The Eye Flashcards
What two features does the motion after (waterfall) effect demonstrate about the properties of the visual system?
Feature detection - specialised circuitry for detection of edges/colour/lines in orientations/movements/faces etc
Adaptation - brain mainly registers change; if feature = constant (even if moving), signals are reduced
What wavelengths are visible to the human eye?
400-700mn = visible light; lower = UV, higher = IR
What are the main functions of the eye?
Regulates amount of light entering - photopic (>30Cd/metre squared), mesopic (0.003-3), scotopic (<0.003); what appears 2x brighter in experience = 10x brighter in lumens
Creation of a sharp image on the retina - sensitivity vs acuity
Conversion/transduction of light-electrochemical signals
What is the iris and how does it work?
Stroma + pigmented epithelial cells
Stroma connected to sphincter muscle responsible for contraction and dilatation of pupil
Regulated by pupillary light reflex ie the amount of light entering the eye
Dark = relaxed = dilated = sensitivity > acuity
Bright = contracted = constricted = sensitivity < acuity
Pigmented layer = melanin blocks light passing through iris to retina
How does the trade off between acuity and sensitivity work?
Larger pupil = more light hitting retina = greater overlap in cells activated = blurring
Smaller pupil = less light hitting retina = smaller, discrete number of cells activated = less blurring
What is the cornea and how does it work?
Transparent covering in front of the lens; helps focus 75% of light
Is fixed (unlike lens)
What is the lens and how does it work?
Transparent, sits behind cornea
Held in place by suspensory ligaments = zonules
Shape is variable - alteration of focal distance = accommodation
How does accommodation work?
Close vision = contracted ciliary muscles = reduced zonula fibre tension = round lens = more refractive power; far sighted people = cant focus near = hyperopic vision
Distant vision = relaxed ciliary muscles = increased zonula fibre tension = flat lens = less refractive power; near sighted people = cant focus far away = mypoic vision
What is the retina and how does it work?
Layer of photoreceptive cells at the back of the eye that converts light-electrochemical signals
What is the macula and how does it work?
Central area of retina
High density of photoreceptive cells
Lost in age-related macular degeneration = a loss of central vision
What is the fovea?
Central area of maucla
Point of highest visual acuity - when focusing on something = foveal vision
No rods or blood vessels, only high density cones
50% of optic nerve fibres supplied by fovea
What is the optic blindspot?
Arises from the optic disc - point where all vessels/nerves exit the eye; no photoreceptors
Vision here is filled in through top down processing = completion
What are the sclera and choroid?
Sclera = white of the eye = connective tissue
Choroid = layer between retina and sclera; highly vascular - origin of the red eye from reflected light from camera flash
What is the anatomy of the retina?
(anterior-posterior) Retinal ganglion cells (amacrine cells) Bipolar cells (horizontal cells) Cones and Rods
Appears to be backwards (ie light sensing far away) as eyes would have to be massive to accommodate
What is the basic structure and function of photoreceptors (for rods and cones)?
Visual transduction through specialised cells - normal Mt/nuclei/synaptic ends
Photoreceptor disks at the terminal end of the cells contain the proteins responsible for phototransduction and are continuously replaced
Photopigment = retinal (vitamin A aldehyde) + opsins which tune which wavelengths are absorbed
Dark = rhodopsin inactive = Na/Ca channels open by cGMP = depolarised cell = continuous release of glutamate
Light = rhodopsin active = cGMP breakdown and Na/Ca channels close = hyperpolarised cell = reduction in glutamate release
What are the functions of rods and cones?
Rods - scotopic vision in low light; monochromatic vision; high convergence = lots of rods:one bipolar cell = sensitivity > acuity; peripheral vision
Cones - photopic vision in good lighting; three colours (RG>B); low convergence - 1:1 = sensitivity < acuity; central vision
How does colour vision work?
Colour requires 2+ photoreceptors to differentiate wavelength from intensity - it is impossible to know whether a single receptor is being activated by exposure to many photos which it is relatively insensitive to or few photons which it is highly sensitive to - through comparison at higher level processing, this can be deduced
How does colour blindness work?
8% men and 0.5% women have a deficiency
Due to a reduced sensitivity or absence of cones
Deutranopic vision = most common - green cone = shifted towards red sensitivity = R:G differentiation is hard
What is the function of bipolar cells?
Act as connections between photoreceptor and retinal ganglion cells
Allow for low level integration of data ie high/low convergence to regulate sensitivity vs acuity
What is the structure and function of retinal ganglion cells?
Are the bridge between sensory organ and CNS
Integration of information from cells within their receptive fields so they become responsible for detection of spots of contrasting light = edges
Two types - on centre/off surround and off centre/on surround - on centre = action potentials when stimulated; off centre = same stimulus reduces APs; they are evenly distributed through the retina
Process mediated by lateral inhibition
What is lateral inhibition?
A process mediated by horizontal cells that acts to enhance areas of contrast detected by photoreceptors - input received by a centre cone is transmitted by horizontal cells to surrounding cones, inhibiting them and so potentiating the effects of the central cone on the on-centre bipolar cell which then feeds to thee on centre RGC
The more the central photoreceptor fires, the more the horizontal cells inhibit its neighbours
The Mach band illusion demonstrates this in action; as does Hermann grid, scintillating grid and Koffka ring illusion
What are the fluid environments within the eye?
Anterior chamber - behind cornea in front of lens; contains aqueous humor produced by ciliary processes in posterior; replaced 12x/day; flows through pupil; drainage failure = glaucoma - increased intraocular pressure = restricts blood flow = retinal neuron damage
Posterior = vitreous humor, gelatinous, contains phagocytes - failures = floaters
What are cataracts?
Visual opacities in the lens associated with age
What is macular degeneration?
Damage of foveal vision
Age related; aetiology unknown
Wet - abnormal blood vessel growth under macular - leaks fluid = damage
Dry (majority) - slow decay of retinal pigmented epithelium leading to photoreceptor loss
What transmitters do on centre and off centre bipolar cells use?
On centre = mGluR6 (G-protein coupled metabotrophic glutamate receptor) = sign-inverting
Off centre = AMPA and kainate (inotrophic) = sign-conversing (polarisation of cell + photoreceptor = the same)
All depolarisation is graded - responding proportionally to luminance rather than as a binary output
