215 Notes

• Oil droplets

Many animals have in their cones oil droplets. In birds and diurnal reptiles oil droplets are coloured. Rods never have oil droplets Coloured oil droplets improve discrimination of colour, reduce quantum catch, reduce the overlap between receptor sensitivities Colour oil droplets were lost, when animals were exposed to nocturnal lifestyle Oil droplets were when animals were exposed to extremely dim conditions

• Explain how the size of the eye is related to its resolution

- Camera-type eyes are small, which means lens have small radius of curvature (so is obligatorily curved) but they overcome the problem of spherical aberration by having graded lenses, hence achieving greater resolution. - E.g. fish eyes have f=2.5 lens radii - Lens size also limit pupil size (pupil cannot be larger than lens); small pupil limit resolution by diffraction, large pupil limit resolution by spherical aberration. - Pupil size matches resolution set by geometric optics of photoreceptor size (which limit resolution by the max sampling frequency)

• Relate the sampling frequency to the size of cones and size of the eye

- Cone diameter is used to calculate the focal length needed to achieve particular resolutions

• Explain how birds of prey achieve high resolution with small eyes

- Despite having relatively small eyes, a hawk is able to resolve 160c/degree while humans is able to resolve only 60c/degree. - A hawk has a much deeper fovea. This foveal pit provides an additional refractive surface and acts as a negative lens. This additional negative lens combined with the actual lens aid in creating a Galilean telescopic system within the hawk's eye. - With this telescopic system in place, the hawk is able to achieve high magnification and greater visual field at the centre of the visual field and is able to achieve a greater effective focal distance without having large eyes. - However this is reduced in the periphery - The retina is also more densely coated with cones so enabling them to resolve fine details

• Relate Stiles-Crawford effect and sensitivity of a camera type eye

- First kind Eye is the most sensitive to rays falling on the centre of pupil The intensity of light reaching pupil is less than that predicted by pupil size Overestimates sensitivity for large pupil size - Second kind Colour depends on the eccentricity Explanation: Angular sensitivities depends on the wavelength of light

• Vertebrate visual pigments

- LWS - reds sensitive visual pigment in birds, all mammalian L pigments belong to this group - RH1 - rods - RH2 - green cone of birds, reptiles, fish - SWS2 - blue cones birds, reptiles, fish - SWSI - two subtypes UV or violet sensitive - Early vertebrates had 4 cone visual pigments

• Explain why many nocturnal insects have superposition eyes. Draw a diagram of image formation in superposition eyes

- Light is collected by many facets and all the light is focused through a clear zone (not present in apposition eye) beneath the cornea to a point on the rhabdom layer - As a result, the superposition eye produces a single deep lying erect image in the vicinity of the retina unlike the multiple inverted image seen in apposition eyes. - Each of the cells in the rhabdom are not fused and they are arranged in a way that light from one direction is collected by the cells in the neighbouring ommatidium and the signal is then neutrally summed. - Superposition eyes are more sensitive than apposition eyes - Tend to have larger aperture (~10 facet wide) which means that it is (100 times) more sensitive than apposition eye to low light condition (suited for nocturnal conditions)

• Relate sampling frequency to the width of the point spread function

- Matched sampling: the width of PSF = 2x distance between sampling points. This is valid for any PSF.

• Origin of trichromacy in primates

- Primates trichromacy evolved as an adaptation to foraging on fruits - Primates evolved trichromacy by duplication of L-pigment gene (LWS gene)

• Describe the main stages of evolution of primate vision

- Primates trichromacy evolved as an adaptation to foraging on fruits - Primates evolved trichromacy by duplication of L-pigment gene (LWS gene) - Primate colour vision is a specific adaptation to finding yellow-orange on the background of green leaves

• Relate sensitivity of apposition eyes to the size of the facet

- Resolution is defined by interommatidal (phi) and acceptance angles (theta). Theta is determined by the airy disc diffraction pattern of the facet and by the geometrical angular width of the rhabdom - Resolution and sensitivity inversely proportional

• Explain how diffraction limits the resolution of the eye

- Resolution is limited by the size of receptors and optics of the eye - Airy disc is a diffraction pattern of a circular aperture (pupil). Large airy disk corresponds to a small pupil. The image cannot be larger than the size of the airy disc. The airy disc size is limited by the width of the aperture.

