Physiology & Behavior, Vol. 23, pp. 391-393. Pergamon Press and Brain Research Publ., 1979. Printed in the U.S.A.
An Accelerometer for Recording Head Movement of Laboratory Animals I W I L L I A M J. M U N D L A N D H E L E N P. M A L M O
Neuropsychology Laboratory, Department of Psychiatry, McGiU University 1033 Pine Avenue West, Montreal, P. Q., Canada H3A 1A1 Received 7 O c t o b e r 1978 MUNDL, W. J. AND H. P. MALMO. An accelerometer for recording head movement of laboratory animals. PHYSIOL. BEHAV. 23(2) 391-393, 1979.--The construction of an inexpensive light-weight accelerometer for use as a head-movement transducer with small animals in brain-recording (or brain-stimulation) experiments is described. The accelerometer can simply be taped to the existing cables, thereby furnishing a sensitive measurement of head movement.
Head movement recording
Accelerometer
Integrator
INCREASING attention is being directed to the necessity of recording movements of animals during recording of brain activity [1]. In recording body movement from rats in our laboratory it was noted that the head often moved independently of the body, and that even our sensitive apparatus for recording body movement [2] could not record these head movements. The head-movement recording system herein described has proved to be a valuable addition to our recording equipment [1], and the application of this technique to other experimental situations where the animal has recording or stimulation cables attached seems promising.
Counter
Brain recording
Aluminum Cover t~r~In
Qnn
x~i,)ng )
Stimulation
~
~....---~ I ner tia Weight IBrass. 4,5 Dia., 7 Long)
c Cortrid(Je
THE ACCELEROMETER
Although light-weight accelerometers are commercially available, their high cost is somewhat prohibitive for use in the behavioral laboratory. Accordingly, Fig. 1 shows how an inexpensive accelerometer can be fabricated by using a phono cartridge. After evaluating different types of ceramic cartridges, type 93-T made by Astatic was found to be the most suitable. Basically, a phono cartridge generates a voltage whenever its ceramic element undergoes mechanical stress. Thus, a force acting on the pick-up stylus is translated into a voltage. The rebuilding of the cartridge to perform as an accelerometer calls for a weight to be affixed to the freemoving end (stylus end) of the ceramic element, while its other end remains attached to the cartridge body. In this way, as the cartridge body follows the animal's head movement, the affixed weight acts as an "inertia force," causing the ceramic element to bend. The procedure for assembly was as follows. The extreme triangular-shaped tips of the plastic cartridge body were cut
i
FIG. 1. Exploded view of accelerometer. The ceramic cartridge is an Astatic type 93-T. The cable is a low-noise type, No. 250-3823, made
by Microdot Inc. The inertia weight must not come into contact with the aluminum cover when assembled.
tSupported in part by Canadian Medical Research Council Grant MA 6438. The critical reading of the manuscript by R. B. Malmo is gratefully acknowledged. Thanks are due S. Aylwin for manuscript typing.
Copyright © 1979 Brain Research Publications Inc.~0031-9384/79/080391-03500.80/0
392
MUNDL AND MALMO LP Filter
Input Amplifier
Rectifier I~K
27 K 5,6K
~
,OK
~
~ t ,3 ~ 3- ~K
A
Integrator A 22
33F - 3 I K ~7
Recorder ,8K
J
~
AmplitudDiscr. e
Discharge MV
Inverter
Counter
v
V
39K
Headmovement maybedisplayed
+24v
33K
--
-15V
FIG. 2. Schematic diasram. on a recorder as continuing trace, or as a summarized count. Power requirements are rcBulated -+]5 V and unregulated +24 V. Operational amplifiers are National Semiconductor's. The counter is a 24 V DC type. Diodes are ]N457, N P N and PNP transistors are 2N5449 and 2N5447 respectively, made by Texas Instruments. A l l resistance values have a 5% tolerance.
Head Movement
Body Movement
MUA in Forebroin(Recorded with Pulse Stretcher)
Integrated MUA I
,
,
,
,
I
5 sec. FIG. 3. Recording of head movement with concurrent body movement and multiple unit activity (MUA) from the right medial paroLfactoriaJ area. The head movement accelerometer was taped to the cable being used for MUA recording. The three major deflections for body movement were produced by brief fast walk, vigorous shift in posture, and slow walk.
