vision Flashcards
what is light?
stream of photons
what are photon?
they rush out from light source at a phenomenal speed and can penetrate transparent materials but bounce back from or get absorbed by opaque materials
how can we see?
eyes must collect photons that bounce off objects and try ot work out prop of objs from those reflected objs
how does the eye project orderly images to the retina at the back of the eye?
the eye exploits the fact that photons travel in rays
discuss the range of wavelengths of photons visible to the human eye
relatively short wavelengths (400-700 nanometers) of photons is visible ot the human eye because photons of these wavelengths are the ‘right size’ to be abdsorbed by photopigment molecules (opsins) in our eyes
What is the approximate distribution of rod and cone cells in the retina, and how do their functions differ in terms of vision?
each retina: ~ 130M photoreceptors
highly light sensitive rod cells:
- ~120M
- night vision
cone cells:
- ~ less than 10M
- daytime vision
rods and cones have different types of opsin
what is iodopsin
iodopsin is a specific type of protein also known as cone opsin
it makes cones sensitive to diff colors of light
long: red
medium: green
small: blue
what is the composition of rods and cones (chem)
high intracellular K+, high extracellular Na+ and cell membrane that is polarized due to K+ leakage
why do photoreceptorshave Na+ leakage channels in their membranes which are open in the dark
so that Na+ can enter the cell (the ‘dark current’), causing photoreceptors to be depolarized, and their synpases to be active, releasing glutamate
how is the amount of darkness measured
when opsins in the photoreceptor cells are stuck by photons, it sets off a biochem cascade
it closes the Na+ dark current channels and causes the cell to become more polarized and its synapses to release less glutamate
the amt of glutamate released by photorecp is the measure of darkness that falls on pixel that recp cell occupies
what is the optic nerve
it carries info from eye to brain
why does the neural circuitry of the retina have to compress info
because the optic nerve only has 1.3M nerve fibers but there are 10M cones + 120M rods in retina
how is info compression achieved
through a network of bipolar, horizontal and amacrine cells which converges onto RGCs, and which uses lateral inhibition to create receptive fields with opposing centre-surround organization (ON-centre or OFF-centre cells)
what is centre-surround receptive field
allows neurons to detect contrast and edges in visual stimuli
what is color-opponent receptive field
make cells sensitive to colour (“chromatic”) contrast rather than brightness (“luminance”) contrast.
which flavors do RGC come in?
M-cells
P-cells
non-M non-P cells
what are M-cells
- project to the magnocellular layers of the lateral geniculate (LGN)
- are colour blind but highly sensitive to luminance contrast
- feed heavily into the brain’s motion centres.
what are P-cells
- project to the parvocellular layers of the LGN
- are often red-green opponent
- project to the parts of the brain most interested in shape and fine detail
what are non-M non-P cells
- project to the koniocellular layers of the LGN
- often yellow-blue sensitive
- project into the parts of the brain most interested in colour
how does retina see?
image upside down + LR reversed (inverted) but then the image is put back up again after it is interpretted in the brain’s visual cortex
describe the visual processing pathway
the optic nerve appraoches the brain along intercepting paths and meets at the base of the diencephalon at optic chiasm
~60% of ganglion cell axons cross over to opp side
~40% cross over to same side
on brain pov: L&R bundles of ganglion cells now called optic tracts & include nerve fibers from both eyes
what is LGN
relay center in the thalamus for visual pathway
axons vip target
what happens to axons in LGN
axons of GC terminating in LGN form synapses with other neurons and X prog further
what happens to neurons that received visual info in LGN
neurons that have received visual info from GC radiate out of thalamus and go directly to VI
why are inputs from left and right eye segregated and terminated in different layers of LGN
the neurons in the LGN and the corresponding cortical neurons are excited by visual input from either the left eye or the right eye, but not both simultaneously
this monocular organization ensures that the visual information from each eye remains distinct and allows for the processing of binocular depth and other visual features later in the visual pathway
what is retinotopy
the mapping of the visual field onto the retina and the subsequent visual processing areas in the brain
describe retinotopic mapping
in V1, retinotopic map is distorted so that the central 10 degrees of visual field occupies roughy half of V1 due to poor acuity in periphery cortical magnification
(cortical magnification)
present functional features of v1
- surface: organized in retinotopic map of visual field
- dark and light stripes: surface view of left & right ocular dom columns
- orient columns: contains cells with similar selectivity for orient of stimulus
- color blobs: represent color recog module of functional org
what is dorsal stream?
- where / how pathway
- responsible for processing visual information related to spatial perception, motion, and action
- emerge from V1 and projects to areas in the parietal cortex, including the PPC
- characterized by its involvement in processing the spatial location of objects, motion perception, and guiding motor actions
ventral stream
- what pathway
- begins with V1 - end at interior temporal cortex
- form recog + obj rep + asso w/ storage of long-term memory
what is binocular vision?
visual field of both eyes overlap extrensively in the ‘binocular field’ - extensive regions of left visual field are seen by L&R eyes + vice versa
what is stereopsis?
depth is a key feautre in determining the shape of an object
an important cue for perception of depth is the diff between two eyes’ view of the world which is computed and reconciled by the brain beginning in v1
