P105 Modulating inter-hemispheric coherence to influence behavior – A TMS-EEG study

Abstract

Inter-hemispheric coherence (IHC) is one of the EEG parameters that measures synchronization of oscillations originating from brain regions of different hemispheres and thereby the functional connectivity between them. We know that IHC is critical for tasks such as midline object recognition that involves merging information obtained from the 2 hemispheres. In a previous study, it was observed that midline object recognition is associated with transient increase in IHC over temporo-parietal areas in the alpha frequency range (Mima et al., 2001). Whether this association is causal, remains unknown. In the current study we aimed at determining the causal association of inter-hemispheric alpha coherence to midline object recognition. Several studies have used TMS as an effective tool to entrain intrinsic brain oscillations. We interfered with the IHC by using unilateral or bilateral, synchronous or asynchronous repetitive transcranial magnetic stimulation (rTMS) pulses. Our hypothesis was that asynchronous rTMS would reduce IHC thereby impairing midline object recognition. We recruited 13 healthy subjects for the study. All subjects first participated in a screening session where the threshold intensity for blocking object recognition and the optimal coil orientation for TMS was determined. In the main experiment session, they were asked to perform the object recognition task in four blocks. During the first block, the individual alpha coherence frequency was determined by recording EEG during object recognition. During the second, third and fourth blocks rTMS was delivered in every trial with simultaneous EEG recording. Trains of 7 TMS pulses were administered over left and/or right lateral occipital gyrus at 80% of threshold intensity blocking object discrimination or 60% maximum stimulator output whichever was smaller. Our primary outcome measure for the object recognition task was d-prime or sensitivity index. Our results have not shown a significant effect of rTMS on object recognition. The EEG data is currently being analyzed to look for entrainment effects. We think that this absence of effect on behavior could be due to a ceiling effect in task performance, wrong stimulation site or sub-optimal stimulation paradigm. We are currently performing additional experiments to address the above issues.