That which fire together wires together… Is it?

I don’t think there is anyone in this entire existence who hasn’t heard this famous quote before. Originally emerged through the lips of the old Hebb himself, but how much of it is really factual in the actual neuroscience? Does our brain really wire when its on fire?

Hebbian theory is a neuroscientific theory claiming that an increase in synaptic efficacy arises from a presynaptic cell’s repeated and persistent stimulation of a postsynaptic cell. It is an attempt to explain synaptic plasticity, the adaptation of brain neurons during the learning process. It was introduced by Donald Hebb in his 1949 classic book “The Organization of Behavior”. Put in another way, the strength of connection between any two neurons increases whenever the postsynaptic cell fires just after presynaptic activation. That’s why this rule is also known as “Spike Time dependent plasticity [STDP]” as the temporal coherence is very crucial for synaptic adaptation.

Synaptic plasticity can be generally categorized under LTP and LTD. Long-term potentiation (LTP) is a persistent strengthening of synapses based on recent patterns of activity. These are patterns of synaptic activity that produce a long-lasting increase in signal transmission between two neurons. The opposite of LTP is long-term depression, which produces a long-lasting decrease in synaptic strength. Both are believed to be consequence of hebbian learning. But there are opposing views as well. It has been studied that hebbian learning requires retrograde signalling for its functioning i.e. backpropagating action potentials in the brain which are highly controversial ideas considering the fact that neuronal activity is forward propagating.

There are studies which has showed that brain can also have non-hebbian synapses. I.E. the synaptic plasticity occurs without following the traditional firing together rule. For e.g. how synaptic modification may not simply occur only between activated neurons A and B, but to neighboring neurons as well. The special case of this is anti-hebbian where the cells which fire together doesn’t wire together instead. From my point of view Hebbian like learning is not a pure learning method at all. By learning it means proper “credit assignment” of synaptic weights leading to the required behaviour. But hebbian learning is more like pruning of the unused neuronal connections leading to optimal usage of brain’s wetware resources.

So not everything which fires together wires together, but its the balance of both types of synaptic activities which keeps our brain dance alive.