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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
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BLOCKS OF TRIALS
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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.
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