Vision 1

How much of the electromagnetic spectrum is visible to humans?

only a narrow part - around 400-700nm

How is an image formed on the retina?

• parallel rays from the same distant point enter the eye

• lens converges these rays so that they intersect at the retina

• image formed upside down on the retina

What happens with myopia?

near sighted - light is focused in front of retina

What lens corrects myopia?

concave

What happens with hypermetropia?

farsighted - light focused behind the retina

What lens corrects hypermetropia?

convex

What is the fancy word for normal eye sight?

emmetropia

What is eye growth and refractive error early in life under the control of + explain?

Under visual control

• Hung et al 1995

  • if experiement with animals, fitting negative power spectacles creating a hyperopic defocus causes faster axial eye growth

  • and vice versa

  • defocus of the retinal image is key variable that regulates eye growth

What is astigmatism?

where there is different focus in different plans due to a rubgy ball shaped cornea

How can astigmatism be corrected?

with lenses that have a cyclindrical component in their surface curvatures

What is the accomodation reflex?

mechanism that allows the lens to focus on an object at different distances

What makes the lens rounder and more convex?

• increase in power of lens caused by contraction of ciliarly muscle

• this reduces tension in radial zonular ligaments allowing lens to relax to more convex state

What stimulates the accomodation reflex?

blurred image in visual cortex via the descending parasympathetic system

What is presbyopia?

• failure of accomodation with age resulting in the inability to focus on objects near the eyes

• 80% of people >45 in North America

• also experiences by monkeys

What causes presbyopia?

lens material becoming stiffer and less elastic with age

What is the role of photoreceptors?

turning light into something the neuro system can deal with - electrochemical energy

Why are photoreceptors found at the back of the retina if other cells in front scatter the light?

want to be as close to pigment epithelium

What does the pigment epithelium do?

supports retinal cells and absorbs scattered light

What does the choroid do?

provides vasculature to retina

What type of light do rods respond to?

dimmer light whereas cones have a higher light threshold

Where is the blindspot located on the retina?

at the optic disc

Where is the fovea located on the retina?

• at the centre of our visual field, where light is focused

• 1.5mm across

What is next to the fovea?

macula - centre of retina with best colour vision

What is the distribution of cells like across the retina?

• more rods than cones

• fovea

  • only cones

  • retinal nerve bodies shifted aside

• no photoreceptors in optic disc

What does having no rods in central foveola mean?

very dim lights can be seen in the dark better in our peripheral vision

rods can be isolated and their responses to weak light recorded in vitro

What is the smallest package of light rods can respond to and how do we know this?

Bayer, Lamb and Yau (1979)

• a single photon

• they recorded response oouter segment to dim light flasahes

• smallest response resulted from single photon of light

What is visual acuity?

measure of spatial resolution

What is the smallest gap that can be resolved?

2 microns which is the same as the width of a cone cell

What is the protein inside rods and cones called and what is it made of?

rhodopsin - retinal and opsin

What happens initially in phototransduction?

• photo-isomeric change of rhodopsin

• 11-cis-retinal changes to all-trans-retinal with light

• reverses with enzymes slower

What is dark current?

• when there is no light sodium channels are open

• when illuminated, photoreceptors hyperpolarise

What is the phototransduction signalling pathway?

• light causes isomeration of photopigment

• photopigment activates transducin (G protein)

• transducin activates a phophodiesterase (PDE)

• PDE reduces level of cyclic GMP

• reduced cGMP causes sodium channels to close

• reduced intracellular Na causes hyperpolarisation of the cell

What does the second messenger cascade cause?

• amplifcation of the response to light

• allows plasticity to become more or less sensitive

What is dark adaptation?

it takes about 20 minutes for sensitivty to reach maximal values affter lights turned off

• corresponds to time needed for rhodopsin to regeneraate

Since rhodopsin regeneration will take longer in rods as they are more sensitive to dim light what does this mean?

more likely to photobleach in bright lighting

What does opsin do?

determine the frequencies of light that are absorbed by rhodopsin

How many different types of cone are there?

3 - each sensitive to a different wavelength of light which is the basis of colour vision

What is the Purkinje shift?

• our spectral sensitivty curve shifts towards yellow on moving from dim (scotopic) light, where only rods active to bright (phototopic) light where cones are used

• at night more sensitive to blues and purples than in day

How many occular muscles do humans contain?

6 - which move the eye so that the image of interest is on the fovea

What is the name for eye movements which fixate scenes of interest onto the fovea?

saccades

What is age-related macular degeneration and why relevant?

• lead to blurred vision over time

• demonstrates how important fovea is