Lateral Specialization of Cognitive Mode: II. EEG Frequency Analysis

ABSTRACT This study reports a frequency analysis of hemispheric EEG asymmetries in normal subjects performing cognitive tasks. Language and arithmetic tasks were expected to engage primarily the left hemisphere; spatial and musical tasks were expected to engage primarily the right hemisphere. Both motor and non‐motor tasks were used, e.g. writing a letter and composing a letter mentally. Recordings from temporal leads (T 3 ,T 4 ) and parietal leads (P 3 ,P 4 ) referred to the vertex C z were subjected to discrete Fourier transforms; ratios of power from homologous leads (T 4 /T 3 , P 4 /P 3 ) were computed in conventional frequency bands. These ratios (right/left) are significantly higher in verbal and arithmetic tasks than in spatial tasks primarily in the alpha band; the beta and theta bands show this effect less consistently. The delta band shows no such systematic effect of cognitive mode. Considering the alpha band alone reveals a task dependence of the asymmetry 2–5 times greater than we reported earlier for whole band power (1–35 Hz). Whenever a significant task dependence of asymmetry appears in any band, it is in the same direction: the hemisphere primarily engaged in the cognitive activity develops proportionately less power. The requirement of motor output increases the task dependence of alpha asymmetry and lowers alpha power levels.