Physiology and Behavior: Voi. I, pp. 147-150. Pergamon Press Lid., 1966 Printed in Or~at Britain

Ionizing Irradiation: Effects of Repeated Low Dose Exposure'

BARRON B. SCARBOROUGH, JOAN MARTIN S A N D WILLIAM A. M c L A U R I N n

Department of Psychology, Florida State University, Tallahassee, Florida

(Received 25 June 1965)

SCARBOROUGH, B. B., JOAN MARTIN AND W. A. McLAtrglN. Ionizing irradiation: effects of repeated low dose exposure. PHYSIOL. BEHAV. 1 (2) 147--150, 1966.--Thirteen male Sprague-Dawley rats were given a total dosage of 1544 r of X-irradiation administered in low daily dosages five days a week for approximately nine months. Twelve control animals were sham irradiated for a like period of time. Both groups were ,equired to learn increasingly complex escape patterns in a water maze. The experimental and control groups did not differ in mean number of errors. In addition, the groups did not differ in weight gain for the period of the experiment nor in the time they could stay afloat.

Low dosage X-irradiation X-irradiation Learning Complex Water maze

NuMERous studies including Furchtgott [2], Garcia et al, [3] and Kaplan [7] have been made in an attempt to evaluate the behavioral effects of radiation exposure. In general, the observations have indicated that appreciable changes in learning ability resulting from radiation occur only after exposure to high energy rates or high total doses. However, Fields [1 ] stated that decrements in learning ability could be observed if the measurement was not confounded by acute radiation sickness and if the measuring instrument used was of sufficient complexity.

Relatively few studies have been conducted to provide information concerning the important question of what changes in complex problem solving ability occur under prolonged sub-lethal exposure to ionizing radiation. There- fore, it was the purpose of this study to evaluate the effects of chronic intermittent low-intensity X.irradiation exposure on the maze learning ability of rats under increasing task complexity and utilizing a non.consummatory stimulus as reinforcement for the response. It was assumed that a non, consummatory reinforcement would decrease the performance dependency on possible gastro-intestinal disturbances resulting from radiation exposure.

METHODS Animals

The subjects were 25 male rats of the Sprague-Dawley strain of a mean weight of approximately 290 g at the start of the experiment. Thirteen rats were randomly assigned to the experimental group and twelve to the control group. Each subject was individually housed in an airconditioned room where the temperature was maintained at 70--75°F.

The animals had free access to food and water during the entire experiment covering a period from November 13, 1962, to July 25, 1963.

Equipment The Hebb-Williams [5] closed.field test, and additionally

modified from that used by Rosvold and Mirsky [12], provided for graded problems of increasing complexity and used escape from the water maze as the reinforcing stimulus. The maze was a water.proofed tank, 30 in. square by 12 in. deep, with an extended arm in opposite comers to provide a start and goal box (see Fig. 1). The maze was filled to a depth of 9 in. A sheet of dear Plexiglas marked into 36 five in. square units, covered the entire maze except for the start and goal boxes. The aluminium barriers were attached to the underside of the plexiglas to change the complexity of the problem as desired. The barriers were 10 in. deep, and the following numbers and lengths were used: 8-5 in. long, 4-.10in., 3-15 in., 2-20in. and 2-25 in. long. A hardware doth ladder provided means of escape from the maze. The water in the maze was replaced on Friday of each week with an added antiseptic (Zephiran chloride) and then permitted to stand over the following two days in order that the <water reach room temperature. T h e maze was not moved in the room during the entire period of the experiment. The light source was constant with only a single overhead fixture.

The irradiation source was a deep.therapy 250kVp, 15 mA X-ray machine with an inherent 2.0 mm aluminium filter. An added 1.0 A1, 0.6 Sn, 0,25 Cu filter with a machine setting of 200kV 10 mA and a target-subject distance of 200 cm yielding an air exposure dose rate of lr/min resulting

1This research was supported in part by the Graduate Research Council of the Florida State University; a Public Health Service Fellowship MPM-17, 507-R1 from the Research Grants and Fellowship Branch; and by NSF Grant NSGP-671 to the Computing Center of the Florida State University.

anew at Duke Medical School, Durham, North Carolina. 3Presently located at Lockheed Aircraft Corporation, Marietta, Georgia,

147

148 SCARBOROUGH, MARTIN AND McLAURtN

in a daily dosage of 5 r a day for problems 1-24. A dose rate of 4 r/min or 20 r/day was administered for problems 25-33 and 20 r/min for a daily dosage of 100 r/day for problems 34 and 35.

