Cooling as Reinforcing Stimulus in Aplysia

Transcription

1 AM. ZOOI.OCIST, 12: (1972). Cooling as Reinforcing Stimulus in Aplysia PAUL DOWNEY AND BEHRUS JAHAN-PARWAR Biology Department, Clark University, Worcester, Massachusetts and Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts SYNOPSIS. An experiment was carried out to investigate the role of temperature in the previously reported reinforcing effect of an increase in sea water level in Aplysia. In the present experiment, it was found that the reinforcing effect of water level change on rod-pressing behavior in Aplysia depends on a decrease in temperature associated with water level change. In order to study modification of rod-pressing behavior produced by contingent increase in water level and decrease in temperature, the rate and latency of rod-press responses in experimental animals were compared with those of yoked control animals exposed to non-contingent water level and temperature change. Higher response rates and shorter response latencies were obtained from experimental over yoked control animals, but only the shorter latencies of experimental animals were attributed to a behavioral change resulting from contingent water level and temperature reinforcement.. INTRODUCTION There is considerable interest in demonstrating learning behavior in the marine mollusc, Aplysia. The major cause of this interest is the relatively small number and large size of neurons in the Aplysia nervous system (Frazier et al., 1967), which makes this animal a promising model for studying the cellular basis of learning. Several investigators have reported classical conditioning in Aplysia (Jahan- Parwar, 1970; Lickey, 1968), but there has been only one report on operant conditioning in this animal (Lee, 1969). It was shown in that experiment that an increase in sea water depth from a level that only half covered the animal to one in which the animal was completely covered could be used to reinforce an operant response in Aplysia. What was not determined was the particular stimulus aspect of changing wafer level that produced the observed reinforcing effect. The possibility investigated in the present experiment is that the reinforcing effect of contingent water level change results from a decrease in temperature associated with an increase in Supported by P.H.S. Grant NS08868 and Careeer Development Award K4-HD to B. Jahan- Parwar and by N.I.M.H. Research Traineeship MH to Paul Downey. 507 water level. In the experiment of Lee (1969), modification in the behavior of Aplysia was determined by using yoked control procedures to compare experimental animals exposed to contingent water level change with yoked control animals exposed to noncontingent changes in water level. Observed differences between experimental and control animals were attributed to conditioning associated with contingent water level reinforcement. In the following experiment, yoked control procedures modeled after those of Lee (1969) were used to further evaluate the conclusion that modification in the behavior of Aplysia occurs when water level reinforcement follows that behavior in time. METHOD Subjects. Thirty Aplysia californica, weighing between 120 and 395 grams, were used in this experiment. Between experimental sessions, the animals were kept in individual living chambers supplied with filtered and aerated sea water, cooled to 14 ± 0.5 C. The animals were fed ad libitum portions of dried seaweed (Dulse) which typically caused the animals to gain weight at an average rate of 15 grams per week (range: 0 to 38 grams per week).

2 508 PAUL DOWNEY AND BEHRUS JAHAN-PARWAR WATER /MLE TS SPOTLIGHT HEAT LAMP FIG. 1. Schematic overhead view of the single compartment experimental enclosure (A), the two compartment enclosure (B), and a side view of the I PROGRAMMING EQUIPMENT Apparatus. The experimental chamber was a plastic enclosure with an open mesh floor which was partially submerged in a 30 X 30 X 18 cm tank filled w th sea water. A water-filled syringe located below the i enclosure was used in the manner of a hydraulic cylinder to raise or lower the enclosure relative to the surrounding tank (see Fig. 1). An infusion-withdrawal pump (Harvard Apparatus Model 902) was used to drive the syringe to produce changes in the depth of sea water in the experimental enclosure between 3 cm and 9 cm in approximately 6 sec. Water in the tank surrounding the experimental enclosure was continuously filtered, aerated, and circulated through the chamber at the rate of 2.5 lifers per minute. The experimental enclosure was located in a temperature controlled room maintained at 14 ±: 0.5 C. Two experimental enclosures were used in this experiment. One consisted of a sinsingle compartment enclosure showing the rod manipulandum and the overhead spotlight and heal lamp (C). gle compartment having curved walls and measuring 18 cm in diameter. Next' to the curved wall, near the sea water outlet, was a vertical rod manipulandum extending down to the floor of the experimental enclosure from a circular contact switch, 12 cm above the enclosure floor. This manipulandum allowed contact closure to be made by displacement of the vertical rod in any direction on the horizontal plane. Each contact closure produced a brief (0.5 sec) flash from a spotlight (American Optical Model 651) located over the manipulandum. This served as a feedback stimulus for rod-press responses. Forty cm above the tank floor was a heat source (General Electric Infrared Lamp). Prolonged use of the heat lamp produced temperatures of 17 to 19 = C above the surface of the water and raised the water temperature at the surface from 14 C to no more than 16 3 C. The heating effect of the lamp on the darkly pigmented skin of Aplysia would be

