Unit 5 the eye SHMD219

 The visual system is made up of: the eyes, visual cortex and
visual association cortex
Each eye is set into protective cavities in the skull called
orbits or sockets
Each eye has its own optic nerve leading to the cortex
The eyelids and a continuous lubrication system that keeps
the eye moist provide for further protection
An eye can be rotated within its socket by a set of six
This rotation allows the eye to scan large portions of the
environment quickly and then focus in order to gather
visual information
Sclera – the tough white outer layer
Cornea – transparent portion of the sclera at the front of the eye
Conjunctiva – thin protective membrane covering the cornea
Aqueous humor – small chamber filled with fluid behind the
Lens – separates the small chamber from the large chamber
Vitreous humor – larger chamber
Iris – a circular diaphragm in front of the lens (gives the eye its
Pupil – the central hole in the iris (light enters the eye through
the pupil)
 Whenever the light reflected off of images in the environment
passes through the pupil and is focused on the retina that is
when stimulation of the photoreceptors occurs
The retina is a layer of light-sensitive cells on the back of the
There are two kinds of receptor cells: the rods and the cones.
They contain chemicals that react to light stimulation
The rods do not require much light to stimulate them while the
cones do require more substantial light
Cones are responsible for the perception of color and detail
Rods support vision in dim light and the detection of motion
 The stimulation of the photoreceptors is transformed into
stimuli and can be transported via the optic nerve to the
visual cortex and visual association cortex
The nerve fibres from the receptors of the retina converge
at a spot called the blind spot and form the optic nerve that
will transport all messages to the brain
No vision is possible when an image hits your blind spot
because there are neither rods nor cones on that spot
At the back of the eye there is one area with a high
concentration of cones called the fovea
Positioning your eyes and head so that the image you are
looking at is focused on the fovea gives the best and most
specific visual information
 The optic nerves from the left and right eyes meet at the optic
at the chiasm about half of the fibres from each eye cross over
the midline to join the remaining fibres from the opposite eye to
form a tract
The nerve fibres that start at the inner or nasal side of each retina
are the ones that cross over at the chiasma
The right optic tract carries messages corresponding to what is
seen on the left side of the field of vision
The left optic tract carries messages corresponding to what is
seen on the right side of the field of vision
From the optic chiasm, the tracts pass on each side to the
thalamus, where interneurons then relay the messages on to the
visual cortex.
The left visual cortex receives information from the left side of
the retina in each eye
The right visual cortex receives information from the right side of
the retina in each eye
 You must not only see what is happening around you but you must also
understand what you are seeing
The interneurons of the visual cortex process the incoming messages
The cortex is able to store a memory or record of ‘rules’ for interpreting
messages in some kind of network of interneurons
These networks are established genetically and through actual
Example :a certain combination of visual messages can be interpreted
by a novice ball player to mean ‘there is a ball flying toward me’
With experience(learning), the interpretation of the same combination
of visual messages is modified to mean ‘there is a cricket ball flying
quickly toward my face
The details of how the brain is able to read these messages is not yet
known, but the reading does occur
The speed and accuracy of message reading can be improved with
 Vision is more than just ‘‘seeing’’ something.
 Vision includes seeing and forming a perception of what
you are seeing
The receptors on the retina will be stimulated by
everything in a movement situation that reflects visible
The patterns of stimulation that reflected light makes on
your retina are called the optic array
The optic array is everything that you can see at a particular
The background can give you a lot of information about
what it is that you are seeing
 The optic array makes a stable picture on your retina
only if you and the environment are stable
 If you change where you are looking to focus on
something different, the pattern of stimulation on
your retina also changes
 The brain learns to read these patterns change of the
optic array
 The flow of the patterns of stimulation across your
retina is called the optic flow
 The optical flow has a size , direction and speed
 The pattern of this flow is used by the brain to
determine the direction and speed of motion of any
object that has stimulated the retina
 The larger the image, the closer you are to the
Visual Acuity
Tracking: Rotation
Peripheral Vision
Perception of size Constancy
Perception of Figure and ground
Depth Perception
Coincident Timing
Visual Memory
 Visual acuity is the ability to see and identify objects
 It is needed to identify details of an object or situation
 The ability to see the details of an object when neither
you nor the object are moving
 The ability to see the details of an object as it moves or
as you move, or when both you and the image are
Tracking: Rotation
 When tracking slow motions, you can use a visual skill
called rotations.
 During rotations, you can lock your focus on an object
an hold it there, thus getting continuous information
about the object.
 The ability to change your focus from near to far
objects then back again
 It allows you to adjust your eyes to see objects clearly
at varying distances by quickly shifting your point of
 Convergence: your ability to focus both your eyes on
the same object and hold that focus as you move
toward the object or as the object moves toward you
 Divergence: is your ability to focus both of your eyes on
the same object and hold that focus as the object
moves away from you or as you move away from it
 Refers to what you can see in the parts of the optic
array that fall outside of your foveal focus
 It is the extent of the optic array you can see without
looking directly at it
 It is critical for your general awareness of what is
happening in a situation and your ability to identify
what is moving outside of your primary gaze
Perception of Size Constancy
 This is the ability to judge accurately the size of
different objects that are varying distances away from
 Allows you to focus on an object or person that is
embedded in a distracting background
 The ability to see objects as three-dimension
 Allows you to accurately judge their size, how far away
they are from you and the distances between them
 The ability to judge when a travelling object is going to
arrive at a certain point in space so that you can meet it
 It involves prediction since you must anticipate when
an object will be at a certain point, so that you can hit
it, catch it etc.
 Your ability to remember what you have seen
 The quality of your visual memory is dependant on
your past experiences
 Your ability to picture or imagine something in your
 It requires activation of your visual cortex

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