Notes

How Does the Brain Do Continual Learning? May these ML methods be relevant for the brain?

The Stability - Plasticity Dilemma In ML, we can either:

• Speed up the learning rate, so we learn task 2 faster - but we shall also forget faster.
• Slow down the learning rate, so we forget task 1 slower - but we shall also learn task 2 less.

Similarly the brain needs to strike a balance between:

• More stable synapses, less sensitive to learning.
• More plastic synapses, whiche are more unstable.

For example, children have more plastic brains, and both learn and forget faster than adults. How can we solve this dilemma?

• Two complementary Learning Systems
• Neurogenesis
• Metaplasticity

Two Complementary Learning Systems As evidenced by experiments in rats, memories stored in the Hippocampus are replayed in the same order during sleep for consolidation.

Neurogenesis The process of growth of new neurons. It is known to happen during development in small children at a high rate. Adult neurogenesis:

• Slow rates.
• Limited to certain areas of the brain.

The Hippocampus is one of the areas with adult neurogenesis. We could guess new neurons to be plastic, and older neurons to be more stable. Research on the role of neurogenesis in the formation of new memories is inconclusive so far.

Metaplasticity

We usually talk about synaptic strength, or "weight", which is changed by plasticity. But previous activity could also change how easily a synapse undergoes plasticity. There are mechanisms to regulate the stability or plasticity of the single synapse, acting on a longer timescale. This is called metaplasticity.