3 COOLING AS REINFORCEMENT IN Aplysia 509 CO 2 O Q. Ui It \N\! AC33»«AC 34o«Ht*T Law V-^-( I \.ktip or*) es.ss; /V 4 AC 37^. AC 38 oo SESSIONS FIG. 2. The response rates (responses per minute) of experimental animals (closed circles) and control animals (open circles) over sessions in which the responses oe experimental animals were followed by one of four changes in heat lamp and water level stimuli: Increase in water level either in the presence (heat lamp on) or absence (heat lamp off) of the heat lamp stimulus, increase in water level and extinction of the heat lamp, or extinction of the heat lamp alone. greater than that indicated by these measurements. The second type of enclosure used in these experiments consisted of two oval anc^ compartments measuring 18 X U cnl each equipped with a rod manipulandum. The surrounding tank and other peripheral equipment (heat lamp, spotlights, etc.) were the same as those described above. Events in the experimental chamber were remotely controlled by solid state programming equipment (Massey Dickinson), and responses were recorded on magnetic counters and cumulative recorders. Procedure. At the start' of daily 1-hr sessions, animals were placed in shallow sea water (3 cm deep) facing away from the response rods. For some animals, sessions began with the overhead heat lamp turned on, and for other animals the heat lamp was turned off. A response was recorded each lime a response rod was pressed, and each response produced a brief flash from the overhead spotlight. Each response also began a 30 sec reinforcement period, and additional responses extended these periods until 30 sec elapsed without a response. Experimental animals were run under one of four experimental conditions of heat lamp and water level reinforcement. In the "heat lamp" condition, water level remained at the 3 cm level throughout each session, and the heat lamp was t'urned off during reinforcement periods. In the second experimental condition the "heat lamp and water level" condition water depth was increased from 3 cm to 9 cm, and the heat lamp was extinguished during reinforcement periods. The third condition was the "water level (heat lamp off)" condition in which water depth was increased to 9 cm while the heat lamp remained off throughout each session. The last' experimental condition "water level (heat lamp on)" was added to control for possible behavioral effects of change in level of illumination associated with extinguishing the heat lamp stimulus. Under this condition, the heat lamp remained on throughout each session. Thus, increase in water level that accompanied reinforcement periods produced a substantial decrease in temperature but had little effect on level of illumination. A separate group of four experimental animals paired with four control animals were run under the "heat lamp and water level" condition. In addition, these animals were run in a series of probe trials used to determine difference in the response latencies of experimental and control animals. In probe trials, experimental and control animals were placed in comparable locations in the two compartments of the experimental enclosure, facing away from the response rods. A relative measure of response latency was obtained by recording which of the two animals made the first response in each probe trial and calculating the percentage of probe trials in which each animal made the first response. These trials were carried out' twice daily: once at the start of each session, and once about 30 min into the session. Two of the experimental animals were run individually in the single compart-

4 510 PAUL DOWNEY AND BEHRUS JAHAN-PARWAR TABLE 1. Average response rates (responses per minute) of experimental animals under response-contingent changes in water level alone, heat lamp alone, or in both heat lamp and water level stimuli; and of control animals exposed to non-contingent stimulus changes. The number of sessions over which each average was taken is shown in parentheses. Animal AC12 AC13 AC33 AC34 AC37 AC38 AC21 AC22 AC23 AC24 AC25 AC26 AC15 AC16 AC17 AC1S AC19 AC20 AC49 AC50 Average Water level (heat lamp off) 1.3 (9) 1.4 (9) 1.35 Contingent Heat lamp 4.5 (6) 2.3 (6) 2.7 (11) 3.5 (7) 2.3 (8) 3.06 ment enclosure shown in Figure 1A. The remaining animals were run in pairs (one experimental and one control) in the two compartment enclosure illustrated in Figure IB. The responses of control animals were recorded, and each response produced a flash of light from the overhead spotlight. But changes in water level and heat lamp stimuli depended on the responses of the experimental animal paired with each control animal. RESULTS Figure 2 shows response rates (responses per minute) obtained under each of the four experimental conditions. The upper graphs show results from experimental animals AC 12 and AC 13 run in the single compartment enclosure. These graphs are divided by vertical lines into segments and labeled as to the reinforcing stimulus conditions used in sessions falling within each segment. The lower graphs are from experimental animals AC33, AC37, and AC 13 (represented by closed circles) and control animals AC34, AC38, and AC44 Heat lamp and water level 3.9 (8) 3.9 (6) 5.2 (2) 4.1 (2) 3.9 (17) 6.1 (9) 2.7 (10) 2.2 (11) 4.0 Non-contingent (control) 0.6 (6) 0.7 (6) 1.3 (11) 1.2(7) 1.6 (8) 3.7 (17) 1.4(9) 1.4 (10) 1.4 (11) 1.26 (represented by open circles). Figure 2 shows that response rates above control levels were obtained from experimental animals under the three conditions that included exposure to the heat lamp stimulus. These were the "heat lamp," "heat lamp and water level," and "water level (heat lamp on)" conditions. In the absence of the heat' lamp stimulus as in the "water level (heat lamp off)" condition, response rates remained near control levels. Listed in Table 1 are the mean response rates obtained from animals exposed to three of the experimental (contingent) conditions, and control (non-contingent) conditions. Included in this table are data from animals shown in Figure 1 and similar data obtained from additional animals. These data indicate the magnitude of difference in response rates obtained from animals exposed to the heat lamp stimulus (columns labeled "Heat lamp" and "Heat lamp and water level") and response rates obtained from animals run under "water level (heat lamp off)" and control conditions. The latter group of animals pro-

