lhe Psychological Record, 1964, 14, 31-36. WORMS IN A STRAIGHT ALLEY: ACQUISITION AND EXTINCTION OR PHOTOTAXIS l STANLEY c. RATNER Michigan State University This study investigated the behavior changes of worms in a straight alley with two different goal conditions and levels of illumination. A straight-alley (plastic tube) was selected to avoid problems that seem inherent in the maze as used with worms, Earthworms, Lumbricus terrestris, were assigned among three groups. Groups 1 and 2 were run into a darkened goal box with different conditions of illumination over the alley. Group 3 was run into an open area. Goal conditions were then shifted so that Groups 1 and 2 ran to the open area while Group 3 ran to the darkened goal box. Worms going to the goal box showed a rapid decrease in time to traverse the alley ( acquisition) , and a rapid increase when goal conditions were shifted (extinction ). Worms going to the open area showed higher and more variable response times until shifted to the goal box condition. Conditions of illumination over the alley had little effect on response times. The behavior changes are discussed in terms of the alternatives of learning and phototactic responses. The majority of studies dealing with learning of segmented worms, Annelida, have used some choice situation such as a T-maze (Datta, 1962; Robinson, 1953; Yerkes, 1912). From the results of these and other studies it has been concluded that worms can learn a position response in such a maze. Review of studies dealing with such learning shows several things that may attenuate the interpretation of the results as learning. The first involves a detail of procedure which is reported by Yerkes (1912) and the subsequent researchers. A quote from Robinson (1953) illustrates the problem: "If the worm did not leave the box and enter the stem within a few seconds, it was touched lightly with a moistened camel hair brush . . . Once the worm entered the maze, reversing in the stem or backing out was discouraged by the use of a long probe applied at the point of maximum effectiveness." Based on attempts to use this procedure in this laboratory, it seemed that some of the results that were interpreted as learning the maze by worms may be accounted for as learning by the experimenter to guide the mooements of a flexible object, the worm. For example, it was found that touching a worm led it to raise its anterior portions and swing them from side to side. This may provide the flexible object for subtle 1 This research was supported by a grant from the National Institute of Health, GM08967, to Dr. D. J. Montgomery of the Department of Physics. Frederick Boersma assisted in the collection of the data. 32 RATNER manipulation, Simple prodding may constitute the unsubtle manipulation. A second problem in interpreting the behavior of the worm in a maze as learning is that receptors to mediate responses to position have not been clearly identified for the species most frequently used in the learning studies, namely, Lumbricus terrestris and Eisenia foetida (earthy, 1958, Ch. 7; Warden, et.al., 1941, p. 492). Thus, it is difficult to conclude that learning of a position response has occurred, particuJarly in the absence of careful controls of other classes of stimuli. The third problem is that none of the variables known to affect rate or amount of learning, such as removing the reward Or spacing of trials, has been reported to affect maze learning of Annelids. Thus, the present study was designed to investigate a situation that would yield stable behavior and could be used to demonstrate the acquisition and extinction of an instrumental type response with worms, To avoid some of the problems which seemed inherent in the maze, a straight alley was selected as the learning situation. The selection was based on 'preliminary work with more than ten worms in a straight alley. METHOD Subjects Nine earthworms, Lumbricus terrestris, were used in the present study. The worms were selected haphazardly from a large shipment received from a commercial bait supply company. After selection, Ss were assigned at random to one of three groups. For the duration of the experiment each 5 was kept in an individual plastic box in a refrigerator. The box had moist sphagnum moss in it at all times. Apparatus. The apparatus for learning trials consisted of a clear plastic tube 12 in. long with a ~ in. inside diameter. The tube was divided into two 6 in. sections by means of a sliding plastic door, the start-box door. The door could be raised when 5 reached that point in his journey through the tube. The second 6 in. section terminated at the goal area. Two goal conditions were used. One condition included a black plastic box, 3 in. square, containing moist sphagnum moss. The top of this box was cut back so that the worm could readily enter the box on leaving the tube. The other goal condition consisted of a square of moistened paper towel resting on a 10 in. square of opaque plastic. The