LECTURE 1.1-Colour vision: signals and visual system
0-Why is important to see colour?
Colour can influence our moods, emotions and sensory perception. More importantly, our ability to see colours has practical
and adaptive functions.
Colour facilitates object detection by increasing figure/ground segmentation, allow us to know when an object ends and another
begins, and how to distinguish one thing from its background. It aids in object recognition by allowing objects to be
distinguished on the basis of colour (e.g. ripe fruit).
1-Electromagnetic radiation
It is out there, the sun radiates it and is measured based on the wavelengths of light. Wavelengths of electromagnetic radiation
could be very short (left-hand side of the picture) or very long (right hand side of the picture):
This little part of the electromagnetic radiation we call the visible spectrum. So, this is the bits you see between blue and red,
which correspond to the wavelengths between 400 and 700 nanometres. 400 short wavelengths to blue end of the spectrum and
700 is the red end of the spectrum.
1.1-Receptors
For us to detect that part of electromagnetic radiation, we need to have receptors.
If you have receptors in ultraviolet, for example, in some of the fish specimen, they are able to see a much wider window of
this electromagnetic radiation.
If, on the other hand, you have got, a single wavelength photoreceptor or something much narrower, then the whole window
shuts into a smaller region.
There is electromagnetic radiation out there and your brain, your nervous system is what interprets it as a colour. Colours do
not exist outside your brain. What's outside your brain is electromagnetic radiation at different wavelengths, at different
distributions.
We have receptors in our retina called rods (night vision) and cones (day vision) in our eyes to detect this electromagnetic
radiation.
Some species have receptors in their iris or they have them inside their brains (for animals like most of the fish species, if they
have got transparent bodies, the photoreceptors are actually inside the brain).
But for us, it's in our eyes only. And therefore, if you shine different lights, or different combination of light on your skin, it is
irrelevant. It´s with eyes that we are detecting lights and we become sensitive to this very small part of the spectrum.
1.2- Newton´s experiment
Newton showed that if you shine a light into a prism, it breaks down to the
spectrum of colours. And again, if you get a spectrum these colours and mix
them up together through the prism, you get back the white light. It works both
ways.
What about surfaces around us? So around us we have multiple surfaces that
can be red, green, blue, or whatever.
,If you look at this slide, you can see that a tomato, for example, look red because it reflects
the red part of the spectrum. So as the light shines, and here it shines on a tomato is reflects
this part and it reflects less and less in the blue and green and yellows. Therefore it will look
red.
A white piece of paper reflects equally across the entire spectrum. Whereas the black piece
of paper absorbs equally across the whole spectrum and very little light gets reflected from
that black surface. That's why it looks black to us.
1.3-Scotopic and photopic spectral sensitivity curves
We have got visible lights and it's got between 400 to 700 nanometres of spectrum that we are seeing, from blue to red. But
overall sensitivity that we have differs depending on the light that is around us.
So, if it's very bright, we are maximally sensitive to around 555 nanometre
or yellowish colours.
We are more sensitive to and under dimmer conditions, we are more
sensitive to more of this bluish colour which it matches that of the spectral
reflectance that you get from the surface of the moon going through the
atmosphere and that of the sun going through the atmosphere.
So, yellows and blues are our maximum sensitivity, for daylight and dim
light condition or lower light levels.
1.4-Photopigments
We start with a very basic photopigment receiving a proton and then we translate this photon into an electric signal. So, the
photopigment is the key, an essential part of seeing, detecting light.
This is the spectral absorption of a photopigment. What does that mean? That
means that if you have got a single photopigment, it is not going to be equally
sensitive to every wavelengths of light.
There are parts of the light spectrum that a pigment is very sensitive to, and
there are others which it is less sensitive to.
So, this imaginary photopigment seems to be very sensitive to 550
nanometres and less sensitive to others.
One thing for sure is that this photopigment cannot tell you whether there is a colour out there or not for different parts of this
spectrum as lights shine, whether it is short wavelength or long wavelength lights.
Because what you could do is to match huge amount of light at this part or that part of the spectrum with a small amount of
light in this part, and the photo pigment would responds by the same thing: just an electric signal saying that there is light.
So, the outcome of a single photopigment is an electric signal that signifies the presence or absence of light.
Each photopigment responds to a preferred wavelength, but the neural response of the photoreceptor does not specify the
wavelength.
Univariance principle tells us that a photoreceptor's response is summarised by one variable that specifies the amount of light
absorbed. It doesn't tell you which wavelength it was. So given appropriate intensity, all wavelengths can affect a single
photopigment in the same way, and therefore a visual system with only one kind of photopigment cannot see colour.
, In the picture, we have got the red berries on the green foliage. Supposing we have a photopigment which is maximally sensitive
to red. Having a red component on this image, you would get lots of output from the reds, and very little from the background
coming from that photopigment. So, the red appears brighter and the green background darker.
If you look at, for example, the green component of something more sensitive to the green part, then huge amount of green in
here will look bright and the red will look dark.
And if you have something, which is more sensitive to the blue end of the spectrum, there's very little blue in here, therefore,
the whole thing will look a lot more dark.
What would happen if you have a visual system with multiple photo pigments?
This time we have two, one maximally sensitive to shorter wavelengths and one to higher wavelengths.
A visual system with two photoreceptor types (such as possessed by
mammals) will be able to extract some wavelengths information.
However, it could lead to colour confusions, because any wavelength can
be matched by a pair of wavelengths.
So, if we have got short wavelength sensitive pigment and a long
wavelength sensitive pigment and we shine a light at 460 nanometre onto
this visual system.
Where there is an intercepts between this line representing the 460
nanometres light and the absorption spectrum gives you how much signal
comes out of that photopigment. So, we have that much signal from the red one or long wavelengths one, and that much signal
from the short wavelength photopigment.
But if we replace this light with two different lights, one much lower, one
much higher, and we look to see how much information's being absorbed or
how much signals being given by these photopigments, again that part is the
intercept from the short wavelength photopigment and for the long
wavelength photopigment a little bit on this one and a little bit on that. That
can add up together.
So, basically, first single light from the long photopigment can be matched by
some of the two last ones together. And this light here from the short
photopigment is equivalent to that amount from there.
So this visual system cannot distinguish between the light from the single light
wavelengths of this colour and the combined light of those two. That's why it
confuses colours.
1.5-Human eye
So, the more photopigments we have, the lower the chances of confusion.
The human eye has three photopigments sensitive under photopic conditions, so it is trichromatic (three-coloured).
Any wavelength will produce three responses, a trio that is less likely to be confused with any other single wavelength.
The wavelength sensitivity of human eye:
We have short wavelength sensitive cones (S), medium sensitive
photopigments (M) and the long sensitive photopigments (L).
So L M S cones in human vision are responsible for our colour vision.
The peak sensitive is given on this picture. L cones are responsible to detect
reds, oranges and yellows; M cones respond optimally to the middle
wavelength sensitive that is, yellows and greens; and S cones are
responsible for blue end of the spectrum.
Pattern of cones in the retina:
