learning sensorimotor transformations
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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
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