Delta-alpha/beta coupling as a signature of visual working memory in the prefrontal cortex.

Executive functions, particularly visual working memory, depend on the prefrontal cortex (PFC). Phase-amplitude coupling (PAC) has been proposed as a measure of synchronized brain oscillations. To study the neural correlates of working memory in cross-frequency interactions, local field potential (LFP) recordings were made in the PFC of two macaque monkeys. PAC analysis revealed that the delta band (1-5 Hz) phase modulated the alpha-beta band (8-33 Hz) amplitude throughout task epochs, in both the pre- and post-training stages. The elevation of δ-αβ PAC in the fixation period during post-training was a signature of task learning. Interestingly, the δ-αβ PAC was not enhanced in error trials compared to correct trials, and the subject's performance was strictly dependent on the orchestration of the delta phase. Furthermore, contrary to the dorsoventral functional specialization of PFC, spatial and shape stimuli induced the same pattern of PAC in PFC subdivisions.