worm could crawl directly onto this towel on leaving the tube. Two conditions of illumination were used, One consisted of a lightto-dark gradient created by shining a 75-watt bulb, mounted 9 in. above the tube, through a graded filter which progressively reduced the level of illumination from the start door to the goal area. That is, the tube was brightly illuminated at the start door and became progressively less well illuminated toward the goal area. The other WORMS: LEARNING OR PHOTOTAXIS 33 condition of illumination was considered as no illumination over the tube. Specifically, a ruby-red dark-room bulb was mounted at the goal end of the alley. This light was reported to be undetectable by Lumbricus terrestris ( Hess, 1924 ) and has been used in other research in our laboratory (Ratner & Miller, 1959). With the exception of the sources of illumination described above, the experiment was conducted in a darkened room with walls and ceiling that were painted flat black. Measurement of running times and intertrial intervals was done with stop watches. Measurement of levels of illumination was made with a Gossen light meter, Model 12.611168. TABLE I DESIGN OF THE EXPERIMENT SHOWING GROUPS AND CONDITIONS OF TESTING INCLUDING TYPE OF ILLUMINATION AND GOAL CONDITION Group Phase A Phase B 1 light-dark gradient goal box light-dark gradient moist paper towel 2 red light goal box red light moist paper towel 3 red light moist paper towel red light goal box P·rocedure The general design of the experiment is shown in Table 1. The Ss in each of the three groups were subjected to two experimental phases. The two phases included a total of 53 trials with 25 in the first phase, phase A, and 28 in the second, Phase B. As seen in Table 1, Group 1 first had 25 trials with the light-dark gradient and went into a goal box (acquisition); they then had 28 trials with the gradient and went onto the moist paper towel (extinction). Group 2 had the same treatment as Group 1, except that the trials were run under conditions of "no illumination," that is, with the red light. Group 3 was also run with "no illumination," but first had 25 trials running to the moist paper towel (control procedure), and then 28 trials running to the dark goal box (acquisition) . On each trial for each worm the animal was transferred in a darkened room from its home box to a moist plastic sheet, and allowed to crawl from the sheet into the starting end of the tube. Approximately 5 sec. after the head of the worm reached the starting door, the appropriate condition of illumination was established, the door was opened, and the timing of the response was begun. Response time was 34 RATNER counted until ~ in. of the worm exited from the tube into the approximate goal area. The worm remained in the goal area for 5 min. before its next trial was begun. An arbitrary maximum response time of 300 sec. was established. If 5 had not exited from the tube by that time it was dropped through the tube and placed in the goal area. The levels of illumination at the goal area (measured with a meter calibrated for humans) were: 40 ft.c and 9 ft.c, on a plastic sheet in the gradient and the red light respectively; 9 ft.c and 3 ft.c in the goal box in the gradient and the red light, respectively. The experimenters rinsed their hands before handling Ss, and holding of Ss in the hand was kept to a minimum since earlier observations had indicated that salts, tobacco and other residues on the hands were extremely detrimental to worms. The 58 were not touched or otherwise probed while they were in the tube. RESULTS The main results of the study are summarized in Figure 1, which shows the medians of the response times for blocks of trials for each group of Ss, Each block corresponds to a day's run and consists of two or three trials. The number of trials within each block was equal for all groups. It can be seen from Figure 1 that during phase A, before the switch in goal box conditions, Groups 1 and 2 showed rapidly decreasing response times and these were maintained through the 10th block of trials. During this phase Groups 1 and 2 were run to goal boxes. Group 3 running under the same conditions of illumination of Group 2, but without a goal box, showed great variability in median response times: at only one point did the median of Group 3 reach a level as low as the highest level reached by either Groups 1 or 2. The results from the last trial of phase A provide an example of the type of data obtained during this phase. The response times in seconds for the individual worms are as follows: Group 1-33, 39, 13; Group 2-20, 18, 11; Group 3-300, 140, and 300. The second half of Figure 1 shows the medians of the response times of phase B, after the switch in goal conditions. As noted in Table 1, Groups 1 and 2 were switched to the moist towel (extinction trials) and Group 3 was switched to the goal box (acquisition trials). It can be seen from Figure 1 that the switch in goals was associated with a rapid change in response times for the various groups. Groups 1 and 2 showed elevated and variable times on successive blocks of trials, whereas Group 3, now running to a darkened goal box, showed response times typical of those shown by Groups 1 and 2 when they were running to a darkened goal box. However, the median response \VORMS: LEARNING OR PHOTOTAXIS ~ 0-0 GP. I GP.II GOAL " ,, :I t- '. " , , z e en '. 