Med. & Biol. Eng. & Comput., 1978~ 16, 203-206

Dual-channel audio monitor for distinguishing action potentials from two different sources Andrew D. McClellan Christopher S. Cohan Department of Biomedical Engineering and Department of Anatomy, Case Western Reserve University, Cleveland, Ohio 44106

A b s t r a c t - - A n audio-monitor design is described which allows action potentials from two different input channels to be heard and distinguished from each other. Two other more conventional modes of operation have also been incorporated into the design, making the device a versatile instrument for monitoring neurophysiological events. The dual-channel mode of operation is most useful in detecting the relationship of activity between the two sources,

Keywords--Electrophysiological audio monitor, Motor activity patterns, E.M, G.s

1 Introduction

AuDio monitors have been used in neurophysiology for many years to free the investigator from con- stant observation of events on the oscilloscope or chart recorder. The devices have generally been used for two conventional applications: (i) nerve or muscle action potentials (a.p.s) are amplified and drive a speaker (Fig. 1B1) such that a 'blip' is heard for each spike (BROWN et al., 1973); (ii) the shift in potential (Fig. 1B2) resulting from the penetration of a cell with a microelectrode is applied to a voltage vA controlled oscillator (v.c.o.) to alter its tone fre- quency (LONGENECKER and LONGENECKER, 1975). B1 The first application is limited since generally only ~s one channel can be heard at a time and, if the a.p.s are long in duration (e.g.e.m.g.s or compound nerve a.p.s) they will have a low frequency content and may be difficult to hear. Furthermore, most designs only perform one of the above operations.

A unique 2-channel audio monitor (Fig. 1A) is B2 described which allows a.p.s from two different sources to be heard and distinguished. It also provides the two conventional modes of operation mentioned above. Spikes from the two channels are converted to tone bursts of different frequencies ~A (Fig. 1B3), added together and then used to drive a speaker. The tone burst frequencies are chosen so v B that a.p.s from the two channels can be easily 83 discriminated. It is even possible to recognise the occurrence of simultaneous spikes. ~s

2 Basic operation

The A-channel half of the 2-channel audio monitor with output summer is shown in Fig. 2. The two channels are identical except for the timing capacitors (C,~=0"0022 pP, CB=0.0016 pF) of Mr. Extracellular and intracellular potentials are

Received 16th June 1979

0140-0118/78/0724-0203 $1.50/0 �9 1978

typically amplified by a factor of 1000 and 10, respectively, before being applied to the monitor. Preamplified a.p.s in the range of approximately 0.02 to 2 '0 V (positive or negative) are acceptable.

A - ~ VB ER

AC DIRECT MODE

F, CHANNE I. GEE

v, ~--A AAAAA ~, ) V V VVVVVV

7\ ~t

VVVV ~ t

DC TONE MOOE

B CHANNEL OFF

AC TONE MODE

Fig. 1 (,4) Equivalent two-channel audio monitor, (B) Idealised representation of available modes of operation: (B1) conventional a.c, mode for hearing action potentials directly; (B2) conven- tional d.c. mode for detecting the penetration of a cell membrane by a microelectrode and recording the resting potential; (B3) dual channel mode for simultaneously monitoring activity from two sources.

Medical & Biological Engineer ing & C o m p u t i n g M a r c h 1978 203

A conventional power stage following the monitor is necessary to drive a speaker (BROWN, et al., 1973). The operation of the audio monitor in Fig. 2 will now be described for the three modes shown in Fig. lB.

In the single-channel a.c. direct mode (Fig. 1B1) the input signal Va is first amplified by a variable gain of 5 to 50 by A1. With S~ in the a.c. position and $2 in the direct position, the amplified signal (VA~) is a.c. coupled to the output summer, A4,

which thus allows intracellular and extracellular a.p.s to be monitored. Since only one channel should be used at a time in this mode, the $2 switch for the unused channel should be placed in the centre off position.

