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Summary Visual System
The system of vision, from the eyes and receptors to the optic nerve and inverted images
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Chapter 6 – The visual systemGeneral information
Migraines affect vision – they cause centred dots that develop into horseshoe looking fortification
illusions in the peripheral gaze.
Light enters the eye and refracts onto the retina, which sends the signals up through to blind spot to
the brain. Animals who can ‘see in the dark’ cannot do so, they can simply see with very dim light.
Humans can see anything on the visual light section of the electromagnetic spectrum. Intensity
depends on the lumens the light shines whereas brightness is subjective to the eye.
The pupil is constricted when a source is nearby and the light is bright, as there is enough light to
enter the eye and create the image. The iris constricts or dilates the pupil depending on the
environmental conditions. Acuity is heightened with closer images, as we see more details, whereas
distant images are seen to those with light and image sensitivity. Acuity is low with distant objects.
There is a greater depth of focus when objects are close-by, as the light focuses easily on the retina
when the pupil constricts. This acuity is sacrificed for distant objects in dim light, but this feature is
essential to see at night.
The ciliary muscles tighten, and the suspensory ligaments relax for close objects, allowing the lens to
thicken. This reduces the diameter of the pupil, allowing focus on closer objects. The opposite
happens for distant objects. The lens flattens and the pupil dilates, allowing higher sensitivity.
Eye position and binocular disparity
Eyes come in pairs and humans have them facing forward to focus better on what is in front.
Squirrels look out for prey, so their eyes are on the side of their heads. We also converge our eyes to
focus on objects in front of us, giving us good visual acuity. This allows us to see the dimensions of
objects. If we hold something out and close one eye, then check the other, we see different parts of
the object. This is called binocular disparity.
Structure of the retina
Ganglion cells take in light initially, passing them onto the bipolar, amacrine, and horizontal cells,
straight to the cone and rod cells. The Bipolar cells pick up information from the cone cells, anything
regarding sensitive lights, so lighter and dimmer colours, are passed to the ganglion by the bipolar
cells. Amacrine cells input the rod signals, horizontal cells focus on managing photoreceptor input
and the ganglion cells take all this information, sending it to the brain via the blind spot. This spot is
here to allow a gap for visual information to pass through. Humans compensate for it by completing
any informational patterns that are not caught by the spot.
Humans also have a fovea, which is a slight dent in the retina
It helps to increase visual acuity by heightening focus.
Surface interpolation – perceiving information about
The surface, colour, and edges of an object to complete it
, Cone and rod vision
Cone cells are photopic, which means they pick up details in bright light and help us see details
during the day. Rod receptors help us see dimly illuminated objects, and they are referred to as
scotopic. It helps to see vague images in dim light, at the sacrifice of acuity. This is collectively
referred to as the duplexity theory. Not many cones are needed to refract onto the ganglion cells,
but many rod cells must converge onto the ganglion for any sort of stimulation. Dim light on the
cones does not summate to much and the ganglion would not fire. There are more rod cells than
cone cells as human vision is very sensitive, with all this information not amounting to high acuity
unless there is high enough intensity for it to be picked up by the cones.
Low convergence and less bipolar cells needed in photopic vision and high convergence in scotopic
vision, with many bipolar cells picking up the sensitive lighting.
There are only cones at the fovea, which helps with visual acuity.
Spectral sensitivity
Same intensity lights presented at different wavelengths – same luminosity, different colours. To
check for photopic spectral sensitivity, different wavelengths of light with the same brightness are
shone on the fovea. For scotopic spectral sensitivity, peripheral lights are shone on the eye at an
intensity low enough to not stimulate the cone cells. People should be able to judge the brightness
of these lights even if they are shone on the side.
Photopic sensitivity – high wavelength lights seem more intense than low wavelength lights. Lower
wavelengths must be more intense for a human to be able to see it.
Scotopic sensitivity – covers lights with lower wavelengths.
Eye movements
The eye is constantly scanning the surroundings and taking in information from many nooks and
crannies’. Our eyes make involuntary fixational eye movements. They tremor, drift, and saccade (jerk
slightly). These movements provide clear images of an always moving environment. Saccade
movements are horizontal, vertical, or oblique.
Visual transduction – conversion of light to neural signals
Transduction is the conversion of one form of energy to another. Light to neural signals. Rhodopsin
is a red pigment which becomes bleached under intense light and could no longer absorb light, but it
returns to normal in the dark. This receptor, when in the dark, keeps sodium channels partially open,
which keeps them slightly depolarised and ensures flow of glutamate. The channels close in bright
light. Rhodopsin is a G protein, which are secondary messengers of the CNS. They help to activate
other mechanisms of the brain, in this case, rhodopsin is a marker of neurodegenerative diseases,
helping to predict their onset.
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