Sensorimotor Integration and Eye Movements

Introduction

Wrapping up the motor system lectures, shifting focus from spinal reflexes and lower motor neurons to upper motor neurons and cortical areas.
Transitioning to eye movements and the role of sensory inputs in movement generation.
Focusing on sensorimotor integration with eye movements as an example.

Sensorimotor Integration

Sensorimotor integration involves the interplay between sensory and motor systems to generate appropriate movements.
It's a continuous loop where sensory inputs are transformed into motor outputs, and the resulting motor behavior provides feedback that influences subsequent motor commands.
This loop is essential for adapting and flexibly changing motor commands based on sensory information.

Example: Inspecting an Object

Goal: Inspect a new object to determine its properties.
State: Current state of the body and environment (proprioceptive, visual, auditory, tactile inputs).
Motor Command: Bring the object closer for better visual inspection.
New State: Object is closer, providing more fine-grained visual information.
Sensory Feedback: Updated information about the object's texture, position, etc.
The loop closes, feeding sensory feedback back into the motor system to generate new motor commands.

Importance of Eye Movements

Eye movements are crucial for various functions, including:
- Visual perception: Sampling high-resolution information from the visual environment.
- Social coordination: Conveying attention and intentions.

Visual Perception

The photoreceptors in the eye are not uniformly high resolution.
Cones are high resolution in the fovea, but resolution decreases in the parafoveal regions.
Eye movements allow us to move the high-resolution fovea to different parts of the visual scene to gather rich information.

Social Coordination

Eye movements are a salient social cue, indicating where others are paying attention.
They play a role in parent-infant interactions and social signaling.

Advantages of Studying Eye Movements

Eye movements are relatively simple to study due to:
- Ease of measurement.
- Restricted degrees of freedom.
- Limited number of controlling muscles (six extraocular muscles).
- Consistent load (eye weight remains constant).
- Smaller number of degrees of freedom (three).

Task of the Oculomotor System

The primary task of the oculomotor system is to estimate the distance and direction the eyes must move to achieve a goal.
This involves converting the difference between the current and intended eye position into a sequence of motor commands that activate the appropriate extraocular muscles.

Eye Movements and Visual Perception

The non-uniform distribution of cones and rods across the retina necessitates eye movements to gather high-resolution information from the entire visual scene.
Without eye movements, visual perception would be significantly different.

Retinal Adaptation

Prolonged stimulation of photoreceptors without eye movements leads to retinal adaptation, where the receptors become less responsive.
Tiny micro-saccades help prevent adaptation by constantly shifting the light falling on the retina.

Eye Muscles

Six extraocular muscles control eye movement:
- Superior oblique
- Inferior oblique
- Superior rectus
- Medial rectus
- Lateral rectus
- Inferior rectus

Muscle Pairs

These muscles work in antagonistic pairs to control eye movements.
Leftward eye movement: Activation of the lateral rectus muscle of the left eye and the medial rectus muscle of the right eye.
Rightward eye movement: Activation of the lateral rectus muscle of the right eye and the medial rectus muscle of the left eye.
Upward eye movement: Activation of the superior rectus muscle and inferior oblique muscle.
Downward eye movement: Activation of the inferior rectus muscle and superior oblique muscle.

Brainstem Circuitry

Extraocular muscles are innervated by lower motor neurons in the brainstem.
Axons from these neurons form three cranial nerves:
- Abducens nerve: Connects to the lateral rectus muscles.
- Trochlear nerve: Connects to the superior oblique muscles.
- Oculomotor nerve: Connects to the remaining muscles.

Types of Eye Movements

Gaze stabilizing eye movements:
- Vestibular ocular reflex (VOR)
- Optokinetic response
Gaze shifting eye movements:
- Saccades
- Smooth pursuit
- Vergence movements

Vestibular Ocular Reflex (VOR)

The VOR is a reflex response that automatically compensates for head movements by moving the eyes in the opposite direction.
Sensory information from the semicircular canals is relayed to the brainstem to generate compensatory eye movements.
It is limited to relatively fast head movements.
Doesn't depend on visual inputs.

Optokinetic Response

The optokinetic system is sensitive to global visual motion produced by slow rotational movements.
It stabilizes the visual image on the retina by producing compensatory eye movements in the direction of visual motion.
Sensitivity profile opposite that of VOR

Complementary Systems

VOR and the optokinetic systems are complementary, with VOR compensating for rapid head movements and the optokinetic system compensating for slow movements.
VOR compensates nearly perfectly for rapid head movements at frequencies around $1$ hertz or above.
Optokinetic is sensitive to global or full field visual motion produced by slow movements.

Smooth Pursuit

Smooth pursuit movements track moving objects.
There is a voluntary and involuntary component.
Metrics include:
- Requires approximately 300ms to initial the smooth pursuit.

Vergence Eye Movements

vergence movements allow us to focus our version in different depths.
They align the fovea of each eye with targets located at different depths.
involves moving both eyes in different directions.

Saccadic Eye Movements

Saccades are rapid eye movements that shift our gaze to different objects of interest.
They occur a couple of times per second and take about 200 milliseconds to initiate.
They shift our gaze to different objects of interest.
They take about 200ms to initiate.
Saccades movements are under voluntary control.
They are ballistic and highly stereotyped movements.
Reflect knowledge of a task domain.

Neural Control of Saccadic Eye Movements

Two separable control tasks:
- Controlling the size of the eye movement (how far).
- Controlling the direction of the saccadic eye movement (which way).
Saccade amplitude(size) is coded by the duration of activity in lower motor neurons in the brainstem.
The direction of the saccade is determined entirely by which muscles are being activated.

Neural Control Diagram

Horizontal and vertical gaze centers in the brain stem control eye movements.
The paramedian pontine reticular formation (PPRF) is the horizontal gaze center.
The rostral interstitial nucleus is the vertical gaze center.

Movement Coordinates

Superior colliculus transforming visual information into a saccade command.

Brain Areas

Superior colliculus (SC) and frontal eye fields (FEF) play important roles in initiating saccades to visual targets.
FEF is a premotor cortex structure that sends connections to the gaze centers.

Organization

When an object appears at a particular location in the visual field, neurons are activated in a particular part of superior colliculus.
The superior colliculus is organized into a visual layer, an interneuron layer, and a motor layer.
Aligned visually and motor maps.

Target Position

Demonstration of tracking ability

Summary

Sensorimotor integration is a crucial process that involves the interplay between sensory and motor systems to generate appropriate movements.
Eye movements are an excellent example of sensorimotor integration, as they require precise coordination between sensory inputs and motor outputs.
The oculomotor system has evolved a variety of eye movements, each with its own specific function and neural control mechanisms.
Understanding the neural control of eye movements can provide insights into the broader principles of sensorimotor integration.