5-6: The Visual System

Plato

Plato

early scientist (400 BCE) that believed that the eye sent out rays, which seized objects (extramission)

Theophrastus

300 BCE scientist who thought eye has internal fire from which the rays emanate (extramission)

Galen

200 CE scientist who thought optical pneuma was emitted from the eye, considered lens to be origin of vision, since cataracts obstruct vision (extramission)

extramission theory

our eyes shooting out energy causes us to see; 50% of Americans believe it & Plato, Theophrastus, and Galen

intromission theory

our eyes absorbing energy causes us to see, supported by al-Razi, Ibn al-Haytham, and Ibn Sina

al-Razi

900 CE scientist that noticed light levels controlled pupils, supported intromission theory

cornea

clear tissue at front of eye, provides 2/3 of eye’s optical power due to refraction

pupil

opening in the iris which expands (for more light in dim situations) and contracts to control the amount of light entering

lens

tissue that focuses incoming light on retina whose refractive power is affected by ciliary muscles; issues cause most vision issues (eg. myopia, hyperopia); thin for distant objects, thick for near

ciliary muscles

eye muscles that adjust the thickness of the lens

retina

light sensing neural tissue at the back of the eye, which signals via graded potentials, regulated by cGMP gated channels

fovea

centre of retina responsible for HD vision, w/ high [cones] and visual acuity

optic nerve

bundle from retinal ganglion cell axons; pathway where retinal signals get sent to rest of the brain

optic disk

blind spot in retina where it meets the optic nerve, no photoreceptors

myopia

images are focused in front of the retina, far images are blurry, affecting ~50% of people, treated w/ concave lenses

hyperopia

images are focused behind retina, close images are blurry, treated w/ convex lenses, includes presbyopia

presbyopia

age related hyperopia

astigmatism

condition arising from spherical aberrations of the eye, leading to different focal points for different parts of the visual field

retinal

molecule in photopigments which converts between cis and trans to active transducin and begin vision pathwat

transducin

g protein activated by conversion of 11-cis to 11-trans retinal, which activates PDE

phosphoesterdiase

compound activated by transducin, which hydrolyzes cGMP into GMP, which closes gGMP gated ion channels

achromatopsia

true color blindness due to a lack of cones affecting 1/30k people

off bipolar cell

ionotropic cell active in the dark, which is activated by photoreceptors and activates ganglion cells; stratify in deeper layers of IPL and connect to ON ganglion cells

on bipolar cell

cell w/ metabotropic glutamate receptors on dendrites, depolarized by photoreceptors in light, hyperpolarized in dark, activates ganglion cells indirectly through A2 amacrine cells

inner nuclear layer

location of bipolar cell soma

inner plexiform layer

location of bipolar cell axons

amacrine cell

retinal cell that connects on bipolar cells to retinal ganglion cells; can provide lateral inhibition to bipolar and ganglion cells

on center ganglion cell

retinal cell activated by off bipolar cells which are excited by light in the center, inhibited by light in the surround

off center ganglion cell

retinal cell inhibited by light in the center, excited by light in the surround

horizontal cell

retinal cell that provides negative feedback to photoreceptors through lateral inhibition

luminance

physical measurement of light intensity

brightness

sensation elicited by light intensity

color oppency

creating ganglia that prefer different colors, includes parasol (magnocellular), midget (parvocellular) and bistratified (koniocellular)

parasol

center surround retinal ganglion cells responsible for detecting luminance & motion w/ its large receptive field

midget

red/green retinal ganglion cell, w/ small receptive field and high acuity due to small receptive field

bistratified

blue/yellow retinal ganglion cell

optic chiasm

where ~60% of retinal ganglion cell axons cross brain hemisphere

nasal

portion of visual field closest to nose that crosses at the optic chiasm

temporal

portion of visual field closest to ear whose retinal ganglion axons stay ipsilateral

