Physiology &Behavior,Vol. 21, pp. 781-784. Pergamon Press and Brain Research Publ., 1978. Printed in the U.S.A.
Capacitive Sensor for Lick-by-Lick Recording of D r i n k i n g I WILLIAM
J. M U N D L
AND HELEN
P. M A L M O
Neuropsychology Laboratory, Department of Psychiatry, McGill University 1033 Pine Avenue West, Montreal, P. Q. Canada H3A 1AI ( R e c e i v e d 19 M a y 1978) MUNDL, W. J. AND H. P. MALMO. Capacitivesensorfor lick-by-lickrecording of drinking. PHYSIOL. BEHAV. 22(4) 781-784, 1979.--An electronic device senses the touching of water by the animal's tongue; and each lick is recorded distinctly as a deflection on paper by an oscillograph. From these traces lick rate and any variations therein may be determined. Capacitive sensor
Lick-by-lick recording
Drinking
T H E device described herein is capable of registering individual licking actions of an animal's tongue on a Richter tube, drinking spout or in a dish of water. Since any electric sensing device capable of serving this purpose necessitates the application of a voltage and subsequent passing of a current via the animal's tongue, the parameter of this current must be carefully chosen so that it is completely imperceptible to the animal. An even more stringent requirement as to the type and magnitude of current is imposed when recording electrophysiological potentials. In this case, any electrical artifacts introduced by the passing of current through the animal would be detrimental to the quality of the recording. A wide variety of commercial lick sensors have been described by Weijnen [7]. They all operate on the principle of passing a direct current via the animal's tongue. Our experimentation with both direct and alternating currents (the latter over a frequency range of 15 kHz to 1.5 MHz) revealed that the use of a 200 k H z sensing current produced the desired artifact-free recordings of brain activity. Lower frequencies were not sufficiently attenuated by the recording amplifiers, while a very high frequency (1.5 MHz) is prone to rectification at the interface of recording electrode and tissue, and thereby introduces artifacts much like a direct current. There is a further advantage in amplifying an AC sensing circuit. It can be based on capacitance-coupling, which eliminates the need for placing a metal electrode into the drinking water. The described instrument passes a current of approximately 7/xA RMS via the animal. Since this current is at a frequency of 200 kHz, it propagates largely on the surface of a conductor (such as the tongue), making it imperceptible to the animal [1]. Figure 1 shows a paper chart recording of a rat licking
water from a dish. Each time the animal's tongue touches the water a distinct deflection occurs. The accompanying recording (shown in Fig. 1) of multiple unit activity attests to the fact that no artifacts were introduced with our recording technique, which utilized Field Effect Transistors (FETs) in pairs on each electrode adapter [2, 4, 5, 6]. Head movement was recorded in order to monitor the animal's behavior more closely. CIRCUIT The sensing electrode is part of a capacitance bridge circuit (see Fig. 2). The secondary winding of the transformer constitutes two arms of the bridge; the other two arms are formed by a variable capacitor (balance) and the capacitance of the sensing electrode. The bridge is balanced to minimum meter deflection under the condition that the water dish or the spout is not being touched, either by the animal or by the experimenter. The sensing arm of the bridge is comprised of the capacitance of the sensing electrode to ground. In balance, i.e., minimum signal appearing at the centre tap of the transformer, the ratio between capacitance of the sensing electrode and balancing capacitor is equal to the turns ratio of the transformer's secondary windings. This is only theoretical, however, since stray capacitances create a considerable inequality in the winding reactances of this handwound transformer. A ratio of 1:1 was chosen and was found to provide sufficient sensitivity of the bridge. An increase in capacitance between the sensing electrode and ground unbalances the bridge with an ensuing increase in signal amplitude at the centre tap of the winding. Such a capacitance increase is created when the animal touches the water, presupposing that the animal is grounded. This condition can be achieved with the use of a reference electrode,
~Supported in part by Canadian Medical Research Council Grant MA 6438. Thanks are due to R. B. Malmo for assistance with development of ground electrodes; and S. Aylwin for manuscript typing.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/040781-04502.00/0
782
MUNDL AND MALMO
Licks
2"ov Pulses of Multiple Unit Activity
Integroted Multiple Unit Activity
Heod Movement (Accelerometer) 0 i
P i
2 i
sec. I
3
