vision Flashcards
how much of the brain is processing visual stimuli
- half your brain
- major perceptual system
half the brain processes visual stimuli.
why are so many areas concerned with visual information?
bc visual information interacts with other skills like:
- memory,
- motor movements,
- attention etc
so visual stimuli are processed also with all of these
what is the current issue?
in sight restoration
when your newborn what is shit
- contrast sensitivity - e.g., colour contrast
- visual acuity - your ability to see fine detail
for a newborn to make sense of anything in a scene it needs to be massive and bright and high contrast
in how many weeks does vision development jump from newborn
okay so visual acuity and contrast sensitivity becomes quite good by 1 (almost adult like) is this enough to interpret an object?
no. still need:
- the everyday experience of what different objects look like from diff angles/diff lighting conditions - kinda an interaction with memory
- the putting together different bits of an image that is handled by higher up areas that develop into the teenage years
- certain assumptions that we form with life e.g., that light comes form above so a disc shaded at the bottom vs the top is interpreted differently after you have formed this assumption
the assumtion that light comes from above is this learned through experience?
we dont know. debate about whether its learned with experience or if its something that evolved
develomental studies tend to show that children dont show this as much as adults
can we see patterns in noise from early
no dawg
this is global processing - dont just look at one part of pattern you need to register it as a whole to detect pattern in noise
something that still developing in childhood
visual stimuli hit eyes then what happens?
hit eyes
- then reach the lateral geniculate nucleus
- first projections in the cortex are to V1
- then information travels further upstream - different visual areas with different processing functions (e.g., v5 does motion)
as it gets to higher areas we are interacting with things like memory, attention and motor control
what do we mean by the selectivity of cortical neurons
neurons in V1 prefer certain things e.g., firing more strongly to certain
- orientations
- spatial frequencies
- colours
- directions of motions
- disparity and depth
this is the first level of visual analysis
what do we mean by hierarchical information processing in vision
neurons in V1 are look at a very small part of an image e.g.,the orientation of a line.
These tiny bits of information are put together by higher up areas
- V1 (basic stuff e.g., orientation)
- V2-V4 - respond to more complex patterns
- anterior infrotemporal cotrex - neurons specialised for detecting places/faces/objects tools
if you link visual analysis with motor control what does this aid
Visual action e.g., reaching, locomotion, navigation
if you link visual analysis with memory what does this aid
recognition of objects and faces
if you link visual analysis with attention what does this aid
visual cognition - physics causality, social cognition
where do we draw the line between vision and cogntiion
hard to say when vision stops and other cognitive skills begin
but some researchers sit quite squarely at the lower level features (e.g., detecting the orientation of a line).
how can we measure visual acuity
- black and white gratings
the test
- start with broad thick stripes, low spatial frequencies
- working up to higher and higher spatial frequencies
- eventually gets to a point it just looks grey
visual acuity tests tell us the finest pattern someone can detect before it just looks grey
what is normal adult visual acuity
- imagine circle around you like a hula hoop - 360 degrees with your thumb sticking out following the hoop outline
- thumbnail = 1 degree
- each degree is divided into 60 minutes
- with normal visual acuity you can just about resolve 1 of those minutes
- means you can resolve 30 cycles per degree
how to we measure contrast sensitiivty
again use the gratings but have the same nnumber of cycles
we vary the whitest white and the blackest black until n only see grey
the lowest level they can see = indicator of contrast sensitivity
what is forced choice preferential looking used to measure
describe it pls
way to how measure visual abilities in children
- for example the teller card procedure
- person holds card with the striped pattern on one side and grey colour
- experimenter peeping through hole in middle to see what baby looks at
- baby isnt given any instructions but anything is more interesting than the grey
- so where it looks indicates the location to the experimenter blind to the where the stimuli is
- so forced-choice but acc its the experimenter whos forced to make a choice
staircase mrethod often use - start at very coarse patterns then work up to finer and finer patterns until they cant see
using the teller card procedure what do we find?
