Instruments that work on the principle of recording metabolic activity include functional magnetic resonance imaging (fMRI) and positron emission tomography (PET). Instruments for recording electrical activity consist of electroencephalography (EEG)/event-related potential (ERP), magnetoencephalography (MEG), stationary topography (SST) and transcranial magnetic stimulation (TMS). Instruments for measuring other physiological activities include gaze tracking, galvanic skin response, facial coding, and facial electromyography. (Agarwal and Xavier, 2015).