• Explain what does undersampling, matched sampling and oversampling mean

- Shannon sampling theorem - If a function f(x) contains no frequencies higher than v, it is completely determined by giving its ordinates at a series of points spaced 1/2v apart. Matched sampling: Optics does not transmit frequency higher than v. Image is sampled by pixels spaced 1/2v apart Undersampling: Optics does not transmit frequency higher than v. Image is sampled by pixels spaced MORE than 1/2v apart. Aliasing = error from undersampling. Oversampling: Optics does not transmit frequency higher than v. Image is sampled by pixels spaced LESS than 1/2v apart.

c) How is eye size related to the radius of the lens in aquatic animals?

Focal length (relates to eye length and size) = 2.5r (r = radius of lens) is designed such that fish do not have spherical aberration even though their focal length is so small.

a) Explain why a good resolution of a pinhole eye requires a small pinhole?

Having a smaller hole where light enters causes the more directional light/paraxial rays to reach the photoreceptors, and allows less marginal rays to enter that would contribute to spherical aberrations, affecting the resolution of the image as rays would not be transformed to a single point on the retina/other light sensitive media. NOTE: small pinhole results in good resolution, however, amount of light entering eye is less and thus, sensitivity is compromised.

2) Relate angular sensitivity of photoreceptors to Stiles Crawford effect

• The Stiles-Crawford is a property of the human eye that refers to the directional sensitivity of the cone photoreceptors. Cones in the fovea respond more than cones in the periphery despite the same amount of light received by both. Perceived brightness is greater in the centre than periphery. • Resolution is limited by size of photoreceptors - we want small photoreceptors. • Sensitivity is maximal for the rays passing through the centre of the pupil. • Colour depends on eccentricity. Angular sensitivity depends on the wavelength of light/colour

b) Why do camera type eyes with a lens have larger apertures than pinhole eyes of the same resolution?

The lens will focus the light to a point onto the retina, so having a larger aperture allows more light to enter the eye, which allows good sensitivity while also having good resolution.

4. Describe the main stages of evolution of vertebrate eyes

a) Bilateral ancestors: both ciliary and microvilli photoreceptors evolve, protostomes separate from our line (deuterostome) b) Protochordates: ciliary photoreceptors and ciliary opsins keep developing, becoming those similar to ascidian larvae. cephalochordates(lancelets) and tunicates(sea squirts) separate from our line c) Ancestral craniates: now there is well-organised outer-segment membranes. Ciliary photoreceptors make synaptic contact to neurons that might have evolved from rhabdomeric photoreceptors. d) Lamprey-like ancestors: highly-ordered disc membranes evolve. Cones develop, optics evolve (lens, accomodation, extraocular muscle). Eyes become similar to those of extant lampreys e) Jawed vertebrates: rods evolve, highly contractile iris evolve, intrinsic muscles develop to permit accommodation of lens, have photopic and scotopic pathways, eyes resemble those of extant fish

5) Explain why tapetum improves sensitivity

• A tapetum is like a mirror at the back of the eye. It is behind the retina. This allows any unabsorbed light from the retina to reflect back to give another chance of being absorbed. The light reflected may not go to the same photoreceptor and can decrease spatial resolution. • In dark we want tapetum (reflect to increase brightness when needed), in night we want RPE (absorb excess so not too bright)

1. Explain the difference between compound and camera type eyes and give examples of animals having different types of eyes

• Camera - Vertebrates e.g. humans - Each eye has one lens - Light is absorbed in the retina • Compound - Insects - non-vertebrates - Compound eyes formed by ommatidia - Each ommatidia acts as a single light guide. It contains one lens and several photoreceptor cells - Light is absorbed in rhabdoms - Have many lenses

4) Exotic camera type eyes: copepod Pontella and scallop

• Copepod Pontella - Parabolic lens arranged to have perfect focusing • Scallop eye: - Many small eyes with high power optics and short focal lengths - Mirror behind retina, focus image from mirror behind retina

6) Explain how the two layers of the outer segments help a deep sea fish improve its sensitivity

• More light is absorbed because the light comes through quite a thick layer of visual pigment. More light is absorbed.