HEAD MOVEMENT RECORDING
393
away with a dental separating disc. This exposed the free end of the ceramic element sufficiently for the inertia weight to be attached with epoxy cement. The cable was soldered to the cartridge terminals, leaving the braided shield protruding so that it provides electric contact with the aluminum cover when assembled. Following this, a narrow strip of insulating tape was wrapped around to cover the terminals. The cartridge was then slipped into the aluminum cover, taking precautions to prevent a short circuit occurring between the center conductor and the cover. At the same time it was necessary to ensure that the cable shield was in contact with the cover to provide for an electrically shielded unit. Epoxy cement was then applied to the terminal end of the cartridge which bonded the cover, cartridge, and cable end into one rigid assembly. During this step, it is necessary to verify that the inertia weight remains free to move within the cover. To complete the assembly, a disc of plastic adhesive tape was used to seal the top of the cover. The accelerometer was taped to the existing recording (or stimulation) cables, at a position close to the animal's head. All cables led to a mercury swivel. For the stationary cable section, i.e., between swivel and amplifier, low-noise cable is not required. THE CIRCUIT
The circuit diagram is shown in Fig. 2. The ceramic element is capable of generating very large voltage spikes that can induce artifacts into recording channels. Two diodes are connected across the amplifier input to counteract such unwanted build-up of voltage. Silicon diodes begin to conduct only when the applied voltage exceeds 0.6 Volts. Since the signal voltages of interest produced by the accelerometer are far below this voltage, the diodes do not degrade the input characteristics of the amplifier. The input amplifier is a field-effect transistor type which has a characteristically high input impedance. This is necessary to prevent the electric charges generated on the ceramic element from being conducted off too quickly, which would severely degrade the low-frequency response of the system. The low-pass filter, rectifier, and integrator A (with a time constant of 50 msec) convert the input signal into a suitable form for recording. Integrator A is continuously discharged by the 220 KFI resistor in parallel with the integrating
capacitor, resuRing in a varying output voltage. Acceleration equals change in velocity over a given time. This infers that a fast movement results in a wider pen deflection than a slow movement if both are viewed over the same time interval. Thus, the output voltage can also be regarded as representing energy expended by the animal since producing a fast movement over a given time requires that more energy be expended than is necessary when producing a slow movement [2]. Certain excessively fast movements, such as tremors, can be produced with little energy expenditure but result in large signal voltages. The low-pass filter is used for suppression of these signals so that they are not over-expressed in the final output. The lower part of the diagram shows the circuitry for driving a counter. Integrator B is discharged every time its output voltage reaches a certain level (adjustable with the 5 Kfl potentiometer of the amplitude discriminator). The discharge sequence is as follows: the output of the integrator triggers the amplitude discriminator which in turn triggers the discharge multivibrator; the latter activates the counter and also triggers the discharge transistor into conduction, thereby discharging the integrating capacitor. It should be noted that integrator B summates and stores all signal voltages, i.e., all movements are processed without information being lost in counting. A SAMPLE RECORDING
Figure 3 presents recordings of head movement, body movement (also recorded with an accelerometer which was affixed to the sprung floor of the test chamber), and multiple unit activity (MUA), which was recorded in two different aspects: (a) in the form of neuronal pulses by means of an analogue pulse stretcher [3] and Co) simultaneously as a continuous trace representing integrated voltage. For the multiple unit recording, bipolar electrodes made from 62.5 ~m platinum-iridium wire were used [1]. The Figure shows that both activity-measuring devices operate without introducing artifacts into recordings of brain activity. Note also that head movement does not necessarily correspond to body movement which, as previously mentioned, is typical, and is the reason that separate recordings of head and body movements are desirable.
REFERENCES 1. Malmo, H. P. and R. B. Malmo. Movement-related forebrain and midbrain multiple unit activity in rats. Electroenceph. clin. Neurophysiol. 42: 501-509, 1977.
2. Mundl, W. J. Aqtivity of small animals measured with accelerometer. Med. Biol. Engng. 4: 209-212, 1966. 3. Mundl, W. J. Stretching of analogue pulses. Electronic Engng. 41: 215--217, 1969.
