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Last update 1 Sep 2007

How precise is the connectivity in the cerebral cortex?

It is common in neuroscience literature to read statements about the precision of the connectivity in the brain. But is the connectivity, especially in the cerebral cortex, really precise?

First, I will define the word 'connectivity'. By 'connectivity of a group of neurons' I mean a listing of all the existing synapses between any pair of them. With this definition the strength of the synapse is immaterial to the connectivity, so potentiation/depotentiation of synapses does not affect connectivity. Secondly, I will concentrate on the cerebral cortex, because it demonstrates the point I make most clearly.

How can we tell, in principle, if the connectivity is precise? The 'obvious' way is to compare it to the plan of the connectivity. The problem is that we don't know what the plan is, and in fact we don't even know that there is a plan. Instead, we can compare between different individual brains. If there is a plan to which the connectivity in the cortex conforms, then the connectivity of each individual cortex should be similar to the connectivity of any other cortex. So how similar are the connectivities of different individual cortexes?

The literature contains many descriptions of projections in the cortex, but these describe projections between regions of the cortex, not between neurons. To compare the connectivity, we need to know not only if there are projections from region A to region B, but also which individual neurons in region A project to region B, and with which individual neurons in region B they synapse. In addition, many (probably most) of the connections in the cortex are short-distance, and we need to know about the precision of these too.

So what do we see when we compare the connections of individual neurons in the cortex between different brains? The literature does not actually contain any data about this, but the reason for this is clear to any neuroscientist: the connections of individual neurons in the cortex are so different between individuals that it does not even make sense to try to compare them. That is true not only in humans, but also in mammals in general. The connectivity outside the cortex is more ordered, but even here the connectivity is not replicated between individuals at the level of individual neurons.

The immediate conclusion is that there is no plan for the connectivity of individual neurons in the cortex. Since there is no plan, the connectivity cannot be precise or not precise: the term 'precision' is not applicable. Thus when neuroscientists talk about 'precise' connectivity in the brain, they do not mean the connectivity of individual neurons inside the cortex.

That is clear to neuroscientists, but is it clear to outsiders, i.e. to researchers is 'neighboring' fields and the rest of the public? My impression is that it isn't. From discussions with cognitive psychologists and other people, and from reading cognitive science literature, it seems that outside neuroscience most people believe that the connectivity in the brain is laid precisely according to a plan (an extreme example, and another ).

Is it important? Yes, because it can help us limit the number of models of the functioning of the brain: any model that requires for its implementation precise connectivity is unacceptable. How many of the current models require precise connectivity can be found only by analyzing each model, which I will not try to do here, but I suspect that most of them do, and hence will fail this test. In this case, it will save a lot of effort that is currently spent in researching models that cannot be correct. However, we are not going to know if this is the case until modelers will become aware of this constraint, and analyze their models to see if they meet it.

Conclusions: Neuroscientists know that the individual connections between neurons in the cortex are not the same between individuals, and hence that there is no plan for them. Outsiders don't know that, but it would be useful if they did. Thus neuroscientists should advertise this point, and make it clear that the term 'precise' is not applicable to the connectivity of individual neurons.

Yehouda Harpaz