Abstract: An eye sensor, system and method for measuring fixational eye movements of an individual's eyeball (e.g., ocular microtremors and microsaccades) to provide a variable voltage biosignal for measuring the individual's brain stem activity. The eye sensor comprises a sensor mounted on the individual's closed or opened eyelid so as to be deflected by the fixational eye movements of the eyeball. A shielded flexible ribbon assembly supplies the biosignal generated by the sensor to an amplifier located on the individual's skin where the biosignal is amplified. The amplifier is interconnected with a signal processor and a display by which graphical and numerical representations of the biosignal are made accessible to an anesthesiologist, intensivist or clinician. A method for analyzing the biosignal to determine the brainstem activity of a patient.