vision Flashcards
light
form of electromagnetic radiation
what our visual system lets us see
wavelength
distance between peaks
in nanometers
visual fields
all you survey without head or eye movement
each eye has its own visual fields, they overlap to some extent, not so much laterally
acuity
sharpness of visions
visual system’s ability to resolve fine detail
sharpest as the center of the visual field - falls off toward the periphery
best in fovea
fovea
place in central vision where greatest acuity is found
high density of cones
photoreceptors
rods and cones
basic light receiving units that line the back of the eye
stimulate other neurons whose axons form the optic nerve which exits the eyeball
blind spot
in the visual field of each eye
corresponds to the location where axons of the optic nerve exit the eyeball (optic disk)
optic disks
where blood vessels and ganglion axons leave the eye
no photoreceptors
meaning there is a blind spot
brightness
(value)
an axis from light to dark
hue
an axis through blue, green, yellow, orange red and the variations in between
the rainbow
colours
explain transduction in rods
light particles are detected in the disks
photo strikes photopigment on disk membrane
rhodopsin splits when hit by a photon into retinal and opsin to capture energy.
this activates a 2nd messenger pathway
cGMP
sodium channels close (normally open when + ions come, but they are not coming)
graded potential causes hyperpolarization which cuses the cell to release less NT
NT glutamate is released stimulating bipolar cell
hyperpolarization reduces transmitter release, signaling a change in light
graded potential goes down bipolar cell (passive) causeing depolarization
NT is released, stimulating ganglion cell
AP propagates down ganglion cell and message is sent to brain
what type of receptor fields do bipolar and ganglion cells have
donut like receptive field
- light falling on whole receptive field exhibits a weak response (center and surround pretty much cancell
off center bipolar cells
glutamate is excitatory
shining light on cell’s receptive field would inhibit
turning off light excites it
lateral inhibition
interconnected neurons inhibit their neighbours, produces contrast. at the edges of regions
Ca++ currents
are altered to change responsiveness
mechanism is unknown
fusion of retinal and opsin
back into pigment is slow - at high intensities, less and less pigment is available
enzyme phosphodiesterase
rate-limiting in the 2nd-messeneger system that open sodium channels
there is limited phosphodiesterase available and ever more scarce at higher and higher intensities
optic nerve
axons from ganglion cells
travels to the base of the brain
optic chiasm
axons from “nasal hermiretina” cross over to the other side of brain
info from left part of both retinas goes to the left hemisphere and vice versa
left hemiretina receives image from the right visual field
point where two optic nerves cross the midline
optic tract
radiation of fibres into the brain from optic chiasm
radiate into the base of the brain
after passing optic chiasm
minority of axons here send info to superior colliculus for rapid movements of the eye
lateral geniculate nucleus
visual part of the thalamus
relay system
where most optic nerve tracts terminate
occipital cortex
at back of brain
striate cortex
inputs from both eyes converge to give binocular vision (depth perception), among other things
retinotopic organization
map of the retina maintained at all levels and projected onto visual cortex (upside and background)
most devoted to fovea - makes for increased acuity
superior colliculus
spatial maps and eye movements
saturation
amount of pigment a given hux
rich - pale
trichromatic hypothesis
the theory that there are three different types of cones (red, blue, green), each excited by a different region of the spectrum and each having a different pathway to the brain
opponent - process hypothesis
different systems produce opposite responses to light of different wavelengths
motion
movement of the eye is controlled by extra-ocular muscles
visual systems are especially tuned to motion
movement in peripheral vision captures attention and shifts gaze
evolutionary significance ie/ bullet time
subjective brightness
the brightness you perceive
personal experience
visual system opperating at only 1/5th of total brightness range
cornea
transparent outer layer of eye
curvature is fixed
bends light rays
primarily responsible for forming the image on the retina
refracts light rays
myopia
eyeball too long
images focus in front of the retina
image that actually reaches the retina is blurred
nearsightedness
difficulty seeing distant objects
accommodation
muscle process of focusing the eye
like a camera lens
lens must be shapes correctly so that the image of an object at a given distance is focused on the retina
- lens round for close up, lens flattens for far away
inaccurate accomation = poor focus = glasses
refraction
the bending of light rays by a change in density of a medium
happens from cornea to lens
lens
helps us focus the image on the retina
changes its shape to fine-tune the image on the retina
photoreceptor adaptation
tendency of rods and cones to adjust their light sensitivity to match current levels of illumination
range fractionalization
handling of different intensities low threshold in rods and high thresholds in cones
scoptic vision in low light, phototopic in bright light
cannot have an extensive range fractionalization, bc we can not afford to have large numbers of receptors inactive under various lighting conditions
iris
light control
coloured part
opens and closes in response to the amount of light entering the eye
controlled by the brainstem
retina
receptor surface inside the eye that contains photoreceptors and other neurons
turns light into a neural signal
ciliary muscles
around iris
pupil dilates when contracted
pupil relaxs - opens
changes lens shape
superior/ inferior rectis
up and down movement
constriction of pupils
in bright light
controlled by parasympathetic
dilation of pupils
controlled by sympathetic division
superior and inferior oblique
rotational movement
medial and lateral rectus
side to side movement
oculomotor nerve
everything except superior oblique and lateral rectus
trochlear nerve
superior oblique
abducens nerve
lateral rectus
ganglion cell
AP
one million
connect with bipolar cells
any cells in retina whose axon forms the optic nerve
tapetum ledum
in many animals (not human)
an eye flash
goes bright when flashlight shines in dark
reflective to the light
amacrine cell
AP
contact with bipolar and ganglion cells
significant in inhibatory interactions in the retina
provide lateral communication with neighbouring retina
horizontal cell
graded
make contact with photoreceptors and bipolar cells
provide lateral communication with neighbouring retina
bipolar cell
graded
photoreceptor cells release here
interneuron
receives from photoreceptors and passes to ganglion cells
rod cell
graded
convergence
several receptors connect to individual bipolar cells
numerous bipolar cells may connect to a single ganglion cell
greater in rods than in cones
scotopic system
works in dim light - where it is most sensitive
insensitive to colour
rods
lower acuity
away from fovea
black and white
lots of convergence
photopic system
needs more light
has a higher threshold
cones
colour - sensitivity to wavelengths
high acuity
near fovea
ganglion cells report from a single cone
suprachiasmatic nucleus
biological rhythms
scotoma
a spot where nothing can be perceived
a region of blindness within the usual visual fields caused by injury to visual pathway or brain
retinohypothalamic tract
tracks light to know when day/night is
parvo system
four outer layers of the LGN contain small parvocellular neurons
relatively small receptive fields
donut shaped receptive fields
sensitive to wavelength (colour)
cone based
receive axons from “P” type retinal ganglion cells, which are smaller, like high contract, notice colour and fire with a tonic (firing at all times just increased or increased) background rate
magno system
inner 2 layers of the LGN contain large magnocellular neurons
larger receptive fields
rods
most are not sensitive to wavelength
receive axons from M type retinal ganglion cells, which are larger, detect low contrast, do not notice colour, anre fire only transiently. they have large extra speedy axons
sensitive to low intensity
what are the four classes of V1 cells
simple
complex
hypercomplex 1 and 2
simple cells
respond and have more APs where there is a bar or edge of a specific width, specific orientation, and specific location in the visual field
complex cells
have elongated receptive fields
like a bard or edge (would be long) of a specific orientation and size but could be at a number of different locations in the visual field
hypercomplex 1 cell
particular emphasis on bar length
hypercomplex 2 cell
like particular angles of intersection of lines
spatial frequency analysis
visual patterns are not perceived as a built up complex of edges and continuous after all
instead cortical cells respond to various spatial frequencies that make up an image
any image can be broken down into a mathematical sum of large number of alternating light and dark grids (fourier analysis)
broad dark areas in a pic have low spatial frequency
areas with fine detail show rapid alterations from light to dark and have high spatial frequency
cortical blindness
lesions in V1
a place in the visual field where nothing is perceived
blindsight
permit some perception of movement
seperate visual systems for seeing things and moving through the world (knowing a stimulus is present)
ocular dominance columns
regualr spaced along the V1 cortex and extending inwards in a column, are found patches of cells that respond to inputs from either the left or right eye
orientation columns
within ODCs
cells within each orientation column respond best to stimuli with a particular angular orientation
Area V2
receives input from V1
has more complex receptive fields
is active in providing contour inferences, may be important in disembedding stimuli (discriminating which parts of the visual scene make up complex items)
has complex interations with V4
passes info to the temporal lobe where object recognition
prosopagnosia
condition in which there is a selective loss of ability to identify faces
lesions are large and bilateral and and believed to involve the systems that receive input from V2
area V3
not well understood
seems to be involved with dynamic form - ability to know it is the same object even though it is changed from when moving
- slightly different forms of the same thing, ie/ person walking, ball being thrown
provides this sort of perceptual lock
s cones
peak response to blue (short wavelength) but all respond to other shorter and longer wavelengths
fewer of them
acuity is much lower
420nm
M cones
have peak responses to medium wavelengths, roughly green but overlaps
530 nm
L cones
have peak repsonses to long wavelengths around yellow but overlaps
560nm
blue wavelength
short and high frequency
red wavelength
long and short frequency
spectral opponency in LGN
neuron that has opposite firing responses to different regions of the spectrum
l, m and s info is fed to retinal ganglion cells and passed to the LGN
+L/-M
yields an orange-red peak (650) nm
+M/-L
yields a blue-green peak (500nm)
+(L+M)/-S
gives far red peak (700nm)
+S/-(L+M)
gives a blue peak (450nm)
Area V5
in medial temporal area
responds to moving stimuli (own motion, eye motion, or head motion)
parvo in higher levels
implicated in colour, form and recognition
magno in higher levels
implicated in depth and object movement
dorsal
object localization and body movements towards things in the environment (like magno)
where
ventral
object recognition (like parvo)
what
optic ataxia
damage to the dorsal parietal cortex
difficulty using vision to reach/ grasp for objects
amblyopia
acuity is poor in one eye, even tho the other eye is normal
caused by lazy eye, misalignment of the two eyes
