VISION AND VISUAL PERCEPTION 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 muscles This 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 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 cornea 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 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 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 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 seeing The 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 Acuity Tracking: Rotation Accommodation Convergence/Divergence Peripheral Vision Perception of size Constancy Perception of Figure and ground Depth Perception Coincident Timing Visual Memory Visualization

VISUAL ACUITY: Visual acuity is the ability to see and identify objects correctly It is needed to identify details of an object or situation STATIC VISUAL ACUITY The ability to see the details of an object when neither you nor the object are moving DYNAMIC VISUAL ACUITY The 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: 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.

ACCOMODATION: 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 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 Constancy This 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-dimension Allows 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 seen The quality of your visual memory is dependant on your past experiences

VISUALIZATION: Your ability to picture or imagine something in your mind It requires activation of your visual cortex