acuity increases with age
- teller (1981)
- y axis on right side - 1 cycle per degree = shit; 30 cycles per degree = adult level
- see a 30 times increase in visual acuity from 1 month to 5years old
why have vision studies used remote eye tracking
to measure visual abilities in children
- jones et al (2014)
- blue dot is the kids eye where theyre looking
- stiped gratings pop up and see if they look at it
- n look until the stripes are so thin they cant see it
- showed similar results to the teller card findings that visual acuity increased with age
How can measure visual processing without n having to make an overt response
visual evoked potentials - ERP
- look at the activity in the primary visual cortex using electrodes to measure an EEG signal
- show n stimuli: phase reversal or pattern reversal. so they switch the pattern across trials
- significant activity in the frequency that the pattern is changing suggests n are resolving the pattern. if not suggest PVC not getting info about the pattern
sweep VEP - what is this
relating brain activity to a specific event - like every time a specific target popped up
e.g., used in phase reversal ERP study to investigate visual abilities. looking at electrical signal w when the stimuli changed its polarity
how can you investigate visual acuity with sweep VEP?
- you look at electrical signal each time it switches its polarity
- but gradually the stripes beome finer and finer
- analysing brain activity through the sequence of this would reveal the point they no longer detect anything at that frequency
using the sweep VEP method to measure the development visual processing in the primary visual cortex
Norcia and Tyler (1985)
- looked at the numebr of cycles per degree their brain could resolve
- again we see dsteady progression in development
- but this is different to the behavioural study
- behaviourally acuity looks 2-to-three times worse
tells us more whats going on. says information is reaching the cortex at an earlier age but doesn’t tells theres any further processing of it/conscious perception or what the baby does about it
why is the behavioural measure so conservative
baby might see the stimuli but decide not to react to them
which development of visual acquity do we trust - behavioru or ERP
behavoiur one is conservative but the ERP one only tells us the information has reached the primary visual cortex, not any further processing that might have been done with that information visual acuity we can say. is soemwhere between both of these
also depends on n how you define visual acuity - is it just detecting the stimuli that reach the PMV or is it the brains ability to do something with that information
visual acuity increases dramatically with age - why?
- doesn’t seem to be the eye it self
- back of the eye - photoreceptors does seem to be it - are changes in the density and efficiency in the photoreceptors that make a difference
- neural development in the eye also makes a difference
shape and layout of photoreceptors undergo quite a lot of development
what parts of photo receptors develop with age
outer segment = contains photosensitive pigment
- that causes it to fire when they pick up light
- start of short and stubby and inefficient at catching light
- then get longer form several years into life
inner segment
- start off fatter - mean they can’t be packed in so densely in the fovea - the central part of the retina that has the sharpest vision
- poor spatial sampling on the image
- development of long fibre = cones displaced to allow dense packing together
could banks and bennet (1988) explain the developmental changes in visual processing looking JUST at changes in the level of the eye
used modeling
- no
- showed improvments in sampling and photoreceptor alone cannot account for visual aquity changes in infancy
- other more central changes must be going on
what changes other than the level of the eye that explain developments in visual acquity in infancy
- myelination of visual pathways
- develoing connectivity in the cortex
key points on visual acuity
- rapid developments in the first year of life
- measured vie preferential looking and EEG/VEP
explaining eye improvements:
- eye - photoreceptors
- brain - myelination and connectivity
cortical selectivity of single neurons to:
orientation, motion and binocularity
cortical selectivity of single neurons to: orientation, motion and binocularity
where in the brain do we see this?
and are babies born with this selectivity?
- seen in PVC and beyond
- develops quickly after birth
- develop with normal visual experience
how do we measure cortical neuron selectivity using extra-cellular single unit recordings?