FIG. 1. Oscillographic recording of a rat licking water from a dish. Multiple-unit activity remains free of artifacts. The accelerometer which detects head motion was taped close to the rat's head onto the recording cables. the bridge arms (and thereby in the animal) is not additionally increased. After some amplification (× 18) the signal is filtered by a twin-T feedback filter [3], rectified and integrated (2.2k resistor and 4700pF capacitor). The amplitude can be read on the meter and it is also fed to the recorder. The operational amplifiers must be properly damped to prevent instabilities occurring at the 200 kHz range. Damping networks are connected to Pins 1 and 8. The bridge transformer was made of a Phillips' type LA
or when the animal is situated on a grounded metal floor. In the case of concurrent electrophysiological recording, the reference electrode must provide an efficient ground connection by being in good contact with brain tissue. In order to avoid artifacts we have found that the ground wire should be made of bare wire, which extends at least several mm into the brain of rats. The signal from the bridge is applied to a source follower, the high input impedance of which ensures that the current in Oscillator 200KHZ +~5V
Bridge Transformer
CapacitanceBridge
Source Follower
~ To Sen,,ogE,..... d,
X 18Amplifier
÷lSV .l i
220K % I
2N3819 Turn~IIi~90 T "~C T
BandpassFilter
Rectifier
68K:
Meter
~ 6 " V e K 0~__ ~ K ~ ~ ~ ' "
T-T
RT°rder
~
2N249743V
+,sv
801) II IOK
8O0 IL~IOK
FIG. 2. Circuit diagram. The oscillator is a type P from Allen Organ Co., Macungie, PA. Operational amplifiers are National Semiconductor's.
CAPACITIVE SENSOR FOR RECORDING DRINKING
783 Richter Tube
Drinking Dish
Por¢eloin Dish (60 dlo
i
Sensing Efe~troae
•
#
Drinking Spout
S0o
FIG. 3. Exploded views showing attachment of sensing electrodes to various drinking vessels. Electrodes are fashioned from brass shim stock (0.25 mm). Jacks for the banana plugs are mounted on the wall of the testing chamber.
2106 pot core, wound with 38 ga enameled copper wire. The number of turns are indicated in Fig. 2. A grounded metal foil between primary and secondary winding serves as a screen. The bobbins were attached to the chuck of a hand drill and the wire guided by hand. With this rather crude procedure it frequently happens that shunt capacitances in the secondary winding are grossly unbalanced, thus making it impossible to zero the bridge. Therefore, it is advisable to wind several bobbins, selecting them for acceptable performance when the circuit is in the breadboard stage. A trimmer capacitor (CT) provides an additional aid for zeroing the bridge. SENSING ELECTRODES Figure 3 shows three possible configurations for attaching sensing electrodes to drinking vessels. In each case the electrode establishes capacitive coupling to the drinking water. As the animal's tongue comes in contact with the water it effectively puts the sensing electrode at ground potential. This accomplishes the desired bridge unbalance. The sensing electrodes are fashioned from thin brass sheet and are cemented at the places shown. The banana jacks are mounted on the sides of the test chamber. F o r the Richter tube, a bracket to secure its top to the wall is necessary. The porcelain dish is cemented to the aluminum frame and the back banana plug is secured to both the phenolic
board and to the small bracket. After assembly of the drinking dish, phenolic boards of appropriate size (not shown in figure) are cemented to the top part and the sloping part of the aluminum frame. All sensing electrodes should be sufficiently covered with insulating material to prevent them from becoming wet and from being touched by the animal; epoxy cement is used on the dish, plastic tape on the Richter tube and vinyl tubing on the spout. A convenient way of testing the apparatus is by touching the water with one's finger. The meter deflection and the recorder write-out can then be conveniently observed. Close approach of the animal or the experimenter to the drinking vessel, or the fluid in it, causes only negligible bridge unbalance. It is not sufficient to register an erroneous lick response. Care must be taken to ensure that the animal makes contact with the water solely through its tongue; touching the water with any other part of its body would cause the sensing circuit to respond. Since the animals frequently place their paws on the rim of the dish, its height must be such that the toes cannot reach the water level. To prevent the paws from touching the water in the spout, the latter may be recessed into the vinyl tubing (Fig. 3), or into a piece of suitable glass tubing [7]. A similar recessing arrangement might be necessary for the Richter tube. The extent of the need for such arrangement would depend on experimental factors, such as trial time and the behavior pattern of the animals.