T/tble 1 shows daily and summated dosages. Concurrent dosage monitoring was performed on each group exposure with a thimble chamber and Victoreen meter. The subjects were confined in a compartmentalized turntable which rotated at 2 rev/min. Sham irradiation of the control group consisted of the same manipulative treatment except the X-ray tube was not energized during the confinement period.

TABLE 1

EXPERIMENTAL TREATMENT

Problem No. days per No. trials No. r's Cumulative number problem per day per day dosage

1-24 5 4 5 604 25-27 5 4 20* 904 28-33 2 4 20* 1144 34-35 2 4 100" 1544

*The higher daily exposure dosages were administered at the same machine setting and dose rate as the 5 r/day dose. The control group received the same manipulative treatment except radiation exposure.

Ten problems devised by Rabinovich and Rosvold [10] as a method of rating intelligence in the rat, twelve additional problems devised by Rishikoff and Rosvold [11], and six devised by the writers were used for this experiment. The

graphical illustration of the maze problems is shown in Fig. 1. All problems except those devised for this study had been ordered according to level of difficulty as revealed by error scores.

Procedures The subjects were given five trials on each of two days with

no barriers in place for exit training. The irradiation treat- ment was begun on the third day. Radiation exposure was accomplished at 1500 each afternoon with testing at 0800 the next morning. One problem per week was given with four consecutive trials a day for each animal for five days. No trials were run over the weekends. For problems 28-35, only two days were given to each, with the mirror image of the problem being presented on the third and fourth day.

The animals were transported in groups of four t o the testing room. The water in the tank was maintained at about 73°F. The relatively cool water may have served to increase motivation and reduce variability [4].

Each subject was held over the start box and released into the maze. The experimenter started a stop watch as soon as the subject was placed in the water and then took up a position on the side of the tank and counted the errors made by the subject. An error was scored if an animal's nose crossed a grid line marking the entrance to a cul-de-sac. Repetitive errors were counted as additional errors. Time was recorded as soon as the subject's front paws touched the ladder. The subject was then given a 30 see rest, and placed in the maze for the next trial. Each subject was dried with a towel and returned to the transporting cage at the end of the four trials.

Two weeks after the end of the experimental period, a metal container (23 in. high, with a diameter of 17 in.) was filled with water to a depth of 13 in. The water was at a room temperature of about 73°F. Each rat was placed in the water and forced to swim until he became exhausted and

I 2 3 4 5 6 7

8 9 I 0 I I 12 13 14

15 16 17 18 19 20 21

22 30 :31 32 33 34 35 FIG. 1. Graphical illustration of the maze problems. Problems 23-29 were repeats

of 11, 21, 19, 9, 12, 11 and 22.

IONIZING IRRADIATION AND BEHAVIOR 149

sank to the bottom of the container. He was then removed and placed in his home cage. The time from placement in the container until sinking was used as the measure of physical endurance.

RESULTS There were no differences between groups in rate of learning

to criterion out of the 35 problems. A graphical analysis showed no apparent difference between groups in terms of mean time and mean error scores for each trial on each problem. The data were recombined and plotted in terms of the mean number of errors per problem over trials, days and subjects (see Fig. 2). This yielded one score per problem for each group. Since it was apparent that no differences were present, no statistical analyses were run on these data, except a t test was run for day 1 on problem 30, the day on which the scores showed the greatest divergence between the two groups. A non significant t-value (t = 1.54, dr= 23) was obtained. It will be noted from Fig. 2 that as the problems increased in difficulty the number of errors increased markedly for both groups.

No difference was found in the physical endurance between groups as measured by the duration of time the subjects were able to remain afloat. The mean time afloat for the experi- mental group was 328 sec and for the control group 366 sec. A rank sum test, [13] on the differences in float time yielded a rank sum of 158 for experimental animals and 167 for control animals.

The weight gain of the experimental subjects at the end of the experimental period was not significantly different from that of the control group. The mean weight gain of the experimental group was 154g and 165 g for the control group. A rank sum test on the differences of the weights resulted in a rank sum of 157 for the experimental group

TABLE 2

DIFFICULTY LEVEL OF PROBLEMS BASED ON CONTROL GROUP PERFORMANCE

Problem Mean No. Problem Mean No, number errors number errors

6 0.29 10 5.46 2 0.71 22 5.89 3 0.98 13 6.14

19 1.08 20 6.41 18 1.10 14 6.79 5 1.18 16 7.02 4 1.33 32 7.65 1 1.77 11 8.33

17 2.19 34 9.46 9 2.64 21 9.47 7 3.08 35 10.31

15 3.56 33 15.66 8 4.06 31 16.04

12 5.12 30 18.27

*Problems 9, 11, 12, 19, 21 and 22 were repeated and 11 was repeated twice.

and 168 for the control group. The probability of a more extreme rank sum is 0.52 under the null hypothesis.