Hubel and Wiesel (1968)
- using electrodes
- measured a neurons orientation selectivity
- can see fires most strongly when line is at certain orientations
Findings
not much of this sort of selectivity is present at birth in kittens and results only in response to normal visual experience. if eyes covered in critical period early life then they dont develop this selectivity.
how do we measure cortical neuron selectivity using optical imaging
image a large area of cortex
- can find (in kitten) areas of visual cortex responding strongly to lines of certain orientations
- look at the map - discrete patches responding to certain orientations
- very interesting - revealed the % of cortex responding to lines of certain orientations. so for a normally reared kitten with normal visual experience they devoting about a quatre of cortex to each orientation
how did atkinson get around the problem of the change in orientation of stimuli also changing the contrast of the lines (black → white)
Had a slightly more complicated sequence where two different things happen during the sequence
- Image, phase shift, phase shift, orientation reversal , phase shift, phase shift, orientation reversal
- 6 different points where we have something related to contrast - black→ white (all things change contrast)
- Only 2 points at which the orientation changes (or 1 because the next one puts it back to how it was at the start)
- So you can see if there’s a difference in activity between detecting contrast and contrast+orientation
- Would tell us something above and beyond the contrast change
what did the braddick et al (1986) find with the orientation reversal and phase shift combined show from ages 2 weeks-3 weeks
- At 2 weeks
- found strong phase reversal signals (top one)
- But no significant signal detecting the orientation change (bottom one)
- So could detect the contrast change but not the orientation change
- At 3 weeks
- Still had strong phase reversal signal
- Now also seeing strong orientation reversal responses too
what does this figure show interms of e development of orientation-selective VEP? (look at the two lines)
- lookin at age in weeks
- and the % of infants that showing sigificant VEP responses
- can see the devleopment of the ability depends on how slow / fast the sitmuli are shown - when 3 reversals per second - easier to detect w/ slow changes
- looking at median age (where 50% of them showed this) we can see the average age of onset for each
investigating cortical selectivity of orientation, direction of motion and binocular correlation and disparity. describe the order of which ability develops first
- Orientation before motion
- Motion before binocular disparity
what mechanisms that allow for cortical sensitivity to orientation
1 idea
- if you take a bunch of cells that are spatially organised like the left image (on center off centre (plus signs) and off center on center (minus signs))
- if you give them. allexcitatory connections to one cortical cell
- then that cell will like bars oriented like the image on the right
- so wirting cells in this way = some more sensitive to certain orientations than others
this comes about via experience-dependent development of connectionsfrom the LGN (hubel and Wiesel 1960)
what mechanisms that allow for cortical sensitivity to binocular convergence
- LGN - the first place information from eyes reach
- we have discrete stripes dedicated to one eye or the other
- so within lGN information is still segregated by eye
- but when we get to the striate cortex (PVC) the information from each eye is combined)
- image on left - 1 column reflects neurons getting information from one eye, another column = neurons getting information from the other eye; in between these some neurons get information from both eyes
- useful, when both eyes work together gives us information on depth
binocularity (input to cortical cells from BOTH eyes) via experience dependent changes in connectivity
global form and motion
step up in the hierarchy of visual processing
to percieve motion what do we need?
the same exact local motions are present in two examples - e.g. spot pointing to top right - but when you look at each example as a whole the global motion is very different
to percieve global motion we need to integrate local motions over a large area
macqueu v5 / MT (dorsal) shows preference towards…?
- random noise of motion
- as noise decreases and pattern increases
- area becomes increasingly responsive
- detects coherent overall motion
behavioural measures: looking at development of global form and motion
investigated children’s abilities to integrate contours, to detect contours in noise
- varied the density of different elements
- n had to find where the contour was
- found right up even the teen years - the density they could tolerate is still developing
- for 5/6 year olds the dots have to be widely paced to find the contour
- even at 13-14 n weren’t at adult levels
why
- probably something to do with the development of long-range neural connectivity within v1, and of the feedback between v1 and v2 - which extends long into childhood (Burkhalter et al., 1993)
- connectivity with higher ventral areas mediating form perception (e.g., v4) maturing also throughout childhood
looking at the ability to detect contours in noise and varying task difficulty
what happens when we add more and more background noise (e.g., dots), how does this affect our ability to detect contours
With little noise we can detect quite rapidly where the contour is but with enough noise you can’t just pick it out and actually have to follow the contour to detect it in noise
whole point of that task - how dense can you make the background noise and people STILL pick out the contour
the fact that we don’t reach adult levels for ages (not even at age 14). so this is more complex a visual skill with its basis higher up in the processing hierarchy. this is something developing way into the teenage years
what is the coherence threshold and how is this used to measure of sensitivity to global form or motion
- asks what % of elements needs to be coherently organised (rather than randomly) in order for the global organisation to be percieved*
- Observers coherence threshold: specifically the lowest percentage at which global organisation can be detected*
what does it mean if an image has 100% coherence
each line in the pattern is coherently oriented to every other line in the pattern
the pattern itself is 100% coherent
Wattam-Bell et al., (2010)
ERP study - what they find?
