Electroencephalograph

A multi-channel instrumentation system is provided wherein signals from selected electrode pairs are sequentially connected to the isolated input of a common signal amplifying path. The electrode pair selecting means is included in a portable head box which may be placed adjacent to the patient to be tested. Included in the head box are facilities for testing the impedance of any selected one of the patient electrodes and providing a visual indication of the magnitude of such impedance. The head box also includes facilities for selecting a group of patient electrodes which are connected together and used as an average or reference potential for one input of the common signal path. Facilities are also provided for calibrating the common signal path by applying a d.c. calibration signal to the input thereof by means isolated from sytem ground. An improved circuit arrangement is provided for limiting the current which can be drawn from the patient ground electrode to a small value to provide for the safety of the patient in the event of malfunctioning of the electrode selecting means. Facilities are provided in the common signal path for removing the d.c. components of the electrode pair signal voltages, and for selectively varying the gain of the common signal path on an individual channel basis.