3) Describe differences between diurnal and nocturnal eyes

• Nocturnal animals have the biggest lenses and diurnal animals have the smallest. Animals active during the night and day have intermediate size lenses. • Pupil size cannot be larger than lens size

3) Amphibian eyes & Graded refractive index lens

• Not fish - not always graded refractive index. Have 2 refractive indices. Single chambered eye, where the image is produced by a concave mirror. Image can be formed in a very small eye but contrast is sacrificed. • Homogenous lens f = 3.4r • Matthiessen's ratio for fish and octopus f = 2.5r • Refractive index different in centre and periphery. Fix abberrations. • Has smaller focal distance in the lens than lenses without refractive index gradient. This is good because you can have smaller eyes with the same good high resolution.

1) Image formation in camera type eyes - Pinhole eye in Nautillus

• Patella (Primitive eye, some directional sensitivity) -> Halitosis (Inside filled with water)-> Nautilus (Developed pinhole eye) • Good pin hole eye must be big but have a small pinhole

4) Find the physical limit for the sensitivity of a camera type eye

• Sensitivity is proportional to the number of photons absorbed by photoreceptors • Photoreceptors more responsive in the fovea. • Physical limit is size of photoreceptors (diameter) • Sensitivity can be improved by widening the aperture

2) Describe the difference between terrestrial and aquatic camera type eyes

• Terrestrial: - Focuses in air - Cannot focus in water. Not enough power of the cornea in the water. Focal point is behind retina. - Main refraction due to cornea • Aquatic: - Focuses in water - Main refraction due to lens

3. Explain why do we think that eyes have evolved independently several times

• There are still simple eyed animals alive today that have not evolved further than just photoreceptors on the surface • We have compound and camera type eyes - these eyes could not have evolved into each other so they must have evolved independently. • Each PR looks through the same aperture in different directions • Cannot develop camera type eyes from compound eyes and vice versa because in between there is a system where you have no spatial vision aka flat surface • We have 2 optical types of eyes that each evolved once Camera cillary Camera rhabdomeric Compound ciliary And compund rhabdom

Describe the difference between apposition and superposition eyes.

• Two types of insect eyes: compound or simple • Compound eyes can be broken into apposition eyes or superposition eyes • Each compound eyes is made up of many individual ommatidia • Each ommatidia has a hexagonal face known as facet Aposition Eyes: • Typically found in diurnal animals • Image formed by each lens of the facet in an apposition eye is inverted - since there are many facets- we see multi-inverted images • image on each facet is focused to the tip of a rhabdom and is trapped by total internal reflection within this crystalline structure • Rhabdom is made up of eight photoreceptor cells that are optically fused • Each rhabdom collects light on its own and project it to the retina • There is no overlap with neighbouring rhabdoms • The image that is perceived by the brain is an average of all the multi-inverted images on the retina projected by each rhabdom • Low resolution as there is more diffraction due to the large number of small apertures • Low sensitivity as each rhabdom has a small aperture. Small apertures only allow a little light into the rhabdom.

Describe the difference between apposition and superposition eyes. (Superposition)

• Typically seen in nocturnal animals • Light is collected by many facets and all the light is focused through a clear zone (not present in apposition eye) beneath the cornea to a point on the rhabdom layer • As a result, the superposition eye produces a single deep lying erect image in the vicinity of the retina unlike the multiple inverted image seen in apposition eyes. • Each of the cells in the rhabdom are not fused and they are arranged in a way that light from one direction is collected by the cells in the neighbouring ommatidium and the signal is then neutrally summed. • Superposition eyes are more sensitive than apposition eyes • Tend to have larger aperture (~10 facet wide) which means that it is (100 times) more sensitive than apposition eye to low light condition (suited for nocturnal conditions)

1) Describe how the acceptance angle of a photoreceptor depend on photoreceptor diameter and pupil diameter

• We want small photoreceptors for better resolution, but photoreceptor size are limited by their diameter, the geometric optics acceptable angle will be limited by the rays coming through the nodal point.

