vision

Question 1

what is light?

Answer 1

stream of photons

Question 2

what are photon?

Answer 2

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

Question 3

how can we see?

Answer 3

eyes must collect photons that bounce off objects and try ot work out prop of objs from those reflected objs

Question 4

how does the eye project orderly images to the retina at the back of the eye?

Answer 4

the eye exploits the fact that photons travel in rays

Question 5

discuss the range of wavelengths of photons visible to the human eye

Answer 5

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

Question 6

What is the approximate distribution of rod and cone cells in the retina, and how do their functions differ in terms of vision?

Answer 6

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

Question 7

what is iodopsin

Answer 7

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

Question 8

what is the composition of rods and cones (chem)

Answer 8

high intracellular K+, high extracellular Na+ and cell membrane that is polarized due to K+ leakage

Question 9

why do photoreceptorshave Na+ leakage channels in their membranes which are open in the dark

Answer 9

so that Na+ can enter the cell (the ‘dark current’), causing photoreceptors to be depolarized, and their synpases to be active, releasing glutamate

Question 10

how is the amount of darkness measured

Answer 10

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

Question 11

what is the optic nerve

Answer 11

it carries info from eye to brain

Question 12

why does the neural circuitry of the retina have to compress info

Answer 12

because the optic nerve only has 1.3M nerve fibers but there are 10M cones + 120M rods in retina

Question 13

how is info compression achieved

Answer 13

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)

Question 14

what is centre-surround receptive field

Answer 14

allows neurons to detect contrast and edges in visual stimuli

Question 15

what is color-opponent receptive field

Answer 15

make cells sensitive to colour (“chromatic”) contrast rather than brightness (“luminance”) contrast.

Question 16

which flavors do RGC come in?

Answer 16

M-cells
P-cells
non-M non-P cells

Question 17

what are M-cells

Answer 17

• 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.

Question 18

what are P-cells

Answer 18

• 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

Question 19

what are non-M non-P cells

Answer 19

• project to the koniocellular layers of the LGN
• often yellow-blue sensitive
• project into the parts of the brain most interested in colour

Question 20

how does retina see?

Answer 20

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

Question 21

describe the visual processing pathway

Answer 21

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

Question 22

what is LGN

Answer 22

relay center in the thalamus for visual pathway
axons vip target

Question 23

what happens to axons in LGN

Answer 23

axons of GC terminating in LGN form synapses with other neurons and X prog further

Question 24

what happens to neurons that received visual info in LGN

Answer 24

neurons that have received visual info from GC radiate out of thalamus and go directly to VI

Question 25

why are inputs from left and right eye segregated and terminated in different layers of LGN

Answer 25

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

Question 26

what is retinotopy

Answer 26

the mapping of the visual field onto the retina and the subsequent visual processing areas in the brain

Question 27

describe retinotopic mapping

Answer 27

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)

Question 28

present functional features of v1

Answer 28

1. surface: organized in retinotopic map of visual field
2. dark and light stripes: surface view of left & right ocular dom columns
3. orient columns: contains cells with similar selectivity for orient of stimulus
4. color blobs: represent color recog module of functional org

Question 29

what is dorsal stream?

Answer 29

• 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

Question 30

ventral stream

Answer 30

• what pathway
• begins with V1 - end at interior temporal cortex
• form recog + obj rep + asso w/ storage of long-term memory

Question 31

what is binocular vision?

Answer 31

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

Question 32

what is stereopsis?

Answer 32

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