Lecture 5 Flashcards
visible spectrum to humans
above and below spectrum?
380- 760 nm
below 380 = UV
above 760 = infrared
2 properties of light
wavelenght = perception of colour intensity = perception of brightness
cornea - what? where?
transparent outer covering of eye
works with lens to bend light into retina
pupil - what were
size changes.
bright - muscles? depth? acuity?
low - muscle? acuity? sensitivity?
hole in iris
regulates amount of light that passes from cornea to the rest of the eys.
bright illumination = constrict pupil, using sphincter muscles, greater depth of focus, higher acuity (on fovea)
low illumination = pupil dilation, using dilator muscles. lets in more light = decresae acuity, increased sensitivity
lens - where what fxn?
accomodation
just behind pupil, focuses light on retina.
accomodation = adjustment of lens for distance
accomodation: nearby object
ciliary muscles contract. less tension on ligaments. lens in natural shape.
accommodation: far object
ciliary muscles relax = more tension on ligaments. flattens lens
human eye position
sacrifice? better for? movement btw 2 eyes?
sacrifice backwards view.
but better for assessment of distance/depth. = predator usually eyes in front, prey eyes on side.
movement of 2 eyes is coordinated = move in unison.
eyes converge to see close object.
binocular disparity - what? why? difference btw close and far?
construct 3D image from 2 slightly different 2D images.
helps with perception of distance.
greater difference btw retinal images for closer objects than father objects.
retina - where, fxn? , how many layers? name them.
located at back of eye
transduced electromagnetic energy into neural energy.
5 layers - photoreceptor (furthest back)
bipolar , retinal ganglion cells (makes up optic nerve).
horizontal and amacrine = lateral communication
2 disadvantages of retina inside out design - how are they fixed
- light distorted because must travel 5 layers.
fixed: thinning of layers over fovea - foveal indentation. reduces distortion for high acuity vision- cones get preferential access to lgith - optic disk has no receptors - blind spot - RGC all bundled up. no light transduced at that location
FIXED: completion, filling in. eyes constantly moving to fill in missing info.
fovea - what, where, types of receptors?
tiny indentatino in center of retina. thinning of layers in front of it.
required for high acuity vision
contains densely packed cones.
surface interpolation - what is it used for?
process to perceive surfaces. extracts edge info and infers appearance of large surfaces.
cones: type of vision in what kind of lighting, what kind of perception. sensitivity to light? convergence
phototopic system - colour vision - only in good lighting. high acuity, fine-detailed coloured perception. not very sensitive to light.
convergence = 1 cones : 1 RGC. high foveal representation in brain. helps brain ID where object is.
3 types of cones. why 3? genetic differences - dichromatic, achromotrypsia
s -cones: short wavelength, blue. high sensitive, least abundant
m-cones : medium wavelength, green.
l-cones: long wavelenght, red
dichromatic vision = colour blind. usually red-green
achromotrypsia = achromatic vision. only black and white. no cones, blinded by bright light bc of highly sensitive rods. - dont detect red light.
rods - type of vision, lgihting conditions? where in retina? convergence?
scotopic visual system - lacks detail and colour.
dimly lit conditions
all over retina except in fovea.
several hundred rods : 1 rgc. => poor acuity.
what photopigment do rods use? how does it work?
rhodopsin - extremely light sensitive to blue-green light.
in the dark, sodium channels are open, constant high NT release.
when light enters rod = conformational change in rhodopsin = change cell conformation to change [ion]. closes na+ channel = activate GPCR = less NT release. hyperpolarization upon light entering.
Purkinje effect in eyes
transition from photopic to scotopic vision as sun sets.
see particular colours better at different types of day. - blue better at night bc rods dominate. brights and red better in bright/photopic.
disk shedding in cones vs rods
based on circadian rhythm - when not in use.
cones - shed in evening
rods - shed in morning
dark adaptation in cones vs rods
cones = rapis with high threshold - doesnt matter bc dont work in dark
rods - slow with low threshold. bleaching with light, takes 45 mins to unbleach
eye movements occur bc of blind spot but also for fovea - explain
temporal integration of eyes
in lgiht conditions, eye must constantly move to get image on fovea for colour and detail.
