[MUSIC]
The back surface of the eye contains layers of neural tissue.
So here's a schematic.
We're looking at this surface here, it's known as the retina.
One of the layers of cells here is a layer of light sensitive cells.
These are the rods and cone cells.
The rods are very sensitive indeed, they allow us to see in dim light levels.
The cones are less sensitive, they allow us to see in daylight levels.
In the retina there are approximately 6 million cone cells.
Most of them are concentrated in the central region of the retina here
where our vision is best.
The cone cells are very small.
They're between two and eight microns in diameter.
That's 2 thousandths of a millimeter or 8 thousandths of a millimeter.
And to see individual cells of that size, we would normally use a microscope.
A sample of tissue would be put on a microscope slide, and we'd
use the microscope to magnify the tissue so that we could see individual cells.
We want to image cells in the living human eye.
We want to be able to see individual cone cells by looking into the eye.
>> Here in the lab we've built a special microscope for
looking in the back of a participant's eye.
But unlike a traditional camera,
we don't a picture of the entire retina all at once.
Instead what we do is focus a laser beam into the eye.
So the laser is directed into the eye via these lenses and mirrors and
via these two special mirrors that scan the beam.
So one moves the beam horizontally, the other moves it vertically, and
together, they trace a rectangular patch on the retina.
Then, some extra mirrors relay the light into the participant's eye.
Some of that light reflects back out and through these lenses and mirrors again.
We collect some of that light and use it to measure the optical distortions in
the eye, which we can correct with this mirror.
Most of the light goes onto this very sensitive detector.
And what we can do is use that to build up an image by measuring
the amount of light at each position as it is scanned.
And using some very clever software, we can reconstruct an image pixel by pixel.
>> We're borrowing techniques from astronomy to be able to do this and
we've worked in collaboration with
researchers in the instrumentation group at Durham University.
The final lens in our microscope is actually the eyes lens, so
it's the lens of a living human eye.
Light has to pass through here so
that we can image the retina at the back of the eye.
That introduces a few challenges, so even for an observer who's got
perfect eyesight, the optical quality of these components is not perfect.
And secondly, if we ask our participants to try to keep their eyes still,
it's not possible to do that.
The eye always makes minute eye movements.
>> One of the key components of this system is this deformable mirror.
This is what allows us to see these really tiny cells at the back of the eye.
This mirror has lots of small actuators behind it's surface, and
these actuators push and pull to change the shape of the mirror surface.
By measuring the optical distortions in the eye, we can calculate the shape
that this mirror needs to have to cancel out those optical distortions.
Allowing us to recover a very detailed picture of the back of the eye.
Okay, Anna, we're going to take a picture of the back of your eye.
So if you'd like to come forward and bite onto the dental impression.
I'm now going to align your eye lens to the system so
that we can get the best possible image that we can.
I'm now going to put a fixation cross on the screen in front of you.
If you could just keep your eye fixated on that while I take some images.
>> So the ability to caption high resolution images of the living eye
has opened up new avenues of research.
Labs around the world are exploiting this ability to
see individual cells in the living human eye.
We can obviously start to analyze cone statistics.
So the density and the packing arrangement of cells within the retina.
And we can think about how that impacts our ability to see.
Each of these blobs is an individual cone cell.
But the cones are not all equal.
So there are three different types of cone cell.
Cones that are sensitive to different ranges of wavelengths.
Some of them are sensitive to short wavelengths, some to middle wavelengths,
and some to long wavelengths.
And its comparisons between these three classes of cone signals that allow us
to see color, to extract wavelength information about the external world.
So it's possible to identify the type of cone of each individual cell here.
They're either short, middle, or long wavelength sensitive.
We can also use the system to image the retina in health and in disease.
And we can look at minuscule changes that occur within disease,
opening up the possibility to track these changes at a very fine scale,
give us early warning signals about change in the damaged eye and to identify
possible candidates for treatment, for example in gene therapy trials.
By adjusting the depth of focus of our microscope,
of our imaging system, we can also look at different layers in the retina.
And here's an example where we're focusing on the blood vessel layer.
And, because we have such high resolution images,
we can see blood flow of individual blood cells moving through these vessels.
We can also focus into different depth planes, and
look at other structures within the retina.
So here's an example where we're focused on the nerve fiber layer.
This is the layer of axons of all those neurons in the retina
before they leave the retina via the optic nerve.
The ability to image at this scale, at this microscopic scale,
also opens up the possibility of stimulating the retina at that scale.
So it's possible to send light to an individual cone cell and
to track the ability to use that signal to elicit percepts, for example.
So we can stimulate the retina at the single cone level.
We mentioned one of the challenges of this sort of imaging being
the fact that the human eye is always in motion.
As well as that being a challenge, it's also an open question in research.
So why does the eye move all the time and
in what way does that impact our ability to see?
One of the fundamental things we know about vision is that if we suppress these
movements, if we stabilize the image on the retina, then vision fades to nothing.
So it's fundamentally important that the eye moves in order for
us to be able to see it all.
Being able to measure eye movements at this scale,
opens up exciting new avenues of research to understand more
about the link between eye movements, the stimulation of the retina, and
the biological properties of the cells in the retina.