Sensory Modality Shapes Early Non-Phase-Locked Alpha Activity Independently of Frequency Definition

Alpha-band activity has been widely associated with sensory and cognitive processing, yet the functional role of non-phase-locked alpha modulation during early sensory stages remains incompletely understood. In particular, it is unclear whether such activity reflects modality-specific neural mechanisms and to what extent its characterization depends on the frequency range used for its estimation. The present study investigated phase-locked and non-phase-locked alpha activity during visual and auditory oddball tasks, focusing on their temporal dynamics and topographical organization. In addition, we examined whether the observed effects depend on the definition of the alpha band by comparing a conventional frequency range (8–13 Hz), subdivided bands (8–10.5 Hz and 10.5–13 Hz), and individualized frequency ranges based on the individual alpha frequency (IAF). Results revealed clear modality-dependent differences, with shorter latencies for auditory responses and a dissociation between anterior and posterior regions, with auditory activity predominating in central areas and visual activity in parieto-occipital regions. Non-phase-locked alpha activity showed a spatial distribution similar to phase-locked activity but with opposite polarity. Importantly, these patterns were consistent across all frequency definitions, indicating that early alpha modulation is primarily determined by sensory modality and cortical organization, rather than frequency definition.