The Visual System Flashcards
how does light affect our perception
wavelength and intensity of a light affects COLOUR and BRIGHTNESS
iris
donut shaped bands of contraticle tissue that affect the amount of light entering the retina
pupil
hole in lens where light enters which adjusts in size to changes in light (sensitivity and acuity)
lens
focuses light on retina; adjusted by cillary muscles
sensitivity
ability to detect presence of an object in dim light
actuity
abilitiy to see details of an object
accomodation
adjusting configuration of lenses to bring images into focus on the retina
label an eye diagram
x
what have verebrates evolved to have
bilateral vision
stereoscopic vision (3D perspection)
bincoular disparity
colour vision
convergence
eyes coordinate so poitns in visual world correspond to the retina
binocular disparity
difference in position of the same image on 2 retinas
label the cells in the retina
x
amacrine and horizontal cells
specialized for lateral communication
fovea
indentation of ceter of retina for ‘high acuity’ vision
this thin layer dilutes incoming light
blind spot
where optic nerve connects= hence no visual processining occurs here
how does the visual system cope with the blindspot
visual information by periperhal receports fill in the ‘gaps’ (completion)
types of photoreceports
rods and cons
cones
phototrophicv vision colour perecpetion high lighting high actuity 1 cone to 1 bipolar cell 3 types (RGB) in centre of the retina few cones converege to each ganglion cell
rods
scotopic vision sensitive vision (shape/detail, etc) dim lighting low acuity several rods converge to 1 ganglion cell (multiple rods: 1 bipolar cell) 1 type peripheray of retina
visual perception
summation of recent visual information
temporal integration
how visual system fills in time-lags during processes
involuntary fixational eye movement
eyes are contionusly moving to allow us to see even duraction fixation
this results in tremors, drifts and saccades
visual transduction
conversion of one form of energy to another: in terms of visual transduction this referes to the conversion of light to neural signals
how does visual transduction occur
rhodopsin absorbs light in visual receptors; this is a G-Protein that responds to LIGHT instead of neurotransmitters which activates chemical events
it acts by INHIBITION
in dark light, rhodopsin… (3 steps)
- rhodopsin is inactive
- sodium channels open
- sodium ions flow into rods= slight depolarization of them
in bright light, rhodopsin… (3 steps)
1, light reaches rhodopsin
- sodium channels close
- sodium ions can’t enter= hyperpolarizes rods
after passing through the retina, what happens to visual information
it is transported vis the retina-geliculate striate pathways to the primary visual cortex
signals from LEFT visual field are transported to RIGHT primary visual cortex
retina reliculate striate pathways mechansims
carry signals from retina to primary visual cortex via the lateral geniculai nuclei in the thalamus using left and right pathways
RGS pathways
6 layers; each layer of each nucleus has different input from all the convulaed visual fields on an eye
retinotopic organization
RGS is retinotopic as each level is organized as a map in the retina
disproportionate amount of each system is dedicaed to analyzing the fovea
what is an edge
perception of a contrast between 1 adjacent areas of a visual field; yields information information about an object
how do we see an edge
- contrast
2. brightness
mach bands
nonexistence stripes of brightness/darkness adjacent to an edge
they enhance the CONTRAST of an edge and allow the nervous system to improve its perception of edges
lateral inhibition
when a receptor fires= it inhibits its neighbours from using lateral networks
greatest when receptor is MOST illuminated
what are the recpetive fields of visual neuron
visual fields where a visual stimulus infleunces the firing of a neuron
who were Hubel and Wiesel
they mapped out the recpetive fields of the visual system to understanding increasing neural respones at different receptor levels
Hubel and Wiesel studies (4 steps)
did studies on cats/monkeys visual systems:
- placed microelextrodes near single neurons
- blocked eyemovement by paralyzing eye muscles
- receptive fields were identified
- the neuron responses were measures by providing stimules where activity was measured
commonalities of retinal ganglion cells, lateral geniculate nuclei and lower neurons (4)
- receptive fields in fovea smaller than in periphera of retina
- all neurons have circular receptive fields
- all neurons are monocular (specific to one eye)
- many neurons have exictatory/inhibitiory areas separated by the ciruclar boundaries
on center cells
respond to light shone on central region of the receptive field