5 COOLING AS REINFORCEMENT IN Aplysia 511 TABLE 2. The percent of probe trials in which experimental animals designated in Column 1 produced a shorter latency from the start of a session to the first response in that session (Probe 1), or from the start of a probe within a session (Probe 2) and the total percent of probes in which shorter latencies were obtained from, experimental animals. Experimental animals AC 71 AC 75 AC 77 AC 81 Number of probe trials Percent of Probe 1 trials Percent of Probe 2 trials duced response rates of 1.7 responses per minute or less (average: 1.26 to 1.35 responses per minute), while animals exposed to the heat lamp stimulus produced rates of 2.2 responses per minute or greater (average: 3.06 to 4.0 responses per minute). Table 2 contains the results of probe trials used to compare response latencies of experimental and control animals after a period of exposure to contingent (experimental) or noncontingent (control) conditions. A difference in response latency is indicated by shorter latencies from experimental animals in more than the 50% of probe trials that would occur on the basis of chance. Table 2 shows that' three of the four of experimental animals in Probe 1 trials and all four experimental animals in Probe 2 trials had shorter latencies in more than 50% trials. DISCUSSION This experiment shows that the reinforcing effect of increase in water level on rod-pressing behavior in Aplysia depends on the cooling effect produced by increasing water level. Under all conditions that included decrease in temperature as one of the stimulus changes following a response, the rate of rod-pressing behavior was substantially higher than that of control animals exposed to non-contingent changes in temperature and water level. Contingent water level change in the absence of a drop in temperature produced response rates near control levels. Extinguishing the heat lamp stimulus in the present experiment produced a decrease in level of illumination as well as a drop in temperature. The "water level (heat lamp on) " condition was included in the present experiment to control for possible behavioral effects of change in illumination. This procedure allowed substantial reduction in temperature (by immersing the animal in sea water), while only slightly decreasing the level of illumination. The high response rate obtained under these conditions shows that the effect of the heat lamp stimulus on the behavior of Aplysia results from its effect on temperature. Evidence for behavior modification in Aplysia was obtained in measures for bofli response rate and response latency in the present experiment. A change in response rate is shown by the higher responses per minute of experimental over yoked control animals. However, it is possible that such differences are the result of an unconditioned change in activity level associated with changes in temperature and water level. For example, increase in activity level occurring at the termination of reinforcement could produce higher response rates in experimental animals. Since the occurrence of reinforcement depends on the proximity of experimental animals to the response manipulandum, the likelihood of random responses resulting from an increase in activity level is greater for experimental than for control animals. Differences between experimental and control animals in response latency, however, could not be attributed to nonspecific effects of reinforcement since the measured behavior (latency) is completed prior t'o reinforcement. Moreover, since the latencies obtained in Probe 1 trials in the present experiment were carried out at the start of each session, the latency measurement was not closely preceded by reinforcement. Thus, the effect of reinforcement on behavior in Probe 1 trials results from conditioning rather than from short' term unconditioned effects of reinforcing stimuli.

6 512 PAUL DOWNEY AND BEHRUS JAHAN-PARWAR REFERENCES Jahan-Parwar, B Conditioned responses in Aplysia californica. Amer. Zool. 10:287. (Abstr.) Frazier, W. T., E. R. Kandel, I. Kupfermann, R. Lee, R. M Aplysia behavior: effects of Waziri, and R. E. Coggeshall Morphologi- contingent water level variation. Conimun. Behav. cal and functional properties of identified neu- Biol. 3: rons in the abdominal ganglion of Aplysia cali- Lickey, M Learned behavior in Aplysia vacfornica. J. Neurophysiol. 30: caria. J. Comp. Physiol. Psychol. 66: