Vision

Different kinds of eyes

  • Pupils

    • Can be round and expanded

      • e.g. humans, birds

      • 180 degree visual field with mostly overlap/depth perception

    • Narrow —> round

      • e.g. cats, dogs

      • Better at adjusting their size

      • Broader visual field with more narrow overlap

    • Horizontal

      • e.g. horses, goats, sheep

      • Very broad visual field (almost 360 degrees)

        • Horses still have 65-80 degree frontal overlap, just like canids.

      • Additional “white fingers” that stick out/forwards of the eye to block sunlight (the shade from this can be visible)

    • “Dots”

      • Very small dots turning into covering the entire iris (basically)

      • e.g. gecko, tarsier


Mammal eye

  • Cornea

  • Lens

  • Iris

  • Ciliary muscle

  • Zonule


How does the mammalian eye work?

Ciliary muscle is a “circle” around the iris, “attached” through the zonule.

  • When the ciliary muscle contracts, the zonule become relaxed and the lens becomes rounded to focus on close objects

  • When the ciliary muscle relaxes, the zonule taut and flattens the lens to focus on more distant objects


Fish eye

  • Spherical Gradient Lens

    • (Lens doesn’t change in shape ?)

  • (Retractor/Protractor Lens in some species)


Reptile eye

  • Bony ossicle

  • Brucke’s muscle


Bird eye

  • Bony ossicle

  • Brucke’s muscle

  • Crampton’s muscle


How do reptile/avian eyes work?

Active changing of the lens, actively pressing to change the shape

In birds, you have the additional Crampton’s muscle to change the size of the cornea

  • Diving birds also use it as a surface for water (?)


Amphibian eye

  • Protractor lentis


How does light reach the retina?

Light must go through all layers of cells before reaching the photoreceptors

  • Light might “change”/reflect throughout this journey


Cones vs rods

Cones see colours and are more “angular” in shape ( > )

Rods are bigger and more light sensitive, have more pigment, and are rod shaped ( ニI )


Explain eyesight (?)

Light must go through all layers of cells before reaching the photoreceptors

  • Light might “change”/reflect throughout this journey

When light is absorbed, there is a hyperpolarization

  • (not depolarisation as in other processes)

  • In the relaxed state, you have a lot of neurotransmitters. When light is absorbed, you get less neurotransmitters.

  • “It is only the outer segment that is light sensitive, A larger outer segment can absorb more light


Ganglion cells

Ganglion cells are the last step before the signals are sent to the brain.

  • When densely packed, there are less photoreceptors

  • = think of them as pixels. When they are smaller (and more) we can see finer details

  • Packed areas of ganglion cells is called the fovea


Fovea

Where the ganglion cells are the most packed

  • in humans, this area only consists of cones (not rods)

  • Horses, wolves, hares etc have rods as well in their fovea

  • Most birds have 2 fovea in each eye (other species only have 1)


Correlation between domestication and vision in dogs ?

  • Dogs with more “wolf-shaped” nose have a broader area of ganglion cells

  • In pugs (and other dogs with flatter noses), they have a very small but very “good”/Intense? spot — just like humans    

    • This spot is called “area centralis”

  • Similar pattern in horses


Spatial resolution

  • Measure of how closely lines can be resolved in an image

    • (e.g. black-white stripes. One black-white is one cycle. Measured as “cycles per degree (at a certain distance)”)

      • dvs how many degrees could one’s eye detect?

  • Is sacrificed in dim light

    • Spatial resolution becomes worse in darker environments


Temporal resolution

  • “shutter time”

  • Photoreceptors can only code information up to a certain temporal frequency

  • Cones are generally better than rods to discriminate fast movements.

  • Imagine a fan:

    • With short shutter time, you can see each individual blade

    • With long shutter time, you see it all as a blur

  • Sacrificed in dim light


Explain how color vision works

  • Colour requires two or more cone types

  • The reflected light from a surface enters the cones and the brain considers which type of cone absorbed the most amount of light, compare them, and then the brain makes us perceive a certain colour based on the cones that absorbed the light.

  • An absorbed photon has no “identity”, it’s just what cone absorbed it

    • Colour is just a hallucination


Humans are trichromats, bees too (but they see UV light instead of red)

Blue-tits have four (ancestral cone photopigments)

Stomatopods have 12 (but they don’t have a higher resolution to actually experience all of this at once, it is thought that they use them for different things)


Ancestral cone photopigments

Most animals have the four ancestral cone photopigments

As mammals, we lost two of these four due to the nocturnal history from the dinosaur ages and whatnot

  • Primates have created a new third one 30 million years ago (unrelated to the four ancestral ones) to experience red colour


Neutral point

Dichromatic visioned mammals have a so-called “neutral point” where they cannot discriminate grey shades from a green-blue shade at 480nm.



Ways to improve night vision in dim light

  • Enlarge pupils

  • Enlarge eyes

  • Short focal length

    • Shorter distance = less “disturbance” —> clearer image

  • Summation in space and time

    • Can extend visual range down to intensities 100 000 dimmer than provided by optics itself


Discuss colour vision in dim lights

Horses, despite having large eyes + tapetum lucidum, they cannot distinguish colours in darkness due to their vision being mostly based on rods.

Nocturnal helmet geckos can see colours in dim light when we can’t.

Several bees and hawkmoths can see colours at night.