rTMS-enhanced gamma oscillation through strengthened network connection.

Gamma oscillations emerge generally in working memory to aspect of higher cognitive functions, and typically with the interaction of brain network. How is gamma oscillatory activity influenced by the functional connection in the brain'a network? How can gamma oscillations be increased by neural modulation to result in potential benefits for working memory? The present study aims to investigate the intensity of gamma oscillations modulated by low frequency repetitive transcranial magnetic stimulation (rTMS), and the possible brain network mechanism of increased gamma oscillations during working memory task in rats. Adult SD rats were divided into two groups: rTMS group receiving low frequency (1Hz) rTMS for 14 consecutive days and control group.16-channel local field potentials (LFPs) were recorded from rat prefrontal cortex while the rats performed a working memory task on Y-maze. Feature frequency oscillations in LFPs were measured by coherence analysis. Furthermore, the values of Directed transfer function (DTF) among LFPs were calculated to describe the function connection quantitatively in LFP network. The results show that the gamma (30-60Hz) synchronization preceded rat's correct choice in working memory task for the two groups. Coherence peaked before the behavior reference point for the two groups. Meanwhile, DTF value peaked before the behavior reference point for control and rTMS groups respectively. The rTMS modulation resulted in a coordinated increase in both gamma oscillation and strength of connection in the gamma band, which showed a significant difference (P<0.01). These findings suggest that gamma oscillations occur (both energy density and coherence in gamma-band) during working memory, which might be led by functional connection in gamma-band LFP network. Furthermore, gamma oscillation can be stimulated by rTMS, and strength gamma oscillation might be caused by the rTMS-mediated enhanced connection in the network.