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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 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.
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.
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.
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
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.
Burst firing in pyramidal cells Burst-timing dependent plasticity
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 2π 0 Phase (rad)
Extra assumptions • Non-associative potentiation (in order to prevent the weights from going to zero).
Bursts and isolated spikes code for different features of a stimulus Oswald et al. 2004
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