melanopsin

photopigment crucial for circadian rhythms, why circadian rhythm is sensitive to blue light

superior colliculus

brain region that coordinates head and eye movements to visual target

pupillary light reflex

both pupils respond to monocular visual stimulation; retinal ganglion cells → pretectum → EWN (midbrain) → oculomotor nerve → ciliary ganglion → constrictor muscles of the iris

saccadic eye movement

ballistic eye movement occuring when focusing

smooth pursuit eye movement

following stimuli w/ eye

optokinetic reflex

gaze stabilization reflex combining both saccadic and smooth pursuit eye movements

primary visual pathway

retinal ganglion → LGN (of thalamus) → V1

pyramidal cell

cell on visual cortex, superficial project to other cortical areas, deep project to subcortical targets, such as LGN and superior colliculus

orientation column

cells immediately above and below each other in visual cortex, which represent same location of visual field and exhibit ~ feature selective responses

orientation pinwheel

cluster of cells w/ similar orientations, repeated every ~1mm

stereopsis

sensation of depth arising from viewing objects w/ 2 eyes located in slightly different locations causing disparity; also controlled by far, near, and tuned zero cells in the visual cortex

strabismus

esotropia or exotropia; where the eyes do not properly align when looking at object; can cause amblyopia if untreated

esotropia

cross eye form of strabismus

exotropia

form of strabismus where the eyes deviate out

amblyopia

untreated strabismus causing the brain to stop processing inputs from one eye; can be correct by weak eye being forced to work in critical period

ventral stream

visual pathway (what) concerned w/ semantic nature of visual scene

dorsal stream

visual pathway (where) concerned w/ moving objects, involves MT

MT

region in dorsal stream involved for motion detection; neurons integrate input from neurons w/ smaller receptive fields to direct overall direction

aperture problem

small V1 receptive fields may give misleading info of movement direction

flash face distortion effect

brain forcibly computes far distances on face proportions when switched quickly, makes them look weird

cataract

clouding of the lens; cause of 50% of blindness and 1/3 of visual impairment; risk factors include diabetes, smoking, and prolonged sun exposure; can be removed w/ artificial lens

glaucoma

group of diseases causing optic nerve damage and vision loss due to raised interocular pressure; 2nd most common cause of blindness; high P causes fluid recycling to be messed up → too much fluid → P on optic nerve → ganglion cell damage → death → blindness

macular degeneration

leading cause of vision loss in people >55, predominantly affects photoreceptors in fovea and macula; loss of high acuity vision; forms include wet and dry

wet AMD

10% of macular degeneration cases due to leaky blood vessels, has treament

dry AMD

90% of macular degeneration cases, opposite of RP, slowly progressing, no good treatments

retinitis pigmentosa

heterogenous group of hereditary disorders causing peripheral retina photoreceptor damage

diabetic retinopathy

leading cause of blindness in people 20-64, affects 80% of diabetes patients who have had it for >20 years due to leaky blood vessels

blindsight

cortical blindness w/ functional eyes due to V1 damage; subconscious vision passing through superior colliculus or LGN can persist

hemispatial neglect

brain damage occurring in R parietal lobe, causing patients to only see R visual field, can see L visual field if pointed out

cerebral akinetopsia

motion blindness caused by MT damage in dorsal stream, where moving images appear as slowly refreshing static images

propsopagnosia

face blindness due to FFA damage in inferotemporal cortex

split brain

severed corpus callosum causing no interocular transfer

cell therapy

use of stem cells to grow and implant new cells to replace diseased or damaged cells

gene therapy

treatment correcting damaging or null mutations via delivery of copies of correct gene, include Luxturna

retinal prosthetic implant

treatment that attempts to restore sight via implantation of a chip that electrically stimulates retinal ganglion cells, w/ goggle camera that converts visual signal into electrical stimulation

optogenetic therapy

treamtnet that uses viruses to get cells to express light and tries to make remaining cells in blind retina light sensitive