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.
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 fish use perturbations of their selfgenerated 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:
granule cell
Parallel fiber
sensory input
Anti-Hebbian STDP:
presynaptic
postsynaptic
• 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…
Synaptic weights
Most depression
Least depression
stimulus
0
π
Phase (rad)
2π
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