Learning sensorimotor transformations

Maurice J. Chacron

The principle of sensory reafference:

Von Holstand Mittelstaedt, 1950

• Movements can lead to sensory reafference (e.g. body movements)

• An efference copy and the reafferent stimulus are combined and give rise to the

perceived stimulus.

• Question: how is the efference copy combined with the reafferent stimulus to give rise to the perceived stimulus?

Mechanical tickling experiment:

Blakemore, Frith, and Wolpert, J. Cogn. Neurosci. (1999)

• Motor command arm movement

• Reafference tactile stimulus • Perceived stimulus tickling sensation

Wolpert andFlanagan, 2001

• The predicted sensory stimulus (efference copy) is compared to the actual stimulus

• If there is a discrepancy, then the subject perceives the stimulus as causing a tickling sensation.

• The efference copy contains both temporal and spatial information about the reafferent stimulus.

Adaptive cancellation of sensory reafference

Motor learning:

Martin et al. 1996

• Sensorimotor coordination does not require the cerebellum.

• Adaptation to novel conditions does require cerebellar function.

• Adaptation is an error driven process.

Cerebellar Plasticity:

Co-activation of parallel and climbing fiber input gives rise toLTD

• How does cerebellar LTD help achieve cancellation of expected stimuli?

Weakly electric Fish

• Electric fish emit electric fields through an electric organ in their tail.

Trout Electric Fish

Anatomy

• The cerebellum of electric fish is very developed.

• Cerebellar anatomy is conserved across vertebrates.

• Electric fish have “simple” anatomy and behaviors.

• Electric fish are a good model system to study cancellation of reafferent input.

Electrolocation

• Electric fish use perturbations of their self-generated electric field to interact with their environment.

• Pulses generated by the animal can activate their own electrosensory system.

• Are there mechanisms by which sensory neurons can “ignore” these reafferent stimuli?

Cerebellar-like anatomy:

Bell, 2001

Bell, 2001

• Changes in the reafferent stimulus cause changes in the efference copy

• What mechanisms underlie these changes?

Plasticity experiment:

Parallel fiber

granule cell

sensory input

Anti-Hebbian STDP:

postsynapticpresynaptic

• Cancellation of unwanted stimuli requires precise timing.

• Anti-Hebbian STDP underlies the adaptive cancellation of reafferent input.

How?

Adaptive cancellation of tail bends

Cerebellar-like anatomy

Anatomy

Burst firing in pyramidal cells

Burst-timing dependent plasticity

Model of adaptive cancellation in the electrosensory system

Model Assumptions: How to “carve out” a negative image

• A subset of cerebellar granule cells fires at every phase of the stimulus

• Probability to fire a burst is largest/smallest at a local stimulus maximum/minimum

• Weights from synapses near the local maximum/ minimum will be most/least depressed

Graphically…

Phase (rad)0 2ππ

stimulus

Most depression

Least depression

Synaptic weights

Extra assumptions

• Non-associative potentiation (in order to prevent the weights from going to zero).

Does the model work?

Bursting is frequency dependent

Bursts and isolated spikes code for different features of a stimulus

Oswald et al. 2004

Adaptive learning

Summary

• Sensorimotor transformations require learning.

• This learning must be adaptive (e.g. adapt to changes during development, etc…)

• Anti-Hebbian plasticity provides a mechanism for adaptive cancellation of reafferent stimuli