The problems were ranked on the mean number of errors per problem for the subjects in the control group. A number of inversions were found for the earlier problems. In general, the later problems were in fair agreement with the order in which they were presented (see Table 2).

The problems which were repeated (Table 2) showed, in

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P R O B L E M FIG. 2. Comparison of the group mean errors on sequential problems.

150 SCARBOROUGH, MARTIN AND McLAURIN

general, an increase in the number of errors over the first presentation, which points up the need for much further standardization in order to make full use of this instrument in non-consummatory reinforcement studies.

DISCUSSION The results of this experiment are in agreement with

Furchtgott's conclusion that doses of over 1000 r's do not interfere with the learning of relatively complex problems in the absence of radiation malaise. No evidence of radiation sickness was in evidence during any phase of the experiment.

The decrement in performance, observed by Jarrard [6], of his irradiated subjects was not paralleled by a similar decrement in performance by the subjects in this study. Nor was a significant weight loss evidenced as reported by Kohn [8] and Nims and Sutton [9]. However, the divergence of their findings with the present study can probably be attributed to the difference in amount and rate of exposure. BecauSe of the intermittent character and the relatively low exposure rate of the present study, no logical comparison can be made to results of studies using a higher energy level. The most

plausible hypotheses is that the amount and/or rate were insufficient to effect a decrement or the recuperative powers were sufficient to overcome any possible decrement in facility.

It must be concluded from the results that under the conditions of this study low intensity, chronic, intermittent X-irradiation exposure has little or no effect on physical endurance, weight gain, or complex problem solving behavior in the rat. However, it would seem somewhat unrealistic to assume that the biological changes which occurin an organism under radiation exposure have no psychological manifes- tations. Perhaps the interval between treatment and testing was too long for behavioral effects to be manifest. It may be that the determination of behavioral changes await a more refined measuring instrument or a longer period of obser- vation of performance of subjects exposed to low intensity intermittent radiation. The possibility remains that the experimental animals could have demonstrated a difference in performance on relatively easy patterns after having been subjected to large accumulated dosage. Increasing complexity of patterns and increase in dosage occurred at the same time for these animals.

REFERENCES

1. Fields, P. E. The effect of whole body X-irradiation upon activity drum, straightaway and maze performance of white rats. J. Comp. Physiol. Psychol. 40: 386-391, 1957.

2. Furchtgott, E. Behavioral effects of ionizing radiations: 1955-1961. Psychol. Bull. 60: 157-199, 1963.

3. Garcia, J., D. J. Kimeldorf and E. L. Hunt. The use of ionizing radiation as a motivating stimulus. Psychol. Rev. 68: 383-395, 1961.

4. Hack, E. R. Learning as a function of water temperature. J. Exp. Psychol. 16: 442--445, 1933.

5. Hebb, D. O. and K. Williams. A method of rating animal intelligence. J. Gen. Psychol. 34: 59-65, 1946.

6. Jarrard, L. E. The effect of long-continued, low-intensity gamma irradiation on learning and body weight in the rat. J, Comp. Physiol. Psychol. 51: 536-540, 1958.

7. Kaplan, S. J. Radiation research in psychology; an analysis of techniques in maze experimentation. Psychol. Bull. 59: 153-160, 1962.

8. Kohn, H. I. Changes in composition of blood plasma of the rat during acute radiation syndrome, and their partial miti- gation by dibenamine and cortin. Am. J. Physiol. 165: 27-42, 1951.

9. Nims, T. F. and E. Sutton. Weight changes and water con- sumption of rats exposed to whole-body X-irradiation. Am. J. Physiol. 171: 17-21, 1952.

10. Rabinovich, M. S. and H. E. Rosvold. A closed-field intelli- gence test for rats. Can. J. Psychol. 5: 122-128, 1951.

I I. Rishikof, J. R. and H. E. Rosvold. Effects of electrocon- vulsive shocks on the performance of the rat in the closed field test. Can. J. Psychol. 7: 29-33, 1953.

12. RosvoJd, H. E. and A. F. Mirsky. The closed-field intelligence test for rats adapted for water-escape motivation. Can. J. Psychol. g: 10-16, 1954.

13. Walker, H. M. and J. Lev. Statistical Inference. New York: Holt, p. 434, 1953.