The disclosure relates to a sensor, a system, and a holder arrangement for biosignal activity measurement. One example embodiment includes a sensor module for brain activity measurement. The sensor module includes a main electrode base. The sensor module also includes a plurality of pins protruding from the main electrode base. The plurality of pins is arranged such that, when applied on a subject, the pins make contact with skin of the subject or are in close proximity with the skin of the subject. The main electrode base comprises electronic circuitry for near infrared spectroscopy (NIRS) measurements and electronic circuitry for electroencephalography (EEG) measurements, both connected to the plurality of pins. The plurality of pins includes electrically conductive pins. The plurality of pins also includes at least one source waveguide pin configured for light emitting purposes or at least one detector waveguide pin configured for light detection purposes.