784
MUNDL AND MALMO
REFERENCES 1. Dalziel, C. F. Electric shock hazard. IEEE Spectrum 9: 41-50, 1972. 2. Malmo, H. P. and R. B. Malmo. Movement-related forebrain and midbrain multiple unit activity in rats. Electroenceph. clin. Neurophysiol. 42: 501-509, 1977. 3. Mundl, W. J. Practical design of low-frequency bandpass filters. Med. Biol. Engng. 6: 203-207, 1968. 4. Mundl, W. J. Stretching of analogue pulses. Electron. Engng. 41: 215-217, 1969.
5. Mundl, W. J. Preamplifier for recording of multiple-unit activity from moving animals. Physiol. Behav. 6: 617-618, 1970. 6. Ranck Jr., J. B. A movable microelectrode for recording from single neurons in unrestrained rats. In: Brain Unit Activity During Behavior, edited by M. I. Phillips. Springfield: Thomas, 1973, pp. 76--79. 7. Weijnen, J. A. W. M. The recording of licking behavior. In: Drinking Behavior: Oral Sthnulation, Reinforcement, and Preference, edited by J. A. W. M. Weijnen and J. Mendelson. New York: Plenum, 1977, pp. 93-114.
Capacitive Sensor for Lick-by-Lick Recording of D r i n k i n g I WILLIAM
J. M U N D L
AND HELEN
P. M A L M O
Neuropsychology Laboratory, Department of Psychiatry, McGill University 1033 Pine Avenue West, Montreal, P. Q. Canada H3A 1AI ( R e c e i v e d 19 M a y 1978) MUNDL, W. J. AND H. P. MALMO. Capacitivesensorfor lick-by-lickrecording of drinking. PHYSIOL. BEHAV. 22(4) 781-784, 1979.--An electronic device senses the touching of water by the animal's tongue; and each lick is recorded distinctly as a deflection on paper by an oscillograph. From these traces lick rate and any variations therein may be determined. Capacitive sensor
Lick-by-lick recording
Drinking
T H E device described herein is capable of registering individual licking actions of an animal's tongue on a Richter tube, drinking spout or in a dish of water. Since any electric sensing device capable of serving this purpose necessitates the application of a voltage and subsequent passing of a current via the animal's tongue, the parameter of this current must be carefully chosen so that it is completely imperceptible to the animal. An even more stringent requirement as to the type and magnitude of current is imposed when recording electrophysiological potentials. In this case, any electrical artifacts introduced by the passing of current through the animal would be detrimental to the quality of the recording. A wide variety of commercial lick sensors have been described by Weijnen [7]. They all operate on the principle of passing a direct current via the animal's tongue. Our experimentation with both direct and alternating currents (the latter over a frequency range of 15 kHz to 1.5 MHz) revealed that the use of a 200 k H z sensing current produced the desired artifact-free recordings of brain activity. Lower frequencies were not sufficiently attenuated by the recording amplifiers, while a very high frequency (1.5 MHz) is prone to rectification at the interface of recording electrode and tissue, and thereby introduces artifacts much like a direct current. There is a further advantage in amplifying an AC sensing circuit. It can be based on capacitance-coupling, which eliminates the need for placing a metal electrode into the drinking water. The described instrument passes a current of approximately 7/xA RMS via the animal. Since this current is at a frequency of 200 kHz, it propagates largely on the surface of a conductor (such as the tongue), making it imperceptible to the animal [1]. Figure 1 shows a paper chart recording of a rat licking
