An electronic open field

Physiology&Behavior,Vol. 30, pp. 635-637. PergamonPress Ltd., 1983. Printedin the U.S.A.

BRIEF COMMUNICATION

An Electronic Open Field D A V I D I. W H I T M O Y E R , * D A N I E L M A S C 0 ~ A N D H U G O F. C A R R E R ¢ 1

*Department of Anatomy and Brain Research Institute, UCLA School of Medicine, Los Angeles, CA 90024 and tlnstituto de Investigaci6n Mddica, Mercedes y Mart{n Ferreyra Casilla de Correo 389, 5000 C6rdoba, Argentina R e c e i v e d 12 N o v e m b e r 1982 WHITMOYER, D. I., D. MASCO AND H. F. CARRER. An electronic open field. PHYSIOL BEHAV 30(4) 635-637, 1983.--An electronic open field made with inexpensive, off-the-shelf materials is described. Its automatic operation frees the researcher from tedious and time consuming direct observations in experiments measuring exploratory behavior, goal preference, etc. Its numeric output can be read from the display or interfaced to printers, recorders, computers, etc. As an example of its operation, experimental results measuring sexual motivation (proceptivity) of female rats are reported. Open field

Sexual motivation

Proceptivity

Exploratory behavior

THE open field is a commonly used apparatus in the behavioral laboratory. Usually it consists of an arena where the test animal can move freely while the investigator watches its behavior. When large numbers of animals must be observed for long periods of time, as is often the case, it becomes a time-consuming technique. In studies of sexual motivation Meyerson and Lindstrom [3] reduced the problems by placing an automatic moving film camera above the arena. Pictures were taken at fixed intervals, allowing study of the film at the investigator's convenience. This technique has its drawbacks, being expensive in terms of photographic equipment, the infrared film used, and the time spent reading the films. Hetta and Meyerson [2] replaced this apparatus with an electronic one in which the floor of the arena was made of multiple independent plates balanced on hinges, so that when the animal stood on one sector it activated a microswitch, thus identifying the location of the animal. The present report describes an inexpensive apparatus based on a similar principle which can be built with standard materials.

between the metalized face of the tensioned Mylar and the aluminum conductors below. When the animal to be tested was placed in the arena, it deformed the Mylar film so that contact was made between the metalized plastic surface and the aluminum foil. Each sector of the open field thus constituted a switch that closed when the animal was standing on it. In the circuit diagram of Fig. 2 this switch is shown interposed between the +5 V power supply and the base of transistor Q1. Since the objective for building this open field was to measure sexual motivation [3] or proceptivity [1], the open field had an elliptical shape with small cages at the ends of the long axis of the ellipse, which housed the stimulus animals. Sectors A and D, approximately 20% of the total surface, were located next to the wire mesh cages housing the stimulus animals. In this way the test animal placed in the arena could smell, hear and see the stimulus animals but had only limited contact with them through the wire mesh. The rest of the arena was equally divided into sectors B and C; the whole was surrounded by a metal barrier approximately 40 cm high.

THE ARENA The floor of the open field was divided into four sectors (A,B,C,D) as shown in cross-section in Fig. 1. Each sector was covered with a sheet of aluminum foil approximately 0.1 mm thick glued to the floorboard. The entire floor was then covered with a single piece of metalized Mylar film which was separated from the aluminum by 5 mm thick standoffs made of strips of rubber foam placed in the boundaries between adjacent sectors. By hanging small weights around the edge of the Mylar covering, a small space was maintained

THE MEASURINGSYSTEM The circuit shown in Fig. 2 was designed to provide a record of the time the test animal spent in each sector and the number of times the test animal moved from one sector to another, thus giving a measure of the animal's exploratory activity. The system works on the principle of directing pulses from a 10 Hz clock to one of four separate counters depending on where the animal is standing at each particular moment. Thus, when the animal is standing on sector A, the

~Requests for reprints should be addressed to Dr. Hugo F. Carrer, Instituto de Investigaci6n M6dica, Mercedes y Martin Ferreyra, CasiUa de Correo 389, 5000 C6rdoba, Argentina.

