Nearly 1/3 of the brain is devoted to processing visual information
If visual sense gives us information that is in conflict with information form
another sense, we tend to bias our trust towards our sense of vision.
Our Visual Sense
primarily an instrument to collect, focus, and sense the light.
there is some initial processing done on the information collected, the
heavy duty processing occurs in the brain.
John is blind b/c of stroke in occipital lobe and can‟t perceive world around him
and maintain his independence
Never been in your house but he can navigate through home easily and avoid a
pile of books in middle of living room hall.
How can he do this if he has no visual perception
The Stimulus: Light
roperties of light: There are three physical characteristics of light that translate into
the three psychological perceptions of our visual world.
Light travels as a wave that moves at about 300 000 km/sec. Light waves can vary
two respects: the height of each wave, called the amplitude, and the distance between the
peaks of successive waves, called the wavelength.
. Variations in amplitude affect the perception of brightness. Generally, the greater the
amplitude of the light wave, the more light is being reflected or emitted by that object,
and so that object appears brighter or more intense to us.
iations in wavelength affect the perception of colour. Wavelength is measured in
nanometers. Smaller wavelengths refer to light waves with a higher frequency, because
there is less distance between successive peaks. Larger wavelengths refer to light waves
with a lower frequency. (See image above, right.) wavelengths of electromagnetic radiation, and this tiny portion that we're sensitive
to is called the visible spectrum. The shortest wavelength that we can see is
around 360 nanometers, which looks violet to us, and the longest wavelength that
we can see is around 750 nanometers, which is red.
example, insects like bees can see wavelengths shorter than 360 nm in the
ultraviolet spectrum, and may perceive differences in the colours of flowers that
all look the same colour to us.
than 750 nm in the infrared spectrum, which allows them to find prey in the dark
by being able to see the body heat that is emitted by the prey.
the saturation, or richness, of colours.
and the perceived colour would be described as completely saturated.
wavelengths this light would be perceived as white and would be described as
wavelengths and are less intense than pure colours.
process. the eye is
covered by the white part of the eye called sclera, a tougher
cornea, light passes through the pupil of the eye, which is the
window that you see as a black dot in the middle of your eye.
The iris is
basically a band of muscles that is controlled by the brain, so that if
not enough light is reaching the retina, these muscles cause the pupil to constrict
into a tiny opening.
transparent structure that does the final focusing of light on the retina.
reversed from left to right. However, the final perceived image is a
-down and reversed, there is a
correction that allows us to see a properly oriented image. by surrounding muscles, allowing it to focus on objects that are close or far away.
produce a clear image, but if the object is far away, the lens of your eye gets
elongated to focus the image on the back of your eye.
is change in the shape of the lens to focus on objects that vary in
distance is called accommodation.
humor, which is the clear, jelly-like substance that comprises the main chamber
inside the eyeball, until it finally lands on the retina, which is the neural tissue that
lines the back of the eye. (See image on next page.)
Neural processing of visual information: the retina, because this is where
the physical stimulus of light is first translated into neural impulses. The Retinal Layer 1:
-thin sheet that covers that back of the eye, and is
made up of a complex network of neural cells arranged in three different layers.
-intuitive; the layer at
the very back of the eye, farthest away from the light, is where the photoreceptors
the physical stimulus of light into a neural signal that the brain can understand.
of retinal tissue, which are transparent.
-out arrangement in the retina has to do with
where the photoreceptors get their nutrients from, which is a layer of cells at the
very back of the eye called the retinal pigment epithelium, or RPE. photoreceptors were located at the front of the retina, facing the light, then they
would not have access to the RPE that they need to live. (See image below.)
Photoreceptors: Rods and Cones
named for their respective shapes. (See image below)
The cones provide us with the sensation of colour and provide good
visual acuity, or sharpness of detail. Cones become more concentrated towards
the fovea, a tiny spot in the middle of the retina that contains exclusively cones.
When you want to see something in detail, we move our eyes so that the image
falls directly onto the fovea.
night vision. They provide no information from which colour can be determined,
and offer poor visual acuity. There are no rods in the fovea itself, with increasing
concentration in the region just surrounding the fovea. This arrangement make
rods very useful for peripheral vision. environment that is dimly lit, you‟re better off looking slightly to one side of the
object as opposed to trying to stare right at it.
in a dimly lit environment. But if you stare to one side of the object, you‟ll be
using your rods and increasing the chance that you will see it.
Response to Light
neural signal that the brain can read?
which is a complex molecule that is sensitive to light.
cones, but they all basically work the same.
t is absorbed, it changes the chemical state of the
pigment and splits into its two component molecules which sets off a
chain reaction leading to an electrical current flowing across the
in a currency that can be understood
processed by the brain.
within the photoreceptor cause the two molecules to recombine, so that the
photopigment is ready to react to light again. However, there is a brief period of
time during which the photopigment will not be able to react to light.
that are ready to react to light depend on the relative rate at which they are being
split and recombined.
is exceeds the rate at which they are being recombined. the dark, a phenomenon called dark adaptation.
movie theatre on a bright day and for the first few minutes, you cannot see much,
but gradually, you are able to see things more clearly.
is is because at the time you enter the theatre, most of your
photopigments, particularly those in the rods, haven‟t had time to recombine yet,
leaving few that are ready to react to light, causing you to see very little.
of your photopigments will have
recombined and be ready to respond to light. That is how the eye becomes
adapted to the dark.