visual perception is summation of recent visual info.
temporal integration = add together foveal images from preceeding fixations => blinks dont interrupt vision.
describe contact lens experiment . proved what?
contact lens with image on it moved with the eyes. after few seconds, participants stopped perceiving image. moved eyes more rapidly to try to bring it back
proved that visual system cares about change in vision , constancy was being ignored.
describe the retinal geniculate striate pathway
how much (in %) of rgc’s go thru this path? what does this mean for foveal representation on brain?
visual space processed where?
retina -> optic nerve -> optic chiasm -> LGN (thalamus) -> striate/ v1 (occipital lobe)
90% go thru this path. => a ton of space in v1 dedicated to foveal representation.
contralaterally - right visual field ( left temporal, right nasal) in left hemisphere.
LGN - how many layers? which ipsilateral? which contralateral? which M? which P?
6 layers ipsi = 2,3,5 contra = 1,4,6 M = 1,2 p = 3-6
retina inverts image.. soo brain ?
brain reinverts image. top of visual field on retina = on bottom of v1.
how 2 adjacent neurons in retina activate in higher levels?
follow adjacency. adjacent in LGN and v1.
M & P layers in LGN
input from which cells?
which layers?
responsive to what stimuli? project to ?
P cells - input from cones. layers 3-6 in LGN. colour, stationary, fine pattern detail. project to bottom of striate layer 4 neurons.
M cells - input from rods. layers 1-2 of LGN. movement. project to top of striate layer 4 neurons
how lateral inhibition has effect on edge perception
perception of contrast between 2 adjacent areas.
neuron inhibits adjacent neurons - contrast in stimulation = increased sensory perception. informative - helps define position of objects.
mach bands illusion
nonexisten stripes, brain perceives edges. neurons laterally inhibit to enhance contrast at each edge to make edge easier to see.
helps for surface interpolation.
studying contrast enhancement in crab model
ommatidium axons.
lateral plexus = interconnected ommatidium axons. when single ommatidium illuminated - fired at rate of intensity striking it. inhibited neighbouring cells via lateral plexus.
lateral plexus in crabs = horizontal/amacrine cells in human.
receptive fields of rods vs cones
rods = larger receptive field than cones because cones have higher convergence. less overlap for higher acuity.
at all 3 levels of RGS path - neurons have what kind of receptive field? what do neurons care about
round receptive field.
care about lines in particular orientation, not images. care about part of visual space and orientation of object in it.
explanation of hermann grid
common illusion.
when image in periphery, some intersection in center = firing, other intersection in surround = less firing = flickering of black, white. but on fovea when you focus on one spot. no flickering because stimulus clearly in center.
receptive fields of all neurons in RGC pathway (to lower layer 4 in v1)
on-center or off-center. circular receptive fields.
simple cortical cells - where? respond to what? why?
primary visual cortex neurons - upper layer 4. respond to straight edges instead of circular edges because have rectangular receptive fields. respond to contrast.
similarities and differences btw simple and complex cortical cells
simlarities
- rectangular receptive fields
- respond best to straight line stimuli in particular orientations.
- respond to movement of straight edge , have preference for direction
- unresponsive to diffuse light - need variation in environment
differences
- larger receptive fields than simple
- no statis on/off region. respond to any orientation, movement in particular direction
- many binocular: receive input from btoh eyes. probs comparing and creating depth
- fire more to natural than artificial
- fire mroe if stimulus presented to both eyes simultaneously
- ocular dominance - one eye a little more important..
columnar organization in V1 vertically horizontally ocular dominance columns binocular regions
6 layers of neocortex.
all 6 layers in one column respond to same type of visual stimulus.
vertically = same general area of visual field
horiontally = slightly different location in visual field. respond to lines of slightly different orientations. for same eye.
alternating ocular dominance.
binocular columns - smushed in between ocular dominance columns.
receptive fields in V1 neurons - highly plastic
depend on stimuli and context.
- care about natural > artificial stimuli.
- change fxn rapidly. as elements become more relevant = more receptive neurons.
