Physiology & Behavior, Vol. 16, pp. 493--495. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A.

BRIEF COMMUNICATION Activity Monitor for Small Animals G. G. MOROSS AND G. I. K A U F MA N

Division o f Laboratories and Research, New York State Department o f Health Albany, N Y 12201 (Received 6 March 1975) MOROSS, G. G. AND G. I. KAUFMAN. Activity monitor for smallanimals. PHYSIOL. BEHAV. 16(4) 493-495, 1976. A photoelectric activity monitor which is easily moveable between cages is described, including a circuit diagram and set-up procedure. In a preliminary study, the number of squares traversed by mice on the cage floor was counted both instrumentally and visually; the instrument count ranged from 105 to 111 per 100 observer counts. In a correlation study, a simulated mouse was moved from square to square both laterally and diagonally while being counted by the instrument and a human observer. They agreed 994 and 977 times respectively in 1000 trials. The instrument should prove particularly useful in studying hyperactive mice, whose activity level is too high for the human observer to count accurately. Behavior monitor patterns may also be studied by feeding the channel outputs to a minicomputer. Activity monitor

Electronic

Small animal

Photoelectric

THE activity level of test animals is a useful indicator of their physiological [2] and psychological [1,3] condition. In our laboratory, mouse activity is currently being measured in studies of the effects due to ingestion of different drugs and compounds and to virus infection. In order to quantify activity, a principal technique has been to divide the floor of the cage into squares and visually count the number of squares the animal enters per unit of time. A square is considered to be entered when the animal moves half of its area into that square. Measurement of such movement becomes tedious over an extended period of time, especially when dealing with hyperactive or hypoactive mice. We therefore decided to build an instrument which could measure the activity o f small animals using the same criterion as does the human observer. To enhance the utility of the activity monitor, we also required that it be easily interchangeable between cages. This would eliminate the delays inherent in using a single monitored cage, i.e., the necessity for sterilizing the cage for each new animal and for allowing the animal to become acclimatized to it. A third requirement was that the determination of patterns of movement be ultimately possible. The use of beam-breaking devices was rejected because of their intrinsic inaccuracy when arranged in a 2 dimensional pattern. An animal moving on a diagonal path between 2 areas always intersects 2 beams, and such an instrument would thus overcount the number o f squares entered. We therefore designed an instrument which has, over each unit of area to be monitored, a sensor capable of detecting the entry of half the animal. This design also allows for direct interchangeability between cages and for the ability to deter-

mine paths by recording against operation of the sensors.

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INSTRUMENTATION The unit (Fig. 1) was developed to study full grown adult white mice approximately 5 cm long in a cage 19 × 38 × 20 cm high. One side of a black floor liner was painted with a grid of white lines which divided it into eighteen 6.3 × 6.3 cm squares. A high-sensitivity silicon Photo-Darlington transistor (Clairex Corporation CLR No. 2180) containing an input lens with a half angle acceptance of 15 ° was mounted above the center of each square as the motion sensor. The sensor was positioned approximately 11 cm above the cage floor so that the diameter of its field of view was approximately equal to the length of the square it was monitoring. When an animal enters the sensor's field of view, the change in fight intensity impinging on the sensor triggers the counting mechanism. The counting logic is such that when half the projected area of the animal enters the sensitive area, the sensor detects its presence, and a count is registered. When the animal moves so that significantly less than half of its projected area is under the sensor, the sensor is released. In this manner small movements, such as shivering, are ignored, as they would be by a human observer. Eight incandescent bulbs (No. 328) were arranged above the cage to generate a uniform light level. The lamps are driven from a variable regulated supply adjusted to approximately 3 V, which provides subambient illumination at the 493

494

MOROSS AND KAUFMAN

FIG. 1. Activity monitor for small animals.