Bipolar and Ganglion Cells:
an electrochemical signal in the photoreceptors.
retina, called the bipolar cells, by means of a transmitter substance.
to the next layer of
cells in the
retina, called the ganglion cells.
Retinal Layer 2 and 3:
retina, and the
axons of these cells all converge on one point in the eye, called the optic disc, and
then leave the eye to join the optic nerve, which travels all
the way to the brain.
this small area contains no photoreceptors at all, and so it
constitutes our blind spot.
The Blind spot: We are not normally aware of our blind spot because our brains fill in
gap by blending the surrounding image most of the time.
To recap, light enters the eye and must pass through the ganglion cells, bipolar cells, and
strike the photoreceptors on the retina at the very back of the eye. At that point the light is
converted into a neural signal that is sent from the photoreceptors to the bipolar cells, and
then on to the ganglion cells, whose axons make up the optic nerve.
Processing in the Retina:
inal layer to
communicate with each other, called horizontal cells and amacrine cells.
Information from over 130 million rods and cones in the retina converge to
travel along only 1 million axons in the optic nerve. What this means is that some
amount of visual processing is done in the retina, before the signal is sent on to
Receptive Field in the Retina:
Think of the photoreceptors in the retina as being divided up into
and the information from each group getting assimilated into
one signal that
affects the ganglion cell down the line. In the fovea, the photoreceptor “group” for a particular ganglion cell
contain one cone, which means the ganglion cell is representing a
very small area
of the image.
Since each cone in the fovea has a direct link with the brain, a lot of the
preserved and more visual acuity occurs in the fovea.
But more often, the input from many rods and cones is combined into
signal for one retinal ganglion cell.
These groups get larger as we move toward the periphery of the eye,
which is one
reason why our visual acuity is so low for peripheral vision.
The collection of rods and cones in the retina that, when stimulated,
firing of a particular ganglion cell is called the receptive field of that
retinal ganglion cell.
These receptive fields in the retina come in a variety of shapes and sizes,
but most of them are basically donut shaped, such that light falling in the center of
the donut will either excite or inhibit the cell, and light falling in the surround part
of the donut will have the opposite effect on the cell. Excitation and inhibition of the cell is determined by the rate at which
that cell fires compared to baseline, or the rate at which the cell would fire
normally, without any light signals.
As you probably remember from our discussion of the brain, all cells
have some baseline rate of firing. So a cell would be excited if the rate of firing of
that cell increased compared to baseline, and it would be inhibited if the rate of
firing decreased compared to baseline.
Suppose we‟re looking at a receptive field on the retinal surface that has
this donut shape, with the center being excitatory and the surround being
This means that if light struck the center of the receptive field, this
an increase in the firing rate of the ganglion cell, but if light struck
the surrounding of the receptive field, this would cause a decrease in the firing
rate of the ganglion cell.
If light covered both the center and surround of the receptive field, these
two effects would basically cancel each other out, and the cell would fire at the
same rate that it does at baseline, when no light is available.
Either way, when a receptive field of a ganglion cell is stimulated, that
ganglion cell sends signals towards the brain. (See image above, right.)
Retinal cells can also affect signaling of adjacent retinal cells through
antagonism or lateral inhibition.
This is done through the horizontal cells, which are activated by the
photoreceptors, and also through the amacrine cells, which are activated by the
Whenever a retinal cell is stimulated by light falling on its receptive
field, that cell
sends signals onto the brain, but it also sends messages sideways
cells that inhibit their activation.
The perceptual result of this kind of a physiological mechanism is that
of objects are easier to detect. This becomes obvious when we look at the illusory Mach bands, strips
of greys that range from dark grey to light. Within each strip, the colour is
constant. Yet it appears as though a darker band exists right at the border of each
strip. This illusion can be explained by lateral inhibition, and this is one way in
which the retina enhances our ability to detect edges in visual images.
To illustrate the point, imagine we have 4 cells A, B, C, and D. Cell A,
B and C are receiving intense stimulation from the same patch of bright light,
whereas Cell D is receiving moderate stimulation from a dark grey patch of light.
Thus, Cell C is on the edge of the bright and grey light, and you‟ll see
that with lateral inhibition, this cell ends up sending more stimulation to the brain
than Cell B, even though both cells receive the exact same input.
So cells A, B and C are strongly stimulated, and since they are also
neighbors, they are inhibiting each other as well. Since Cell D is only moderately
stimulated by the dark grey patch, and it sends less inhibition to its neighbour Cell
C than Cells A, B and C do to each other.
As a result, Cell B receives a lot of inhibition from the intense
stimulation of both of its neighbors, whereas Cell C only receives strong
inhibition from one of its neighbors.
Because of this, Cell C sends out a stronger signal to the brain than Cell
B does, even though Cell C and B are receiving the same input from the world.
The perceptual result for us is that edges look more distinct. other areas for processing. Instead, cells just a few synapses away from our visual
receptors are already beginning to process the incoming information by accentuating
certain features of a stimulus, like its edges, while placing less importance on other
features of the stimulus, like areas that are uniformly stimulated.
rised of a set of assembly lines. Areas along the visual
pathways process parts of the visual input before sending those partially-
processed bits of information on to the next set of areas down the line for further
Visual Fields and Hemispheres: