PMID-24711417 Evidence for a causal inverse model in an avian cortico-basal ganglia circuit
- Recorded an stimulated the LMAN (upstream, modulatory) region of the zebrafinch song-production & learning pathway.
- Found evidence, albeit weak, for a mirror arrangement or 'causal inverse' there: neurons fire bursts prior syllable production with some motor delay, ~30ms, and also fire single spikes with a delay ~10 ms to the same syllables.
- This leads to an overall 'mirroring offset' of about 40 ms, which is sufficiently supported by the data.
- The mirroring offset is quantified by looking at the cross-covariance of audio-synchronized motor and sensory firing rates.
- Causal inverse: a sensory target input generates a motor activity pattern required to cause, or generate that same sensory target.
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- Similar to the idea of temporal inversion via memory.
- Data is interesting, but not super strong; per the discussion, the authors were going for a much broader theory:
- Normal Hebbian learning says that if a presynaptic neuron fires before a postsynaptic neuron, then the synapse is potentiated.
- However, there is another side of the coin: if the presynaptic neuron fires after the postsynaptic neuron, the synapse can be similarly strengthened, permitting the learning of inverse models.
- "This order allows sensory feedback arriving at motor neurons to be associated with past postsynaptic patterns of motor activity that could have caused this sensory feedback. " So: stimulate the sensory neuron (here hypothetically in LMAN) to get motor output; motor output is indexed in the sensory space.
- In mammals, a similar rule has been found to describe synaptic connections from the cortex to the basal ganglia [37].
- ... or, based on anatomy, a causal inverse could be connected to a dopaminergic VTA, thereby linking with reinforcement learning theories.
- Simple reinforcement learning strategies can be enhanced with inverse models as a means to solve the structural credit assignment problem [49].
- Need to review literature here, see how well these theories of cortical-> BG synapse match the data.
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