- receptive fields change rapidly with experience ex: wild vs lab monkeys.
seeing colour - due to what?
what is black? white? grey? colour?
due to what wavelengths are reflected
black = lack of light, no wavelengths reflected
white = mix of intense wavelenghts all reflected at equal proportions
grey = mix of wavelengths in equal proportions at lower intensity.
colour = usually mix of wavelengths - mostly the colour reflected is what we see.
component procesing
trichromatic theory
problem?
- 3 different colour receptors. colour of stimulus encoded by ratio of activity of 3 receptors.
- any visible colour matched by mixing together 3 diff wavelenghts of light in 3 diff proportions.
- complementary colours can’t exist in same colour AND doesnt explain afterimage effect
opponenet processing
explain afterimage
2 classes of cells that encode colour, third encodes brightness.
- red/green
- blue/yellow
- black/white = brightness.
afterimage: saturate receptor of colour youre looking at so it stops responding to that colour. when you look away see opposite colour because of absence of response to stimuli.
reality of component and opponent?
both exist.
component = 3 cones, correct at low visual areas.
opponent = RGC and up. receptive to one colour, turns off to opponent colour.
colour constancy
colour stay some colour despite change in enviro lighting.
advantageous: always know what youre seeing.
requires: all 3 wavelengths reflected to some degree.
Land’s retinex theory
- cells that do this?
dual opponent colour cells.
colour is determined by reflectance. visual system compares light reflected by 3 wavelengths. cortex maintains the constant colour.
blobs = columns of cells betewen ocular dominance columns - preserve colour.
dual opponent - subtraction for colour constancy.
- need two requirements for on and off.
on = + colour AND - opponent colour. opposite for off.
scotoma - where is damage? effect of damage?
test?
area of blindness in contralateral visual field of both eyes.
usually damange in one hemisphere in V1 – hard to damage.
test: perimetry test- determine area of blindness. patient stares at fixation point with one eye open. point moves, respond when see dot.
completion - report seeing whole objects.
blindsight - what? how?
seeing without conscious awareness. complete lesion of V1.
still prefrom visually guided tasks because part of 10% that isnt going to RGC pathway is bypassing v1 and going to superior colliculus in midbrain to orient eyes in space, after which is fed forward to v2 which mediates spared visual fxn.
v2 - what are hypercomplex cells?
where does it project to?
hypercomplex = respond to stimuli of certain size/shape/length and movement.
neurons have larger receptive fields here than earlier in hierarchy.
projects to visual association areas - multi-modal
association visual cortex - fxn?
higher visual functions such as giving meaning. identity detection.
cells respond to compelx pattern of structure. respond to certain stimuli in particular way.
dorsal stream - visual path function forward path receives info from? perception of what stimulus? alternative theory
“where”
visual spatial perception in visually guided behaviour
pathway: V1 –> dorsal -> v2 –> PPC
fed by M pathway
motion perception (V5 crucial for processing motion)
“control of behaviour” appropriately, consciously interact.
damage to PPC
difficulty interacting with object.
difficult to accurately reach and/or identify object
ventral stream of visual path function forward path fed by? perception of what stimulus althernative theory:
“what”
visual pattern recognition in conscious visual perceptions
v1 –> ventral V1 –> inferotemporal cortex.
fed by p pathway
perception of shape & colour - responds to objects that theyve experienced.
(v4 - crucial for colour processing and shape info
“ conscious perception theory” mediates conscious perception of objects they’ve experienced)
v5 = important for what? damage =?
important for processing motion
damage = akinetopsia. inability to perceive fluid motion. see snapshots.
v4 = important for? damage = ?
important for processing colour and shape information
damage = achromotopsia = inability to see colour.
visual agnosia = interact with object properly, but cant name/ID it.
prospagnosia - what is it? damage to what? autonomic responses?
failure of facial and minute detail recognition
cant see faces, cant detect differences in familiar objects.
damage to inferior v2, and inferotemporal cortex, including fusiform face area.
autonomic responses to familiar faces = increased skin conductance.
what is fusiform face area?
what happens when you TMS?
increases activity when viewing familiar face. help recognize one another.
also helps with hobbies/jobs that rely on minute differences ex: door making.
TMS = inhibit facial recognition
akinetopsia
damage to v5 - inability to perceive fluid motion
