Event-related gamma and beta oscillations analyzed over supratemporal, central and precentral gyri during the auditory-motor task​​​​​​​
Presenter: Eysteinn Ívarsson
Authors(s): Eysteinn Ívarsson1,2, Sara Marcu2,3, Aron D. Jónasson1, Stephen A. Shaw2, Paolo Gargiulo2, Magnús Haraldsson4, Eric Wassermann5, Sigurjón Stefansson1, Ovidiu C. Banea1,2
1Clinical Neurophysiology Unit, Neurology Department, National University Hospital of Iceland, Reykjavik, Iceland 
2School of Science and Engineering, Reykjavik University, Reykjavik, Iceland 
3University of Padua, Padua Italy 
4Department of Psychology, School of Business, Reykjavik University, Reykjavik, Iceland, 
5Behavioral Neurology Unit, National Institute of Neurological Disorders and Stroke, Bethesda MD, U.S.A. 
Email: eysteinn.ivarsson@gmail.com
Rationale: Functional magnetic resonance imaging (fMRI) have suggested that external auditory-verbal stimuli are processed in bilateral superior temporal gyri but predominantly on the left side in approximately 95% of right-handed healthy individuals (Pujol et al. 1999). In a study with intracranial recordings, auditory-verbal stimuli elicited augmented gamma-oscillations in the posterior portion of the superior temporal gyrus, whereas hand-motor responses elicited gamma-augmentation in the pre- and postcentral gyri (Nagasawa et al. 2010). The magnitudes of such gamma-augmentation in the superior temporal, precentral and postcentral gyri were significantly larger when the hand contralateral to the recorded hemisphere was required to be used for motor responses, compared to when the ipsilateral hand was used. However, it remains unclear whether cortical activation in the superior temporal gyrus during an auditory-motor task is affected by laterality of hand-motor responses. Correlations between gamma frequency range (30–80 Hz) and beta 1 range (12–20 Hz) activity revealed a high degree of interdependence of synchronized oscillations in these bands in the human EEG. Evoked (stimulus-locked) gamma oscillations preceded beta 1 oscillations in response to novel stimuli, suggesting that this may be analogous to the gamma-to-beta shift observed in vitro. Our aim was to investigate how the cortical activation related to the auditory-motor task is affected by laterality. We looked to relative power of gamma and beta frequencies in four regions of interest: left superior temporal gyrus, right superior temporal gyrus, left hand sensory-motor cortex and right hand sensory-motor cortex in healthy subjects. 
Methods: Participants: Eight healthy subjects were selectively recruited through convenience sampling. 
Design and Procedure: An EEG recording was collected during 3 minutes of two-auditory motor task with interstimulus interval of 3 seconds (“press”, “do not press”). The experiment was conducted in a sound-attenuated room, while the participants were awake, unsedated, and comfortably seated in a chair in front of a computer screen. Participants held a button-controller in one hand seated on the thigh. During the task, auditory-verbal commands were presented, where the auditory command was adjusted to a comfortable hearing level (Nagasawa et al. 2010). Each subject was instructed to press the button using the thumb when a pre-recorded verbal command saying “Press” was given and not to press the button when a verbal command saying “Do not press” was given. Each subject completed the auditory-motor task for both hands. The task contained 40 trials; 20 auditory-verbal commands saying, “Press” and 20 commands saying “Do not press”, presented in a pseudorandom order. 
EEG recording and preprocessing: The EEG was recorded using high-density 256 channels EEG with a sampling-rate of 1024 Hz. ANT Neuro system with an offline common reference, and electrooculogram (EOG) electrode below the right eye, and a ground electrode on the left neck. Raw EEG was exported and further analyzed using EEGlab toolbox in Matlab. EEG signals were first filtered between 10 and 80 Hz (Hall et al. 2011). The raw EEG was processed by digital bandpass filtering (Wang et al. 2015) and separated into different frequency categories: beta 1 (13–20 Hz), beta 2 (20–30 Hz), low gamma (30–45 Hz) and high gamma (45-80 Hz). Epochs for the auditory cortical activation and analysis were selected between +100ms until 1100ms from auditory command (1000 ms). For the movement related gamma and beta oscillations the epoch segmentation was between -500ms to +500ms (1000ms) relative to the onset of the button press code seen in the EEG trace. The reference period (baseline period) was set between +1500ms to +2500ms post stimulus onset (1000ms). To measure the event-related oscillations augmentation and attenuation, we calculated the average amplitude percentage change relative to the reference period (Pfurtscheller et al., 1977; Pfurtscheller and Lopes da Silva, 1999). Epochs containing artifacts ±50 μV at the regions of interest (STG, M1 and their surrounding areas) were then removed. Finally, the remaining artifact-free trials were analyzed. 
Statistical analysis: Averages in each condition and for both groups were measured and compared. To assess the event related gamma and beta oscillations, linear regression estimation and paired sample t-tests were calculated. The values were later averaged for each participant and then for both groups (Patients vs. Control). Pearson correlation analysis was used for individual EEG measures. 
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