If both $2 switches are placed in the tone position, 2-channel a.c. tone operation (Fig. 1B3) is possible. Considering the A-channel half of the device, the input signal (intracellular or extracellular) is amplified by A1 and then a.c. coupled to A2, a full-

GAIN Ik lOk lOOk

VA, / |VVA, s, ~176 +lSV r~

t I I look A C , ~, oc L d

/

~ I T +ISv~

~ l k IOuF 100k +Jl - - I-'--1 - _

oo=o,T •,o. • ,0o r

1Ok

Fig. 2 The A-channel haft of the 2-channel audio monitor with the output summer. Channel B, which is exactly the same as channel A except for the timing capacitors of M1 (CA = O.O022 pF, CB=O'OO16 tzF), is connected through its own $2 switch to the output summer. A l l 776 operational amplifiers have 2 MQ

,o l: I lk ! ='~ 200k ~

Ok

~

A3

TONEQO DIRECT

SUMMER 1M

V s

. . . . ooo . . . . X 200k

. ~ . . ~ O 2 FROM ~. D ~ CHANNEL B

I AI--A 4 776 555

-: J Qll, Q2 2N3704 D I N4001

from pin 8 to the negative supply (V- = -- 15 V) for low power operation and general-purpose diodes in series with the power supply leads for transient protection. (*decoupled through 100 ~ and O. I /~F to earth to prevent false triggering).

204 Medical & Biological Engineering & Computing March 1978

wave rectifier (GRAEME, 1977). If the rectified signal VA 2 is greater than the selected threshold level, the output of A3, a special comparator (SToTT and WELLER, 1976), goes low and turns the shunt-type transistor switch (Q1) off. Q2 is off continuously and not used in this mode. The tone is generated by M1, a v.c.o. (S~GNETICS, 1973) which operates at its free-running frequency when no signal is applied to the modulating input (pin 5). During the time when the a.p. is greater than the threshold and Qt is off, the v.c.o, tone can pass through to the output summer, resulting in a tone burst. At any other time when the recorded potential is below threshold, Q1 is on and prevents the tone from reaching the output. The v.c.o, for each half of the monitor has i t s own specific free-running frequency resulting in tone burts of different frequencies. The free-running frequencies are chosen so that at least 5 cycles of the tone frequency occur during the duration of the a.p. In Pleurobranchaea, a marine gastropod mollusc used to test the monitor, a.p.s had minimum dura- tions of approximately 5 ms and values of 1-1 kHz and 1,5 kHz were selected for the v.c.o., by use of the appropriate value of timing capacitance C. In general, C = 2 - 4 x l 0 - 6 / f v ..... whele C equals CA or CB and fv ..... equals the free-running frequency of the v.c.o. Single-channel operation is also possible with this mode and may be preferred over the a.c. direct mode if the a.p.s have long durations.

In the single-channel d.c. tone mode (Fig. 1B2) the gain of Ax must be set at its lowest value ( = 5) assuming that intracellular potentials have already been amplified by a factor of 10. A positive offset is added to VA through S~ so that their sum remains positive even after recording the negative resting potential. Thus, upon penetration of the cell, VAt decreases and modulates (through pin 5) the v.c.o, frequency to a higher value. With S~ in the d.c. position and $2 in the tone position, Qz is on, keeping Qx off continuously and allows the tone frequency to reach the output. The $2 switch for the unused channel should be placed in the centre off position.

3 Evaluation

The audio monitor has been tested using extra- cellular and intracellular nerve and e.m.g, recordings from Pleurobranchaea. The three modes in Fig. tB have been tested and shown to operate as described. With the a.c. direct mode many of the a.p.s did not sound particularly distinct because of their long durations; however, the a.c. tone mode eliminated this problem.

