological constraints that the neural system employs to solve the false matching problem?
Surface opacity and surface continuity
Describe the steps in the Marr-Poggio model
- Make all correct and false matches
- Apply constraints
- Stable solution emerges
What is aliasing
When a high-frequency signal is sampled at a rate that is insufficient to capture the changes in the signal.
