Titolo della tesi: Physiological Consequences of the Vertex Potential
Salient events in the sensory environment that may require immediate behavioural reactions elicit a large, transient, biphasic negative-positive response in the time domain in the ongoing EEG: the vertex potential (VP). This response has been shown to affect cortical motor output in a variety of tasks and facilitate purposeful and rapid behavioural responses. In this thesis, I investigate physiological consequences of the vertex potential. In chapter I, I provide a general introduction to the topic. In chapter II, I report the results of a first human experiment investigating whether the VP results in a domain-general modulation of cortical activity. To do so, I tested if the VP modulates also early modality-specific lemniscal somatosensory processing. The results show that the magnitude of the P45 somatosensory evoked potential, which reflects intracortical processing, was modulated by the phase of the concomitant VP. In chapter III, I showed that the results described in chapter II can be replicated in rodents, as the effect of the VP on the lemniscal processing is phylogenetically preserved. In chapter IV, I tested whether the VP differentially modulates lemniscal somatosensory processing at thalamic vs cortical level. To do so, I analysed the high frequency oscillations elicited by the somatosensory stimulus. Indeed, high frequency oscillations can be divided in two components: an early component reflecting thalamic activity and a later component corresponding to the spiking activity generated within S1. The results obtained in this study suggest that the VP influences somatosensory processing selectively at cortical level. In chapter V, I explored VP-dependent modulation of cortico-spinal output when probed directly. The preliminary results of this experiment suggested that the amplitude of the motor evoked potentials elicited using single pulse TMS was overall suppressed during the concomitant VP. Finally, in chapter VI, I discuss (i) the VP as a result of a transient activation of the extralemniscal system; (ii) the similarities between the VP and sleep slow waves, and (iii) the generality of the effects of the VP on the ongoing cortical activity across different brain systems.