The a.c. tone mode was found to be most useful in monitoring two channels whose patterns of activity were out-of-phase (e.g. sequential or an- tagonistic motor activity). In particular, this mode was extensively used to monitor e.m.g.s from muscles having a characteristic sequence of activa- tion. It was quite easy to distinguish the two channels

and qualitatively detect the relative phasing of activity. Conceivably, more channels could be added to the audio monitor to detect the phase relationship of activity from three or even four inputs provided that their activity did not appre- ciably overlap.

Some preliminary recordings from channels whose activity did overlap suggested an additional application for the a.c. tone mode of operation. Often, a spike in one channel was followed shortly by a spike in the other channel, suggesting a causal relationship between the two sources. This was most noticeable when monitoring nerve a.p.s in one channel and e.m.g, spikes in the other channel. When monitoring e.m.g.s in both channels, simul- taneous a.p.s were sometimes heard, suggesting a common input to the two muscles. A systematic study was not undertaken to determine how accu- rately the human ear could discriminate closely spaced tones; however, it appeared as though the ear could selectively 'tune-in' the causally related a.p.s and filter out the noncorrelated a.p.s. Although the above interpretations are speculative, this mode of the audio monitor provides additional informa- tion which can be substantiated by quantitative methods.

It is not our intention here to suggest and evaluate all the possible applications of this dual channel mode of operation. In fact, the type of recorded potentials and the variable human element may have some influence on the full range of possible applications.

4 Conclusion

A 2-channel audio monitor has been described which allows activity from two sources to be easily distinguished from each other. Two more conven- tional modes of operation (Fig. 1B1 and 1B2) have also been incorporated into the design result- ing in a versatile neurophysiological instrument.

Acknowledgments--One of the authors (A.D.M.) is supported by NIH (PHS) grant GM-01090. The other author (C.S.C.) is supported by the Sloan Foundation. This work was supported by NSF grant BNS 76-81233 awarded to George Mpitsos.

References BROWN, P. B., MAXFIELD, B. W. and MORAFF, H. (1973).

Electronics for Neurobiologists MIT Press 394--398. GRAEME, J. G. (1977) Designing with Operational Ampli-

fiers McGraw-Hill 126-127. SmNETICS Linear Data Book (1973) NE/SE 555 Timer

data sheet. LONGENECKER, H. E. and LONGENECKER, G. L. (1975)

An audio monitor of resting potentials. J. Electro- physiol. Tech., 5, 46--50.

STOTT, F. D. and WELLER, C. (1976) Biomedical ampli- fiers using integrated circuits, Med. & Biol. Eng & Comp., 14, pp. 684-687.

Medical & Biological Engineering & Computing

H

M a rch 1978 205

M o n i t e u r de sons & deux canaux pour d i f f6rencier le potent ie l act i f de deux sources d i f f6rentes

Sommaire--I1 d6crit un module de moniteur de sons, qui permet d'entendre et de diff6rencier ]e potentiel actif de deux canaux d'entr6e diff6rents. Deux autres modes de fonctionnement plus traditionnels sont 6galement inclus dans ce module, faisant de ce dispositif un instrument versatile de contr61e des 6v6nements neurophysiologiques. Ce mode d'op6ration ~, deux canaux est des plus utiles pour 6tablir le rapport d'activit6 entre les deux sources.

Zweikan~i l iger A u d i o m o n i t o r zur Erkennung der Funkt ionspotent ia le aus zwei verschiedenen Quel len

Zusammenfassung--Es wird eine Audiomonitor-Konstruktion beschrieben, durch die Funktionspotentiale aus zwei verschiedenen Eingangskan/ilen gehOrt werden und Voneinander unterschieden werden kOnnen. Diese Konstruktion enthiilt auch zwei andere mehr konventionelle Betriebsarten, die das GerAt zu einem vielseitigen Instrument zur Beobachtung neurophysiologischer Vorgiinge machen. Die zweikaniilige Betriebsweise eignet sich sehr zur Erkennung der Beziehung der Funktion zwischen den beiden Quellen.

206 Medical & Biological Engineering & Computing March 1978