An Accelerometer for Recording Head Movement of Laboratory Animals I W I L L I A M J. M U N D L A N D H E L E N P. M A L M O
Neuropsychology Laboratory, Department of Psychiatry, McGiU University 1033 Pine Avenue West, Montreal, P. Q., Canada H3A 1A1 Received 7 O c t o b e r 1978 MUNDL, W. J. AND H. P. MALMO. An accelerometer for recording head movement of laboratory animals. PHYSIOL. BEHAV. 23(2) 391-393, 1979.--The construction of an inexpensive light-weight accelerometer for use as a head-movement transducer with small animals in brain-recording (or brain-stimulation) experiments is described. The accelerometer can simply be taped to the existing cables, thereby furnishing a sensitive measurement of head movement.
Head movement recording
Accelerometer
Integrator
INCREASING attention is being directed to the necessity of recording movements of animals during recording of brain activity [1]. In recording body movement from rats in our laboratory it was noted that the head often moved independently of the body, and that even our sensitive apparatus for recording body movement [2] could not record these head movements. The head-movement recording system herein described has proved to be a valuable addition to our recording equipment [1], and the application of this technique to other experimental situations where the animal has recording or stimulation cables attached seems promising.
Counter
Brain recording
Aluminum Cover t~r~In
Qnn
x~i,)ng )
Stimulation
~
~....---~ I ner tia Weight IBrass. 4,5 Dia., 7 Long)
c Cortrid(Je
THE ACCELEROMETER
Although light-weight accelerometers are commercially available, their high cost is somewhat prohibitive for use in the behavioral laboratory. Accordingly, Fig. 1 shows how an inexpensive accelerometer can be fabricated by using a phono cartridge. After evaluating different types of ceramic cartridges, type 93-T made by Astatic was found to be the most suitable. Basically, a phono cartridge generates a voltage whenever its ceramic element undergoes mechanical stress. Thus, a force acting on the pick-up stylus is translated into a voltage. The rebuilding of the cartridge to perform as an accelerometer calls for a weight to be affixed to the freemoving end (stylus end) of the ceramic element, while its other end remains attached to the cartridge body. In this way, as the cartridge body follows the animal's head movement, the affixed weight acts as an "inertia force," causing the ceramic element to bend. The procedure for assembly was as follows. The extreme triangular-shaped tips of the plastic cartridge body were cut
i
FIG. 1. Exploded view of accelerometer. The ceramic cartridge is an Astatic type 93-T. The cable is a low-noise type, No. 250-3823, made
by Microdot Inc. The inertia weight must not come into contact with the aluminum cover when assembled.
tSupported in part by Canadian Medical Research Council Grant MA 6438. The critical reading of the manuscript by R. B. Malmo is gratefully acknowledged. Thanks are due S. Aylwin for manuscript typing.
Copyright © 1979 Brain Research Publications Inc.~0031-9384/79/080391-03500.80/0
392
MUNDL AND MALMO LP Filter
Input Amplifier
Rectifier I~K
27 K 5,6K
~
,OK
~
~ t ,3 ~ 3- ~K
A
Integrator A 22
33F - 3 I K ~7
Recorder ,8K
J
~
AmplitudDiscr. e
Discharge MV
Inverter
Counter
v
V
39K
Headmovement maybedisplayed
+24v
33K
--
-15V
FIG. 2. Schematic diasram. on a recorder as continuing trace, or as a summarized count. Power requirements are rcBulated -+]5 V and unregulated +24 V. Operational amplifiers are National Semiconductor's. The counter is a 24 V DC type. Diodes are ]N457, N P N and PNP transistors are 2N5449 and 2N5447 respectively, made by Texas Instruments. A l l resistance values have a 5% tolerance.
Head Movement
Body Movement
MUA in Forebroin(Recorded with Pulse Stretcher)
Integrated MUA I
,
,
,
,
I
5 sec. FIG. 3. Recording of head movement with concurrent body movement and multiple unit activity (MUA) from the right medial paroLfactoriaJ area. The head movement accelerometer was taped to the cable being used for MUA recording. The three major deflections for body movement were produced by brief fast walk, vigorous shift in posture, and slow walk.
