Objective: Spectral harmonicity of the ictal activity was analyzed regarding two clinically relevant aspects, (1) as a confounding factor producing 'spurious' phase-amplitude couplings (PAC) which may lead to wrong conclusions about the underlying ictal mechanisms, and (2) its role in how good PAC is in correspondence to the seizure onset zone (SOZ) classification performed by the epileptologists. Methods: PAC patterns observed in intracerebral electroencephalography (iEEG) recordings were retrospectively studied during seizures of seven patients with pharmacoresistant focal epilepsy. The time locked index (TLI) measure was introduced to quantify the degree of harmonicity between frequency bands associated to the emergence of PAC during epileptic seizures. Results: (1) Harmonic and non harmonic PAC patterns coexist during the seizure dynamics in iEEG recordings with macroelectrodes. (2) Harmonic PAC patterns are an emergent property of the periodic non sinusoidal waveform constituting the epileptiform activity. (3) The TLI metric allows to distinguish the non harmonic PAC pattern, which has been previously associated with the ictal core through the paroxysmal depolarizing shifts mechanism of seizure propagation. Conclusions: Our results suggest that the spectral harmonicity of the ictal activity plays a relevant role in the visual analysis of the iEEG recordings performed by the epileptologists to define the SOZ, and that it should be considered for the proper interpretation of ictal mechanisms. * Corresponding authors. Highlights 1 • Harmonic and non harmonic phase-amplitude couplings coexist in epilep-73 nusoidal waveform constituting the seizure activity as a mechanism originating 74 PAC, referred as 'harmonic PAC'. Implications of the coexistence of harmonic 75 and non harmonic PAC patterns during epileptic seizures regarding underlying 76 neural mechanisms and improved therapy are also discussed.77 3 2. METHODS 78 2.1. Patients 79 The seven patients with pharmacoresistant focal epilepsy included in this 80 retrospective study were consecutively selected from the candidates for surgi-81 cal treatment between 2012 and 2015 at the Epilepsy Center -"Ramos Mejía" 82 Hospital and El Cruce "N. Kirchner" Hospital -Universidad Nacional Arturo 83 Jauretche, Argentina. The patients underwent chronic invasive EEG procedure 84 guided by clinical criteria to help identify the epileptogenic zone (EZ) for sub-85 sequent resection. The characteristics of the patients are provided in Table 1 86 and the schematic view of the SOZ, as identified by the epileptologists (NC and 87 SK), is shown in Figure 1. 88 * No, Number of implanted unipolar electrode contacts; * * Number of seizures analyzed in this study; 5 6 2.2. Localization of electrode sites and imaging for three-dimensional reconstruc-89 tion 90 Electrode placement considered not only the final target but also the pre-91 cise trajectory. Surgical planning of electrode placement in specific anatomical 92 structures was thus dependent on the pre-established hypotheses of p...