vision Biology

8/11/2019 vision Biology

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Exam 3 on Vision: Thursday, April 17

Resources that are or will be available on Oncourse:

Focusing worksheet and answer sheet

Last semesters vision exam and answer key

Vision Study Guide

Question & Answer Review Sess ion:probably

Wednesday night

Assigned readings: Urban Eyes, Ch. 8 of text and

What Birds See


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VISION IN OTHER

VERTEBRATES

ASSIGNED READING:

What Birds See by Timothy H. Goldsmith


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Some Animals Have Foveas, Others

Have Visual Streaks:

darker gray = higher photoreceptor density

Retina with Fovea Retina with Visual Streak

Which arrangement a species has depends on its l i festyle

(such as its habitat & whether its most often predator or prey)


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Fovea has extremely high density of cones

Animal constantly moves eyes & head to directimages onto foveas

Typical of animals that live within trees or thick

forests (with no visible horizon), and/or animals

whose lifestyles require excellent visual acui ty

Visual systems of humans, birds, &

various other animals are specialized to

view sing le po ints using a fovea:


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In other species, the visual system is

set up to view the whole horizon:

Highest density of cones in horizontalregion called the visual streak

Found in animals in habitats with visible horizon(ex. open plains, underwater)

Helps animal identify prey or predators from alllocat ions along the horizon

Photoreceptors in visual streak not as dense as withina foveavisual acuity typically not quite as good as

that from a fovea, but still much higher than that of the

rest of the retina


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Some animals retinas are hybrids

between foveas and visual streaks*:

darker gray = higher photoreceptor density

Fovea Visual Streak

Dogs and cats have intermediate retinas like this

Hybrid


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Eye placement also varies

w/ habitat and lifestyle:

most animals (ex. fish, rabbits, squirrels, birds, horses)haveeyes on sidesof their headsthe advantage of a

huge visual f ield ( or panoramic vision)

these animals tend to live in uncluttered

environments (ex. water, open plains) and/or need

this panoramic vision to detect predators

animals with more forward -facing eyes

lack this panoramic vision, but view

more of their visual field with both eyes

They often are top predators.


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Rabbit:eyes on sides of head

huge visual field

(panoramic vision)

most of visual field monocular

(= seen by just one eye)

Human:

eyes in front of head

much smaller visual field

more of visual field binocular

(= seen by both eyes)

Eye placement involves tradeoff btw.

Size of visual field & binocular vision


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1. Excel len t depth perception2. Enhanced ability to discriminate objects from

background and see past ob jects in cluttered

environments (ex. forests)

Advantages of binocular vision

from forward-facing eyes :

because each eye views the scene from a slightly

different angle, binocular vision provides more visual

input to help distinguish objects of interest (food,

predators, etc.) from the background and allows animals to see past leaves or other

obstacles to detect food, predators, etc. This only

works for larger animals, because eyes have to be far

enough apart that one eye looks past the obstacle.

Sometim

es

calledx

-ray

vision


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Woodcock: side-facingeyeswont be

surprised by predator

approaching from behind

when probing the groundfor worms!

Owl: forward-facing eyesexcellent depth perception

for swooping down &

catching prey & x-ray

vision for detecting prey

Forward vs. side placement of eyes

matches animals lifestyle:


11/46American Woodcock, feeds by probing ground with bill

This eye placement is great for keeping an

eye out for predators above or behind you*


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Young green heron photographed from below

This eye placement is great for scanning the

water below*:


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WHICH ANIMALS HAVE

THE BEST VISION?Trade-offs between:

ACUITY(detecting details in good light)& SENSITIVITY(night or low-light vision)

Features that provide the best acuity tend

to reduce sensitivity and vice-versa

Each species visual system matches their

day vs. night vision needs.


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Visual Acuity

= ability to make out details (in good light)Excellent: humans, apes & monkeys

(human standard for acuity = 20/20; horse 20/33

(meaning that we can clearly see a detail from 33 ft that a

horse can only see from 20 ft. away or less), dog 20/50 to

20/75, cat 20/100)*

Absolute best: most birds , especially

raptors (hawks & eagles)


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Songbirds use their excellent eyesight to find tinyseeds & insects to eat:

Hawks & eagles use eagle-eyes to spot prey

from a distance and chase it down:

Birds With Exceptional Visual Acuity


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Hawks visual acuity ~ 8x better than ours

(20/2 vision: hawk can see detail from 20 ft that we can only see at

2 ft. or less)

Hawk can spot a mouse from height of one mile

Kestrel (type of small hawk) can detect grasshoppers

in meadow grasses from 100 ft above

Hummingbird can spot red flower from 1/2 mile away

Examples Of The Amazing Visual

Acuity Of Birds*:


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Features that increase birds acuity:

1. More Photoreceptors TO MORE FINELY

DISSECT RETINAL IMAGES (so that small detailsfall on separate photoreceptors/ganglion cell receptive

fields and can be captured by the visual system)

a) Relatively large eyes can have large retinalimage that falls on lots of photoreceptors even

w/ a relatively small head

- Human eye 2% mass of head

- Starling eye 15% mass of head


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1. MORE PHOTORECEPTORS TO MORE FINELY

DISSECT

RETINAL IMAGES (so that details fallon separate photoreceptors)

a) Relatively large eyes

b) higher photoreceptor densi ty

Species Cones/mm2in fovea

Human 200,000

House sparrow 400,000

Hawk 1,000,000



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2. Pecten : Special structure to supply oxygen &nutrients to ret inadontneed light-scattering bloodvessels in front of retina, so get sharper retinal image

3.Unusual lens placement / eye shape*:that acts like telescope to magnify images on

central fovea (in hawks, eagles & some owls)

Sketch of eagles weirdly-

shaped eye:

pecten


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4. Two foveas in each eye:

mainly in birds that pursue prey in flight(includinghawks, eagles, kingfishers, terns, swifts, swallows,

owls, & hummingbirds)

one fovea receives light from straight ahead and

the other from the side

5. Lots of ret inal neurons and brain devoted

to visual processing


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Sketch of bird eye with pecten and 2 foveas:


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Night -active and deep -sea animals are the

best at seeing in low-light conditions All mammals are descended from a nocturnal

ancestor, and many living species retain

characteristics that help them see at night Human visual system is set up for acuity, our

sensitivity is very poor compared to most

other mammals

bi -e e fishfl in s uirrel

Visual Sensitivity

bush-babyowl


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Examples Of Amazing Night Vision in

Other Animals*:

Housecat needs only 1/6 of the light we need

to see things at night (owls 1/10-1/100th

)

Owl can spot a mouse creeping in the

underbrush >100m away on a moonless night

Barn owl can detect prey illuminated by the

light of a single match from 1 mile away


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Features that increase sensitivity:

1.Giant eyes that bulge out of the head to

gather light: protruding corneas bend lighttowards center of eye & deliver it to pupil

Eyes of owl fill more than

of its skull

Tarsier has largest eyes relativ

to body size of any mammal


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Features That Increase Sensitivity:

2. Pupils that can open really wide tolet in lots of light

Human: fully open pupil 16X larger

Cat: fully open pupil 100X larger

Tarsier: even more


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Eyes bigger than brain!

Tarsier w/ dilated pupils Tarsier w/ constricted pupils

Communicates in ultrasound!


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3. Retina with a very high density of RODS(and w/ few or no cones), often even in

the fovea

Human: 150,000 rods/mm2

Owl: 1,000,000 rods/mm2


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4. Reflect ive membranebehind retina:(often called reflective tapetum*)

- Acts like a mirror to reflect light back into the

photoreceptor layer

- Gives light thats not absorbed by rhodopsin the

first time thru a second chance to be absorbed

- Comes at cost of reducedvisual acui ty in good

light

(which is probably why some day-active animals,

like humans, dont have it)



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Animals with reflective membranes include owls, nightjars,

dogs, cats, rabbits, racoons, cows, horses, deer, kangaroos,

crocodiles & alligators, dolphins, many fish and even

spiders

Animals With Reflective Membranes

Have Strong Eyeshine At Night:


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Animals With High Visual Sensitivity

Often Have SLIT PUPILS:

Copperhead

snake Frog Cat

Slit pupils can close more rapidly & completely than

round pupils, protecting sensitive eyes from bright

light (& probably improve color vision in low light*)

COLOR VISION IN


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COLOR VISION IN

OTHER ANIMALS

lots of variation among animals in number of

conetypes &exactly how the visual pigments

respond to wavelengths

probably also lots of variation in how retina

& brain process color information, & how color is

perceived, but we dont know much about this. 31

M t l i l l k


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Many nocturnal animals lack

color vision:

Color vision requires 2 or more types of cones (sobrain can compare relative responses)

Many strictly nocturnal species have retinas devoted

mostly to the rod system & only one type of cone

(ex. Virginia opossums, kinkajous, some bat

species). Most marine mammals (ex. seals,

dolphins, & whales) also have only one type of cone.

32


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Geckos are an exception to this rule*:

geckos are nocturnal but have only cones in their

retinas (no rods)

their cones are 350 times more sensitive than ours

(and they have special lenses that allow sharp colorvision with a wide pupil)*

so they can see colors in the dark!