HEAD MOVEMENT RECORDING
393
away with a dental separating disc. This exposed the free end of the ceramic element sufficiently for the inertia weight to be attached with epoxy cement. The cable was soldered to the cartridge terminals, leaving the braided shield protruding so that it provides electric contact with the aluminum cover when assembled. Following this, a narrow strip of insulating tape was wrapped around to cover the terminals. The cartridge was then slipped into the aluminum cover, taking precautions to prevent a short circuit occurring between the center conductor and the cover. At the same time it was necessary to ensure that the cable shield was in contact with the cover to provide for an electrically shielded unit. Epoxy cement was then applied to the terminal end of the cartridge which bonded the cover, cartridge, and cable end into one rigid assembly. During this step, it is necessary to verify that the inertia weight remains free to move within the cover. To complete the assembly, a disc of plastic adhesive tape was used to seal the top of the cover. The accelerometer was taped to the existing recording (or stimulation) cables, at a position close to the animal's head. All cables led to a mercury swivel. For the stationary cable section, i.e., between swivel and amplifier, low-noise cable is not required. THE CIRCUIT
The circuit diagram is shown in Fig. 2. The ceramic element is capable of generating very large voltage spikes that can induce artifacts into recording channels. Two diodes are connected across the amplifier input to counteract such unwanted build-up of voltage. Silicon diodes begin to conduct only when the applied voltage exceeds 0.6 Volts. Since the signal voltages of interest produced by the accelerometer are far below this voltage, the diodes do not degrade the input characteristics of the amplifier. The input amplifier is a field-effect transistor type which has a characteristically high input impedance. This is necessary to prevent the electric charges generated on the ceramic element from being conducted off too quickly, which would severely degrade the low-frequency response of the system. The low-pass filter, rectifier, and integrator A (with a time constant of 50 msec) convert the input signal into a suitable form for recording. Integrator A is continuously discharged by the 220 KFI resistor in parallel with the integrating
capacitor, resuRing in a varying output voltage. Acceleration equals change in velocity over a given time. This infers that a fast movement results in a wider pen deflection than a slow movement if both are viewed over the same time interval. Thus, the output voltage can also be regarded as representing energy expended by the animal since producing a fast movement over a given time requires that more energy be expended than is necessary when producing a slow movement [2]. Certain excessively fast movements, such as tremors, can be produced with little energy expenditure but result in large signal voltages. The low-pass filter is used for suppression of these signals so that they are not over-expressed in the final output. The lower part of the diagram shows the circuitry for driving a counter. Integrator B is discharged every time its output voltage reaches a certain level (adjustable with the 5 Kfl potentiometer of the amplitude discriminator). The discharge sequence is as follows: the output of the integrator triggers the amplitude discriminator which in turn triggers the discharge multivibrator; the latter activates the counter and also triggers the discharge transistor into conduction, thereby discharging the integrating capacitor. It should be noted that integrator B summates and stores all signal voltages, i.e., all movements are processed without information being lost in counting. A SAMPLE RECORDING
Figure 3 presents recordings of head movement, body movement (also recorded with an accelerometer which was affixed to the sprung floor of the test chamber), and multiple unit activity (MUA), which was recorded in two different aspects: (a) in the form of neuronal pulses by means of an analogue pulse stretcher [3] and Co) simultaneously as a continuous trace representing integrated voltage. For the multiple unit recording, bipolar electrodes made from 62.5 ~m platinum-iridium wire were used [1]. The Figure shows that both activity-measuring devices operate without introducing artifacts into recordings of brain activity. Note also that head movement does not necessarily correspond to body movement which, as previously mentioned, is typical, and is the reason that separate recordings of head and body movements are desirable.
REFERENCES 1. Malmo, H. P. and R. B. Malmo. Movement-related forebrain and midbrain multiple unit activity in rats. Electroenceph. clin. Neurophysiol. 42: 501-509, 1977.
2. Mundl, W. J. Aqtivity of small animals measured with accelerometer. Med. Biol. Engng. 4: 209-212, 1966. 3. Mundl, W. J. Stretching of analogue pulses. Electronic Engng. 41: 215--217, 1969.
