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 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 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 to LTD

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

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 2π π 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