The N30 component of the somatosensory evoked potentials: a new tool for EEG dynamic exploration of human brain in space

Somatosensory evoked potentials are robust indicators of the afferent information at cortical level. In particular, the frontal N30 component of SEP can serve as a reliable physiological index of the dopaminergic motor pathway (Insola et al., 1999, Pierantozzi et al., 1999). Its properties in sensory-motor gating and cognitive processes make its fine analysis particularly interesting. The physiological interpretation and the origin of the frontal N30 are still debated (Allison et al., 1991, Cheron et al., 1994, Karnovsky et al., 1997, Balzamo et al., 2004, Barba et al., 2005).

In this thesis we have investigated the mechanisms generating the N30 SEP component produced by electrical stimulation at median nerve at wrist, with reference to the current questioning of the additive and oscillatory models of the ERP (Sayers et al., 1974; Basar et al., 1980).

We have applied analysis of the spectral content of neuronal oscillatory activity recorded in electroencephalographic (EEG) in order to study of dynamic brain processing underlying the N30 component. Concretely for studying whether the occurrence of the N30 related input induce amplitude modulation and/or reorganization of EEG rhythms we have analyzed separately power perturbation and phase synchrony of single EEG oscillations trials by means of event-related spectral perturbation (ERSP) and intertrial coherence (ITC) measurements. In addition, in order to model brain localizations of phase synchrony and power enhancement and to compare them to model localization of the N30 SEP we used swLORETA, a distributive method of source analysis.

We have demonstrated that:

(1) Ongoing EEG signals contribute to the generation of the N30 component (Cheron et al., 2007).

(2) Dynamics of ongoing EEG signals underlie the specific behavior of the N30 during gating produced by movement execution (Cebolla et al., 2009).

(3) Localization of brain sources generating the N30 SEP component overlaps those generating beta-gamma ongoing oscillations at the same short latency (Cebolla et al., 2010).

Additionally the work developed during this thesis has served to develop a comprehensive, pragmatic paradigm to identify, evaluate and understand the somatosensory alterations in defined contexts, as illustrated by our recent work on perturbations and adaptations in astronauts over long term microgravity stay. We think that addressing this topic is essential in order to optimize and objectively evaluate adaptation to microgravity. We therefore proposed a detailed project to European Space Agency entitled “The frontal N30 somatosensory evoked potential for the study of sensory-motor and cognitive adaptations in weightlessness: NeuroSEP” (ILSRA 2009) in which we also proposed direct applications for quality of life aboard International Space Station, for the medical field and industry.