Compound Eye & Sight as a Sense Flashcards
Describe 3 constraints on eye design
Limited by radiation available
Availability of detection mechanisms (photopigments)
Animal dimensions:
eye size/position (resolution/FoV/BV)
brain size (speed/ability to process info)
Can humans use IR radiation instead of visible light?
Body emits thermal IR radiation (800nm-1mm) as heat at 37* (310K)
Thermal energy from each other impairs VA so no suitable
Can humans use UV radiation instead of visible light?
damages ocular tissue (delicate photoreceptors)
most absorbed by cornea (protects photoreceptors)
over-absorption of too many high energy photons damages corneal tissue (snow reflection ~ photokeratitis)
Can humans use radiowave radiation instead of visible light?
radio wavelength = 5 cm
with theoretical angle of resolution (30.75 arc secs) ~ pupil diameter would need to be 409.12m!
not suitable
Describe the eyespot design
single, small flat spot of photo-sensitive pigments
crude light detection (on or off)
very crude direction sensitivity (amoeba moves away from light in absolute opposite direction towards darkness)
Explain the eyepit/cup design
light sensitive cells arranged into depression
slight improved directional senses over eyespot
curved retina receives rays from multiple directions ~ overlapping images so results in very poor sensitivity/resolving power
Explain the design of the pinhole eye
single rays on particular retinal points form inverted image requiring more neural processing
simple, better direction discrimination
diffraction limits resolving power, FoV limited, faint image, poor vision in scotopic
Explain the design of the vesicular eye
epithelia cuticle grows over pinhole protects from FBs
irregular lens and fluid filled cavity surrounded by photosensitive cells (associated nerve fibres) in a cupped plane
Explain the design of the lens eye
Cornea (evolved from cuticle)
Small regular Lens
Fluid filled spaces (humours)
Photosensitive cells
Adaptable pupil (iris)
Great direction sensitivity
Sharper images (lens focus/iris adapted)
Complex images requires large neural processing to take relevant info
Why does neural processing take longer in humans?
latency of visual cortex’s neuronal response ~ 80ms
Explain the design and benefits of the compound eye design
multiple optical systems sample a VF focussing light from different points in space (typically convex)
collection of photosensitive detectors (each based on an eyespot at the base of a pit)
Explain compound design in an apposition eye
Diurnal insects
Combines multiple ommatidia orientated in different directions (no need for scanning whole field)
Explain the microstructure of the apposition eye (ommatidium)
Cornea, Lens (gradient n)
Rhabdom (transparent rod shapes structures enhance photo absorption) diverts light outwards to
surrounding Reticular cells (contains rhodopsin photopigments) ~ transmits signals to nerve fibre to CNS
Describe how a gradiented-index cone lens works
relative increase in angle of acceptance of light so ‘off axis’ rays brought into focus
What are the advantages and disadvantages of combining several ommatidia?
less need to scan FoV with head movements, upright image, motion detection from multiple feeds
individual cone lens feeds into 1 rhabdom limiting eye sensitivity (suitable for diurnal)
Explain the design of a superposition eye
multiple cone-lens array redirects light to given rhabdom array to improve sensitivity
air gap between
2 piece gradiented cone-lens twice the length of appositions ~ re-collimates light rays (focussed towards single rhabdom)
higher angle of acceptance of light over ommatidia face
Compare 3 benefits of superposition eyes compared to apposition
Multiple longer cone lenses provide:
larger angle of acceptance of light
enhanced sensitivity idea for nocturnal conditions
better motion detection
Which factors govern optimal ommatidia size/number?
size of head
assumption of hemisphere eye with given radius and diameter ~ calculate the optimal number
