In the past, many neuroscientists have dismissed field effects as having a function role in the CNS because they were too “weak” to trigger action potentials. So while classical synaptic transmission is taught extensively in basic neuroscience and medical textbooks, field effects are often wholly omitted from the discussion, or like gap junction coupling [link to non-synaptic] are briefly presented as secondary and ambiguous phenomena. Through our research we have become increasingly convinced that field effects in fact play a unique and defined role in CNS function – specifically we have considered how the fact that endogenous fields both reflect aggregate population activity and simultaneously coherently effect a neuronal aggregate may result in field effects playing a pivotal role in coherent activity and timing-dependant information processing.
As indicated above, field effects may play a critical role in modulating network spike timing as a result of several unique features of field effect interactions. Field effects are uniquely both rapid, like synaptic chemical transmission or gap junctions, and also global and cumulative, like chemical neuro-modulators. The magnitude of induced extracellular fields are additive reflecting the cumulative activity of neurons, and they have the potential to simultaneously polarize a large number of neurons exposed to generated electric fields. This stands in contrast to synaptic coupling, which involves only localized and salutary interactions between pairs of neurons. Moreover, field effect coupling is not affected by axonal propagation or synaptic transmission delays. Because they are cumulative, with the potential for feed-back unto the same population generated them, the system dynamics of field effects are unique. Finally, field potentials are generally localized, and so it is only regional spike timing that may be affected. Thus, for coherent firing, or oscillations of local populations, field effects could play a distinctive and unique role.
Additional work is evidently required before field effects can be established as a pivotal mechanism for information transfer and integration in the brain (and so essential neuroscience learning!) and we expect to continue to contribute to in this area – this type of analysis require leveraging of traditional neural engineering skill sets.
Despite only “weakly” polarizing membranes, we have continually emphasized that 1) weak signal can profoundly modulate CNS function; and 2) the non-linear nature of the CNS means that small field effect may be “amplified”. Working closely with colleagues, our efforts have therefore focused on developing a precise and quantitative framework to address this modulation and amplification.