There is described an electroencephalography (EEG) device. The EEG device comprises one or more inputs for connecting via electrodes to a subject, and one or more DC/AC EEG amplifiers for amplifying electric potential signals from a subject. Each DC/AC EEG amplifier comprises a preamplifier connected to the one or more inputs and configured to amplify a signal received at the one or more inputs, the preamplifier having a differential gain set so as to have a common mode rejection ratio (CMRR) of at least 100 dB. Each DC/AC EEG amplifier further comprises an amplifier circuit connected to the preamplifier and configured to further amplify the signal and, based thereon, output an output signal comprising a DC component having a frequency of less than 0.02 Hz. There is also described a device for monitoring a subject using near-infrared spectroscopy (NIRS) and electroencephalography (EEG), comprising: one or more light emitters each configured to emit toward the subject light having a wavelength in a range of 650 nm to 1,100 nm； one or more modulators configured to modulate the emitted light using one or more carrier frequencies； one or more light detectors configured to detect modulated light reflected from the subject； one or more demodulators configured to demodulate the detected light, based on the one or more carrier frequencies； and one or more EEG electrodes configured to measure electric potentials of the subject.
