perception Flashcards
What is contrast and how does it track over the lifespan
It declines with age - around or 60s
Differences in luminance over a scene
Can be very debilitating if lost
Where is contrast processed
Retina
LGN
Cortex
What is luminance and how is it measures
Measure of energy emitted or reflected by a light source
Measured by candelas per sq metre (CD/M2)
What is michelson contrast
Maximum luminance - minimum luminance / average luminance
What is a linear receptive field
The region of space where changes in luminance influence the activity of a single neutron
It is linear as excitatory and inhibitory are summed
Neurone respond to stimuli they are tuned in for
Why do we get the Hermann grid illusion
We see the little grey dots in the intersections as there is a smaller neuronal response
When we focus on the intersection the smudge is not visible- we can resolve details with more accuracy at the centre of our vision
What are v1 simple cells tuned in for
Tuned in for orientation
v1 complex cells?
2 linear filters are squared and summed
What is the LGN characterised by
Gain control mechanisms
What is cortical processing characterised by
Nonlinear processsing
Hierarchy of processing
Receptors
Neuron transmit info
Thalamus (relay station)
Cortical processing
What is electrophysiology
Fine wires are inserted into areas of interest
It had high spatial selectivity
We can measure cells in real time
Ventral pathway?
“What”
LGN to temporal lobe
Object processing and fine detaol
Dorsal pathway?
“How”
LGN to parietal lobe
Motion and spatial processing
Where is colour coded
V4
Where is motion coded
V5/MT
What happens if there is damaged to v4
Can’t perceive colour
What happens if there is damage to V5/Mt
Can’t process motion
Couldn’t cross road
Experimental evidence for maps in the brain?
A dye was attached to structures to show where a stimulus is represented in an area of the brain
Monkeys
What are some experimental techniques used when investigating cognitive processes
Psychophysics Single cell recording (electrophysiology) fMRI EEG Adaptive optics Optical imaging
What type of data comes from psychophysics and some techniques used?
Behavioural data not neural
Method of constant stimuli, method of limits, method of adjustment, staircase procedure
What can the method of constant stimuli tell us / what term is used to describe this
It is measuring how well people can discriminate between 2 lines
The threshold for detecting differences in the position of 2 lines was smaller than the width of a photoreceptor
Termed hyperacuity
Why continue to do psychophysics instead of collecting neural data
Neural data is expensive and takes time
Linking propositions more complex for neural data
Psychophysics and neuro imaging are complementary
Paradigms for fMRI
Block design: neural activity is compared when stimulus is present or absent
Event related: neural activity at a certain time after stimulus is presented. Compared across conditions
Combined: make inferences about substrates of human behaviour
2 cortical surfaces
Sulci- dark grey concave
Gyri- light grey convex
Periperternal space?
50-100cm in front of you- you can discriminate object in depth really well
Within this zone you are using binocular disparity
What is occlusion
We know 2/3 of things don’t exist - it is hiding behind another object
What is size constancy
You know how big an object should be
Don’t think someone is shrunk just because they are fair away
What is the Ames room
An optical illusion
We used cues from the room and it shows us the kids are two different sizes when they are actually the same size
What is the ponzu illusion
We assume the lines are getting smaller as they are further away but they are actually the same size
We misapply size constancy and what we know about depth
Kids in western societies more likely to see this than non western
Occlusion and size constancy are what type of clues
Contextual clues - create context about a scene
What are monocular vs binocular cues
Monocular- we use one eye
Binocular- Info is combined from both eyes
What type of cue is motion parallel and what does it do
It is a monocular cue
Used for depth ordering
Objects move around us as we move
Angle small for distant object, wide for close object
What is binocular disparity
The difference between the left and right eye relative to the plane of fixation (heropter)
What is crossed / uncrossed disparity
Objects prior to our plane of fixation have disparity crossed
Objects beyond our plane of fixation have disparity uncrossed
Name 2 sensor intro cues
Accomodation and vergence
What is accomodation
Changing focal length using ciliary muscles to calculate distance of object
Relaxed muscles for things further away
Tightened muscles/ squeezed to focus on something closer to you
What happens with accomodation and age
As we age our lens becomes less flexible, so we can’t adjust our focal length
Can’t accomodate to objects of different distances
If you can’t accommodate your lens you may end up with
Long sighted vision (hyperopic eye)
Short sighted vision (myropic eye)
What lens would u use to correct for a hyperopic eye
Convex
What lens would u use to correct for a myopic eye
Concave
What is convergence
Angle of gaze of two eyes
When something is further away eyes move inward, angle of convergence smaller
When something is closer, angle of convergence is larger
What is strabismus
Misalignment of eyes often caused by different muscle tension
Exotrope- eye moves outward
Esotrope- eye moves inward
Can be fixed with surgery
If not fixed within a critical period can result in damages to contrast sensitivity and binocular depth perception
What is stereo good for
Fine motor control
Sewing surgery picking berries
What is disparity good for
Useful for testing stereo vision
VR
Movies
Hugh risk jobs
2 limits imposed by vision
- People use binocular and monocular cues. But VR relies on binocular cues so people with compromised stereo may just see 2 displaced images
- We only have high resolution at the place we are looking at
In natural environments we are always changing our focal planes as we move around but you can’t do that with VR. This causes headaches eye fatigue bc our vision is forced to combine cues that aren’t normal
What does Reinhardt model comprise
Spatial and temporal filters
Why is energy model better than reichardt
It has an extra step which allows quadrature pairs to be squared and summed to calculate motion energy and have responses oriented in time and space Less fussy than reichardt mode More specific More flexible Can detect more complex motion
Why does the aperture problem happen
Completion between local and global features of a scene
The motion is featureless - any pattern looks like any other part
How to solve aperture problem
Orientation info can disambiguate edges
Changing the orientation of aperture can change its direction of motion
If vertical, lines go up and down
If horizontal lines go left to right or right to left
What can form information do
Can generate or disambiguate motion
How does form information generate perception of motion
Glass pattern- dots have no correspondence (no motion)
Within 10-15 frames people detect motion
Thought to happens bc dot pairs simulate motion streaks
Biological motion?
