Eye Movements and Sensory Motor Integration

Saccade

Rapid eye movements

Reflexive or voluntary

How eyes move from A to B

Single movement and can not see anything in between

Foveation

Directing the fovea to different places in order to scrutinize information

Why we have saccades

Eye Muscles

Six pairs of muscles (three on each eye)

Help move eye in all directions and around

Eye muscles are innervated by lower motor neurons from the brainstem

Vergence

Independent movement of eyes that provide depth of field

Also adjusts pupil to let more or less light in

Categories of Eye Movement

Shift direction of gaze (saccades, smooth pursuit, vergence)

Stabilize Gaze

Stabilize Gaze

Vestibulo-ocular and Optokinetic system

Stabilizing movements to prevent slipping of gaze during movement

Timing of Saccades

It takes time to adjust eyes with movement.

about 200 ms

After 200 ms, eyes will adjust and continue with object

Timing of Smooth Pursuit

Initial delay, but when object is reached eyes adjust velocity

Vestibulo-ocular System

Help counter act rotational head movement

Used when you are moving and object is stagnant

Responds faster, transient movements

Optokinetic Systems

Following an object while it is moving

Responds to slower movements (change in visual scene)

Small variation in movement with less than 1 Hz

Vibrations in Eye

Eye is never stagnant, constantly moving in tiny vibration

Allows you to make contrast from the image

Average of systems makes it seem like you see in a straight line

Neuron Activity in Saccades

Saccade causes strong activity in neurons.

Neurons fire faster with saccade, which moves the motor neurons.

Stops when saccade stops

Amplitude of saccade determined by the duration of increase or decreased activity of the motor neuron

Neurons fire more the more lateral you go, slows down as you get more towards the center

Why does motor neuron firing decrease between saccades

During saccade you are producing a force to maintain eye position

Once position is reach, activity is reduced because you are no longer using force to move the eye

Muscles are not getting signal to contract

Circuitry of Eye Movements

Medial and lateral must be innervated the same way to allow eye to move together

Lesioning different places of the eye can cause lazy eye, could be muscles or neurons

Just like vison, medial rectus of one eye means the lateral of the other (muscles)

How do you move eye to the right

Muscles in the right side of your right eye will contract to pull the eye towards the right

Muscles in the left will relax

This movement is controlled by a neuron

Gaze Centers

Superior Colliculus (midbrain)

Frontal Eye Fields (motor cortex of frontal lobes)

Have overlapping functions

Organized by movement vectors (direction of movement controls which neurons fire)

Superior Colliculus

One of the targets of the optic tract

Coordinates head and eye movements to focus on visual targets

Control express saccades

Gives the final modulation of movement

Sensory Motor Integration in the Superior Colliculus

Receives direct input from the retina

Divided into layers

Input from retina goes to visual layer

Information is then sent to motor neurons in motor layer that will modify behavior

Order of Sensory Motor Integration

Retinal Neurons → activate visual neurons → upper motor motor neurons → lower motor neurons (gaze center)

Horizontal and Vertical Gaze Centers

Target of the lower motor neurons

Located in the Pons

Where the saccades are produced

Frontal Eye Fields

Subsection of premotor cortex (planning of movement)

Projects directly to gaze centers and indirectly to gaze centers via the superior colliculus

Information is coming from premotor cortex to superior colliculus to gaze centers

Make voluntary saccades

Upper Motor Neurons

Send information to horizontal gaze center

Connections of Sensory and Motor Structures

Sensory and motor centers of the brain contribute to saccades

Frontal lobe, parietal lobe, basal ganglia (caudate and putamen), superior colliculus, reticular formation etc

Net effect of all of the inputs gives movement

Basal Ganglia Loops

Occulomotor

Direct innervation from substantia nigra pars reticulata to mediodorsal and ventral nuclei which send projections back to the prefrontal cortex

Output of Direct Pathway in Basal Ganglia

Substantia nigra pars reticulata is tonically inhibiting the superior colliculus at rest

During movement substantia nigra is inhibited by caudate and putamen

Disinhibition of Superior Colliculus

Caudate projects GABA neurons to substantia nigra reticulata (direct pathway)

GABA neurons from substantia nigra project to superior colliculus

Upper motor neurons from superior colliculus then project to the gaze centers then to lower motor neurons and then to muscles for movement