week 3 - from photos to phenomena Flashcards
from eye to cortex
1) reception
2) transduction
3) coding
from eye to cortex
1) reception
absorption of physical energy (photons)
hits the retina and interacts with photoreceptors
from eye to cortex
2) transduction
physical energy converted into an electrochemical pattern
(done by photoreceptors)
from eye to cortex
3) coding
electrochemical pattern sent to the brain
one-to-one correspondence between aspects of the physical stimulus and aspects of the resultant nervous system activity
the eye and the retina
cones
colour, vision, sharpness of vision
6 million in retina
most in the FOVEA (the place where you are looking)
the eye and the retina
rods
vision in dim light and movement
125 million in outer regions of retina (peripherals)
no real colour processing in the periphery
perception is a….
constructive process
colour vision
visible light
electromagnetic spectrum
human eye most sensitive to the green range
colour vision
trichromatic theory
thomas young 1802
all colours by mixing the 3 primary colours
hermann von halmholtz - 3 types of colour receptor in human eye
- short (blue)
- medium (yellow-green)
- long (red)
colour vision
opponent-process theory
hering 1878
sighted people dont see eg blueish yellow
colour perception assumed to have three opponent processes
dual process theory (hurvick + jameson, 1957) linked these processes to combinations of inputs from the 3 cone types
- inputs from 3 different cones are processed in an opposition manner
- the difference between cones = different electrical signal
colour vision
colour consistency
the tendency for a surface to appear to have the same colour despite a change in the wavelengths contained in the light sauce
- evolutionarily helpful (sun)
——-> top-down influences
after the retina
what happens?
signals travel down two parallel pathways
after the retina
what are the two pathways?
parvocellular (P) pathway
magnocellular (m) pathway
after the retina
parvocellular (p) pathway
sensitive to colour and fine detail
most input from cones
after the retina
magnocellular (m) pathway
most sensitive to motion
most input from rods
after the retina
which direction do the pathways travel
info goes down optic nerve towards the back of the brain down the p and m pathways
the pathway from the eye to the brain
stages
retina —> optic nerve —> optic chiasm —> lateral geniculate nucleus (GN) —> cortical ares V1 (the primary visual cortex)
the pathway from the eye to the brain
direction of signal
the signals reaching the left visual cortex come from the left SIDE of the TWO retinas
(cross over at chiasm)
see notes for diagram
properties of visual neurons
retinotopy
things that are near each other are processed in cells physically near each other
properties of visual neurons
receptive fields
the region of the sensory space (ie retina) within which light will cause the neuron to fire
properties of visual neurons
lateral inhibition
a reduction of activity in one neuron that is caused by a neighbouring neuron
useful for enhancing contrast at the edges of objects
lateral geniculate nucleus
part of the thalamus
cells have a centre-surround receptive field
–> responds to different light
maintains a retinotopic map
correlates signals from the retina in space and time
—> rapid perception of if something is moving
primary visual cortex (V1)
back of the brain
extracts basic into from visual scene
sends info to later stages of processing
maintains retinotopy
primary visual cortex (V1)
single cell recordings
hubel and wiesel, 1979
- indicates some cells respond to simple features and others combine those features into more complex ones
damages to V1?
blindsight
patient cannot consciously report objects presented in this region of space (cortical blindness)
visual processing beyond V1
info sent one step to next (hierarchal)
to recognise the range of evidence for distinct visual processing modules
visual processing beyond V1
functional specialisation theory
zeki, 1992, 1993
different parts of visual cortex specialised for different visual functions
visual processing
stages
V1+V2
early stage of visual perception
eg, shapes
visual processing
stages
V3 + V3a
responsive to form
(moving objects)
visual processing
stages
V4
colour
visual processing
stages
V5 / MT
visual motion
visual processing beyond V1
functional specialisation theory
central assumption by zeki
colour, form and motion processed in anatomically separate parts of the visual cortex
brain imaging (PET position emission tomography) study - in humans
- V4 more active for coloured than greyscale images
- V5 more active for moving than static dots
V4 - colour centre of the brain
cortical achromatopisa
cant see colours due to V4 damage (macaque monkeys)
(also V2 and V3 - despite working retina)
can perceive things they know the colour of (implicit colour processing)
link not perfect
V5 / MT - motion centre
akinetopsia
damage = akintetopsia
patient LM
- bilateral damage to V5
- good at looking at stationary objects
- good colour vision
- motion perception grossly deficient
challenge for functional specialisation
the binding problem
colour not separate to shape
how are different features bound together?