'I LIJ IIJ (/) 200 LIJ a: ~ "'I, o , I'" I' ,.'.' " " ' , , '" II " , , , , ',I, "'. ,,\i '. ~• ' I " . , , z 10 2 SWITCH •• .-. GP m .-.--. ,, ," 300 35 ' , ~."",-o 2 VJO\ -.>~ 4 -- ,...J) " ._-..........-_.......---~ 8 10 2 4 BLOCKS OF TRIALS 6 6 8 Fig. 1. Medians of running times for blocks of trials for each group. Trials before goal switch constitute Phase A and after goal switch Phase B. times for Group 1 during phase B suggests that they may have been approaching those of Group 3. Examination of the individual scores for the Ss for the last trial of phase B illustrates the characteristics of the responses of the groups. The individual response times in seconds are as follows: Group 1-300, 300, 300; Group 2-203, 76, 29; Group 3-28, 30, 28. DISCUSSION Several generalizations seem reasonable on the basis of the data presented in Figure 1. One is that the goal conditions at the end of the tube affect the performance of the worms when they are in the tube. As compared with an open goal area the goal box is associated with systematic decrease in medians of response times and reduced variability among blocks of trials. The open goal area is associated with slower approach movements and considerable variability in response times among blocks of trials. The conditions of illumination over the tube seem to have a small effect on the performance in the tube. The Ss tested with the gradient had shorter running times than those tested with the red light. But under both conditions learning seems to occur when the goal box is used at the end of the alley and extinction seems to occur when the goal box is replaced with an open area, the moistened paper towel. 36 RATNER The rapidity of the change in behavior in response to the goal conditions suggests caution in interpreting the results as learning. It can be seen in Figure 1 that during phase A the response times for the groups going toward the goal box decreased within three to six trials for one group and started very low for the other group. During phase B, following the switch in goal conditions, the response times for the group switched to the goal box dropped within six trials, while the times for the groups going toward the open area rose within three to six trials. Thus, an explanation other than one in terms of learning may be necessary. An alternative to the learning explanation is one in terms of phototactic responses associated with differential intensities of illumination reflecting into the alley from the different goal areas. Unfortunately, the second interpretation of the behavior changes shown by the various groups was not as easy to check as it first seemed. A "worm's eye view" down the tube under the conditions of red light revealed no apparent differences in levels of illumination between the goal box and the open area, the moist paper towel. This was verified by a light meter (calibrated to the human range) that yielded readings of 9 foot candles of incident light with the open area and 9 foot candles with the goal box in 'place (these readings could be changed slightly by tipping the sensitive head of the meter in different directions). The worm's eye view and light meter readings indicate that differential intensities were not present as we saw the situation, but they do not tell us how the worm saw it. In summary the straight alley "vas found to lead to locomotion and changes in the rate of locomotion as a function of the goal conditions. Some of the problems associated with learning in the T-maze seem to be circumvented by the use of the straight alley with worms. Behavior changes can be interpreted as acquisition and extinction of nmning responses or changes in phototactic responses that arise from differential levels of illumination into the alley. Research to separate these interpretations is under way. REFERENCES eARTHY, J. D. An introduction to the behavior of invertebrates. London: Allen and Unwin, 1958. DATTA, LOIS-ELLEN. Learning in the earthworm, Lumbricus terrestris. Amer. ]. Psycho I., 1962, 75, 531-553. IIESS, W. N. Reactions to light in the earthworm, L. Terrestris. ]. Morph. Physiol., 1924, 39, 515-542. RATNER, S. C. & MILLER, K. R. Classical conditioning in earthworms, Lumbricus terrestris. ]. comp, physiol., Psychol., 1959, 52, 102-105. ROBINSON, J. S. Stimulus substitution and response learning in the earthworm, ]. comp, physiol. Psychol., 1953, 46, 262-266. 'VARDEN, C. J., JENKINS, TN., & WARNER, L. H. Comparative psychology, Vol. II. New York: Ronald, 1941. YERKES, R. M. The intelligence of earthworms. ]. animo Behao., 1912, 2, 332-352.