• Describe the main stages of evolution of vertebrate vision

- Stage 1: evolution of cone photoreceptors - Stage 2 : First vertebrates had tetrachromatic vision. First terrestrial vertebrates had 4 visual pigments + coloured oil droplets. - Stage 3: Coloured oil droplets and visual pigments were lost by many animals, when they were exposed to dim light conditions. Once coloured oil droplets and/or visual pigments are lost they cannot be regained. Oil pigments initially became colourless and then disappeared entirely. SWS1, SWS2, RH2 genes lost(depending on species) - Stage 4: Primates evolved trichromacy from dichromacy by duplication of LWS gene. - Stage 5: arrangement of the photopic vision pathways allowing colours to be distinguished neurally

• Explain how the sensitivity and resolution depend on spatial and temporal summation

- Summation means that we have many photoreceptors per ganglion cell, this means we have a large receptive field area. Sensitivity is proportional to the number of photos absorbed by photoreceptors, and hence it increases by increasing the area (of summation), it is can be increased my increasing the time, more time means more photons absorbed. - Resolution is inversely proportional to the time and hence it's a trade-off between sensitivity and resolution. - Resolution also is inversely proportional to the receptive field area.

• Coevolution

- The influence of closely associated species on each other in their evolution. Primates trichromacy evolved as an adaptation to foraging on fruits Non-primate mammals are dichromats, because they do not eat colourful food Birds have tetrachromatic vision, because they have colourful plumage and eat colourful fruit Bees have colour vision, because they forage on colourful flowers

d) Relate the upper limit of pupil diameter to the size of aquatic camera type eye.

Aquatic animals have graded RI lens and as long as pupil is smaller than lens, they can have relatively small eyes (f=2.5r). So they are able to have enlarged pupils and still see clearly, without spherical aberration limiting due to graded RI lens. But pupil still has to be smaller than lens.

e) Why do amphibian eyes typically have flat cornea?

So they are able to see underwater without resolution being compromised. Amphibians live on land and water and have a flat cornea, thus, lower curvature (which is essentially a 'fixed' curvature) thus, lower power so that light is not mainly refracted at the air/water-corneal interface, so it doesn't matter what medium they are in; they can still see as light refraction is mainly at their spherical lens, which they can accommodate in air or water accordingly. Thus, resolution is not compromised. NOTE: humans (terrestrial) have a very curved cornea, and so majority of the refraction of light occurs at the air-cornea interface. However if we go underwater then the difference in refractive index of water and air will affect the refraction and the light rays will not bend as much into the eye, causing an image to be formed behind the eye -> limits our resolution

Describe the main stages of evolution of animal eyes.

Stage 1 =rhabdomeric and ciliary photoreceptors evolved Stage 2 =ciliary PRs and opsins continue to evolve Stage 3 =ciliary PRs develop well organised outer segment membranes and make connections with neurons that may have been descendents of rhabdomeric PRs Stage 4 =development of optics, iris, cones, highly ordered disc membrane and extraocular muscle Stage 5 =development of rods, highly contractile iris, intraocular muscles for accommodation, retina containing both rods and cones, differentiation of photopic and scotopic pathways, colour vision

2. Explain the difference between rhabdomeric and ciliary photoreceptors and relate them to two lineages of animals (protostomes and deuterostomes)

Talk about signalling, microvilli vs cilia, opsins, gastrulation. *Rhabdomeric photoreceptors - Gt mediated signalling cascade - depolarisation of photoreceptor membrane - Rhabdomeric opsins - Use microvilli Hair-like structures for absorption and secretion on epithelial surface. P Primary component actin filaments - Protostomes Mouth first. Site of gastrulation initiation (blastopore) -> mouth Anthropods, Molluscs, Annelids • Ciliary photoreceptors - Gt mediated cascade - hyperpolarisation - Ciliary opsins - Have visual pigment in a modified calcium - Cilia contains tubulin. Motile in trachea, non-motile in olfactory neurones. - Deuterostomes Mouth second Blastopore -> Anus Chordates, Echinoderms, Hemichordates