Vision

VISION AND VISUAL PERCEPTIONThe visual system is made up of: the eyes, visual

cortex and visual association cortexEach eye is set into protective cavities in the skull

called orbits or socketsEach eye has its own optic nerve leading to the cortexThe eyelids and a continuous lubrication system that

keeps the eye moist provide for further protectionAn eye can be rotated within its socket by a set of six

musclesThis rotation allows the eye to scan large portions of

the environment quickly and then focus in order to gather visual information

THE STRUCTURE OF THE EYE:Sclera – the tough white outer layerCornea – transparent portion of the sclera at the front of the

eyeConjunctiva – thin protective membrane covering the corneaAqueous humor – small chamber filled with fluid behind the

corneaLens – separates the small chamber from the large chamberVitreous humor – larger chamberIris – a circular diaphragm in front of the lens (gives the eye

its colour)Pupil – the central hole in the iris (light enters the eye

through the pupil)

HOW THE EYE FUNCTIONS: 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 eyeball

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 detailRods 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 chiasm

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 experience(learning)

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 experience

HOW VISION WORKS...Vision is more than just ‘‘seeing’’ something.Vision includes seeing and forming a perception of

what you are seeingThe receptors on the retina will be stimulated by

everything in a movement situation that reflects visible light

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 moment

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 apparatus

VISUAL SKILLS AND ABILITIES:Visual AcuityTracking: Rotation AccommodationConvergence/DivergencePeripheral VisionPerception of size ConstancyPerception of Figure and groundDepth PerceptionCoincident TimingVisual MemoryVisualization

VISUAL ACUITY:Visual acuity is the ability to see and identify

objects correctlyIt is needed to identify details of an object or

situationSTATIC VISUAL ACUITYThe ability to see the details of an object

when neither you nor the object are movingDYNAMIC VISUAL ACUITYThe ability to see the details of an object as it

moves or as you move, or when both you and the image are moving

Tracking: RotationWhen 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.

ACCOMODATION:The ability to change your focus from near to

far objects then back againIt allows you to adjust your eyes to see

objects clearly at varying distances by quickly shifting your point of focus

CONVERGENCE/DIVERGENCE: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

PERIPHERAL VISION: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 ConstancyThis is the ability to judge accurately the size

of different objects that are varying distances away from you.

FIGURE – GROUND PERCEPTION:Allows you to focus on an object or person

that is embedded in a distracting background

DEPTH PERCEPTION:The ability to see objects as three-dimensionAllows you to accurately judge their size, how

far away they are from you and the distances between them

COINCIDENT TIMING: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.

VISUAL MEMORY:Your ability to remember what you have seenThe quality of your visual memory is

dependant on your past experiences

VISUALIZATION:Your ability to picture or imagine something

in your mindIt requires activation of your visual cortex