inhibit when light in peripehary
off center cells
respond to inhibition when light in center
exicte when light in peripeheray
how does light influence neurons
turn neuron firing on or off (inhibition or exitaction)
Hubel + Wisel on functions of neurons
functions of neurons in retina geniculate striate pathways are to respond to the degree of brightness contrast in 2 areas of the receptive fields
2 classes of V1 receptive fields
- simple cortical cells
2. complex cortical cells
simpel cortical cells
- unresponsive to difference light
- receptive fields in ‘on/off’ areas with straight lines
- all are monocular
complex cortical cells
- more numerous
- rectangular receptive field + straight line stimuli
- unresponses to difffuse light
- larger receptive field
- dont divide into off/off regions
- cells response/difre contniously to light
- can be bincoluar
Hubel-Wiesel: organization of the V1 (4)
- organized in functional vertical angles/columns
- location of various functional columns influenced by location on retina in dominant eye
- columns prefer straight line angels
- preferences of neurons become more complex down the visual pathway
Two theories of seeing oclour
- component theory
2. opponent processing theory
component theory
‘trichromatic’ idea that 3 colour cones each pair with different spectral sensitivies
opponent processing theory
2 classes of cells encode for colour or orientation;
- 3 cells encode for TWO colour perceptions when cones are hyperpolarized (red) or depolarized (green)
- second class of cells encode for blue and yellow
component theory thinker
Young
opponent processing theory thinker
Herring
3 visual cortex classes
primary
secondary
visual association
primary visual cortex (what and where)
recieves input from visual relay nuclei in thalamus
found in posterio region of occipital lobe
secondary visual cortex (what and where)
recieve input from V1
in prestriate/inferotemporal cortex
visual association (what and where)
recieve input from V2 and other sensory systems in posterior pareital cortex
scotoma
an area of blindness resulting from damage to V1: however patients often not aware due to ‘completion’ mechanisms filling in blindsight
damage to dorsal stream
neurons respond to SPATIAL stimuli (direction and location; WHERE)
damage to ventral stream
neurons respond to CHARACCTERSITCS of objects; colour, shape
the ‘WHAT’
bilateral lesions in ventral stream might cuase…
can INTERACT with world but not identify (control of behaviour but not conscious pereception)
patients with lesions in dorsal stream might…
can IDENTIFY but not interact with world (no control of bheaivour but conscious perception yes)
prosopagnosia
visual agnosis of faces
agnosia
failure of recognition
akinetopsia
inability to see movement progressions due to damage in middle temporal area (V5/Mt5)
affects pereption ofmotion
cornea
focuses light
why is the fovea thinner
to allow for light to pass through more precisily
organization of the visual system
- retina
- LGN= lateral geniculate nucleus
- primary visual cortex in thamalas
- secondary areas (v2, v3, etc)
what does the occipital lobe process
low level proccessing of colour, orientation and shape
what does the temporal lobe process
high level processing of objects and faces
how are sensory systems organized
- HIERARHICAL
- FUNCTIONAL
- PARALLEL PROCESSING
functional segregation of visual system
from photorecepotr to thalamus (lgn) to v1 to v2 to asociation cortex
current model of organization
not hierarhical but still functionalyl segreated/parallel
what are saccades
ever 3-4 second rapid eye movement= hence there are distinct moments when our eyes get no informatino that our brain ‘fills in the gaps’
what is drift
fixational eyemovement when eye slowly hovers/moves around an object
what is fixtaion
period of time when you reyes is alighed and kept on target to proces information
what is the visual field
total area at which an object can be seen by your eye
what is the visual receptive field
portion of a visual system that a neuron responds to; increaes along visual hierarchy
what is convergence
compression of visual information into an optic nerve by photoreceports (multiple inputs into one output)
what do illusions demonstrate
our eyes are never still!
saccades
rapid eye movement as brain is always updating itself and filling in gaps
waterfall effect
motion ‘after effect’ where comparator neuron percieves motion in opposite direction as it is inhibited and continues moving dowardds
what is light
waves of electroagnecy energy split into WAVELENGTH (perception of colour) and INTENSITY (perception of brightness)
5 layers of neurons in retina
recetpors horizontal cells bipolar cells amacgrine cells retinal ganglion cells