cage floor. A shield prevents normal laboratory lighting from affecting the mouse. The sensor and light sources, mounted on a single sheet of 0.6 cm transparent Plexiglas, form the complete sensor package, which is suspended into the cage. This unit is quickly and easily transferred between cages. The circuit for one channel plus additional electronics is shown schematically in Fig. 2. The voltage signal from the sensor is fed to a comparator, whose reference is set by a trimpot divider. Positive feedback is used around the comparator to eliminate crossover noise and to provide a few mV of hysteresis. The outputs of all channels are fed through inverters to assure uniform TTL levels and are capacitively coupled (wired-OR'd) to the positive input of a nonretriggerable one-shot multivibrator. The one-shot multivibrator and AC coupling eliminate the complexity of an N-input ' O R ' gate (N = 18 for the present monitor) and missed counts, which can occur if the mouse enters a new area prior to vacating the old one, unless the sensor sensitivities are very carefully set. The output of the multivibrator, a 0.05 sec wide TTL pulse, is used to drive a mechanical counter through a driver circuit. A maximum of 20 counts/sec can thus be detected. Activity rates are obtained by recording the counter values at predetermined intervals. The set-up procedure is as follows: (1) Adjust the height of the sensor tray so that it surveys the desired area. (2) Select an illumination level based upon needs of the experiment (e.g. nonintrusive). (3) Set the reference level trimmer so that all the comparators are just off (as evenly as possible). (4) Move a model mouse around the cage to ensure that (a) all sensors are equally sensitive, (b) the mouse provides a minimum signal of 100 mV under the sensor, and (c) the hysteresis is 50 mV or less. The components for this unit cost approximately $250. It took a competent technician 60 hr to assemble and troubleshoot the first unit.

RESULTS The unit was evaluated in 2 ways: (1) by moving a model mouse through different paths between squares and correlating the human observer's score with the machine score; and (2) by monitoring a real mouse and determining the agreement between an observer's visual counts and the total machine count. The use of a model mouse for the former test was dictated by our inability to find mice sufficiently cooperative to move at a reasonable pace through the desired paths. The model mouse was moved 1000 times from the center of a square to the center of an adjacent square in a path perpendicular to the dividing line. The instrument and human observer agreed 994 times; the machine doublecounted 3 times and missed a count 3 times. When the model was moved 1000 times in diagonal paths, the instrument and observer agreed 977 times; the instrument double-counted 23 times. In the test with a live mouse, a single observer monitored a single normal m o u s e for 100 visual counts daily for 7 days. The instrument's output for each of these 7 time periods was recorded independently. It indicated between 5 and 11 more counts than the observer on each 100 counts run (mean: 107). DISCUSSION The activity monitor functions well when the mouse is moving horizontally in a monotonic fashion. It is not designed to and does not discriminate among rearing, normal moving, and head motions. The results obtained were higher than those of the human observer, presumably because random movements into adjacent squares are registered by the counter but not by the human observer. Typical results have been more than 2000 counts per hr with a hyperactive mouse versus approximately 400 counts per hr with a normal mouse and less than 100 counts per hr

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with a hypoactive mouse. The accuracy and reproducibility of this instrument are more than sufficient for this type of discrimination. The advantages of the system include: the ability to determine accurately the translational activity of hyperactive small animals over extended periods of time; less intrusion by the system on the animal's behavior, since a floor grid is not necessary and a human observer does not hover over the cage; and elimination of the variability in observer counting, which was found to be significant. The monitor can be used with dark animals by painting the cage white and adjusting the trip points of the sensors. For use with different size animals, the square size, sensor height, and light output can be appropriately altered. The unit might be further developed to:

Determine patterns of behavior, such as circling and sleeping, without human monitoring. This could be done by directing the data from the sensor channels to a minicomputer and creating suitable algorithms. Study activity in the dark. The sensors have a high sensitivity in the near-infrared region of the spectrum, and the visible light could be filtered at the lamps and sensors. Eliminate recording of the data by a technician. The mechanical counter could be replaced by a printing counter. ACKNOWLEDGEMENT

We would like to acknowledge helpful discussions with Dr. Brian Bush and Dr. Richard Seegal and the technical assistance of Mr. Alan Kozakiewicz.

REFERENCES 1. Pinel, J. P. and R. F. Mucha. Incubation and Kamin effects in the rat: Changes in activity and reactivity after foot shock. J. comp. physiol. Psychol. 84:661-668, 1973. 2. Silbergeld, E. K. and A. M. Goldberg. A lead induced behavioral disorder. Life Sci. 13: 1275-1283, 1973.

3. Whimbey, A. E. and V. H. Denenberg. Two independent behavioral dimensions in open-field performance. J. comp. physiol. Psychol. 63: 500-504, 1967.

Activity monitor for small animals.

Physiology & Behavior, Vol. 16, pp. 493--495. Pergamon Press and Brain Research Publ., 1976. Printed in the U.S.A. BRIEF COMMUNICATION Activity Monit...
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