The ‘secret connections’ of the brain: a connectomic reserve for neuroplasticity?

We here propose a new concept linked to neuroplasticity and cognitive reserve: Connectomic reserve. It is suggested and discussed as a hypothesis, aiming to stimulate experimental testing, and taking as an example the exuberant circuits formed along development by commissural neurons and their projections. Upon reviewing the developmental strategies of formation of callosal circuits, from axonal emission by cortical cell bodies to the choice of trajectories within the ipsi and contralateral hemispheres, we identified the formation of many ectopic and heterotopic projections. Ectopic projections were first described in cases of congenital malformations, but were later shown to exist also in typical adult brains. Heterotopic projections are usually presumed to be eliminated and give way to the more restricted set of connections of adult brains. However, many experiments have shown that they, in fact, remain "hidden" in the brain throughout life in typical subjects of many species. Morphological shaping (pruning or enlargement), and physiological modulation (inhibition or enhancement) of these circuits would be resources to optimize the function of the alternative bundles and preterminal arbors, typical of neuroplasticity. The former mechanisms would take place during development, the latter in adulthood. Connectomic reserve, thus, would consist of a pervasive set of connections broadly distributed throughout the brain, susceptible to selective neuroplastic shaping or modulation depending on the requirements of the external or internal environment. Of course, being a hypothesis, it requires experimental testing in brain circuits other than commissural tracts, and functional validation using emerging imaging techniques with great resolution.