1
Q
how much of the brain is processing visual stimuli
A
- half your brain
- major perceptual system
2
Q
half the brain processes visual stimuli.
why are so many areas concerned with visual information?
A
bc visual information interacts with other skills like:
- memory,
- motor movements,
- attention etc
so visual stimuli are processed also with all of these
3
Q
what is the current issue?
A
in sight restoration
4
Q
when your newborn what is shit
A
- 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
5
Q
in how many weeks does vision development jump from newborn
A
6
Q
okay so visual acuity and contrast sensitivity becomes quite good by 1 (almost adult like) is this enough to interpret an object?
A
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
7
Q
the assumtion that light comes from above is this learned through experience?
A
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
8
Q
can we see patterns in noise from early
A
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
9
Q
visual stimuli hit eyes then what happens?
A
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
10
Q
what do we mean by the selectivity of cortical neurons
A
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
11
Q
what do we mean by hierarchical information processing in vision
A
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
12
Q
if you link visual analysis with motor control what does this aid
A
Visual action e.g., reaching, locomotion, navigation
13
Q
if you link visual analysis with memory what does this aid
A
recognition of objects and faces
14
Q
if you link visual analysis with attention what does this aid
A
visual cognition - physics causality, social cognition
15
Q
where do we draw the line between vision and cogntiion
A
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).
16
Q
how can we measure visual acuity
A
- 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
17
Q
what is normal adult visual acuity
A
- 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
18
Q
how to we measure contrast sensitiivty
A
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
19
Q
what is forced choice preferential looking used to measure
describe it pls
A
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
20
Q
using the teller card procedure what do we find?
A
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
21
Q
why have vision studies used remote eye tracking
A
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
22
Q
How can measure visual processing without n having to make an overt response
A
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
23
Q
sweep VEP - what is this
A
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
24
Q
how can you investigate visual acuity with sweep VEP?
A
- 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
25
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
26
why is the behavioural measure so conservative
baby might see the stimuli but decide not to react to them
27
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
28
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
29
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
30
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
31
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
32
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
33
cortical selectivity of single neurons to:
orientation, motion and binocularity
34
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
35
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.
36
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
37
so kitten with normal visual experience devotes a quatre of cortex to detecting stimuli of each orientations. what happens when kittens are raised only to see vertical stimuli?
see differences in the way the brain adapts to see orientations of the ones they see and less so for the ones they dont see
38
human study - development of orientation-selective cortical neurons - orientation reversal VEP. what do. we need to be careful of with this?
jan atkinson
* *Orientation-reversal VEP*
* *Change the orientations ±45 degrees*
* *Might think that if we get a response form the PVC, at that frequency (4 times a second) it is evidence of neurons processing this change of orientation*
*But it’s not that simple because as you do this a lot of locations in the display also reverse their contrast – go from black to white*
* *So could be getting brain response from this change in orientation just from this change in contrast*
* *VEP could arise from non oriented neurons*
39
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*
40
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*
41
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
42
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*
43
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)
44
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*
45
briefly, how do neural mechanisms develop to give rise to orientation selectivity and binocularity
development in connectivity postnatally gives PVC neurons selectivity in detecting certain orientations and certain depths
46
key points on corticalselectivity
* cortical selsectivity - went through example of orientation reversal VEP measure
* orientation, motion, disparity - different types of cortical selectivity develop post natally at different ages
* neuronal mechanisms - experience-dependent changes in brain connectivity
47
global form and motion
step up in the hierarchy of visual processing
48
to perceive shapes what do we need?
we need to look for useful changes in orientation over a large area. its not enough to detect the local pattern but have to see how they go together over a larger area
49
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
50
why is motion perception important?
cant tell looking at local motion whether the global motion is expanding outwards or shrinking inwards - something very important to day to day life because if the world is getting biger and bigger suggests something is coming close to your faceo r maybeyour falling face first
also the ability to detect global motion helps us stay balanced - visual system tells you your slightly toppling forwards and to lean backwards
51
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
52
macqueu v4 (ventral) shows preference towards…?
* responds to concentric or radial configurations
* detecting the overal configuration of a shape
53
hierarchial information processing: detecting. acontour
* *So one V1 neuron is looking at one element in this scene*
* *So, one v4 neuron sensitive to contour needs to be “looking at” many V1 neurons.*
54
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
55
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
56
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*
57
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
58
different measures to global form or motion
* looking at n's abilities to detect contours in noise
* identifying their coherence threshold
59
whats different between motion and form coherence
form coherence refer to how coherent each part of a pattern is with the rest
motion coherence refers to mow many stimuli are moving together compared to the rest
60
Global form and global motion development - what have we learned?
**Using behavioural and VEP measures**
* with infants, we learned
* local form emerges earlier than local motion
* become sensitive to global form and global motion by 4-6 months (Braddick and Atkinson 2007)
**Using psychophysics - button-pressing responses**
* button-pressing responses at older ages
* both take a long time to mature - not until teenage years
* global form thresholds reach levels later than global motion (Gunn et al., 2002)
* but these kind of studies are hard to determine when n reach adult levels. could be the sample size isnt big enough for the test isnt sensitive enough to tell small differences
61
Wattam-Bell et al., (2010)
ERP study - what they do?
