vision biology
TRANSCRIPT
-
8/11/2019 vision Biology
1/46
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
-
8/11/2019 vision Biology
2/46
VISION IN OTHER
VERTEBRATES
ASSIGNED READING:
What Birds See by Timothy H. Goldsmith
-
8/11/2019 vision Biology
3/46
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)
-
8/11/2019 vision Biology
4/46
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:
-
8/11/2019 vision Biology
5/46
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
-
8/11/2019 vision Biology
6/46
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
-
8/11/2019 vision Biology
7/46
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.
-
8/11/2019 vision Biology
8/46
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
-
8/11/2019 vision Biology
9/46
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
-
8/11/2019 vision Biology
10/46
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:
-
8/11/2019 vision Biology
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*
-
8/11/2019 vision Biology
12/46
Young green heron photographed from below
This eye placement is great for scanning the
water below*:
-
8/11/2019 vision Biology
13/46
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.
-
8/11/2019 vision Biology
14/46
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)
-
8/11/2019 vision Biology
15/46
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
-
8/11/2019 vision Biology
16/46
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*:
-
8/11/2019 vision Biology
17/46
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
-
8/11/2019 vision Biology
18/46
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
Features that increase birds acuity:
-
8/11/2019 vision Biology
19/46
Features that increase birds acuity:
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
-
8/11/2019 vision Biology
20/46
Features that increase birds acuity:
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
-
8/11/2019 vision Biology
21/46
Sketch of bird eye with pecten and 2 foveas:
-
8/11/2019 vision Biology
22/46
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
-
8/11/2019 vision Biology
23/46
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
-
8/11/2019 vision Biology
24/46
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
-
8/11/2019 vision Biology
25/46
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
-
8/11/2019 vision Biology
26/46
Eyes bigger than brain!
Tarsier w/ dilated pupils Tarsier w/ constricted pupils
Communicates in ultrasound!
-
8/11/2019 vision Biology
27/46
Features That Increase Sensitivity:
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
-
8/11/2019 vision Biology
28/46
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)
Features That Increase Sensitivity:
-
8/11/2019 vision Biology
29/46
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:
-
8/11/2019 vision Biology
30/46
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
-
8/11/2019 vision Biology
31/46
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
-
8/11/2019 vision Biology
32/46
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
-
8/11/2019 vision Biology
33/46
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
-
8/11/2019 vision Biology
34/46
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
-
8/11/2019 vision Biology
35/46
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 )
-
8/11/2019 vision Biology
36/46
Many animals detect ultraviolet (u.v.)
light: = photons w/ wavelengths a little shor ter thanthose humans can detect
36
uv A
300
-
8/11/2019 vision Biology
37/46
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
-
8/11/2019 vision Biology
38/46
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
-
8/11/2019 vision Biology
39/46
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
-
8/11/2019 vision Biology
40/46
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
-
8/11/2019 vision Biology
41/46
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
-
8/11/2019 vision Biology
42/46
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
-
8/11/2019 vision Biology
43/46
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 -
8/11/2019 vision Biology
44/46
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 *
-
8/11/2019 vision Biology
45/46
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:
-
8/11/2019 vision Biology
46/46
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.