Visual Tracking and Fixing

Visual Tracking

• Why Visual Tracking ?– To keep the image in the Fovea which is less than 1mm size and

detect visual field of less than diameter of full moon.– It keeps the eye still when the image is still and stabilizes the

image when the object moves in the world or when the head itself

moves.

• How Visual Tracking?– Oculomotor system: moves the eye in the orbit

– Head movement which moves the orbit in the space

– Information of head motion processed by the vestibular system

Three Axes of Eye Rotations

1. Ductions refer to monocular movements of each eye

2. Versions refer to binocular conjugate movements of both eyes

3. Vergences refer to binocular disjunctive movements

Extra Ocular Muscles

Extra-ocular muscles nerve supply

Actions of Extra-ocular muscles

Laws of Ocular Motor Control

1. Sherrington's law of reciprocal innervation

2. Hering's law of equal innervation

Six Cardinal Position

"Visual Accessing Cues"

Richard Bandler and John Grinder "Frogs into Princes: Neuro Linguistic Programming (NLP) "

Up and to the Left Indicates: Visually Constructed Images (Vc)

If you asked someone to "Imagine a purple buffalo", this would be the direction their eyes moved in while thinking about the question as they "Visually Constructed" a purple buffalo in their mind.

Up and to the Right indicates: Visually Remembered Images (Vr)

If you asked someone to "What color was the first house you lived in?", this would be the direction their eyes moved in while thinking about the question as they "Visually Remembered" the color of their childhood home.

To the Left Indicates: Auditory Constructed (Ac)

If you asked someone to "Try and create the highest the sound of the pitch possible in your head", this would be the direction their eyes moved in while thinking about the question as they "Auditorily Constructed" this this sound that they have never heard of.

To the Right Indicates: Auditory Remembered (Ar)

If you asked someone to "Remember what their mother's voice sounds like ", this would be the direction their eyes moved in while thinking about the question as they "AuditorilyRemembered " this sound.

Down and to the Left Indicates: Feeling / Kinesthetic (F)

If you asked someone to "Can you remember the smell of a campfire? ", this would be the direction their eyes moved in while thinking about the question as they used recalled a smell, feeling, or taste.

Down and To the Right Indicates: Internal Dialog (Ai)

This is the direction of someone eyes as they "talk to themselves".

Brain area participate in Visual Fixation

The cerebral structures involved in fixation are:� Parietal eye field (lateral interparietal area and area 7a in monkeys)� V5 and V5A (MT and MST in monkeys)� Supplementary eye field� Dorsolateral prefrontal cortex

The brainstem structures involved in fixation are:� Substantia nigra pars reticulata in the basal ganglia� Rostral pole of the superior colliculus

An Active Fixation System Keeps the Eyes on a Stationary Target

The Visual Fixation System

•The fixation system holds the image of a stationary object on the fovea when the head is immobile. •Steady fixation is actually an illusion. •Normal fixation consists of three distinct types of physiological miniature movements that are not detectable by the naked eye

Type of Eye MovementFunction of Eye Movement

"Version" (Conjugate) "Vergence" (Disjunctive)

"Holding" (slow)•Smooth Pursuit•Optokinetic Nystagmus(slow phase)•Vestibular Nystagmus

•Convergence•Divergence•Accommodative Vergence

"Catching" (fast)•Saccades•Optokinetic Nystagmus(quick phase)

"Sustaining" (miniature) •Microsaccades •Tremor

•Drift

voluntary eye movement; involuntary eye movement

Eye Movement

The Vestibular and Optokinetic Systems

• The vestibulo-ocular and optokinetic reflexes are the

earliest eye movements to appear phylogenetically

• The vestibulo-ocular reflex (VOR) stabilizes retinal

images during head motion by counter-rotating the eyes at

the same speed as the head but in the opposite direction

• Optokinetic eye movements stabilize the eyes during

tracking of a large moving visual scene, which causes an

illusionary sensation of self rotation (circularvection) in the

opposite direction

Types of Head Motion

1. Rotation : A change in orientation2. Translation: A change in position

Characteristics of the VOR

The VOR stabilizes retinal images during brief head movements bycounter-rotating the eyes at the same speed as the head but in theopposite direction

The Horizontal VOR Pathway

The Vertical and Torsional VOR Pathway

The Otolith-Ocular Pathway

VOR Adaptation and the Cerebellum

The Optokinetic System

•Optokinetic nystagmus is induced reflexively by motion of a large visual scene, which causes an illusionary sensation of self-rotation (circularvection) in the opposite direction

•Whereas the angular VOR responds best to brief, high-frequency head rotation, the optokinetic system maintains retinal image stability during sustained, low-frequency rotation

The Saccadic System Points the Fovea Toward Objects of Interest

Saccads during reading

Pulse-Step of Innervation for Saccadic Eye Movement

Extraocular Motor Signal Eye Position and Velocity

Central control of Horizontal Saccad

Brainstem Generation of Vertical and Torsional Saccades

Cortical Pathway for Saccades

Summary of Central Control of Saccades

The Smooth Pursuit System Keeps Moving Targets on the Fovea

Gaze Involves Combined Head and Eye Movements

Cortical control of Pursuit Eye movement

Summary of Central Control of Pursuit Eye Movement

Vergence Eye movment

Neural Substrate of Vergence Eye Movements

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