The labile brain. I. Neuronal transients and nonlinear coupling

In this, the first of three papers, the nature of, and motivation for; neur onal transients is described in relation to characterizing brain dynamics. This paper deals with some basic aspects of neuronal dynamics, interactions , coupling and implicit neuronal codes. The second paper develops neuronal transients and nonlinear coupling in the context of dynamic instability and complexity, and suggests that instability or lability is necessary for ada ptive self-organization. The final paper addresses the role of neuronal tra nsients through information theory and the emergence of spatio-temporal rec eptive fields and functional specialization. By considering the brain as an ensemble of connected dynamic systems one ca n show that a sufficient description of neuronal dynamics comprises neurona l activity at a particular time and its recent history. This history consti tutes a neuronal transient. As such, transients represent a fundamental met ric of neuronal interactions and, implicitly a code employed in the functio nal integration of brain systems. The nature of transients, expressed conjo intly in distinct neuronal populations, reflects the underlying coupling am ong populations. This coupling may be synchronous (and possibly oscillatory ) or asynchronous. A critical distinction between synchronous and asynchron ous coupling is that the former is essentially linear and the latter is non linear. The nonlinear nature of asynchronous coupling enables the rich, con text-sensitive interactions that characterize real brain dynamics, suggesti ng that it plays a role in functional integration that may be as important as synchronous interactions. The distinction between linear and nonlinear c oupling has fundamental implications for the analysis and characterization of neuronal interactions, most of which are predicated on linear (synchrono us) coupling (e.g. cross-correlograms and coherence). Using neuromagnetic d ata it is shown that nonlinear (asynchronous) coupling is, in fact, more ab undant and can be more significant than synchronous coupling.