It’s Professor Dave, let’s do some processing.
We’ve learned about how neurons transmit information, as well as the way that neurons
are organized so as to integrate that information.
So now let’s start to look at some of the specific information that is to be integrated.
We receive information about our surroundings through the senses.
We see objects with the eyes.
We hear sounds with the ears.
We smell and taste things with the nose and mouth.
How does this information get to the brain, and what does the brain do with it?
Let’s get a closer look at this process now, starting with vision.
We got a basic introduction to the structure of the eye in the anatomy and physiology course,
when we looked at the nervous system.
We will assume that knowledge here, so make sure to check out that tutorial if you need
to review that terminology.
With the structure of the eye understood, we will now get a closer look at the mechanism
by which visual information is transmitted to the brain, and how that information is
interpreted.
So let’s start at the beginning.
Vision is based on light interacting with the eyes.
There is plenty of information on light in my physics tutorials, but if you’d rather
not get into all that, let’s just begin with the knowledge that light can behave either
as a wave, or as a particle, which is called a photon.
What we refer to as visible light is actually electromagnetic radiation, just like microwaves,
and radio waves, and X-rays, and all the rest, it just happens to have a wavelength between
about 400 and 700 nanometers.
This is the portion of the electromagnetic spectrum that we have evolved the ability
to perceive.
If the wavelength is just a bit longer, in the infrared portion, our eyes can’t pick it up.
A little shorter, in the ultraviolet range, again we can’t pick it up.
But right in this thin band, we are in business.
Other species have slightly different ranges to work with, but this one is ours.
Within that range, the specific wavelength determines the color of the light, and the
intensity determines the brightness of the source.
Light passes through the pupil to enter the eye.
The iris, or the colored part surrounding the pupil, regulates the amount of light that
passes through.
This can be seen when the pupil changes size to adjust to the intensity of available light,
which we call constriction if it gets smaller, and dilation if it gets bigger.
Behind the pupil sits the lens, which focuses the light onto the retina.
We do this with two eyes at once, and the brain is able to interpret the binocular disparity,
which is the difference in the position of a particular image on the two retinas.
The greater the disparity, the closer the object is, so having two eyes is what gives
us our depth perception.
Now, the process by which light hitting the retina is converted into neural signals is
quite complex to say the least.
To begin to understand this, let’s zoom in a bit on the retina.
We see five layers of different types of neurons.
There are receptor cells, horizontal cells, bipolar cells, amacrine cells, and retinal
ganglion cells.
Interestingly enough, the photoreceptors, which are referred to as rods and cones, depending
on their shape, are located at the back of the eyeball, so light must reach these first,
and then the resulting signals propagate out towards the retinal ganglion cells.
The signal continues along the axons from these retinal ganglion cells to leave the
eye, and the disruption in the receptor layer that this bundle of axons creates is what
results in a blind spot in our vision.
Let’s now get a closer look at these rods and cones.
Looking at their shapes, the origin of their names becomes quite obvious.
So how do they differ?
As it happens, they mediate different kinds of vision.
Photopic vision is what happens when there is plenty of light, which allows us to perceive
our surroundings in great detail.
This is mediated by cones.
Scotopic vision is what happens when things are very dim, and there is not enough light
to excite the cones.
This is mediated by rods, and it produces a kind of perception that lacks certain details.
This is better understood when looking at how these neurons converge.
Looking at cones, only a few of these converge onto one retinal ganglion cell.
So the input received here is from just a few photoreceptors.
By contrast, several hundred rods converge onto a single retinal ganglion cell, so the
input received in this case is from a great many photoreceptors.
This is why the rods can take a very dim light source and amplify the stimulation, whereas
this would not work with cones.
This system trades sensitivity for acuity, because an amplified signal means the information
will be received, but it can’t be pinpointed to a single rod, so it is less clear where
the signal is coming from.
With cones, there is less sensitivity, but there is little ambiguity.
The stimulus is coming from a distinct place, so with a brighter light source, this is more
effective.
In terms of the distribution of photoreceptors on the retina, we must be aware of the fovea.
The fovea, which means “pit” in latin, is a small indentation at the center of the
retina, and there are no rods at all in the fovea, only cones.
Because this region is all cones, this is the part of the retina that specializes in
high-acuity vision, which means it can resolve small details.
Then moving out from the fovea, there are fewer cones and many more rods.
The rods reach a maximum at twenty degrees away from the fovea on either side, and then
begin to decline as well.
The sensitivity towards particular wavelengths of light is slightly different for these two
receptor cells as well.
With photopic vision and the cones, maximum sensitivity happens at around 560 nanometers,
which is roughly yellow light.
As it deviates from this, it will have to be more and more intense in order to be perceived
with the same brightness.
With scotopic vision and the rods, maximum sensitivity happens around 500 nanometers,
sort of a bluish-green light.
As light dims and our eyes switch from photopic to scotopic vision, an interesting phenomenon
can be observed that relates to this discrepancy in sensitivity, which is called the Purkinje effect.