Copyright © 1983 Pergamon Press Ltd.--0031-9384/83/040635-03503.00

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Q1 transistor whose base is connected to that sector will conduct, and the input to the 7413 Schmitt trigger gate goes from high (H) to low (L). As the output of this gate changes state, one of the four inputs to the 74154 4-line to 16-line decoder/multiplexer also changes state. Since the 10 Hz clock is connected to the G inputs of the 74154, one and only one output will follow the H - L transitions of the clock. This output is directed to a decade divider and then to a four-digit counter. When the animal moves to another sector the clock is redirected to a different four-digit counter, while the first one holds its accumulated counts and serves as a memory. The 16 output lines of the 74154 represent all possible combinations of sector occupancy. At present, we use only the four lines related to occupation of single sectors. More counters could be added to log the time spent straddling sector boundaries. The outputs of all counters are connected to the 74125 tri-state bus buffer gates (one for each digit) whose outputs are connected through a parallel bus to the 7448

decoder drivers and display. At the end of the test period the four bus buffer gates belonging to each sector can be simultaneously enabled, so that the display shows the accumulated counts for that sector. By means of a rotary switch the corresponding set of bus buffers can be selected so that the accumulated counts for each sector can be sequentially displayed and recorded by the investigator. The output of the clock also is fed to a four-digit counter which accumulates the total elapsed test time. The output of this counter is similarly bus-buffered so that the elapsed time can be displayed also. To measure the boundary crossing or exploratory activity, the circuit depicted in Fig. 2 was used. The outputs from each 7413 (one for each sector) are fed through a 7475 clocked latch to the A inputs of a 7485 4-bit magnitude comparator. The B inputs of this comparator receive their signals directly from the 7413's. Therefore, when a change occurs in the output pattern of the 7413's as a consequence of the animal changing sectors, the A and B

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FIG. 3. (A) Location of ovariectomized rats (N = 13) in the vicinity of identical stimulus animals (intact males). Ordinate represents the percent time (mean-+SE) spent in the sector close to each stimulus animal. (B) Location of cycling female rats (N= 13) in the vicinity of an intact male (sector A) or a castrated male (sector D). Ordinate represents the percent of total time (30 min) spent in the corresponding sector; abscissa represents the day of the cycle (D=diestrus; P=proestrus; E=estrus). inputs to the comparator are different until the next clock cycle resets the latch. Pulses so produced are accumulated in a 3-digit counter which is also bus buffered to the display. The measuring system's control panel has a push button for resetting the counters, a switch for interrupting the clock input to the multiplexer at the end of each test session, and a 6-position rotary switch to select the sets of bus buffers so that the accumulated numbers (A, B, C and D sector times, number of boundary crossings and elapsed time) will be shown in the LED display. RESULTS AND DISCUSSION The apparatus has been used for several years in different experimental designs. As an example, the data summarized in Fig. 3 are presented here. Rats ovariectomized 3 weeks earlier were placed in the open field for 30 rain. Experiments were conducted during the dark phase of an inverted lighting cycle of 12:12 hr. Figure 3A shows the data obtained in a situation where no preference was to be expected. Both stimulus animals were intact males and they were changed

FIG. 4. Activity index (counts/time in sec) for the experiment shown in Fig. 3. Points represent mean-+SE. from day to day. The time spent in sector A near one male was not different from the time spent in sector D near the other male. On the other hand, Fig. 3B shows results of an experiment in which a preference was to be expected [3] since intact cycling females were used and their choice between an intact and a castrated male was studied. The time spent near the intact male the night of proestrus is significantly longer than the time spent near the castrated male, while the differences are not significant during diestrus. Also the time spent near the intact male during proestrus is significantly greater than the time spent near the intact male during the first or second day of diestrus. Overall differences were tested by Friedman's two-way ANOVA; a posteriori tests were made with Wilcoxon's matched pairs signed ranks test. Data collected using this open-field apparatus allowed calculation of a general activity index. This index was computed by dividing total elapsed time of a test session into the boundary crossing number for that session. This was done for the experiment shown in Fig. 3A; the results can be seen in Fig. 4. The first day the animals were placed in the field, they showed more activity than the following days; after day 4 the index shows that exploratory activity was reduced and remained stable for the rest of the experiment. This open field has several advantages. It is easily built with off-the-shelf materials of low cost. Since the location and activity data are derived from electrical signals, ambient light conditions are immaterial. Its various outputs can be tape recorded or used to drive a printer or a computer system if further automation is required. The number of sectors can be expanded simply by adding "sector modules" to the measuring system. The size and shape of both the arena and the sectors can be changed to accommodate smaller or larger animals and to study other forms of behavior. ACKNOWLEDGEMENT The Instituto de Investigaci6n M6dica Mercedes y Martin Ferreyra is supported by the Consejo Nacional de Investigaciones Cientificas y T6cnicas of Argentina. This work was supported in part by NIH grant NS01162 and the Ford Foundation.

REFERENCES 1. Beach, F. A. Sexual attractivity, proceptivity and receptivity in female mammals. Horm Behav 7: 105-138, 1976. 2. Hetta, J. and B. J. Meyerson. Sexual motivation in the male rat. Acta Physiol Scand Suppl. 453: 1-68, 1978.

3. Meyerson, B. J. and L. H. Lindstrom. Sexual motivation in the female rat. Acta Physiol Scand Suppl. 389: 1-80, 1973.