vision Flashcards
visible spectrum
the small portion of the electromagnetic spectrum we can see
aspects of LIGHT
4
electromagnetic radiation
rays
particle (quanta)
wave
what is a wave measure in
nanometres
luminance scale
not all wavelengths are visible, this scale takes visibility into acount
high contrast
high difference between light and dark
cornea
transparent ‘window’ through which light enters eye
[curved and acts as lens]
pupil
dark circle opening where light enters
lens
adjustable
focus light on retina (ciliary muscles)
they have at least 1 curved surface
light travels slower thorugh lens [compared to air]
retina
back of eye contains the photoreceptors, sends the image to the optic nerve
iris
coloured part
aqueous/vitreous humor
squishy bits behind the corner
where does focussing occur?
recombining rays from various directions to form a single point on the imaging surface
3/4 of eyes focussing power comes from cornea
1/4 lens
emmetropia
normal refractive condition; appropriate focus
myopia
short-sightedness (good short vision)
- focal length is too short
- light is focused in front of retina
- need concave corrective lenses; diverging lens [reduces power]
hyperopia/hypermetropia
long sightedness
- focal length it too long, lens too weak
- light focused behind retina
- need convex corrective lens; converging lens
presbyopia
old age
lens looses its natural elasticity, inability to change accomodation
astigmatism
different focal lengths for different orientations
e.g., ok for vertical lines but myopic for horizontal lines
rods
more rods then cones
high sensitivity, NIGHT VISION
cones
lower sensitivity, DAYTIME
comes in 3 sorts; red, green, blue. refer to WAVELENGTH
scoptic
only rods are active
photopic
cones active
rods momentarily blinded
mesopic
in between, both rods and cones
blind spot/optic disk
where optic nerve leaves the eye
no photoreceptors
visual transduction
rods/cones pass electrical impulses to ganglion cells (via bipolar/amacrine/horizontal cells)
- Ganglion cells have long axons that exit the eyeball via a bundle called the optic nerve
- optic nerve carry info from eye to visual cortex
fovea/macula
the thing you’re looking at is imaged here
many receptors, no blood vessels
ganglion cell activity
one ganglion cell receives input from many photoreceptors
retinal field =
where light falls on retina
receptor field for foveal vision
smaller rf.
more densely packed
greater acuity
fovea and cortical magnification and acuity
larger area of cortex for processing foveal vision (+ smaller rf) than for peripheral.
explains why foveal vision has greater acuity/precision
2 characteristic of retinal ganglion cells
- small receptive fields
- centre sorround antagonism receptive fields
what happens when the retinal ganglion cell axons (bundle=optic nerve) leaves the eye
they terminate in LATERAL GENICULATE NUCLEUS (LGN)
- > crossover at optic chiasm (partial decussation)
- > LGN projects to V1 via OPTIC RADIATIONS
extrastriate cortical visual areas
30+ visual areas beyond V1
- each areas specialised for particular aspect of vision (v4=colour)
- each is RETINOTOPIC (except MST)
ON centre
- tell us how bright an area is
- detect luminance increments
OFF centre
tell us how dark an area is
help us detect local luminance decrements
centre-sorround antagonism
ganglion cell receptive fields
excitation and inhibition
why have 2 sets of cells?
compliment each other
make sure dark spots are detected as easily as light ones
if there is light all over, or no light at all [on ganglion cell rf]
spontaneous activity only - no response
WHY centre sorrround antagonism?
tells us where CHANGES are in the image - exaggerates EDGES
& sensitivity to CONTRAST between light and dark
describe the herman grid illusion (what you see)
illusory dark patches at the intersection, less noticeable close to the fovea
explain herman’s grid illusion
RFs of periphery are large and less dense creating the illusory dark patches. move our eyes to get better acuity of object in foveal vision where RFs are small & densley packed.
centre sorround antagonism; intersection, more bright sorrounds so middle apears darker
dark sorround will make an area appear lighter
sensitivity to contrasts. effect on cells
most visual cells increase activity in response to increases in contrast
LGN properties (5)
- 6 layers
each is retinotopically organised
all cells are monocular
both eyes have inputs to LGN, but each eye goes to diff layerLGN receives input from diff sides
neurons in LGN
MAGNUCELLULAR = large RFs, process motion PARVOCELLULAR = small RFs, process colour
Koniocellular = betwee the M/P layers
role of optic radiations
carry neural signal from LGN to V1
properties of V1
- retinotopic
- cortical magnification
- channels selective to orientation/diff angles
- selectivity for eye of origin / ocular dominance
filtering in vision
filter for many different properties
orientation tuning; selectivity
reduced activity as orientation departs from preferred cells
range of orientations to which the cell fires lots is a measure of
its bandwidth
V1 organisation
orientation columns
& columns of ocular dominance
vertical and horizontal
hypercolumn
= collection of orientation columns go round, 180 degress, back to original orientation
types of V1 cells
simple cells
complex cells
hypercomplex cells (end-stopped cells)
simples cells
respond to an orientated stimulus in a certain location within their RF
- can be bar or edge detectors
complex cells
respond to an orientated edge ANYWHERE within the receptive field
- do not have ON and OFF areas
- phase insensitive; similar response across their RF (unlike simple cells)
building of complex cells
connect several simple cells with same orientation preffered
hypercomplex cells / end-stopped cells
prefer stimuli with an END within their receptive field.
hypercomplex cell building
connect several complex cells to construct an end-stopped cell
how do we detect an illusory edge?
signalled by interconnected hypercomplex cells
low-pass filter
removes high frequency
reduces detail
high-pass filter
remove low frequency
more detail
size of stripes for what spatial frequency
fat stripes = low spatial frequency
thin stripes = high spatial frequency
range of visible spectrum
400-700 nm
how is spatial frequency measured
in cycles per degree of visual angle (the size of an object)
how does the lens adjust/accomodate
cilary bodies
optical power measure in…
diopters
retina is a layered network contain __ diff types of cell, whose nuclei are grouped in __ layers
5 types of cell
3 layers
binocular neurons
receive signals from both eyes and compare the images from the left and right eyes
most common V1 cells
complex cells
