The Eye

Question 1

What two features does the motion after (waterfall) effect demonstrate about the properties of the visual system?

Answer 1

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

Question 2

What wavelengths are visible to the human eye?

Answer 2

400-700mn = visible light; lower = UV, higher = IR

Question 3

What are the main functions of the eye?

Answer 3

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

Question 4

What is the iris and how does it work?

Answer 4

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

Question 5

How does the trade off between acuity and sensitivity work?

Answer 5

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

Question 6

What is the cornea and how does it work?

Answer 6

Transparent covering in front of the lens; helps focus 75% of light

Is fixed (unlike lens)

Question 7

What is the lens and how does it work?

Answer 7

Transparent, sits behind cornea

Held in place by suspensory ligaments = zonules

Shape is variable - alteration of focal distance = accommodation

Question 8

How does accommodation work?

Answer 8

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

Question 9

What is the retina and how does it work?

Answer 9

Layer of photoreceptive cells at the back of the eye that converts light-electrochemical signals

Question 10

What is the macula and how does it work?

Answer 10

Central area of retina

High density of photoreceptive cells

Lost in age-related macular degeneration = a loss of central vision

Question 11

What is the fovea?

Answer 11

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

Question 12

What is the optic blindspot?

Answer 12

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

Question 13

What are the sclera and choroid?

Answer 13

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

Question 14

What is the anatomy of the retina?

Answer 14

(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

Question 15

What is the basic structure and function of photoreceptors (for rods and cones)?

Answer 15

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

Question 16

What are the functions of rods and cones?

Answer 16

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

Question 17

How does colour vision work?

Answer 17

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

Question 18

How does colour blindness work?

Answer 18

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

Question 19

What is the function of bipolar cells?

Answer 19

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

Question 20

What is the structure and function of retinal ganglion cells?

Answer 20

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

Question 21

What is lateral inhibition?

Answer 21

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

Question 22

What are the fluid environments within the eye?

Answer 22

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

Question 23

What are cataracts?

Answer 23

Visual opacities in the lens associated with age

Question 24

What is macular degeneration?

Answer 24

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

Question 25

What transmitters do on centre and off centre bipolar cells use?

Answer 25

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