Sensorimotor Control: Internal Models and Motor Function

Sensorimotor Control

Introduction to Sensorimotor Control

Sensorimotor control refers to the brain's ability to convert sensory information—both stored and real-time—into purposeful movement. This process is fundamental to biological psychology and underpins how humans interact with their environment through voluntary actions.

• Key Point: The brain must determine which muscles to activate to achieve a desired movement.

• Internal Representation: To generate an initial movement trajectory, the brain requires an internal representation of body geometry and the interaction of motor systems (e.g., eye, arm) with the environment.

• Development: These representations likely develop through experience, forming connections between brain areas responsible for motor behavior.

• Example: Infants learning to reach for objects are developing these internal models through trial and error.

Information Processing Model in Motor Control

Stages of Information Processing

The information processing model describes how sensory input is transformed into motor output through a series of stages:

• Stimulus Identification: Recognizing relevant sensory information.

• Response Selection: Deciding which movement to make.

• Movement Programming: Planning and executing the chosen movement.

Additional info: This model is foundational in cognitive psychology and is often used to explain reaction time and decision-making in motor tasks.

Internal Models for Motor Control

Types of Internal Models

Internal models are neural mechanisms that predict and control motor behavior. There are two main types:

• Forward Model: Predicts the behavior of the motor system in response to a command and estimates the current and future state of the effector (e.g., arm, leg).

• Function: Forward models can predict sensory consequences from efference copies of issued motor commands.

• Inverse Model: Calculates the necessary feedforward motor commands from desired motor output information, using an 'inverse model' of the musculoskeletal system.

• Function: Inverse models determine what muscle activations are needed to achieve a specific movement goal.

Forward Model Example

• Example: When reaching for a cup, the forward model predicts the sensory feedback (e.g., hand position) that should result from the movement command.

Inverse Model Example

• Example: To move your hand to a target, the inverse model calculates which muscles to activate and in what sequence.

Adaptation and Error Correction

Role of Adaptation in Internal Models

Internal models are updated through adaptation when discrepancies arise between desired and actual motor output.

• Discrepancy Resolution: The adaptation component compares desired motor output to actual output and updates the internal model accordingly.

• Equation: $\text{Error} = \text{Desired Trajectory} - \text{Actual Trajectory}$

• Example: If you miss catching a ball, your brain updates its internal model to improve future attempts.

Combined Use of Forward and Inverse Models

Integration in Motor Function

Both forward and inverse models are used together to achieve precise motor control.

• Process: The motor target and current position are input into the inverse model to determine necessary muscle activations.

• During Movement: The forward model estimates movement end-point based on sensory inflow and efference copy.

• Discrepancy: Any difference between predicted and actual movement leads to modification of ongoing motor commands.

Neural Basis of Internal Models

Brain Regions Involved

Internal models for motor control are believed to exist in specific brain regions:

• Parietal Cortex: Involved in integrating sensory information and spatial awareness.

• Premotor Cortex: Responsible for planning and selecting movements.

• Cerebellum: Critical for fine-tuning movements, procedural learning, and ongoing movement monitoring.

Additional info: Neuroimaging studies show activation in these areas during motor learning and execution tasks.

Summary Table: Forward vs. Inverse Models

Model Type	Function	Example
Forward Model	Predicts sensory consequences of motor commands	Anticipating hand position when reaching
Inverse Model	Calculates necessary motor commands for desired movement	Determining muscle activations to grasp an object

Conclusion

Forward and inverse internal models are essential for sensorimotor control, allowing the brain to plan, execute, and adapt movements. These models are supported by neural circuits in the parietal cortex, premotor cortex, and cerebellum, and are refined through experience and procedural learning.