Internal Models and Interaction Torques

What is the main function of a Forward and Inverse Internal Model working in series?

They predict the trajectory resulting from a motor command to the motor plant, enabling fast, accurate movement control.

True or False: Forward and Inverse Internal Models can only operate independently.

False—they can operate in series for zero-lag predictive control.

What does "zero-lag feedback control" mean?

It refers to predictive control that compensates for plant properties without waiting for delayed sensory feedback.

Compare zero-lag and delayed feedback control.

Zero-lag uses prediction; delayed feedback depends on actual sensory input from the periphery.

Which internal model predicts the sensory outcome of a movement?

Forward internal model.

Which internal model generates the necessary motor commands for a desired movement?

Inverse internal model.

True or False: An inverse model allows fast and accurate feedforward control.

True.

What is feedforward control?

Movement planned and executed in advance using internal predictions instead of feedback.

According to Kawato (1999)

what does a forward internal model provide?,Predicted sensory feedback or trajectory resulting from motor commands.

According to Kawato (1999)

what does an inverse internal model provide?,The motor command necessary to produce a specific desired movement.

Why can't we directly observe internal models in the brain?

Because they exist as distributed neural computations, not as physical structures visible in imaging.

How can scientists infer the presence of internal models?

By studying how the CNS compensates for the body's physics and interaction torques during movement.

What are interaction torques?

Forces generated at one joint due to movement at another joint in a multi-link limb system.

True or False: Interaction torques simplify dynamic analysis.

False—they make it more complex.

During what kind of motion are interaction torques most influential?

During rapid, multi-joint movements.

What does it mean that the CNS plans for interaction torques?

The CNS anticipates joint coupling forces and activates muscles to counteract them before they occur.

Which brain region is most associated with storing internal models of intersegmental dynamics?

The cerebellum.

True or False: The cerebellum projects to the Primary Motor Cortex (M1).

True.

What is the role of the Primary Motor Cortex (M1) in relation to internal models?

M1 sends descending motor commands informed by cerebellar predictions.

What evidence supports the idea that the cerebellum encodes internal models?

Cerebellar neurons show activity that predicts interaction torques and adjusts muscle commands accordingly.

What is cerebellar ataxia?

A movement disorder resulting from cerebellar damage causing poor coordination and inaccurate movements.

List common causes of cerebellar ataxia.

Stroke, tumor, multiple sclerosis, infection, or chronic alcohol use.

What is dysmetria?

A cerebellar symptom where movements overshoot or undershoot their target due to poor coordination.

What is an intention tremor?

A tremor that increases as a person nears their movement target, often seen in cerebellar damage.

What did Bastian et al. (1996) observe in cerebellar patients?

They produced inappropriate shoulder torques and failed to adjust elbow torques for interaction forces.

What does the Bastian et al. (1996) study suggest about cerebellar function?

That the cerebellum helps compensate for dynamic interaction torques, supporting internal model theory.

True or False: The cerebellum directly produces movement.

False—it refines and coordinates movement via predictive control.

What type of neural control compensates for interaction torques between segments?

Feedforward control based on internal models.

What evidence supports inverse-forward model cooperation during grip control?

Grip force increases slightly above load force to prevent slippage, predicted by forward-inverse coordination.

In grip force control

what does the forward model predict?,The necessary load force based on arm dynamics and object weight.

In grip force control

what does the inverse model compute?,The motor commands to achieve the desired lifting trajectory.

Why does grip force slightly exceed load force?

To provide a safety margin against slipping while lifting.

What is meant by "the CNS stores intersegmental dynamics"?

The brain predicts how motion in one segment affects others and compensates automatically.

True or False: Internal models are thought to reside primarily in the spinal cord.

False—most evidence points to the cerebellum and cortical circuits.

Compare the roles of the cerebellum and motor cortex in internal modeling.

The cerebellum predicts and adjusts forces; M1 executes motor commands based on those predictions.

What does the term "interaction moment" refer to?

A torque produced at one joint due to movement of another joint in a connected limb system.

Which theoretical framework explains anticipatory muscle activation before joint coupling?

Internal model theory.

In rapid arm movements

why must shoulder muscles activate before elbow movement?,To counteract predicted interaction torques from elbow acceleration.

What is one major criticism of internal model theory?

That it imposes an engineering metaphor onto biology, assuming computational order where none may exist.

Who criticized internal model theory as "the illusion of internal models in biological movement"?

Mangalam (2025).

True or False: Critics argue internal models overcomplicate biological control.

True.

What is a central argument of internal model critics?

That observed behaviors may arise from simpler dynamics, not from explicit neural "models."

