Lecture #9

L&N ch 4 : Grand challenges in aquatic vision

2/21/13

Topics for today

• You are now experts in the physical principles that underlie visual system design

• We’re going to present you with several challenges and see what sorts of solutions you can come up with

• We’ll discuss some examples and key points as we go

Challenge 1

• How do you design an aquatic eye when the index of refraction of tissue is nearly the same as water?

Eye designs

Fig 1.9

a pitc lensg single chamber mirror

b compounde appositionf superpositionh compound mirror

Compound

Single chamber

Solution 1a) Pinhole eye

• Acceptance angle is about 10°

• Limits amount of light coming in

• Can form an image

Abolone

Leonardo Da Vinci - camera obscura

• Image passes through pinhole

• Keeps all in focusInverts the image

Nautilus• Largest pinhole eye, 1

cm diameter• Pinhole size can vary

0.4 to 2.8 mm• Pinhole is actually oval

Build model of Nautilus eye to test resolution

• Image falling on Nautilus retina shows blurring as pupil gets larger

1) Incident on eye2) 1 x 0.4 mm pupil3) 2 x 1 mm4) 2.8 x 1.7 mm

1 2

3 4

Acceptance angle is 2.3° for smallest pupil

Nautilus vision

• Low light collection efficiency of Nautilus eye makes the world a darker place

• 400 times dimmer than fish eye

Optomotor response to measure visual angular resolution

• Rotate grating• Nautilus swims to

keep image of world constant

• Decrease grating - determine when nautilus can’t resolve it - stops swimming

Angles < 11°

Solution 1b) Add a lens - Molluscs

• Helix has a blob of jellyIncreases light collection – originally no image!But eventually evolved to focus light onto retina

Single chambered aquatic eye – if life evolved on land, the lens might not have been invented!

Molluscs

Vertebrates

What kind of lens do you want?

• Lens focal length determined by curvature of lens surface

Shortest radius of curvature is a sphere

• Sphere defining the radius of curvature is the spherical lens itself

r

Shortest possible focal length

Lens focal length

• Lens maker’s equation

Lens focal length• If n2=1.53 (lens crystallin) and n1=1.34 (water)

Focal length is 4*radius or 2*lens diameter

How does focal length depend on lens material, n2

Many fish and cephalopods have lens focal length of 2.5r

This is Matthiessen’s ratio

1c) Flat cornea – no contribution to focusing

1d) Compound eye with individual lens for each receptor set

1e) Collect light with mirror

Challenge 2

• Water limits the transmission of light

2a) Spectral properties of

water shape the evolution of life

Animals came out of the water onto land with evolution of amphibians about 350 Mya

Vertebrate eyes evolved >500 MYa

Aquatic eye sets the course of the vertebrate eye

Several families of opsin genes

LWS

SWS1

SWS2

RH2

RH1 - rods

cones

Light spectrum into clear shallow water

Visual pigment diversity in vertebrates matches light transmitted by water

2b) Deep sea tubular eyesDeep sea fish - Scopelarchus

• No light from belowOnly look up and to side

• Tubular eyesLarge binocular overlap

Fig 4.9

Fish of the deep…

http://www.youtube.com/watch?v=RM9o4VnfHJU

2c) Compound eyes

• Can make large lenses to collect lots of light

• Crab – surface vs deep sea isopod

2d) Phosphorescence and detection Malacosteus niger

• Deep sea dragonfish

• Red emitting photophorePrivate communicationOr Head light

Malacosteus niger• Deep sea dragonfish

• To detect red light put chlorophyll into photoreceptors• Chlorophyll absorbs

red light and transfers energy to visual pigment

Challenge #3

• Eyes are complicated to make. If you can only afford to have one receptor for a lens, how can you learn about your environment?

Dual element lens – scan receptor across image

• Copepod SaphirinaLarge lens on topSmall lens above retina with 6-7 receptors

• Acceptance angle about 3° • Scan small lens and retina about 15° at 0.5-10 Hz

Fig 4.11

Sea urchin tube feet

• Early photoreceptors were single detectors which collect light in many directions

Sea urchin tube foot is a photoreceptor

3c) Compound eye: Take lots of single receptors and put them together

Challenge 4

• Eyes have to trade off sensitivity and resolution. Is there another way to get around this trade off?

Lens eye - cnidarians

Neither lens eye focuses an image

Challenge 5

• A single chambered eye has high resolution. Is it possible to evolve this eye more than once? If two organisms had a single chambered eye, how would you know if they were from a common ancestor?

Fish and cephalopod eyes

Fig 4.6

SimilaritiesCephalopods and fish

Eye Single chamber, camera like

Lens Spherical, f=2.5rGraded index

Pupil Variable iris

Muscles 6 muscles move eye in 3 axesOther muscles focus lens, and adjust irisSaccades - rapid eye movements

Adaptation for aquatic lens

Fig 4.3

Homogeneous lens

Actual fish lens does not show aberration

Spherical homogeneous lens - focuses too much on edges: spherical aberration

Graded index lens - refractive index decreases at edge so longer focal length on edges

Fig 4.3

Fish and cephalopod eyes

Fig 4.6

DifferencesCephalopods Fish

Receptors Microvillar Ciliary

Visual pigments

One Several

Receptors Point to front Point to back

Neural processing

In brain In retina

Lens Two halves separated by living cells

Single structure surrounded by living cells

Giant squid eyes

http://www.youtube.com/watch?v=JSBDoCoJTZg

Squid lens is in two halves

Why do squid have such large eyes?

Detect predators which induce phosphorescence – low photon vision

Summary

• Lens eyes evolved from pigmented eyes multiple timesEvidence for convergent evolution

• Most aquatic lenses have graded index of refraction

• Retina can be specialized• Can have multi-element lenses