visual physiology and behavior

Question 1

What do photoreceptors detect

Answer 1

• Frequency – short wavelength = high freq
• Intensity – proportion to photon arrival rate and energy
• Polarisation – orientation of electric field component (if oscillations in single plane then linearly polarised)
• Direction of light

Question 2

How do eyes form images?

Answer 2

1) Very small aperture
a. Pinhole eye eg. Nautilus

2) Refractive lens
a. Terrestrial camera eye
b. Aquatic camera eye
c. Apposition compound eye
d. Superposition compound eye

3) Reflective mirror eg.

Evo convergence of eyes – constraints mean look similar even on another planet

Question 3

Physical constraint of eyes

Answer 3

Resolution: fineness you view environment
Sensitivity: sensitivity to the amount of light

Visual environment: Sea
- Only blue, 480nm wavelength light enters deep seas
- V deep meso/bathypelagic only light is bioluminescence

Solution: Tubular eyes
- > Increase resolution (long tube) and sensitvity (large) over narrow field so can spot silhouettes against downwelling light
- > no good for lateral bioluminescence so laterally directed accessory eyes w/ concave mirror for bioluminescence in spookfish and
rotatary tubular eyes in Barreleye fish

Resolution vs sensitivity trade off:

Sensitivity Improves with
- Increased angle of light on each photoreceptors
- increased aperture
- Absorption efficiency of photoreceptors

Resolution improves with
- Decreased angle of light achieved by smaller photoreceptors (Sampling resolution)
- increased aperture (optical resolution)

Question 4

Resolution vs sensitivity trade off solutions: superposition eyes -> insects

Answer 4

Nocturnal insects -> superposition eyes

• Photoreceptors get light from multiple facets
• Each facet gets light from different direction so get optical cross-talk between rhabdomeres
• High sensitivity
• Low resolution

Spherically constrained due to regular arrangement of ommatidial lattice –> no specialisation of different regions of eye

Question 5

Resolution vs sensitivity trade off solutions: apposition eyes -> insects

Answer 5

Diurnal insects -> apposition eyes

• Each rhabdomere gets light from own face
• adjacent ommatidia are optically isolated
• Low sensitivity due to small aperture
• High resolution – different areas of eye can specialise for different tasks

Not spherically constrained and get local acute zone of enlarged ommatidia

1) Dragonfly – dorsal stipe of high acuity to detect prey above
2) Blowfly – forward looking acute zone as fast fliers
3) Surface dwelling insect eg. pond skater – horizontal acuity stripe to look across surface

Question 6

Photo transduction

Answer 6

Occurs in rod and cone cells

1) Opsin is bound to inactive chromophoe molecule (most commonly retinal) -> make up a g- protein coupled receptors
2) Absorbtion of light causes cis retanol to transition to trans retanol causing a conformational change in opsin making it active.
3) triggers intracellular signalling cascade through G proteins resulting in gating of ion channels and hyperpolarsation.

slow but one activated receptor protein activates many G-proteins so amplification + persistence of response

Question 7

Colour vision

Answer 7

Colour quantified by comparing response of different photoreceptors w/ different spectral sensitivities

Opsins w/ different spectral sensitivities arose via multiple gene duplication events (long, medium and short wavelegnths)

Mammals lost several visual pigments in nocturnal phase of evo 200-150mya
- Most mammals = dichromats
- Aquatic = monochromats
- Some primates + humans = trichromats as extra gene duplication
- > Primate colour vision reflects their genetics, ecology, and evolutionary history.
- > e.g. Old World monkeys + apes that needed to detect fruit against foliage as is main diet

Question 8

Polarisation vision

Answer 8

moonlight and sunlight is origonally unpolarised and is polarised due to scaterring in atmosphere/ reflection

Allows detection of:
* Water bodies
* Axis sun / moon lies on (dictates direction for honeybee waggle dance or dung beetle rolling poo in straight lines under stars)

Mechanism:

• Each photoreceptor responds more to particular direction of polarisation depending on its position in the eye
• where light oscillates parallel to chromophores long axis = most photoisomerization
• Degree + direction of polarisation compared across different rhabdomeres by interneurons

Insect polarisation neurons located in dorsal rim.

Question 9

Motion vision

Answer 9

Insect optic lobe maintains retinotopic organisation meaning the layout of the visual field is preserved in the brain’s visual processing pathways/ architecture.

Insect exmaple

local motion encoding:
- Elementary motion detectors compare the outputs of adjacent photoreceptor cells with a slight delay, allowing comparison of visual inputs from slightly different points in time.
- The output of the EMDs is processed by sets of 4x T4 (singllaing brightness increments) and 4x T5 (signalling brightness decriments) cells
- Each set of T4 and T5 cells within each ommatidial unit encodes motion in one of the four cardinal directions

Wide field detection:
- The next layer of the motion vision system integrates local input from across the visual field.

Question 10

Overview

Answer 10

phsyical constrains
- environment (solution: tubular eyes)
- refinement vs sensitvity trade off (solution: superposition/ apposition eyes)

Types of vision:
- colour vision
- polarisation vision
- Motion vision