Yellow and red objects will appear brighter than green and blue ones, and as the light
dims, this relative brightness will appear to reverse.
To get even more specific with cones, we must understand that there are three cone types.
These are red, green, and blue, which are also called L, M, and S, which stand for long,
medium, and short wavelength.
Trichromatic color theory explains how differential activation of these three types is responsible
for our perception of all colors.
This is why a computer monitor can use only red, green, and blue pixels to represent any color.
This can be combined with opponent process theory, which outlines opponent systems that
describe how the cones connect to the ganglion cells.
In this model, a combination of excitatory and inhibitory responses controls what color
combinations can be seen.
A ramification of this mechanism is the notion of the negative afterimage that can be seen
after staring at something for an extended period of time, whereby the afterimage is
always the complementary color.
This is because staring at something of a particular color will cause certain cones
to become fatigued, while others do not fire at all, so when shifting focus to a blank
surface, only the opposing cells will fire.
So now we have a basic grasp of how light is received by the retina, but how is this
converted into information for the brain?
Let’s talk a bit about visual transduction.
Take a look at this protein, a pigment molecule called rhodopsin.
The absorption spectrum of this molecule is nearly identical to what we saw for the scotopic
spectral sensitivity curve.
This is no coincidence.
This protein is a G-protein-coupled receptor that is found in all of the rods, and the
degree to which it absorbs light of different wavelengths informs our ability to detect
light of these wavelengths.
The way this works is that in the dark, when rhodopsin is not activated by light, sodium
channels are open, which keeps the rod slightly depolarized, generating a graded potential
which results in the continuous release of glutamate.
This is a neurotransmitter that has an inhibitory effect, keeping the bipolar cell hyperpolarized
and inhibiting neurotransmitter release towards the ganglion cell.
When exposed to light, rhodopsin becomes bleached, initiating a cascade of events that results
in the closure of the sodium channels.
The rod becomes hyperpolarized and the release of glutamate is minimized, so the inhibition
of signal transmission ceases.
The bipolar cell can now depolarize and release neurotransmitters towards the ganglion cell,
which will generate an action potential that can propagate along the optic nerve.
We should note that rods contain just this one pigment, rhodopsin, while cones contain
three, which can act together in varying ways to allow for the perception of any color in
the visible spectrum.
Zooming out a bit, action potentials that start at the retinal ganglion cells will travel
from the retina, along retina-geniculate-striate pathways.
These begin with axons from the ganglion cells, which leave the back of the eyeball and form
the optic nerve.
This will pass through the lateral geniculate nuclei, found in the thalamus, and then continue
all the way to a region of the brain called the primary visual cortex, which is divided
into left and right sections, found in the left and right hemispheres of the brain.
Notice that all the inputs from the left visual field are carried to the right primary visual
cortex, and inputs from the right visual field are carried to the left primary visual cortex.
Within these pathways we can also identify a few layers of neurons.
There are parvocellular layers, also called P layers, which are comprised of neurons with
smaller cell bodies.
These respond to color, fine details, and objects that are still or moving slowly.
Then there are magnocellular layers, or M layers, comprised of neurons with larger cell
bodies, which are more responsive to objects in motion.
These layers project to different areas of the visual cortex.
Another aspect of organization to the visual cortex is the way we can identify vertical
columns, perpendicular to the cortical layers themselves, which each correspond to a specific
area of the retina from a particular eye.
What other areas of the brain are involved here?
Aside from the primary visual cortex, which receives most of its input from the lateral
geniculate nuclei, we also have the secondary visual cortex.
This is named as such because it receives input from the primary visual cortex.
Then there is the visual association cortex, which receives input from the secondary visual
cortex as well as other areas of the brain.
To help distinguish the locations of these regions within the brain, we can see that
the primary visual cortex sits in the posterior region of the occipital lobes.
The secondary visual cortex can be found in the prestriate cortex, which surrounds the
primary visual cortex, as well as the inferotemporal cortex, which is found in the inferior temporal lobe.
The visual association cortex is found largely in the posterior parietal cortex, and also
distributed around other areas of the cerebral cortex.
As we said, the hierarchy is such that visual information goes from the primary cortex,
to the secondary, then to the association cortex.
These other two areas of the brain are responsible for visual analysis, or the interpretation
of the information brought to the primary cortex.
This information travels to these regions via the dorsal stream and the ventral stream.
Roughly speaking, neurons in the dorsal stream interpret spatial information, like the location
of an object and its motion, while the ventral stream is for object characteristics, like
color and shape.
The dorsal stream flows from the primary visual cortex to the dorsal prestriate cortex to
the posterior parietal cortex, while the ventral stream flows from the primary visual cortex
to the ventral prestriate cortex to the inferotemporal cortex, as is shown here.
There is much more to discuss regarding how the brain processes visual information, and
we will elaborate on all of this in the future, but for now we will have to stop here and
move on to some other topics, so let’s check out the other senses next.