Sensory Systems: Processing and Integration in Motor Control

Skilled Performance and Motor Learning

Introduction to Sensory Systems in Motor Control

Understanding how sensory information is processed and integrated in the brain is essential for controlling skilled movement. Sensory systems provide critical input for motor commands, error correction, and movement planning.

Visual System

Function of the Visual System

• Exteroceptive Sense: Enables identification of objects in space and determination of their movement.

• Visual Proprioception: Provides information about the body's position in space, the relation of body segments, and motion of the body.

• Example: Catching a ball requires both exteroceptive (where is the ball?) and proprioceptive (where is my hand?) visual input.

Higher Order Visual Cortex

Visual information from the cortex is distributed to different brain regions via two main pathways:

• Dorsal Stream ("Where"): Processes spatial visual cues, projects to the posterior parietal region, and is concerned with guiding actions.

• Ventral Stream ("What"): Responsible for pattern discrimination and recognition, projects to the temporal cortex, and is concerned with perception and recognition of objects.

• Example: The dorsal stream helps you reach for a cup, while the ventral stream helps you recognize the cup.

Vestibular System

Function and Anatomy

• Balance Control: The vestibulo-spinal pathway helps maintain balance.

• Head and Neck Positioning: Vestibulo-colic reflexes adjust head and neck position.

• Eye Movement Control: Vestibulo-ocular reflex stabilizes gaze during head movement.

• Anatomy: Includes 3 semicircular canals and 2 otolith organs.

• Signal Transmission: Vestibular signals are carried via the vestibulo-cochlear nerve (CN VIII) to the ipsilateral vestibular nuclei.

Vestibular Nuclei Outputs

• Signals travel from the brainstem to the cortex via the thalamus, where they are integrated with visual and somatosensory information.

• Reciprocal connections with the cerebellum support balance, eye control, and movement coordination.

Somatosensory System

Proprioception and Kinesthesia

• Proprioception: The sense of limb position and movement.

• Kinesthesia: Awareness of movement of body parts.

• First-order Sensory Neurons: Transmit information from peripheral receptors to the spinal cord, converting stimuli into electrical signals.

Dorsal Column-Medial Lemniscus System

• First-order (Primary) Neurons: Ascend in the dorsal column (ipsilateral) from the periphery to the ipsilateral brainstem (medulla).

• Second-order (Secondary) Neurons: Medial lemniscus system, cross to the contralateral thalamus.

• Third-order (Tertiary) Neurons: Project from the thalamus to the ipsilateral somatosensory cortex.

Anterolateral Pathway

• Composed of multiple ascending pathways: spinothalamic (to thalamus), spinoreticular (to pons), and spinomesencephalic (to midbrain).

• Primary Sensory Neuron: Synapses in the ipsilateral dorsal horn.

• Second-Order Sensory Neuron: Crosses the midline over 2-3 segments, ascends in anterolateral white matter to the thalamus.

• Third-Order Neurons: Project to the ipsilateral somatosensory cortex.

Somatosensory Cortex

• The most complex somatosensory processing occurs in the cerebral cortex, integrating input from the contralateral side.

• Electrical stimulation of the primary somatosensory cortex can cause somatic sensation (Penfield, 1930s).

• Neighbouring neurons represent neighbouring body parts, forming a somatotopic map (homunculus).

Processing Somatosensory Information

• Information from the primary somatosensory cortex is relayed to secondary somatosensory and parietal cortices for integration.

• Parietal Cortex: Multimodal association area for spatial analysis and integration, combining visual, proprioceptive, vestibular, and other sensory inputs for movement planning.

Sensory Contributions to Motor Control

Role of Sensory Input

• The nervous system uses sensory input to control movement by:

  • Controlling parameters of motor commands

  • Triggering commands

  • Correcting errors

• Timing and location of sensory processing are crucial for effective motor control.

Integration of Sensory Information

Incorporating Visual and Somatosensory Information into Movement

• Both visual and somatosensory inputs are used to control motor commands, trigger movements, and correct errors in trajectory.

• For movements longer than 200 ms, both inputs are used to correct errors (e.g., visual input after ~200 ms, somatosensory after ~120 ms).

Visual, Somatosensory, or Both?

• Vision and somatosensory inputs guide reaching movements.

• The system uses whichever input is better for the task:

  • Somatosensory for fine control (e.g., handwriting, piano playing)

  • Visual for movements to targets in extra-personal space

• When both are available, a weighted combination is used for tasks like reaching and posture.

Sensory Organization Test

This test assesses the contributions and integration of different sensory systems during balance tasks.

• Six conditions vary the quality of sensory information from different systems (normal, unreliable, or absent inputs).

• Sway, an indicator of stability, increases from condition 1 to 6.

Condition	Visual Input	Somatosensory Input	Vestibular Input	Stability (Sway)
1	Present	Present	Present	High
2	Absent	Present	Present	Moderate
3	Unreliable	Present	Present	Moderate
4	Present	Unreliable	Present	Lower
5	Absent	Unreliable	Present	Low
6	Unreliable	Unreliable	Present	Lowest
Additional info: Table entries inferred from standard Sensory Organization Test protocol.

Somatosensory vs. Vestibular Inputs

• Different sensory inputs are used for different components of postural responses:

  • Somatosensory information triggers the response (timing).

  • Vestibular information scales the response (amplitude).

Summary

• Movement requires monitoring the position of body segments.

• Visual information is processed in distinct pathways for action (dorsal stream) and recognition (ventral stream).

• Somatosensory information travels to the spinal cord for reflexes and to higher centers for integration.

• Vestibular information is processed in the brainstem, cerebellum, and cortex for balance and coordination.

• Sensory integration is essential for skilled movement, and the reliance on different inputs varies with the task and timing.