findings
- adult and infants look different to each other
- form and motion look different to each other in both groups
- how form and motion differ is different for adults vs children
- but EEG is the best technique for location specificity
- measuring signals in the sclap that bounce around in the head a lot before reaching the scalp before that
- and the structure of the head is different for adults and infants
- so study cant draw such firm conclusions about exactly where the signals are coming from
- but at least they can see that the topography of responses look very different from infants and adults
- suggests even thorough infants are seeing the global form and motionand processing it, where in the brain is processing it - where in the brain is changing and being reorganised during development
How did hubel and Weisel investigate the effects of monocular deprivation on kitten eye? Single neuron study
- single neuron study – measure the strength to which neurons in the PVC respond to inputs from either eye
- graph x axis – 1 (responds ONLY to left eye) to 5 (responds ONLY to right eye). 3 if they respond equally to both eyes
typical development
Typical cat shows the graph on the left with lots of neurons responding to inputs from both eyes – useful for binocular vision. Some responding mainly to one.
atypical development
If n grew up with one eye closed. Then neurons in the PVC don’t exist e.g., those (responding to both eyes). Even if both eyes work fine now in adulthood.
poor visual experience affects the synaptic connections in the visual system such that some neurons respond most strongly now to one eye over the other
how does exeprience affect synaptic connections
the way experience shapes these synaptic connections is through competitive interactions
- its the balance of activity not the absolute level that determines functional connectivity
- so the difference between the normal and the cat completely in the dark is nothing. because there was an equal balance in the information being processed to each eye
- its the relative amount they get from the two eyes not the absolute level
describe patching treatment of amblyopia on animals/humans
- Patching treatment – if the cortex is devoting less resources to taking in inputs from one eye then patch the better one up for few hours a day and kind of force the worse one to compute information
- Done with animals and humans
- Big limitation in this approach because still promotes use of the eye on its own rather than the two working together
Some research has went into what the optimal amount of patching there is to do improving the visual acuity of the bad eye
why deos early deprivation have worse effects and why cant intervention later help ?
developmental changes in neurotransmitter levels regulate the critical period for ocular dominance plasticity (Huang et al., 1999)
Changes in how active different neurotransmitter systems are. changes in these have been linked to brian plaasticity
cateracts
But this study looks just at your ability to see low level aspects of vision (contrast sensitivity)
Face perception
- Find children with cateracts removed from early e.g. 3/4 years
- Long term deficits where they never really catch up with normal configural face processing
case study example what happened to MM?
patient who lost sight early but had it restored in adulthood
- extreme but rate case of deprivation
- damage to corneas in both eyes
- then at 40 he got a corneal graph and both eyes restored to perfect working order
- in the meantinme what happened with his normal visual brain
findings
- regardless of it being 5, 11, 17 or 22 months after having sight restored he still showed a very poor ability to see fine detail
- similar soty with fMRI looking at brain responses to different frequencies
in what ways did MM differ from controle in visual skills
all experienced in both
- shaping the brains ability to do these kinds of visual analysis
- but also in maintaining them
as lot of these are skills he would have formed but after not using them for so long he lost them
Prosthetics
- N with retinal disease – that makes photoreceptors degrade over time
- Trialling things where n wear a camera with electrodes at the back of the eye (or some diretly in the visual cortex)
- provide electrical signals with the camera instead of the faulty photoreceptors
