Computational Analysis of Motor Learning

Motor Learning

Three paradigmsForce field adaptationVisuomotor transformationsSequence learning

Does one term (motor learning) fit all?

How to determine similarities/differences?

Acquisition: On-line vs. KR feedback

Generalization: Transfer

Motor Learning

Three paradigmsForce field adaptationVisuomotor transformationsSequence learning

Does one term (motor learning) fit all?

Neural systems: Do these tasks engage common regions?

Motor Learning

Three paradigmsForce field adaptationVisuomotor transformationsSequence learning

Does one term (motor learning) fit all?

Neural systems: Do these tasks engage common regions?

Test case: Cerebellum

Smith and Shadmehr, 2005

Force field adaptation is impaired in patients with cerebellar degeneration and not basal ganglia degeneration.

Prism adaptation impairment in patient with bilateral cerebellar stroke.

Control Participant Cerebellar Stroke

Martin et al., 1996

Gomez-Beldarrain et al., 1998

Sequence learning is absent (ipsilesional) in patients with unilateral cerebellar stroke.

S S S R S S S R

Classical conditioning of eyeblink responseClassical conditioning of eyeblink response

Cerebellar lesions selectively abolish the learned response.

McCormick et al., 1984; many others

Model system of motor learning

1. Dissociation of performance and learning.

2. Many species.

3. Specification of US and CS pathways.

-- US, CS simulations. -- Reversible lesions

4. Genetic manipulations.

Cerebellar lesions selectively abolish the learned response.

But not all forms of classical conditioning.

McCormick et al., 1984; many others

Pre-training lesions

Specifying domain of function.

Task domain: Cerebellum for motor learning.

Computation: Cerebellum for learning precise timing between stimulus events.

Specifying domain of function.

Task domain: Cerebellum for motor learning.

Computation: Cerebellum for learning precise timing between stimulus events.

Timing hypothesis accounts for dissociation of heart rate and eyeblink responses.

Lesions of the cerebellar hemisphere do not abolish eyeblink response but do disrupt the adaptive timing.

Prelesion Postlesion

Trained with 200 ms ISI

Trained with 500 ms ISI

Perrett et al., 1993

Multiple levels of learning.

Simple associations at DCN

Precise timing from cerebellar cortex to flexibly make response adaptive.

Motor Learning

Three paradigmsForce field adaptationVisuomotor transformationsSequence learning

Neural systems: Do these tasks engage common regions?

Revisit role of cerebellum:

Do these tasks require precise timing?

Sequence learning as test case.

Stimuli appear at one of four positions.

Press response key in corresponding position.

Stimuli follow sequence or are chosen at random.

Sequence learning as test case.

Stimuli appear at one of four positions.

Press response key in corresponding position.

Stimuli follow sequence or are chosen at random.

Learning series of spatial associations.Stimuli, responses lack precise timing.Transfer indicates effector-independent learning.

Computational analysis: Cerebellum not essential for sequence learning.

Avalanche of patient and imaging studies with SRT task

central question: Is Structure X involved in learning?

Extensive focus on basal ganglia and cerebellum given hypothesized role in skill, procedural learning, and automaticity.

Patient Groups Degree of Learning Compared to Controls Normal Attenuated None

Basal gangliaParkinson’s: 2 4 1Focal BG lesions 2 1 0 Cerebellum 0 1 5

Patient Groups Degree of Learning Compared to Controls Normal Attenuated None

Basal gangliaParkinson’s: 2 4 1Focal BG lesions 2 1 0 Cerebellum 0 1 5

Imaging results Directional Change in Activation with Learning Increases No Change Decreases

Basal Ganglia 5 6 0Cerebellum 0 6 4

Patient Groups Degree of Learning Compared to Controls Normal Attenuated None

Basal gangliaParkinson’s: 2 4 1Focal BG lesions 2 1 0 Cerebellum 0 1 5

Imaging results Directional Change in Activation with Learning Increases No Change Decreases

Basal Ganglia 5 6 0Cerebellum 0 6 4

If both BG and Cerebellar lesions impair SRT learning, why are imaging results so different?

If cerebellar lesions are so devastating to learning, why decrease with learning, esp. when motor cortical areas show increase w/ learning?

Sequence learning as test case.

Performance-based hypothesis:

Cortical-cerebellar interactions to maintain S-R mapping.

Non-cerebellar systems for forming sequential associations.

Patient deficit: Poor sequence learning because noisy S-R codes provide weak input for associative mechanisms.

Prediction: Deficit in SRT learning will be reduced in patients with cerebellar lesions if S-R coding requirements are minimized.

Prediction: Deficit in SRT learning will be reduced in patients with cerebellar lesions if S-R coding requirements are minimized.

Reach to target Reach to target associated location with central color

Successive targets follow 8-element sequence or selected randomly.

Symbolic cues tax S-R coding system (e.g., Wise premotor)

Patients selectively impaired with symbolic cues.

Consistent with performance problem rather than sequence learning per se.

Experiment 2:Use more traditional keypressing SRT taskCompare with patient control: PD patients

Experiment 2:Use more traditional keypressing SRT taskCompare with patient control: PD patients

Ataxia group shows selective impairment with symbolic cues.

Controls and PD learn with both cues.

Results bring together patient and imaging work.

LesionAll human cerebellar studies have used symbolic cues.One monkey lesion study used direct cues: normal performance

Results bring together patient and imaging work.

LesionAll human cerebellar studies have used symbolic cues.One monkey lesion study used direct cues: normal performance

ImagingConsistent with imaging studies showing reduction in cerebellar activation over the course of learning.

Demands on maintaining S-R mapping will decrease over time as mapping becomes well-learned.

Directional Change in Activation with Learning Increases No Change Decreases

SRT learning 0 6 4

Results bring together patient and imaging work.

LesionAll human cerebellar studies have used symbolic cues.One monkey lesion study used direct cues: normal performance

ImagingConsistent with imaging studies showing reduction in cerebellar activation over the course of learning.

Demands on maintaining S-R mapping will decrease over time as mapping becomes well-learned.

Directional Change in Activation with Learning Increases No Change Decreases

SRT learning 0 6 4

Prediction: Weak cerebellar/prefrontal activation with direct cues.

Motor Learning

Cerebellar role in learning

Sequence learning Indirect (no timing)

Force field adaptation likely (on-line timed error signal)

Visuomotor transformations ???

Motor Learning

Cerebellar role in learning

Sequence learning Indirect (no timing)

Force field adaptation likely (on-line timed error signal)

Visuomotor transformations ???

Imaging and patient work suggests yes.

Working memory account:

Two S-R maps required in transformed environment S-R Map 1: Old MapS-R Map 2: Hypothesis of New Map

Motor Learning

Cerebellar role in learning

Sequence learning Indirect (no timing)

Force field adaptation likely (on-line timed error signal)

Visuomotor transformations ???

Compare single- and multi-step transformations.Single: 25 deg displacement in one stepMulti: 5 deg every 20 trials

Predictions?

Classic model of cerebellum and error detection and correction.

Parallel fibers: Simple spikes indicate context.

Climbing fibers: Complex spikes indicate error.

Complex spike activity leads to weight change.

“Constant” somatosensory input that is either expected or unexpected

Unexp. Exp. Unexp

Unexp. Exp. Unexp Exp Unexp

Errors of Omission and Commission

Actual Yes No

Expected Yes Correct OmissionNo Commission Correct

Errors of commission are well-timed.

Errors of omission lack precise timing.