People see a person walking just bc of light points of joints
Used high level constraints (our idea of what a person should look like) on low level info (dots)
How much of visual cortex devoted to visual processing
1/3
2 components of hand movements?
Planning phase
Guidance phase
What happens in planning phase
Visual info about target and hand plot course of action
Plot action trajectory, velocity movement plan to make movement as accurate as possible
What happens in guidance phase
You compare your hands location to the target, which you can use to update the movement plan to be more precise
In acceleration stage, movement plan dominanates velocity and trajectory
In deceleration phase, plan is subservient to updating information
4 limiting factors on performance
Bio mechanical costs
Risk
Visual error
Motor error
What are bio mechanical costs
Energy expenditure involved when making movements and muscle stress involved
Risk
Sometimes a cost to making an inaccurate movement
Surgery picking berries
Motor error/ Fitts law
Speed accuracy tradeoff
Less accurate with fast movements
More accurate with slow movements bc you have time for feedback
What does movement depend of
Visual quality and associated error
2 things to minimise visual error
Light is better to see the target
Still targets- but doesn’t happen much in real life
What is online control of movement
When u alter motor plan based on the discrepancy between predicted and real feedback
How many milliseconds to update a slow reach
110-150
Are movements ballistic
No - we can use visual information throughout our reach to update performance to be more accurate
How do we know the posterior parietal cortex plays a role in online correction
Through TMS experiments (trans cranial magnetic stimulation)
Electric magnetic coil placed on scalp where the target region lies
It disrupts nerve cells in target region
Movements and points are less accurate
What is developmental coordination disorder
Clumsy kids have trouble with fine motor skills- sport and writing
What causes it unclear
Probs due to deficits in visual and motor mapping
Maybe Dorsal impairment
What happens with parietal damage
Often caused by stroke
Sometimes can lead to visual neglect
You only attend to one side of the visual field
You are unaware of this tho
2 common eye movements ?
Saccades and smooth pursuit
Reaches are planned in …. coordinates??
Eye centred not arm centred
What are saccades
Eye movement characterised by rapid acceleration to a new location
Occur 3 x per second
Allow us to go rate points of interest
What are smooth pursuit eye movements
Tracking movement
Characterised by constant velocity at the same speed of target
Allow us to track an item of interest (often seen with a saccades) and then tracked with smooth pursuit
What drives saccades to be the same
The spatial temporal tuning of perception and mechanisms
Why should we move our eyes
The fovea (part of retina) contains central vision that has the highest density of photoreceptors and largest overlap of receptive fields The more photoreceptors to receive sensory input, the more info is available to process, results in high acuity
What did Yarbus 1967 find
Participants looked at photos and were given instructions such as “look at how weatlthy the ppl are in the photo”
Questions guided where people’s fixated on
Ppl looked at faces then objects in room
Showed saccades are not random
What is the sequential information maximisation model
After each fixation the next fixation will be at a location that minimises uncertainty and maximises info
Distance between 1st saccades and hand?
1.5 degrees
How is the superior collicukus involved in selecting a new target
It mediates target selection
Mid brain structure, integrates visual, auditory and somatosensory spatial info to intimate orienting movement of hand and eyes
Part of circuits for visual processing and deployment of saccades
Info is combined to become a single type of info
What happens when frontal eye fields are stimulated
Results in an automatic deployment of covert attention which improves performance
Ability to select a target is decreased is superior collicukus is inactive
5 key points about bottom up attentional deployment
- Stimuli depends on context e.g. colour can depend on what colour is next to it
- Saliency map plausible and efficient bottom up strategy
- People tend to move on from a spot and not return (inhibition of return)
- Attention and eye movements tightly coupled- poses computational challenges to coordinate system used to control attention
- Seeing and understanding an object makes you look it it for shorter time or longer time (longer if u don’t recognise it)
Top down pathways?
You have a cognitive goals in mind to do something e. G. Pick up water bottle
Bottom up control
A stimulus had a feature that captures our attention
You make a saccade and attend to it