Compare supporters and critics of internal models.

Supporters emphasize predictive computation; critics emphasize emergent, distributed dynamics.

What does cerebellar dysfunction reveal about internal models?

Without cerebellar predictions, movement becomes delayed, inaccurate, and poorly coordinated.

If the cerebellum fails to account for interaction torques

what happens?,Movements overshoot or undershoot, showing lack of predictive compensation.

What type of control allows the brain to anticipate body dynamics?

Feedforward predictive control via internal models.

True or False: Interaction torques only occur in slow

isolated joint movements.,False—they are strongest in fast, multi-joint movements.

Which experimental finding best supports internal models of dynamics?

Preemptive muscle activation at one joint to offset expected torques from another joint.

What role does feedback play when predictive control fails?

It provides corrective information after errors occur.

What brain structure provides "zero-lag" movement correction?

The cerebellum, through its predictive output to M1.

Compare feedforward and feedback roles of the cerebellum.

Feedforward prediction minimizes error; feedback updates the model after movement.

What evidence from grip and load force coordination supports internal model theory?

The brain anticipates load forces before they occur and adjusts grip accordingly.

Why is the cerebellum essential for smooth

coordinated movement?,Because it predicts the dynamic effects of body motion and updates commands in real time.

True or False: Cerebellar damage leads to exaggerated reliance on sensory feedback.

True—patients rely more on slow feedback due to loss of predictive control.

Why is studying internal models scientifically difficult?

Because they cannot be observed directly, only inferred from behavior and neural activity.

If a person catches a ball before seeing it clearly

which type of control are they using?,Feedforward control through predictive internal models.

A patient with cerebellar ataxia overshoots when reaching for a cup. What does this indicate about their internal models?

Their forward model predictions are impaired, leading to poor movement calibration.

Compare feedforward and feedback control in terms of timing.

Feedforward is predictive and fast; feedback is reactive and slower due to sensory delays.

Why would shoulder muscles activate before elbow movement during a rapid reach?

To counteract predicted interaction torques from elbow acceleration.

True or False: Forward models are primarily responsible for predicting sensory outcomes of movement.

True.

A person lifts a heavy object smoothly on the first try—what does this suggest about their internal model?

They have an accurate inverse model that computes the correct motor commands.

In cerebellar damage

why does intention tremor worsen near a target?,Because predictive control weakens and the person relies on delayed feedback.

Compare the main functions of forward and inverse internal models.

Forward models predict consequences; inverse models compute required commands.

If two people perform the same task with different joint angles

what principle does this demonstrate?,Motor redundancy—multiple solutions can achieve the same goal.

When someone adjusts their grip strength to prevent an object from slipping

what internal model process is involved?,Forward-inverse coordination predicting load and friction forces.

True or False: Feedback control is sufficient for accurate high-speed movements.

False—feedback is too slow, so prediction is needed.

Why do cerebellar patients show inappropriate shoulder torque when moving their elbow?

They fail to compensate for interaction torques due to impaired internal models.

Compare how a healthy person and a cerebellar patient adapt to new force fields during reaching.

Healthy individuals adapt predictions quickly; cerebellar patients rely on feedback and learn slowly.

If you change from a light to a heavy racket

which internal model needs to update?,The forward model, to predict new dynamics of limb and object.

True or False: Internal models are thought to reside primarily in the cerebellum.

True.

Why does grip force slightly exceed load force during lifting?

The forward model predicts load and adds a safety margin to prevent slip.

In a delayed visual feedback experiment

why might a subject's movements become unstable?,Because the forward model cannot rely on timely sensory information for correction.

Compare how feedforward and feedback systems handle unexpected perturbations.

Feedforward predicts and prevents errors; feedback detects and corrects them after occurrence.

What evidence supports cerebellar involvement in internal modeling of dynamics?

Preemptive muscle activations compensating for expected interaction torques.

True or False: Interaction torques only occur during slow

isolated joint motions.,False—they are strongest in rapid, multi-joint movements.

How does the cerebellum communicate predictive corrections to the motor cortex?

Through projections to M1, influencing descending motor commands.

When a person adapts to a rotating visual field while throwing darts

which internal model is being modified?,The forward model predicting altered sensory consequences.

Compare internal model theory and dynamical systems theory in one sentence.

Internal model theory emphasizes predictive computation; dynamical systems theory emphasizes emergent coordination.

If a person with cerebellar damage must rely on feedback

how does this affect movement?,Movements become delayed, less smooth, and less precise.

Why do critics argue internal models are "engineering metaphors"?

Because they impose computational order onto biological systems that may rely on emergent, not modeled, behavior.