water from a dish. Each time the animal's tongue touches the water a distinct deflection occurs. The accompanying recording (shown in Fig. 1) of multiple unit activity attests to the fact that no artifacts were introduced with our recording technique, which utilized Field Effect Transistors (FETs) in pairs on each electrode adapter [2, 4, 5, 6]. Head movement was recorded in order to monitor the animal's behavior more closely. CIRCUIT The sensing electrode is part of a capacitance bridge circuit (see Fig. 2). The secondary winding of the transformer constitutes two arms of the bridge; the other two arms are formed by a variable capacitor (balance) and the capacitance of the sensing electrode. The bridge is balanced to minimum meter deflection under the condition that the water dish or the spout is not being touched, either by the animal or by the experimenter. The sensing arm of the bridge is comprised of the capacitance of the sensing electrode to ground. In balance, i.e., minimum signal appearing at the centre tap of the transformer, the ratio between capacitance of the sensing electrode and balancing capacitor is equal to the turns ratio of the transformer's secondary windings. This is only theoretical, however, since stray capacitances create a considerable inequality in the winding reactances of this handwound transformer. A ratio of 1:1 was chosen and was found to provide sufficient sensitivity of the bridge. An increase in capacitance between the sensing electrode and ground unbalances the bridge with an ensuing increase in signal amplitude at the centre tap of the winding. Such a capacitance increase is created when the animal touches the water, presupposing that the animal is grounded. This condition can be achieved with the use of a reference electrode,
~Supported in part by Canadian Medical Research Council Grant MA 6438. Thanks are due to R. B. Malmo for assistance with development of ground electrodes; and S. Aylwin for manuscript typing.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/040781-04502.00/0
782
MUNDL AND MALMO
Licks
2"ov Pulses of Multiple Unit Activity
Integroted Multiple Unit Activity
Heod Movement (Accelerometer) 0 i
P i
2 i
sec. I
3
FIG. 1. Oscillographic recording of a rat licking water from a dish. Multiple-unit activity remains free of artifacts. The accelerometer which detects head motion was taped close to the rat's head onto the recording cables. the bridge arms (and thereby in the animal) is not additionally increased. After some amplification (× 18) the signal is filtered by a twin-T feedback filter [3], rectified and integrated (2.2k resistor and 4700pF capacitor). The amplitude can be read on the meter and it is also fed to the recorder. The operational amplifiers must be properly damped to prevent instabilities occurring at the 200 kHz range. Damping networks are connected to Pins 1 and 8. The bridge transformer was made of a Phillips' type LA
or when the animal is situated on a grounded metal floor. In the case of concurrent electrophysiological recording, the reference electrode must provide an efficient ground connection by being in good contact with brain tissue. In order to avoid artifacts we have found that the ground wire should be made of bare wire, which extends at least several mm into the brain of rats. The signal from the bridge is applied to a source follower, the high input impedance of which ensures that the current in Oscillator 200KHZ +~5V
Bridge Transformer
CapacitanceBridge
Source Follower
~ To Sen,,ogE,..... d,
X 18Amplifier
÷lSV .l i
220K % I
2N3819 Turn~IIi~90 T "~C T
BandpassFilter
Rectifier
68K:
Meter
~ 6 " V e K 0~__ ~ K ~ ~ ~ ' "
T-T
RT°rder
~
2N249743V
+,sv
801) II IOK
8O0 IL~IOK
FIG. 2. Circuit diagram. The oscillator is a type P from Allen Organ Co., Macungie, PA. Operational amplifiers are National Semiconductor's.
