This episode was brought to you by squarespace If I looked down at an ant by my foot, an
eagle would be able to see it in the same resolution from the top of a first storey
building.
And if I took off my glasses, I would see just about the same as a rat!
Those are pretty cool facts about animal vision, but how would you go about testing how clearly
an animal sees in the first place?
[Intro by Caro Waro & Cristina de Manuel]
There are three ways you could estimate an animal’s visual acuity, or in technical
terms, how sharp or clearly an animal can see the world around them.
The first would be to simply look at the number of photoreceptors in the retina.
This can give you an idea of the maximum resolution available, although in most cases you will
find that actual visual acuity is less than predicted.
There are a many reasons for this, and several relate to the post-processing that happens
after the retina detects light and before it is consciously perceived by the brain.
One example of this is spatial summation, whereby signals from individual photoreceptors
converge or are bundled together into a single ganglion meaning multiple photoreceptors end
up sending a single signal through the optic nerve.
For instance, our fovea may have a 1:1 ratio of photoreceptors to ganglia entering the
optic nerve, whereas the macula may have a 7:1 ratio and the peripheral retina a 35:1
ratio, leading to dramatically different resolutions across our field of view!
The second would be to look at the optic qualities of the eyeball - eyes are subject to refraction,
diffraction and aberration which affect how the light is bent and focused in the back
of the eye and eyes aren’t always shaped perfectly for perfect focus.
In fact, in my video about cephalopod vision I actually explain how a squid’s eye defocuses
and bends light on purpose to detect colour!
Our own eyeshape is not perfect either, and it's the reason why some people are able to
achieve a higher than 20/20 vision after LASIK surgery, because their cornea is reshaped
to focus light better than it would do so naturally.
Now, whilst the previous options give you the maximum possible visual resolution an
animal could have, they don’t really inform us about how they actually see - and the best
way to find that out would be to use an actual eyetest, and there are several ways of doing
this!
In humans we just use a traditional Snellen eye chart, the Landolt test or the Illiterate
test, and the way they work is very simple, you stand at a set distance and try to identify
the different symbols, and the point at which you can no longer identify them marks the
limit of your visual acuity.
However, animals aren’t very cooperative when it comes to calling out letters, so in
order to test their eyesight we need to be a little bit more imaginative!
For animals such as rats and mice, it is possible to set up elaborate behavioural experiments
by taking advantage of the fact that we can train them to learn things in exchange for
rewards and in this case, we want to teach them to choose black & white gratings over
a plain grey display to find out how fine the gratings need to be before they can no
longer tell them apart.
And some researchers actually taught the rodents to do just that by swimming in a special tank
up to a choice line, where they had to then swim towards the black and white grating as
beneath it was a hidden underwater platform that would provide them with a reward.
By using this set-up, scientists were able to discern that rats have a visual acuity
of about 1 cycle per degree.
To give you an idea, 20/20 vision is around 30 cycles per degree, and 1 cycle per degree
is about as well as I can see without my glasses on...
All this being said, as you may imagine, there are plenty of sighted creatures out there
that won’t pander to a scientist’s every whim and fortunately, there are other ways
to measure acuity.
One method that is frequently used takes advantage of what is known as the optokinetic response.
The eyes of many animals naturally track moving objects from side to side following a horizontal
saccadic motion, or horizontal nystagmus, or, the optokinetic response.
Therefore, by placing the same gratings moving across a display or on rotating devices, it
is possible to monitor whether an animal is tracking the grating’s movement or not.
If they are, it means they are capable of seeing the individual lines, and if they don’t,
they probably can’t.
So there you go!
I hope this gave you some clear insight into seeing clearly!
I’d also really like to thank our sponsor for today's episode which is Squarespace.com.
If you’re seeking to make the next move in your career - you probably want a website,
and as you may know, I’m planning to transition into science communication full time after
completing my PhD so I’m currently building my own professional website for my next move
on squarespace.com using their beautiful and easy to use templates, which is allowing me
to showcase the type of work I do to those who may want to hire me!
So if you want a website as snazzy and as simple as mine head on to squarespace.com/drawcuriosity
and use the code 'DC' to get 10% off at check out.
And if you enjoyed today’s video and want to find out more, check out the links in the
description because there's plenty of cool research and very kind researchers who let
me use their footage to illustrate some of the points in this video!
And as always, thank you so much for watching me, and I’ll see you in the next one!
Bye!
[Music: Thastor & CryoSleepKitten] [Channel Art: Caro Waro & Cristina de Manuel]
[Scripting, Hosting, Editing: Inés Dawson] [Sponsored by SquareSpace]
[Translated by {your name} into {Spanish}]