reorganisation of cortical visual processing in the development of global form and globa motion
* was a high-density ERP recording study with 5 month olds and adults
* n investigated 2 kinds of stimuli
1. stimuli switch from being random, then coherent, random, coherent
then here we can look at brain areas that are responding to something happening at this frequency - area is processing the change in coherence
2. motion stimuli same thing but with dots, all random, then 100% coherent, all random, then 100% coherent
62
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
63
Development of the visual processing depends on having normal visual input. If you are deprived of vision how does this affect the visual system
It doesn’t develop normally
e.g., if born with congenital cataracts and they’re NOT operated on then even after theyre removed the visual system doesn’t develop typically in the way others without cataracts developed
64
what kind of effects do we see with early visual deprivation
* low level effects – e.g., poor visual acuity
* higher level effects – e.g., face perception
65
Name one of the systems that need normal input to develop
the convergence of information from both eyes. in the striate cortex (primary visual cortex) some neurons respond to input from left eye, and others respond to inputs from the right. some neurons here however respond to inputs from both eyes. and the developed ability of these dual-processing neurons depends on normal visual experience
66
describe a study showing the effects of monocular deprivation
Hubel. etal., 1977
when monkey brain stained to see neurons recieving input from one eye vs the other
* monkey normally reared get a 50/50 split of neurons receiving input from one eye vs the other
* monocularly deprived monkey (raised with one eye uncovered in early life ) - even if its uncovered have this persisting organisation where the visual cortex is mainly taking its input from one eye
67
if raised with one eye -→ affects organisation of the visual system to take more information from that eye. is this a good or bad thing?
both!
* makes sense - the system represents itself via the way it sees the world. pointless devoting a lot of cortex to something that isn't useful if one eye isnt providing any information
* but bad too - bc hard to reverse. ones you reach a certain age even if the eye works fine the visual cortex doesn't go back to representing it
68
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.*
69
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
70
if raised with prism between eyes. so never share the information between eyes how does this affect the neurons in the primary visual cortex and which eye they respond to
animals that don't experience correlated input - dont develop these kinds of neurons that take in information from both eyes (hubel and wiesel)
71
how are humans affected by a lack of visual input?
and when are they most vulnerable to this effect?
lack of visual input leads to poor later vision in the deprived eye
Most vulnerable during a critical periodin infancy and early childhood
72
Key points of deprivation and aatypical development
* Neurons in V1 have little input from one eye if it didn’t provide vision during the critical period
* theres a limit on the visual processing from that eye
* The effects of this are strongest in the early critical period
* See both lower and higher effects on visual processing
* Prompts the need for early correction for things like cataracts, strabismus , large refractive error
73
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
74
Binocular treatment for monocular depritvation: tetris
* N had to combine the information from both eyes to see what the elements were
* Make input to the good eye low contrast so this information doesn’t domainate
* Now both stimuli are equal in terms of the contrast of stimuli to each eye
* Foce n to use both eyes together in a goal directed way
75
sensitive/critical periods what have they taught us?
early deprivation has large and permentance effects (reconfiguring visual cortex) , later deprivation doesnt
early treatment can reverse deprivation effects, later treatment cant
76
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
77
ocular dominance plasticity
* ocular dominance is the extent to which one eye dominates (eihter V1 as a whole or 1 neuron)
* ocular dominance plasticity is your eability as an experimenter to show its possible to reverse this at any given age
78
what evidence is there that its the changes in NT's in infancy that make n so vulnerable. tovisual deprivation
* pharmacological studies - fluoxitine could reinstate ocular domainance plasticity in adult rats (Vetencourt 2008).
* achieved via reducition of intracortical inhibition
79
environmental enrichment and physical exercise - how does this affect plasticity
* Giving mice an enriched environment/more physical exercise – also allows for more plasticity.*
* Causal mechanism – neurotransmitter levels are changed by this and effectively the brain is kept younger at a later age.*
79
what things increase ocular dominance plasticity
* NT's present in early years/ with pharmacological manipulation
* environmental enrichment/ physical exercise
80
so what are we doing with this knowledge on things improving plasticity
*well currently the move is trying to apply these findings to help treat humans with amblyopia. Over time the animal research is hoped to lead to pharmacological and behavioural interventions that could increase plasticity in humans (Levi 2012)*
81
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*
82
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
83
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
84
different treatment ideas for sight restoration
* *Prosthetics*
* *Optogenetics*
* *Gene therapy*
*Check out review article with more information – Fine et al., 2015*
85
*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*
86
gene therapy
* N with inherited retinal disease can have the faulty gene replaced*
* Some success in genetic problems with photorecpetors fixed at the level of the eye*
87
challenges with things like prosthetics or gene therapy in restoring sight in adulthood
limiitations specific to prosthetics
* *early devices have very low spatial resolution*
* *Image like A – once its downsampled (B) its very hard to make out what it is unless its very clear*
* *Need to devlop this so its fine enough to provide useful information (C + D)*
*limitations to both*
* *Will this actually be experienced as real vision*
* *to what extent can people who’ve went without vision for so long can learn to use it*
*challenges to both*
* *Technological challenges – getting tech to work*
* *Limitations with plasticity in brain – if been without vision for so long (past critical period ) can it even use this information / or what can we do to help it use this information*
* *→ Will this actually be experienced as real vision*
* *→ to what extent can people who’ve went without vision for so long can learn to use it*
88
visual development key points
* visual acuity develops rapidly in early life
* this and other basic functions e.g., orientation - depend on development of the primary visual cortex
* one step up in complexity is global form and global motion processing - handles by higher coritcal areas.
* experience is critical and deprivation leads to atypical devleopment (poor vision)
* new research on prospects. for restoring vision, the limitation of plasticity and ways to overcome them
current issues
flashcards
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