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Preamplifier for recording of multiple-unit activity from moving animals
PhyMo/ogyand Bchav/or, Vol. 6, pp. 617-618. Pergamon Pte~, 1971. Printed in Great Britain
BRIEF COMMUNICATION Preamplifier for Recording of Multiple-Unit Activity from Moving Animals I W I L L I A M J. M U N D L
Allan Memorial Institute, McGill University, Montreal, Quebec, Canada (Received 19 October 1970)
MUNDL, W. J. Preamplifierfor recording of multiple-anit activityfrom moving animals. PHYSIOL.BEHAV.6 (5) 617-618, 1971.--Recording of multiple-unit activity from moving animals is improved by employing a preamplifier of special design which is insensitive to electrical artifacts generated by the animal's movements. The shields of the input cables are driven in synchrony with the input signal in order to reduce shunt-capacitances. Multiple-unit activity
Preamplifier
THE PREAMPLIFIERdescribed herein greatly reduces or eliminates the frequent artifacts encountered in recording multipleunit activity from unrestrained laboratory animals. Insensitivity to such artifacts is achieved by (a) employing bipolar electrodes connected to the differential input of the amplifier with the two connecting wires being separately shielded, and (b) driving the shields with a voltage sychronized with the input signal [1]. The differential input provides rejection of unwanted in-phase signals which are usually generated when the animal is chewing, sniffing or moving and which may also be caused by electrostatic charges building up on the fur. The driven shields of the input cables minimize shunt capacitances which would reduce signal amplitude, especially at higher frequencies. The voltage amplitudes which drive the shields are those of the input signal, because of the unity gain of the amplifier. Thus, positive feedback can not exist and capacitance reduction is active over the complete frequency range of the amplifier. This type of capacitance reduction should therefore not be confused with "capacitance compensation" frequently employed in recording from microelectrodes, a method for increasing the high-frequency response of the amplifier by positive feedback. Figure 1 shows the response in the lateral geniculate body of a rat to light flashes. The bipolar recording electrode was made from 0.010 in. dia. stainless steel wires. Since an ungrounded animal may accumulate static charges, the use of a separate grounded reference electrode is recommended. The trace of Fig. 1 was obtained by rectifying the output of the high-gain amplifier (which follows the preamplifier) and integrating it with a "phase-lag" circuit, incorporating an operational amplifier [2]. This method is similar in action to
the integrator described by Weber and Buchwald [3] but has the advantage of voltage and power gain. The time constant of the integrating circuit was 100 msec. The trace of Fig. 1 was recorded while the animal was grooming and it illustrates the insensitivity of the preamplifier to movement artifacts and its capability of handling neuronal signals with minimum attenuation. Figure 1 should not be regarded as proof of any physiological phenomenon; it merely shows the amplifier's performance in a behavioral test situation.
CIRCUIT
The circuit of the preamplifier (Fig. 2) is made up of two identical X1 amplifiers, one consisting of stages Qa and Qa and the other of Qi and Q,. The outputs of the X 1 amplifiers are brought together across a transformer winding, thus achieving rejection of common-mode signals. Since maximum commonmode rejection in an amplifier can only be attained when it takes place at the input (either an input transformer or a long-tailed pair), the circuit of Fig. 2 represents a compromise, insofar as the rejection element is placed farther back from the input thus allowing the shields of the cables to be driven separately. A common-mode rejection ratio of approximately 5000:1 is obtained. Each X 1 amplifier consists of a field-effect transistor followed by a common-emitter stage whose output signal is fed back to the source of the field-effect transistor. Bootstrapping the gate to the source of the input stages with 0.1 gF capacitors results in an input impedance of 100 Mt~. Input capacitance is 2 pF. Differential input impedance is therefore 200 Mt'l shunted by 4 pF. Low-frequency response is purposely limited
aThe development of this apparatus was supported by grants to Dr. R. B. Maline from National and Medical Research Councils and Defence Research Board of Canada; and U.S. National Institute of Mental Health and National ScienceFoundation. The recording shown in Fig. 1 was made by G. Blevings. 617
618
MUNDL
I sec.
t
4O
FIG. 1. Integrated multiple-unit activity in the lateral geniculate body during its response to light flashes (indicated by arrows). The vertical scale denotes the amplitude in per cent of a sinusoidal calibration signal. The rat was grooming during the entire length of this trace.
I
T
~,~P.- + 5.4 V
~,
.~
(,.25,.A)
Cable Shield
>_,o
i
!
t
In r~--t~- '
220 1
IOk
~4k
I
I% 250
IM ~
.47p
IM ~
<< 4k,1% '
' Common Mode Rejection
FIG. 2. Circuit diagram. Maximum rejection of con'u-non-mode signals is achieved by adjusting the 250 E~ potentiometer and by changing the values of the resistors marked ,. Power is supplied by a Mercury battery. Transistors are Texas Instrument's. The transfo~er is from United Transfomler
Company, New York. Coupling diodes are general purpose Silicon types.
by the choice of the values of coupling capacitors leading to the transformer. RECORDING WITH THE PREAMPLIFIER
If a rotating commutator is employed in the connections to the animal, then the preamplifier can simply be suspended from the rotor of the swivel, thereby permitting it to rotate with the input cables. In this case the transformer is placed at the input of the high-gain amplifier. Input cables should be of the low-noise type, such as manufactured by Microdot Inc., South Pasadena, California. Frequently, additional short lengths of wires are used as
adaptors between animal and input cables to facilitate connecting the animal. In such arrangements, plasticcovered wire should be avoided because of its tendency to accumulate static charges. Wire with rubber insulation was found to work well. The efficacy in capacitance reduction is indicated by measuring an input shunt capacitance of 4 p F at the end of a 40 in. (100 crn) long shielded cable. Without driving the cable shield, this length of cable has a capacitance of 90 p F between conductor and shield. This measurement was carried out with single-ended input, having the other input terminal grounded.
REFERENCES 1. Morrison, R. Grounding and Shielding Techniques in Instru3. Weber, D. S. and J. S. Buchwald. A technique for recording and mentation. New York: Wiley, 1967, pp. 129-130. integrating multiple unit activity simultaneously with the EEG 2. Philbrick/Nexus Research. Application Manual for Operational in chronic animals. Electroenceph. clin. Neurophysiol. 19: Amplifiers. Dedham, Mass.: Philbrick/Nexus Research, 1968, 190-192, 1965. p. 46.
BRIEF COMMUNICATION Preamplifier for Recording of Multiple-Unit Activity from Moving Animals I W I L L I A M J. M U N D L
Allan Memorial Institute, McGill University, Montreal, Quebec, Canada (Received 19 October 1970)
MUNDL, W. J. Preamplifierfor recording of multiple-anit activityfrom moving animals. PHYSIOL.BEHAV.6 (5) 617-618, 1971.--Recording of multiple-unit activity from moving animals is improved by employing a preamplifier of special design which is insensitive to electrical artifacts generated by the animal's movements. The shields of the input cables are driven in synchrony with the input signal in order to reduce shunt-capacitances. Multiple-unit activity
Preamplifier
THE PREAMPLIFIERdescribed herein greatly reduces or eliminates the frequent artifacts encountered in recording multipleunit activity from unrestrained laboratory animals. Insensitivity to such artifacts is achieved by (a) employing bipolar electrodes connected to the differential input of the amplifier with the two connecting wires being separately shielded, and (b) driving the shields with a voltage sychronized with the input signal [1]. The differential input provides rejection of unwanted in-phase signals which are usually generated when the animal is chewing, sniffing or moving and which may also be caused by electrostatic charges building up on the fur. The driven shields of the input cables minimize shunt capacitances which would reduce signal amplitude, especially at higher frequencies. The voltage amplitudes which drive the shields are those of the input signal, because of the unity gain of the amplifier. Thus, positive feedback can not exist and capacitance reduction is active over the complete frequency range of the amplifier. This type of capacitance reduction should therefore not be confused with "capacitance compensation" frequently employed in recording from microelectrodes, a method for increasing the high-frequency response of the amplifier by positive feedback. Figure 1 shows the response in the lateral geniculate body of a rat to light flashes. The bipolar recording electrode was made from 0.010 in. dia. stainless steel wires. Since an ungrounded animal may accumulate static charges, the use of a separate grounded reference electrode is recommended. The trace of Fig. 1 was obtained by rectifying the output of the high-gain amplifier (which follows the preamplifier) and integrating it with a "phase-lag" circuit, incorporating an operational amplifier [2]. This method is similar in action to
the integrator described by Weber and Buchwald [3] but has the advantage of voltage and power gain. The time constant of the integrating circuit was 100 msec. The trace of Fig. 1 was recorded while the animal was grooming and it illustrates the insensitivity of the preamplifier to movement artifacts and its capability of handling neuronal signals with minimum attenuation. Figure 1 should not be regarded as proof of any physiological phenomenon; it merely shows the amplifier's performance in a behavioral test situation.
CIRCUIT
The circuit of the preamplifier (Fig. 2) is made up of two identical X1 amplifiers, one consisting of stages Qa and Qa and the other of Qi and Q,. The outputs of the X 1 amplifiers are brought together across a transformer winding, thus achieving rejection of common-mode signals. Since maximum commonmode rejection in an amplifier can only be attained when it takes place at the input (either an input transformer or a long-tailed pair), the circuit of Fig. 2 represents a compromise, insofar as the rejection element is placed farther back from the input thus allowing the shields of the cables to be driven separately. A common-mode rejection ratio of approximately 5000:1 is obtained. Each X 1 amplifier consists of a field-effect transistor followed by a common-emitter stage whose output signal is fed back to the source of the field-effect transistor. Bootstrapping the gate to the source of the input stages with 0.1 gF capacitors results in an input impedance of 100 Mt~. Input capacitance is 2 pF. Differential input impedance is therefore 200 Mt'l shunted by 4 pF. Low-frequency response is purposely limited
aThe development of this apparatus was supported by grants to Dr. R. B. Maline from National and Medical Research Councils and Defence Research Board of Canada; and U.S. National Institute of Mental Health and National ScienceFoundation. The recording shown in Fig. 1 was made by G. Blevings. 617
618
MUNDL
I sec.
t
4O
FIG. 1. Integrated multiple-unit activity in the lateral geniculate body during its response to light flashes (indicated by arrows). The vertical scale denotes the amplitude in per cent of a sinusoidal calibration signal. The rat was grooming during the entire length of this trace.
I
T
~,~P.- + 5.4 V
~,
.~
(,.25,.A)
Cable Shield
>_,o
i
!
t
In r~--t~- '
220 1
IOk
~4k
I
I% 250
IM ~
.47p
IM ~
<< 4k,1% '
' Common Mode Rejection
FIG. 2. Circuit diagram. Maximum rejection of con'u-non-mode signals is achieved by adjusting the 250 E~ potentiometer and by changing the values of the resistors marked ,. Power is supplied by a Mercury battery. Transistors are Texas Instrument's. The transfo~er is from United Transfomler
Company, New York. Coupling diodes are general purpose Silicon types.
by the choice of the values of coupling capacitors leading to the transformer. RECORDING WITH THE PREAMPLIFIER
If a rotating commutator is employed in the connections to the animal, then the preamplifier can simply be suspended from the rotor of the swivel, thereby permitting it to rotate with the input cables. In this case the transformer is placed at the input of the high-gain amplifier. Input cables should be of the low-noise type, such as manufactured by Microdot Inc., South Pasadena, California. Frequently, additional short lengths of wires are used as
adaptors between animal and input cables to facilitate connecting the animal. In such arrangements, plasticcovered wire should be avoided because of its tendency to accumulate static charges. Wire with rubber insulation was found to work well. The efficacy in capacitance reduction is indicated by measuring an input shunt capacitance of 4 p F at the end of a 40 in. (100 crn) long shielded cable. Without driving the cable shield, this length of cable has a capacitance of 90 p F between conductor and shield. This measurement was carried out with single-ended input, having the other input terminal grounded.
REFERENCES 1. Morrison, R. Grounding and Shielding Techniques in Instru3. Weber, D. S. and J. S. Buchwald. A technique for recording and mentation. New York: Wiley, 1967, pp. 129-130. integrating multiple unit activity simultaneously with the EEG 2. Philbrick/Nexus Research. Application Manual for Operational in chronic animals. Electroenceph. clin. Neurophysiol. 19: Amplifiers. Dedham, Mass.: Philbrick/Nexus Research, 1968, 190-192, 1965. p. 46.