Learning sensorimotor transformations
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Learning sensorimotor transformations. Maurice J. Chacron. The principle of sensory reafference:. Von Holst and 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

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Learning sensorimotor transformations

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Learning sensorimotor transformations

Maurice J. Chacron


The principle of sensory reafference:

Von Holst

and Mittelstaedt, 1950


  • Movementscan 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 and

Flanagan, 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 copycontainsboth temporal and spatial information about the reafferent stimulus.


Adaptive cancellation of sensory reafference


Motor learning:

Martin et al. 1996


  • Sensorimotor coordinationdoes not require the cerebellum.

  • Adaptation to novel conditionsdoes 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.


Anatomy

Trout

Electric Fish


  • 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 fishuseperturbations 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

    causechanges in the efference copy

  • What mechanisms underlie these changes?


Plasticity experiment:

granule cell

Parallel fiber

sensory input


Anti-Hebbian STDP:

presynaptic

postsynaptic


  • Cancellation of unwanted stimuli requires precise timing.

  • Anti-Hebbian STDPunderlies 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…

Synaptic weights

Most depression

Least depression

stimulus

π

0

Phase (rad)


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


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