Driving associative plasticity through cortico-cortical paired associative stimulation (ccPAS) shapes cortical connectivity

Neural plasticity refers to the human brain's ability to adjust its functioning, structure, or efficiency in specific neural circuits in response to environmental demands. These adjustments can lead to changes in perception, emotion, cognition, and behavior. Use dependent plasticity mechanisms drive structural or functional changes, resulting in short- or long-term alterations in the morphological or functional connectivity between cells or neural networks. Repeated activation of neuronal circuits can cause lasting changes in synaptic responses as a key adaptive mechanism (Bliss & Collingridge, 1993; Hebb, D. O., 1949; Kalia, 2008; Xerri, 2012). The efficiency of neuronal signal transmission can improve when the activity of the presynaptic cell consistently supports the activation of the postsynaptic cell (Markram et al., 1997; Bi & Poo, 2001; Jackson et al., 2006). As proposed by Hebb (1949), this form of plasticity is known as spike-timing-dependent plasticity (STDP) and is based on strict timing rules. Several studies induced this type of plasticity through a double-coil transcranial magnetic stimulation (TMS) protocol, called cortico-cortical Paired Associative Stimulation (ccPAS) (Arai et al., 2011; Borgomaneri et al., 2023; Chiappini et al., 2018, 2020; Johnen et al., 2015; Sel et al., 2021; Veniero et al., 2013). These experiments demonstrated the strengthening of connectivity between stimulated regions using TMS and electroencephalogram (EEG) integration to track this modulation. We focused on driving plasticity in specific visual networks involved in the processing of emotional features of human faces (chapters I, II, and III), demonstrating the possibility of enhancing perception and processing. We also investigated pathways underlying decision-making, impulsivity, and inhibition (chapter IV), and finally, we modulated interhemispheric inhibition (IHI) at the motor level (chapter V).