Role of Beta and Gamma Oscillations in Working Memory Functions

Working memory, the brain's ability to retain information that is not directly present in the sensory systems, underlies many higher cognitive functions. The putative neurobiological basis of working memory is the self-sustained spiking activity of neurons in the associative regions of the cortex. In addition to the firing rates, collective oscillatory activity of neural ensembles in various frequency ranges is modulated during working memory tasks. In this review, we discuss the existing experimental evidence for the possible roles of beta and gamma oscillations in implementation of working memory functions. We specifically focus on the role of these oscillations in the different phases of the experimental tasks; in particular during the presentation of the to-be-memorized stimuli and during retention of the stimuli in the working memory. We demonstrate that the various studies provide a converging evidence toward the role of the prefrontal gamma oscillations in stimulus encoding and for the prefrontal beta oscillations in working memory retention. We also discuss the reviewed data in a more general framework that implies specific roles for the beta and gamma oscillations in the organization of neural activity. The framework suggests that gamma oscillations are related to the bottom-up propagation of information, as well as to changing the states of the neuronal populations. At the same time, the beta oscillation are presumably related to top-down influences and to maintaining the status quo. Finally, we discuss the main problems of proving the causal roles for prefrontal beta and gamma oscillations in stimulus encoding and retention, as well as lacunea in our understanding of the mechanisms via which beta oscillations influence the activity of the working memory networks. We discuss the potential role of experiments with invasive and non-invasive cortical stimulation, as well as the role of computational modeling of the neural activity in solving the aforementioned difficulties.