Effects of Transcranial Magnetic Stimulation on Object Recognition

Inter-hemispheric coherence (IHC) is one of the electroencephalography (EEG) parameters that measures synchronization of oscillations originating from brain regions of different hemispheres and thereby the functional connectivity between them. We know that object discrimination is associated with transient increase in IHC. In this study we will interfere with the transient IHC that is associated with midline object discrimination by using rhythmic repetitive transcranial magnetic stimulation (rTMS). We will study the effects of entrainment by unilateral or bilateral, synchronous or asynchronous TMS pulses on behavior. The goal of the study is to determine if entrainment by TMS will affect task performance. If the results show that TMS by entrainment influenced behavior, then we can expect TMS to be a useful technique to explore the behavioral impact of specific neuronal oscillations under physiological as well as pathological conditions. Further, TMS could also find applications in rehabilitation or treatment of behavioral symptoms in patients with neuropsychiatric diseases. STUDY POPULATION: We intend to study 11 adult healthy volunteers on an outpatient basis. STUDY DESIGN: All the subjects will first participate in a screening session where the threshold TMS intensity for blocking object discrimination and the optimal coil orientation will be determined using single pulse TMS. Later they will attend the main experiment session where they will be asked to perform the object discrimination task in four blocks, of which the second, third and fourth will be in random order. During the first block, the individual coherence frequency will be determined by recording EEG during the task. During the second, third and fourth blocks, the subjects will receive either unilateral, bilateral or sham rTMS in a random order while EEG is being recorded. Short trains of 7 TMS pulses will be delivered during each trial. TMS pulses will be administered over left and/or right LOG at 80% of threshold intensity blocking object discrimination or 60% maximum stimulator output whichever is smaller. OUTCOME MEASURES: We expect that short trains of TMS will affect IHC depending on whether the pulses are synchronous or asynchronous. By modulating IHC, we expect changes in object discrimination task performance. The primary outcome measure in this study includes object discrimination task performance. During the study, we will also compute the individual frequency at which maximum coherence occurs, the threshold intensity and optimal coil direction for blocking object discrimination and the spatio-temporal characteristics of EEG waves during the rTMS trains to confirm that entrainment is occurring.