Open lead testing system for EEG

Abstract

A system for continuous monitoring for open circuits in multiple lead EEG systems during patient testing. Means are provided for repetitively applying a low current signal, having a distinctive waveform and frequency, to each electrode of the system. In the event an electrode is detached from the patient's scalp or otherwise open, the distinctive signal is apparent in the recorded output. Otherwise the distinctive signal is of such low magnitude that it is below the signal-to-noise ratio of the EEG amplifier. In systems including multiple leads, the test signal is successively applied to each electrode in the system in a time-controlled sequence. In one preferred embodiment a distinctive signal is used comprising a four pulse burst of 25 Hz frequency.

What is claimed is: 
     
       1. A test system for identifying open circuit electrodes of a multi-electrode EEG system during patient testing comprising pulse generating means for producing an alternating electrical signal,   counter means responsive to the alternating electrical signal for producing first and second pulse signals,   logic .[.ciruit.]. .Iadd.circuit .Iaddend.means responsive to said first and second pulse signals for selectively producing a plurality of identical test signals each corresponding to a different EEG electrode, said test signals occurring at different times within a predetermined test sequence, and   circuit means for conducting each test signal to said corresponding EEG electrode.   
     
     
       2. A test system as described in claim 1 wherein said counter means include binary counter means for producing said first pulse signal and decade counter means for producing said second pulse signal, with said first pulse signal having a repetition rate higher than said second pulse signal, and wherein said logic circuit means comprise selectively operable gate means connected to said binary counter means and said decade counter means for producing a plurality of test signals each comprising a burst of pulses of predetermined frequency and amplitude. 
     
     
       3. The test system of claim 2 further including means for maintaining the amplitude of pulses from said gate means at a level sufficiently low that said test signals are lower than the signal-to-noise ratio of the amplifier of the EEG system whereby the test signal remains undetected in the output of the EEG system unless an open circuit condition occurs therein. 
     
     
       4. A test system for identifying open circuit electrodes within a multi-electrode EEG system during patient use comprising pulse generating means for producing an alternating electrical signal,   .[.first.]. .Iadd.binary .Iaddend.counter means connected to said pulse generating means for producing first and second pulse signals,   logic circuit means responsive to said first and second pulse signals for producing corresponding sets of test signals of identical form, said first and second sets of test signals comprising bursts of pulses of predetermined frequency, with the bursts of the first set of said signals occurring at a different time than the bursts of pulses within said second set,   decade counter means connected to said binary counter means for producing sequencing pulse signals, and second logic circuit means responsive to said first and second sets of test signals and said sequencing signals for producing a plurality of bursts of test signals of identical form, with each burst of signals corresponding to a different EEG electrode and occurring at a different time within a predetermined test sequence, and   circuit means for conducting each burst of signals to said corresponding EEG electrode.   
     
     
       5. The test system of claim 4 wherein said second logic circuit means produce a first plurality of test signals of positive amplitude and a second plurality of test signals of negative amplitude whereby test signals applied at the same time to adjacent electrodes of the EEG system are not canceled due to the common mode rejection characteristics of the EEG amplifiers. .Iadd. 6. A test system for identifying high impedance electrodes of a multi-electrode EEG system during patient testing comprising: (a) means for generating first test signals of distinctive form whereby said first test signals will not be mistaken for an EEG signal when they appear in the recorded output, said first test signals being of current amplitude sufficiently small that the voltage developed across a normal electrode having low impedance is below the signal-to-noise ratio of the EEG amplifier but large enough that an electrode presenting a high impedance develops a voltage producing a clear indication on the recorded output; and,   (b) means connected to said generating means for automatically applying said first test signals to the EEG electrodes in a predetermined test sequence. .Iaddend..Iadd. 7. A test system as claimed in claim 6 wherein:   (a) said means for generating test signals includes means for generating second test signals being said first test signals of opposite polarity; and,   (b) said means for automatically applying said test signals includes means for applying said first test signals to one of the EEG electrodes and said second test signals to another of the EEG electrodes simultaneously. .Iaddend..Iadd. 8. A test system as claimed in claim 6 wherein said test signals are generated in bursts of four 25 Hz frequency pulses. .Iaddend.