CAPACITIVE SENSOR FOR RECORDING DRINKING
783 Richter Tube
Drinking Dish
Por¢eloin Dish (60 dlo
i
Sensing Efe~troae
•
#
Drinking Spout
S0o
FIG. 3. Exploded views showing attachment of sensing electrodes to various drinking vessels. Electrodes are fashioned from brass shim stock (0.25 mm). Jacks for the banana plugs are mounted on the wall of the testing chamber.
2106 pot core, wound with 38 ga enameled copper wire. The number of turns are indicated in Fig. 2. A grounded metal foil between primary and secondary winding serves as a screen. The bobbins were attached to the chuck of a hand drill and the wire guided by hand. With this rather crude procedure it frequently happens that shunt capacitances in the secondary winding are grossly unbalanced, thus making it impossible to zero the bridge. Therefore, it is advisable to wind several bobbins, selecting them for acceptable performance when the circuit is in the breadboard stage. A trimmer capacitor (CT) provides an additional aid for zeroing the bridge. SENSING ELECTRODES Figure 3 shows three possible configurations for attaching sensing electrodes to drinking vessels. In each case the electrode establishes capacitive coupling to the drinking water. As the animal's tongue comes in contact with the water it effectively puts the sensing electrode at ground potential. This accomplishes the desired bridge unbalance. The sensing electrodes are fashioned from thin brass sheet and are cemented at the places shown. The banana jacks are mounted on the sides of the test chamber. F o r the Richter tube, a bracket to secure its top to the wall is necessary. The porcelain dish is cemented to the aluminum frame and the back banana plug is secured to both the phenolic
board and to the small bracket. After assembly of the drinking dish, phenolic boards of appropriate size (not shown in figure) are cemented to the top part and the sloping part of the aluminum frame. All sensing electrodes should be sufficiently covered with insulating material to prevent them from becoming wet and from being touched by the animal; epoxy cement is used on the dish, plastic tape on the Richter tube and vinyl tubing on the spout. A convenient way of testing the apparatus is by touching the water with one's finger. The meter deflection and the recorder write-out can then be conveniently observed. Close approach of the animal or the experimenter to the drinking vessel, or the fluid in it, causes only negligible bridge unbalance. It is not sufficient to register an erroneous lick response. Care must be taken to ensure that the animal makes contact with the water solely through its tongue; touching the water with any other part of its body would cause the sensing circuit to respond. Since the animals frequently place their paws on the rim of the dish, its height must be such that the toes cannot reach the water level. To prevent the paws from touching the water in the spout, the latter may be recessed into the vinyl tubing (Fig. 3), or into a piece of suitable glass tubing [7]. A similar recessing arrangement might be necessary for the Richter tube. The extent of the need for such arrangement would depend on experimental factors, such as trial time and the behavior pattern of the animals.
784
MUNDL AND MALMO
REFERENCES 1. Dalziel, C. F. Electric shock hazard. IEEE Spectrum 9: 41-50, 1972. 2. Malmo, H. P. and R. B. Malmo. Movement-related forebrain and midbrain multiple unit activity in rats. Electroenceph. clin. Neurophysiol. 42: 501-509, 1977. 3. Mundl, W. J. Practical design of low-frequency bandpass filters. Med. Biol. Engng. 6: 203-207, 1968. 4. Mundl, W. J. Stretching of analogue pulses. Electron. Engng. 41: 215-217, 1969.
5. Mundl, W. J. Preamplifier for recording of multiple-unit activity from moving animals. Physiol. Behav. 6: 617-618, 1970. 6. Ranck Jr., J. B. A movable microelectrode for recording from single neurons in unrestrained rats. In: Brain Unit Activity During Behavior, edited by M. I. Phillips. Springfield: Thomas, 1973, pp. 76--79. 7. Weijnen, J. A. W. M. The recording of licking behavior. In: Drinking Behavior: Oral Sthnulation, Reinforcement, and Preference, edited by J. A. W. M. Weijnen and J. Mendelson. New York: Plenum, 1977, pp. 93-114.
