Opposite Optimal Current Flow Directions for Induction of Neuroplasticity and Excitation Threshold in the Human Motor Cortex

Hebbian and Anti-Hebbian Spike-Timing-Dependent Plasticity of Human Cortico-Cortical Connections

“…Although there is some indirect evidence that STDP is a key factor in governing some mechanisms of associative learning in humans (McMahon and Leopold, 2012), its existence is still matter of debate. Moreover, the experiments performed in brain slices cannot take in account the modules of large-scale connectivity that are crucial in ruling cognition (Varela et al, 2001). Here, by using bifocal transcranial magnetic stimulation (TMS) (Rizzo et al, 2009(Rizzo et al, , 2011Arai et al, 2011;Buch et al, 2011), we provide novel strong evidence supporting the existence of concomitant antithetic forms of STDP mechanisms in largescale connections in humans.…”

“…It has been suggested that the AP orientation would favor the activation of more apical neuronal populations and be more prone to induce stronger bAPs. In contrast, the PA orientation would induce a current flow directed downward from layer 1 to layer 6 that would be more depolarizing at deeper sites (Sommer et al, 2012). This background could explain why PPC-M1 AP PAS induces STDP following the temporal rules of classical STDP.…”

“…In the M1, TMS with AP and PA orientations is known to induce opposite current flows that activate a set of neurons likely located in different cortical sites (Ni et al, 2011). These opposite AP and PA current flows are supposed to induce upward-or downward-directed current along the dendritic tree of the pyramidal neurons in the anterior bank of the central sulcus (Sommer et al, 2012). It has been suggested that the AP orientation would favor the activation of more apical neuronal populations and be more prone to induce stronger bAPs.…”

“…The mean peak-to-peak amplitude of the conditioned MEP at each ISI was expressed as a percentage of the mean peak-to-peak amplitude size of the unconditioned test pulse in that block. SLAI measures were studied and recorded using the technique that has been described previously (Sailer et al, 2003). CSs consisted of single pulses (200 s) of electrical stimulation applied to the right median nerve at the wrist through bipolar electrodes (proximal cathode).…”

“…Here, by using bifocal transcranial magnetic stimulation (TMS) (Rizzo et al, 2009(Rizzo et al, , 2011Arai et al, 2011;Buch et al, 2011), we provide novel strong evidence supporting the existence of concomitant antithetic forms of STDP mechanisms in largescale connections in humans. We used a novel paired associative stimulation (PAS) protocol (Stefan et al, 2000) to repeatedly activate the short-latency connection between the posterior parietal cortex (PPC) and the primary motor cortex (M1) of the left-dominant hemisphere (Koch et al, 2007) (see Fig. 1A).…”