the binding problem
possible solution
coherent perception depends on synchronised neural activity between brain areas (attention needed)
beyond visual cortex
parietal (dorsal) pathway
where pathway
movement processing
along top of brain
beyond visual cortex
temporal (ventral)
what pathway
colour and form processing
along bottom of brain
vision for perception / vision for action
patient DF
lesion to lateral occipital cortex (what pathway)
trouble locating and identifying objects
however pereption fine (has where)
letter box task
- cant match
- can put through
object recognition
a model
1) early visual processsing (V1, V2, V3, V4, V5/MT)
2) perceptual segregation - grouping of visual elements
3) structural descriptions - matchiing grouped visual description onto a representation of the object stored in the brain
4) attaching meaning to an object - based on prior knowledge
object recognition
2) perceptual segregation
seperatiing visual input into individual objects
thought to occur before object recognition
Gestalt psychology
fundamental priciple
law of Pragnaz
perceieve whats in front of you as the simplest possible solution
assumes a set of rule sthat operate early in visual processing
Gestalt law of perecptual organismation
1) law of prosmimity
2) law of similarity
3) law od good continuation
4) law of closure
figure - ground segregation
faces
goblet illusion
- on in front of the other
assume figure > ground
Gestalt problems
segmentations processes not always bottom up
- x in letters or shape
- faster to fidn x if in letter because we know letterrs
- so top-down influences
most evidence descriptive
relies heavily on introspection and evidence from 2D drawings
object recognition and the brain
where?
happens in temporal (what) pathway
object recognition and the brain
object recognition deficits
AGNOSIA
impairment in object recognition
different kinds of impairments depending on where in pathways
object recognition and the brain
object recognition deficits
APPERCEPTIVE AGNOSIA
impairment in process which constructs a perceptual representation
seeingn parts on whole
lateral occipital lobe damage (the side)
paient HSA (1987) –> bilateral ventral medial occipital damage
- can recognise from touch
seeing paintbrush seperately
cannot group objects
object recognition and the brain
object recognition deficits
ASSOCIATIVE AGNOSIA
impairment in teh process which maps a visual representation onto knowledge of the objects function
seeing the whole thing but not knowing its meaning
occipio-temporal damage (where occipital and temporal lobe meet)
Patient LH: can copy drawings but cannot name them
conisitant with a hierarchal multistage process
object recognition and the brain
object recognition deficits
APPEREPTIVE OR ASSOCIATIVE AGNOISA further along temporal pathway?
associative agnoia is further along
cultural differenes and biases in psychological research data
most from white western researchers and participants
cukture plays a significant role
- westerners prioritise processing / categorising objects
- east asians proitise relationships between objects
sso cant assume psychological “truths” apply to all humanity
the problem with faces
is a within category discrimination (look similar)
other objects recognition is between categories
different type of processing altogether?
are faces special?
neuropsychological evidence
propsopagnosia
impairment of face processing (late stage –> for V1 ect)
de renzi (1986) patient cant recognise families faces but can by clothes and voice
impairment at the stage of matching to stored info
are faces special?
neuroscience evidence
fusiform face area -> underside of temporal love
part of ventral stream
responds to faces more than other objects (in functional imaging)
why are faces special
just more difficult?
holistic / contijural processing ?
visual expertise ?
domain specificty
why are faces special
holistic processing
relationship between features
features of faces are processed less than other types of objects (eg homes)
upsidedown objects
- face recognition distruped by inversion
sighted people tend to recognise upsidedown faces slower
typrically interpreted as evidecne for holistic processing
why are faces special
visual expertise
have become experts because within category discriminations
- greeble discrimination (1999+2000)
- can learn a good level of Greeble distinction
- make use of fusiform face area
so perhaps fusiform face area for complex visual processing (not just faces)
why are faces special
visual expertise
critisms
not all prosopagnosic patients are impaired at within category discrimination
WJ: flock of sheep
RM: miniture cars
so not just fusiform face area needed for this
