Neurosynaptic plasticity — The cell assembly theory
Continuing on the paradigm of the Neuro mechanics functioning theories of the brain. Another star in the universe twinkles quite bright, the cell assembly and correlation theory. It was proposed by Donald O Hebb.
Synaptic plasticity basically says if axon of cell A is near enouth to excite cell B and persistently fires it, some growth process or metabolic change takes place in one or both cell such that A’s efficiency, as one of the cells firing B, is increased. “Fire together, wire together”.
The cell assembly theory posits that groups of interconnected neurons are formed through a strengthening of synaptic connections that depends on correlated presynaptic and postsynaptic activity.Now the question that you must be asking is, what is pre and post synaptic activity.
Above diagram shows the propagation of the action potential(electric impluse ) from the axon terminal of one neuron called pre-synaptic cell and the one that receives the action potential is called post-synaptic cell.
Pre-synaptic cell has calcium channel at the end of the axon terminal, as the neurotransmitter, the action potential carrier reaches the axon terminal, it depolarizes the membrane and Ca2+ flows in. The Ca2+ influx triggers synaptic vesicles to release neurotransmitter. Neurotransmitters are either small amine molecules, amino acids, or neuropeptides. Neurotransmitters bind to receptors on target cell i.e. the post synaptic cell and propagates the signal.
The cell assembly theory postulates that groups of interconnected neurons are formed through a strengthening of synaptic connections that depends on pre and post synaptic activity. Thus, this enables brain to take pre-defined axonic paths in the brain for same, repetitive and interpolating tasks.This gives rise to the correlation theory which is theoretical treatments of synchronised brain activity and its role is in associating or binding attributes to specific objects or causes. This is called synaptic efficacy.
The synaptic efficacy coincides with the free energy principle(refer my blog on free energy principle) as to optimise the encoding of the connection strength in the brain which results in the expectation of observation due to a cause in the environment.
Gradient descent on free energy (that is, changing connections to reduce free energy) is formally identical to Hebbian plasticity.
Let’s summarise what we wrote above with an example. Let’s take up our cat example again. Suppose you observe a cat, it becomes a cause of sensory input. The sensory input is the light waves reflecting off the surface of the cat to your eyes. The sensory input traverses through the neurons in your brain which in mathematical formulation is a generative probabilistic model(Bayesian Brain Hypothesis). Now to encode precise and efficient representation of the image of the cat, the brain model would like to take a minimal and most correct synaptic path in the brain. This would lead to minimisation of the free energy in terms of the brain’s internal modelling of the cat and the external/environmental evidential encoding of the cat.
While writing this blog, the lottery ticket hypothesis came to my mind, which rides in the paradigm of the network pruning methodology. It states there are sub-networks to be found inside parent network which performs as par with the parent network. This synaptic plasticity and correlation theory tells somewhat the same story. Imagine a child who opens up his eyes for the first time as and when he is new born. The brain all together is a parent network and the environment around the child causes sensory input, which triggers almost all the neurons randomly in the brain and gradually over a period of time with multiple iterations an efficient synaptic path is defined in the child’s brain which becomes the efficient encoding of the objects of concern in his brain.
It simply amazes me, how we as human race became this level of sophisticated bio engineered product that too through natural selection phenomenon.
Leaving on this note, The next blog will be on the theory of Neural Darwinism. Till then stay safe!!