M t th l f l


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Most other mammals see fewer colors

than we do

most mammals, including dogs and cats, have onlytwocone types (a S-cone and a ML-cone), resulting

in color vision somewhat similar to that of a human

with severe red-green colorblindness

tests with dogs show theyre good at distinguishing

among shades ofblue but cantdistinguish reds, yellow,

and greens

Humansview

Dogs

view

34

B th f th d t


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Both of the dogs cone types

respond to these wavelengths, so

relative cone responses can be

used to distinguish hues

Only one of the dogs cone types

(the ML cone) responds to these

wavelengths, so hues cant be

distinguished

A blue or white frisbee may give

better contrast against green

grass than an orange or red

frisbee . 35

Many animals detect ultraviolet (u v )


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Many animals detect ultraviolet (u.v.)

light: = photons w/ wavelengths a little shor ter thanthose humans can detect

36

uv A

300


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Some rodents have a UV-cone & a M-cone:

rats, mice, gerbils, hamsters, gophers, degus (and

probably many other rodents) use a UV-cone to

perceive colors based on reflected ultraviolet light

with just 2 types of cones, they probably see fewer

colors than we do, and their UV-based color

perceptions must be quite different from ours

UV- reflecting

belly fur

Being able to see ultraviolet allows


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Being able to see ultraviolet allows

perception of patterns and markings

that others do not see

These birds have ultraviolet patternswe cant see but the birds can.

normal

light

UV light

Many flowers have ultraviolet

patterns visible to insect pollinators38


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Apes, some monkeys, & marsupials

have 3 cone types like we do:

Apes and old-world monkeys have 3 cone typesthatare similar to ours and probably see similar colors

Many marsupials (kangaroos & relatives) have 3 cone

types so they may distinguish a similar number ofcolors, but their 3rdcone detects ultraviolet light, so the

actual colors they see must be quite different

Fat-tailed Dunnart:

a trichromatic marsupial

39

M i l h f


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Many animals have____types of cones

and perceive____colors than we can:

Day-active____, lizards, turtles, and many____have

S-cones, M-cones, L-cones,and a___-_____

These animals can see ultraviolet light & a whole

_________ ____ of colors that we cant even imagine

Their cones wavelength response curves are also

narrower

more distinct sets of relative coneresponsescan probably more finely __________

among different colors

40

See assigned reading What Birds See


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light wavelengths (nanometers)

Coneresp

onses

human:

S-cone, less

distinct M- &L-cones

bee:

far UV cone,

S-cone, ML

cone

bird:

UV-cone,S-cone, more

distinct M- &

L-cones


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birds that appear camouflaged to mammals mayappear brightly colored to other birds (same with

fish)

flowers, fruits, and berries probably better stand

out because they reflect UV light while the green

leaves in the background absorb UV light.

the ability to see more colors can also help

predators find prey: ex. some rodents mark theirterritory by laying down urine or scent trails that reflect UV

lighthawks can see these and know where a rodent is

likely to be

Animals with 4 cone types see many

things differently than we do for example,

Visual perceptions can differ among


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Visual perceptions can differ among

species in_____ different ways:Well illustrate this by comparing human

v is ion to dog v is ion*

humans acuity at least 2 times better

dog fovea is ~10% cones, 90% rods

& is shaped differently (intermediate btw.

fovea & visual streak)

dogs have reflective membrane

dogs cant round lens up as much

cant focus well on nearby objects

(even when they are young!)

Dog:

better

night

vision

Us:

better

dayvision

More human versus dog vis ion *
http://videos.howstuffworks.com/discovery/29039-a-dogs-life-dog-vision-video.htmhttp://videos.howstuffworks.com/discovery/29039-a-dogs-life-dog-vision-video.htmhttp://videos.howstuffworks.com/discovery/29039-a-dogs-life-dog-vision-video.htmhttp://videos.howstuffworks.com/discovery/29039-a-dogs-life-dog-vision-video.htm


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More human versus dog vis ion

dogs have more side-facing eyes

(dogs visual field ~240 vs. humans 200)

and view the world from closer to the ground

More hum an versus dog vis ion *


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More hum an versus dog vis ion

dogs see fewer distinct colors (similar to a

severely red-green colorblind human)

dogs are more visually attuned to motion, while

we are more attuned to visual detail

dogs have faster updating of retinal imagesdogs see TV (at

least older sets)

as series of static

images

( flicker-fusion*:

tends to be

faster in faster

moving species)

Summary:


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Summary:

tradeoffs between great acuity at a single point (fovea) &decent acuity over whole horizon (visual streak)

tradeoffs between panoramic vision (side-facing eyes) &

excellent depth perception and ability to see past obstacles

(front-facing eyes)

tradeoffs between visual acuity (cone system) & visual

sensitivity (rod system, reflective membrane)

1. Tradeoffs limit what a single visual system can do -

each species has system that best suits its lifestyle:

2. Species with more types of cones see more colors.Most mammals see fewer colors than we do, but many

other vertebrates (including birds) see many more

colors than we do, including colors based on UV light.

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