Our Visual Sense
The important role of visual information is reflected in the many visual references we use in our language.
When visual information is in conflict with information from another sense, you tend to bias your trust
towards your sense of vision-for most people, seeing is truly believing.
The eye is primarily an instrument to collect, focus, and senses the light stimulus. However, this is only the
beginning as the heavy duty processing occurs in the brain.
John was rendered blind an occipital lobe stroke at the age of 30. Although he is now unable to perceive the
visual world around him, he struggles to maintain his independence.
The Stimulus: Light
Light travels as a wave and can vary in two respects: the height of each wave, called the amplitude, and
the distance between the peaks of successive waves, called the wavelengths.
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.
Variations in wavelength affect the perception of colour. Wavelength is measured in nanometers, or millionth
of a millimetre.
o Smaller wavelengths refer to light waves with a higher frequency, because there is less distance
between successive peaks.
o Larger wavelengths refer to light waves with a lower frequency.
Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation.
We refer to this tiny portion that we`re sensitive to as the visible spectrum.
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The shortest wavelength that we can see is around 360 nanometers, which looks violet, and the longest
wavelength that we can see is around 750 nanometers, which looks red.
There are other species which can see the light outside our visible spectrum.
o Insects like bees can see the wavelengths shorter than 360 nm in the ultraviolet spectrum, and
perceive differences in the colours of flowers that all look the same colour to us.
o Some species like snake can see light made up of wavelengths longer 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 two physical characteristics of light, amplitude and wavelength translate into our perceptions of
brightness and colour, respectively.
A final physical characteristic of light we will consider is purity, which affects the perception of the
saturation, or richness of colours.
A light that is made up of a single wavelength is a pure light, and the perceived colour would be described
as completely saturated.
However, natural light you experience will more likely be a combination of many wavelengths. This light
would be described as desaturated.
o Most of the colours we see in our everyday life are not pure but a mixture of wavelengths and thus
are less intense than pure colours.
Light first passes through the curved cornea, which begins the focusing process. The cornea is a
transparent window at the front of the eye. The rest of the eye is covered by the white part of the eye called
sclera, a tougher membrane.
After the cornea, light passes through the pupil, which is the round window that you see as a black dot in
the middle of your eye.
The iris, or the coloured part of your eye, controls the size of the pupil. The iris consists of a band of
muscles that is controlled by the brain; if not enough light is reaching the retina, these muscles cause the
pupil to dilate into a larger opening, whereas if too much light is entering they eye then these muscles cause
the pupil to constrict into a tiny opening.
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After passing through the pupil, light passes through the lens, a transparent structure that does final
focusing of light onto the retina at the back of the eye.
The curvature of the lens causes images to land on the retina upside-down and reversed from left to right.
However, the final perceived image is a product of brain activity.
o Thus, rather than seeing everything upside-down and reversed, there is a correction that allows us
to see a properly oriented image.
The lens is a flexible piece of tissue, the shape of which can be altered by surrounding muscles, allowing it
to focus on objects that are close or far away.
If the object is close, the lens of your eye gets fatter or rounder to 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.
o This change in the shape of the lens to focus on objects that vary in distance is called
After travelling through the lens, light passes through the vitreous humour, which is the clear jelly-like
substance that comprises the main chamber inside the eyeball.
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o The light finally lands on the retina, which is the neural tissue that lines the back of the eye.
Retinal Layer 1: Photoreceptors
The retina is a paper-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.
The organization of these layers may seem counter-intuitive; the layer at the very back of the eye, farthest
away from the light is where the photoreceptors are located.
Photoreceptors are cells in the retina that are responsible for translating the physical stimulus of light into a
neural signal that the brain can understand. To reach the photoreceptors, light must pass through the other
2 layers of retinal tissue which are transparent.
The reason for this inside-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
(RPE). The photoreceptors would die without access to the RPE cells, and if the photoreceptors were
located at the front of the retina, facing the light, then they would not have access to the RPE that they need
Photoreceptors: Rods and Cones
Cones are designed to operate at high light intensities and are primarily used for day vision. 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
Rods are designed to operate at low light intensities, and are primarily used for night vision. They provide no
colour information and offer poor visual acuity. There are no rods in the fovea itself, with an increasing
concentration in the region just surrounding the fovea.
When you`re trying to see an object in an 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.
o When you stare right at it, the image is focused on the cone rich fovea which doesn’t work well in a
dimly lit environment.
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By staring at one side of the object, you’ll be using your rods and increasing the chance that you’ll see it.
Introduction to Visual Pathways
The place where visual perception all comes together is the brain. You can think of the visual system as
being comprised of a set of assembly lines.
o Areas along the visual pathways process parts of 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 processing.
Main Pathway: Occipital Lobe
After the optic chiasm, the information from each visual field arrives in the opposite hemisphere, at which
point the optic nerve fibres split and travel along two pathways.
Most of the retinal of ganglion cell axons travel along the main pathway and synaplateralhe
geniculate nucleus (LGN), which is a part off the thalamus that receives visual information.
o After being processed here, the visual signals are sent to areas in the occipital lobe that make up
the primary visual cortex.
There are over 20 cortical areas that process visual information, but most of the research done on visual
processing has concentrated on area V1 of the occipital lobe, otherwise known as the primary visual cortex.
o Collectively, the visual processing areas in the occipital lobe outside the striate cortex are known
as extrastriate cortex.
Primary Visual Cortex
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Just as many photoreceptors may synapse onto a single ganglion cell in the retina, the receptive field of the
LGN is made up of many ganglion cells.
The receptive field of a single V1 cell is a combination of the receptive fields of many LGN cells. So again,