The role of the CS and its relationship to the CR in avoidance conditioning

LEARNING

The

AND

MOTIVATION

Role

( 1971)

of the

2,

12-25

CS and

Its Relationship

in Avoidance

to the

CR

Conditioning1

MICHAEL CASSADY, MICHAEL COLE,~ MICHAEL HALL, AND TIMOTHY WILLIAMS University of California,

Irvine

Two studies are reported which are concerned with the role of the CS and its relationship to the CR in the standard avoidance learning situation. ’ Study 1 demonstrates that the decrement associated with a trace CS is greatly reduced in one-way avoidance as compared with two-way. The suggestion is made that the differences in CS effects is attributable to differences in task complexity. Study 2 varied CS duration and the CS-CR termination contingency. Rapid acquisition of the avoidance response was obtained only when the CS remained on for the entire CS-US interval unless terminated by a response. The results were discussed with respect to the theoretical aspecti of maintenance and emergence of the avoidance response.

Evidence regarding the role of the CS as a discriminative stimulus and the way that it comes to exert control over the conditioned avoidance response (CAR) is generally lacking, perhaps because the dominant view for so many years emphasized motivational aspects of the CS (cf. Mowrer, 1960; Rescorla & Solomon, 1967). One of the many pieces of evidence used to support the motivational position is the importance of CS termination for the acquisition of the CAR (Kamin, 1956, 1959). The interpretation given this result is that CS termination maintains the CAR by reducing acquired fear or anxiety occasioned by the CS, thus reinforcing the response. Recently, a number of studies have indicated that CS termination is no more effective in the acquisition of the response than other stimuli which provide the animal “feedback” concerning his successin avoiding the UCS (cf. Bower, Starr, & Lazarovitz, 1965; D’Amato, Fazzaro, & Etkin, 1968; Belles & Grossen, 1969). The hypothesis growing out of this work is that feedback stimuli, of which CS termination is but an example, serve as safety cues whereas the initiation of the CS acts as 1 This research was supported in part by grant PHS-7ROl-MH13073-02 Cole. The authors thank M. Grinder and D. Wahlsten for their assistance. ‘Requests for reprints should be sent to M. Cole, The Rockefeller New York, N. Y. 10021. 12

to Michael University,

CS-CR

RELATION

IN

AVOIDANCE

LEARNING

13

a warning cue. The main functions of CS onset and termination are of a discriminative nature. The chief opponents of this latter view, while admitting the informational value of the CS, argue that the informational function alone cannot explain the maintenance or the emergence of the CAR. A different tack is taken by Herrnstein ( 1969) who provides evidence that the necessary and sufficient condition for acquisition of the CAR is that the response produce a reduction in the local probability of noxious stimulation. Herrnstein’s experimental situation, however, was quite unlike the standard experiments employed in the study of avoidance in recent years. In particular, there was no standard CS and the subject could not avoid the US, only delay it. Bolles and Grossen (1970) have verified his result in the standard avoidance procedure by varying the length of the intertrial interval. The authors concluded that the .‘ . . . effectiveness of a safety signal is largely a function of how long a period free of shock it consistently predicts.” This result indicates that the avoidance procedure can be viewed as a successive discrimination task where salient information about alternative events provides for optimum learning conditions. A major difference between these two positions lies in their interpretation of the function of the CS in the avoidance situation. The following studies were conducted to provide additional information about the way in which the CS comes to exert control over responding in a typical avoidance situation. EXPERIMENT

1

If, indeed, the true function of the CS in avoidance learning is to provide the animal with a reliable cue for successfully avoiding the US, rather than to provide motivation and a source of reinforcement for the CR, differences in rate of learning ought to be related in some way to the differential effectiveness of various CSs in providing the animal with the requisite information, In general, it might be expected that learning complex tasks would require CSs which are in some sense more salient or informative in order to bring performance to a level comparable to learning a simple task. It is the purpose of this first experiment to give substance to these ideas about the relation between CS informativeness and task complexity. In this experiment, task complexity is manipulated by requiring animals either to run only in one direction to avoid shock, or to shuttle back and forth between two compartments. Although there is no doubt that, in some global sense, shuttling is a more difficult avoidance response to learn, the cause of the additional difficulties is still a matter of debate.

14

CASSADY

ET

AL.

The early hypothesis that animals tended to avoid returning to a compartment in which they had been shocked previously (Theios & Dunaway, 1964) has recently been contradicted by Wahlsten and Sharp (1969) who provide evidence indicating that the one-way procedure is easier because the spatial cues are reliably related to the avoidance response while for the two-way animals they are conflicting. Stimulus “informativeness” is manipulated by using either a trace procedure with a brief CS or a delay procedure in which the CS terminates with completion of the response. A stimulus is said to be informative in this context if its termination reliably signals a “successful” response, i.e., lack of shock. The previous discussion generates a clear-cut hypothesis about the relation between learning procedures and stimulus conditions; for the one-way animals, there will be little difference between trace and delay procedures, but the trace procedure will produce a decrement in twoway avoidance. The superiority of delay over trace avoidance conditioning in the shuttle box is well known (Black, 1963). However, for direct comparisons with the one-way avoidance procedure, it is necessary to modify the shuttle procedure to control for differential handling of the animals which has been shown to substantially improve shuttle avoidance ( Wahlsten & Sharp, 1969). Method Subjects. The Ss were 40 naive albino rats of the Sprague-Dawley strain obtained from Simonsen Laboratories. They were approximately 60 days old upon arrival at the laboratory and 70-75 days old at the time of training. Apparatus. The experiment was conducted with a two-compartment shuttle box, 50.8 cm long X 17.8 cm wide X 25.4 cm high. The two 25.4-cm compartments were separated by a guillotine door which could be manually raised or lowered by a cable attached at the top. The unit was constructed of PIexiglas with the top and one side transparent and the remainder translucent. The floor of the unit was constructed of 6.3-mm steel bars separated by 15.8-mm spaces and hinged in the middle so that when S crossed the box a microswitch under one end of the box was operated. A 25-W bulb was mounted on the outside wall at the end of each compartment; the light from these bulbs was sufficient to illuminate the corresponding compartment. Shock was provided by the output from a 24-V ac transformer passed through a 320-Kohm resistor to provide an approximately constant current of .75 mA. Adjacent bars of the grid floor were of opposite polarity;

G-CR

RELATION

IN

AVOIDANCE

LEARNING

15

in order to keep Ss from learning to bridge the shock, the current was pulsed at a rate of lOO/min. Programming of event sequences and contingencies was carried out \vith operant relay equipment and electric timers. Response latencies were measured to the nearest .OI set with a Standard Electric timer. Procedure and experimental design. Each trial began when S was placed in the starting compartment facing away from the door. After approximately 15 set (timed with a stopwatch), the guillotine door was raised and the light attached to the starting compartment was illuminated. This event constituted a compound CS. For all Ss the interval between CS onset and US onset was 5 sec. A total of 50 trials was administered to each S with an intertrial interval of 30 sec. When the US occurred, it remained on until S crossed into the safe compartment. A response completed prior to US onset avoided the shock. Independent groups of 10 Ss each were differentially treated with respect to CS duration and start box placement on successive trials. For half of the Ss (Delay Condition) the CS light remained on until S had crossed into the safe compartment, while for the remainder (Trace Condition) the CS light remained on for 1 see unless S responded in less than 1 set, in which case the CS terminated with the response. Half of each of these two groups (One-Way Condition) always began their triaIs in the same compartment. FolIowing each trial S was detained for 15 set in the safe compartment and then placed in the starting compartment for 15 set, at which time the next trial was initiated. The remainder began each trial after the first in the compartment into which they had run on the previous trial (Two-Way Condition). To control for the effects of handling, Ss in the two-way groups were picked up 15 set following each trial and placed back in the same compartment facing away from the door. These procedures resulted in a 2 x 2 factorial design; trace and delay procedures (CS Duration) being orthogonally combined with one-way or two-way avoidance condition ( Task Requirement ) .

Results and Discussion A transformed score for the number of avoidances in successive blocks of 10 trials is shown in Fig. 1. From the figure it is apparent that learning for the one-way groups was rapid regardless of CS Duration, but that for the two-way Ss, the trace procedure produced a large deficit. This visual impression is supported by an analysis of variance of the transformed (z/x & 1) scores. The one-way groups learned more rapidly than the two-way groups (F = 111.1, df = l/36, p < .Ol), the trace groups learned more slowly

16

CASSADY

FIG.

ET

AL.

1. Mean learning curves for groups in Experiment

1.

than the deIay groups (F = 10.1, df = l/36, p < .Ol), and the interaction between CS Conditions and Task Requirements was marginally significant (F = 3.94, df = l/36, .lO > p > .05). Each of the independent variables interacted significantly with trials (p < .05) and more importantly, in view of the hypotheses motivating this study, the interaction between Trials, CS Duration, and Task Requirements was significant (F = 4.15, df = 41144, p < .Ol) in the manner suggested by Fig. 1. Although care should be taken in assessingthe main effects because of the higher order interactions, Fig. 1 shows little difference between trace and delay conditioning for the one-way groups at any point in learning, but a large difference developing between the twoway groups. Thus some initial support is provided for the prediction generated by the notion that the CS serves as an informative cue and that this information is not necessary if the task is sufficiently simple. Bolles and Grossen (1969) have shown that the deleterious effect of delayed CS termination appears in the two-way but not in the oneway avoidance procedure. They suggest that this decrement can be attributed to stimulus consequences of the response (i.e., loss of intrinsic and/or extrinsic information). This explanation is compatible with the one presented above, if it is assumed that task complexity is a function of the amount and saliency of the feedback cues. Using this interpreta-

CS-CR

RELATION

IN

AVOTDANCE

LEARNIKG

17

tion, avoidance in the one-way situation is easily acquired because of the many redundant, salient stimulus changes that occur with the response (e.g., spatial cues, direction, as well as possible olfactory cues). This explanation permits acquisition of the avoidance response by subjects selecting any one or a combination of the available feedback cues. If there is only one reliable feedback cue available (i.e., a complex task), early trials are occupied in part with selection of this cue, resulting in slower acquisition. In order to examine more closely the way in which the experimenterdesignated CS controls avoidance responding and to assess more accurately the factors producing the trace deficit, a second experiment was conducted using the two-way avoidance procedure. EXPERIMENT

2

The experiments of Solomon and his associatesduring the early 1950’s will serve as a model for this experiment. Church, Brush, and Solomon (1956) using dogs, and combining their data with data collected earlier by Kamin ( 1954), demonstrated that avoidance learning was independent of the CS-US interval when a delay procedure was used, but that learning was a decreasing function of CS-US interval when a trace procedure was used. In the trace conditions of these experiments the CS was of fixed duration (.9 set) and the US followed at various intervals. These findings were replicated by Black (1963) using rats as Ss. Black also pointed out the di5culties in interpretation resulting from the usual procedures for comparing performance under different interstimulus intervals and suggested that the performance of the trace animals at the longer interstimulus intervals might actually be caused by “spurious avoidances.” Bitterman (1965) considers the problem of confounding GS-US interva1 with opportunity to respond so serious that he advocates the use of a transfer design in all such studies. This methodological problem is relevant to the present experiments which seek to compare trace and delay conditioning under procedures which allow for varying opportunities to terminate the GS by anticipatory responding. The procedure adopted was used previously by Cole and WahIsten (1968) in the study of leg flexion avoidance and Kamin (1965) in the study of GER; the CS-US interval is held constant, and CS duration is studied by allowing the GS to remain on for varying amounts of the interval. The question of response-produced CS termination is investigated by comparing groups which (in principle) can or cannot terminate the CS by their responses. This procedure has the added advantage of disentangling CS duration and the termination factor which were confounded in the earlier study of Brush (1957).

18

CASSADY

ET AL.

Method Subjects. The Ss were 54 albino rats of the Sprague-Dawley strain obtained from Simonsen Laboratories. Some of the Ss were used previously in a study involving food reward, the remainder were experimentally naive. The Ss ranged in age from 60 to 100 days at the time of training. All Ss were maintained on an ad libitum feeding schedule and were haphazardly assigned to experimental groups with the restriction that an equal number of naive and previously used Ss were assigned to each group. Apparatus. The apparatus was a modified shuttle box with inside dimensions 76.2 cm long X 17.8 cm wide X 15.2 cm high. The two B-cm compartments were separated by a guillotine door that was operated manually. The top, back, and ends of the box were painted flat gray; the front was made of transparent Plexiglas. The floor of the box was made of 3.2-mm steel bars at 12.7-mm intervals. A 15-W frosted light bulb was mounted in the center of the back wall of each compartment and speakers through which white noise could be delivered were mounted adjacent to each light. A l.l-mA shock (650 V through 600 Kohms resistance) could be delivered to the floor of the chamber. The current was discontinuous, occurring in 306msec pulses, 100 pulses per minute, and was applied so that adjacent bars were of opposite polarity. The floor was balanced so that when S crossed from one side of the chamber to the other, a microswitch was tripped. A Standard Electric timer recorded latencies to the nearest .Ol sec. Design and procedure. There were six experimental groups with nine Ss per group. Groups differed with respect to the amount of time the CS was presented in a fixed-length CS-US interval and whether or not a response which occurred during the CS served to terminate the CS. The two contingencies for terminating the CS (Terminate and Nonterminate) were combined factorially with three CS durations (2, 4, and 8 set). For all Ss the CS-US interval was 8 set and the intertrial interval was 30 sec. At the start of the experiment S was placed in one of the compartments and left undisturbed for 2 min. The first trial began with the lifting of the door, the onset of the light in both compartments, and the onset of the white noise. The noise plus light CS remained on for the scheduled time or (for the Terminate groups) until S responded, If S failed to respond in less than 8 set, the shock came on and remained on until S did respond. A standard discrete-trial shuttle procedure was employed. All Ss were given 50 trials in this manner.

CS-CR

RELATION

IN

AVOIDANCE

19

LEARNING

In addition to latencies on each trial, a distance measure was also recorded. The distance of S’s head from the center door was measured using a l-in. interval scale marked on the front wall of the chamber. Distances were recorded separately at CS onset, CS termination, and US onset. If S had already passed the center door when one of these events occurred, a distance of zero was recorded. This measure introduced by Wahlsten, Cole, Sharp, and Fantino (1968) has been found useful in pinpointing differences in the ease of avoidance learning attributable to nonassociative factors such as freezing. Results

and Discussion

Table 1, Row A shows the mean total avoidances for each group. An analysis of variance indicated that there was a significant difference attributable to the effects of CS termination (F = 5.46, df = l/48, p < .03), as well as CS duration (F = 12.71, rlf = 2/48, p < .Ol), and their interaction (F = 4.14, df = 2/48, p < .Ol). Post-hoc analysis of the differences between treatment means using the Neuman-Keuls procedure (Winer, 1962, p. 77) indicated that the only reliable difference among treatment means was attributable to the group with an 8-set terminable CS (the traditional delay procedure) which differed significantly from all other groups, none of which differed from each other. Analysis of changes in response probability across trials was consistent TABLE 1 Summary of Major Performance Characteristics Duration Response measure A. Mean T NT B. Mean T NT C. Mean T NT D. Mean CS T NT Note:

2

of CS (SW) 4

8

total avoidances 9.22 8.67

12.00 10.67

36.67 16.67

escape latencies 3.046 2.208

1.879 1.816

1.482 1.457

3.584 2.152

3.127 2.883

2.955 4.248

1.642 1.866

2.119 2.082

2.693 3.255

avoidance latencies total movement during activation (escapes only)

T = terminate. ?r’T = non-terminate.

20

CASSADY

with

the results

ET AL.

in Table

1. The overall effect of Trials was significant p < .Ol). Greatest improvement occurred in the 8-set CS-terminate group, although only the interaction between CS Termination and Trials attained statistical significance (F = 2.55,

(F = 12.62, df = 4/142,

df = 4/ 192, p < .05). Consequently, we are led to conclude that for the conditions included in this experiment both a delay procedure and the possibility of terminating the CS with the response were required to produce adequate avoidance learning. In order to determine the source of the deficit encountered in the remaining conditions, the latency and distance measures were analyzed in some detail. Table 1, Row B presents mean escape latencies. The trend toward shorter escape response latencies with increasing CS duration was significant (F = 4.51, df = Z/48, p < .05) but no other differences reached significance. An analysis of variance for the avoidance latencies (Table 1, Row C) yielded no significant main effects, but the interaction between CS

CsFIG.

2. Distribution

of avoidance

us

lNlmv~L

latencies

(SECONDSI

for terminate

groups

in

Experiment

2.

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and CS Termination was sign&ant (F = 6.71, df = 2/43, p < .Ol), (Ss who failed to avoid were not included in the analysis; an unequal n analysis of variance was computed according to the procedure suggested by Scheffe, 1959.) Th is interaction reflects the tendency of the latencies for the nontermination groups to increase with increasing CS duration whereas latencies for the termination groups decrease. Figures 2 and 3 clarify the origin of this interaction. From Fig. 2, it will be seen that for all CS-Terminate Groups the mode is constant with a mode at 2 sec. For the Nonterminate Groups (Fig. 3), the mode increases with CS duration peaking at 2 set for the 2-set group, a dual peak at 2 and 3 set for the 4-set group, and a multiple mode from 2 to 6 SW for the 8-set group. It appears very much as if CS duration is differentially affecting some aspect of S’s responding, which is not clearly reflected in the response frequencies. Evidence of rather specific CS control over S’s responsescomes from an analysis of the distance measures. First, an analysis of S’s position at CS onset indicated that there were no differences associated with Duration

/

2

3

cs-us FIG.

3. Distribution

4

INTERVAL

of avoidance late&es

5

6

-77

(SK~NCS)

for nont erminate groups in Experiment

2.

22

CASSADY

ET

AL.

experimental treatment (all p > .25). This result indicates that no group had an advantage over any other by virtue of its initial distance from the center door. Next, by taking the absolute value of the difference between s’s position at CS termination from his position at CS onset, a measure of the amount of movement during the CS can be obtained. Similarly, the absolute difference between S’s position at US onset and CS termination provides a measure of movement in the absence of the CS. An analysis contrasting these measures (CS activation movement vs CS deactivation movement) for the trace groups only indicated that there was significantly more movement during the CS than during its absence (F = 25.27, df = l/32, p < .0005). Only the escape trials were included in this analysis to avoid spuriously high movement scores in the CS activation categories. The finding seems even more impressive when it is recalled that the CS remained on for half or less of the CS-US interval. Neither CS duration nor the CS-CR contingency affected this relationship. To obtain more information on the CS and movement, the mean amount of total movement during the CS is shown for each of the groups in Table I, Row D. The trend toward more movement for more CS ontime is significant (F = 58.8,& = 2/48, p < .Ol) but no other differences reached significance. The relationships between the CS and movement appear to be the following: (a) more movement occurs as the CS increases in duration and (b) relatively little movement occurs when the CS is inactivated during the CS-US interval. Together these factors suggest that the subject’s running response is controlled by the CS. With this as our hypothesis, we can reexamine the relationship between movement, response latency, and adequate avoidance in Experiment 2. Direct observation during testing indicated that Ss in both NT-8 and T-8 groups kept moving until the CS was inactivated; but in the NT-S group, this was true even if the avoidance response had been made early during the CS-US interval. Not all of this movement is reflected in Table 1 because we did not make a formal measurement of retracing within the goal box. In terms of our hypothesis, the nontermination of the CS results in more overall activity and worse performance by the NT-8 Ss because the termination of the tone does not signal successful avoidance. By dividing the CS-US interval into like periods (in terms of CS duration), the number of avoidances that were made during the CS can be compared to those that were made when the CS was off. Nearly half of the avoidances (70/150) of the NT-8 group occurred in the last 4 set of the CS-US interval (during which time the CS was on). In contrast, 0nIy 13% (12/95) and 16% ( 171107) of the avoidances occurred

CS-CR

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IN

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in this same period for the NT-4 and T-4 groups, for which the CS was not on. The same type of analysis can be made for shorter duration trace groups. A greater number of avoidances was produced in the first 2 set of the CS-US period by subjects in 2 set than in the following the NT-2 (46, 26) and the T-2 (40, 29) groups, for which the CS is on only during the first 2 sec. This is not true for the comparable 2-set periods for the subjects in the NT-4 (32, 51) group in which the CS was on for the entire 4 sec. All of these results support the generalization that the CS exerts specific control over the running response of the subjects; CS onset initiates movement and when the CS is terminated, Ss tend to stop running. If CS termination consistently signals successful avoidance no deficit can occur (T-8). When CS termination does not consistently signal successful avoidance (NT and T groups 2 and 4), poor avoidance results. Neither of the theoretical views which provided the setting for this research can be comfortable with all of these data without some elaboration. In particular, the failure of the T-2 and T-4 groups to produce performance superior to their NT comparisons groups needs further consideration. Without pretending to a complete theory of learning in this situation, we think that an elaboration of the informational approach can comfortably be amended to include the present data as follows (perhaps the same can be done for the motivational theory) : Given a simple task (e.g., one-way avoidance), selection of the experimenter-designated CS by S is not necessary for good avoidance behavior because of the number of redundant, valid cues inherent in the situation. As the task becomes more complex (e.g., standard two-way delay conditions with CS termination of the response), selection of the CS by S becomt,s more important for good avoidance behavior because its termination is the only cue that is available to S which provides consistent information about the result of alternative behaviors. As a direct consequence of this proposal, it is assumed that under the standard delay condition, S learns to select the most effective cue. Slower acquisition can be expected in the usual discriminated avoidance situation than in a redundant cue situation because the number of trials required to select the relevant CS should depend at least partially on its saliency. Unfortunately, there are no data available on the effects of additivity of cues for avoidance learning. Some support for the role of stimulus selection under discriminated avoidance procedures is contained in the paper by Bolles and Grossen (1970), where Ss who could respond at any time during the trial did significantly better than those who could avoid only during the last 10 set of a trial (i.e., CS-US interval).

24

CASSADY ET AL.

This interpretation provides a framework for understanding the additional trouble that besets those Ss run under conditions which provide poor feedback or invalid information (e.g., two-way trace, nonterminable delay): selection of the CS by S has been made, but CS termination is inconsistently contingent on the running response. This hypothetical account of discriminated avoidance is consistent with the views of Wagner (1969) who concludes that “ . . . the degree to which a potential cue will come to be reacted to as a signal for another event depends on the cue’s validity, relative to that of other concomitant cues during training.” Our view of avoidance conditioning regards the CS as an important discriminative stimulus which, once selected by S, informs him when to respond, and whose termination provides S with information regarding the effect (i.e., shock or no shock) of particular responsesin a successive discrimination task. This explanation accounts for the maintenance of the CAR, but leaves unnoted any description of the mechanisms which elicit the response initially (i.e., emergence of the CAR). It is suggested that the CAR is initially elicited by the interaction of at least two mechanisms which do not necessarily correspond to traditionally proposed factors. On escape trials, nonrunning responses are followed by shock “punished”) while running responses are followed by nonshock ; p; “rewarded”). It is suggested that this procedure leads to the buildup of responses occurring just before US onset. This corresponds to the classically conditioned reponse and is the proposed fist mechanism. At the same time, there is a tendency for responses to occur at CS onset, because of the generalization of the running response associated with a stimulus change (at US occurrence) to the next occurrence of stimulus change (at C’S onset). This constitutes the proposed second mechanism. The delineation of these proposed mechanisms lies in the realm of CAR emergence, yet studies on emergence are sadly lacking in the literature. REFERENCES

BITTER-MAN, M. E. The

CS-UC interval in classical and avoidance conditioning. In W. F. Prokasy (Ed.), Ck.~sicuE conditioning. New York: Appleton-CenturyCrofts, 1965. BLACK, A. H. Effects of the CS-US interval on avoidance conditioning in the rat. Canadian Journal of Psychobgy, 1963, 17, 174-182. BOLLES, R. C., & GROSSEN, N. E. Effects of an informational stimulus on the acquisition of avoidance behavior in rats. Journal of Comparative Physiobgical Psych& ogy, 1969, 68, 96-99. BOLLES, Ft. C., & GROSSEN, N. E. Function of the CS in shuttle-box avoidance leaming by rats. Journal of Comparative Physiological Psychology, 1970, 70, 165-169. BOWER, G., STARR, R., & LAZAROVIIZ, L. Amount of response-produced change in the

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CS and avoidance learning. Journal of Comparative Physiological Psychology, 1965, 59, 13-17. BRUSH, E. S. Duration of the conditioned stimulus as a factor in traumatic avoidance learning. Journal of Comparative Physiological Psychology, 1957, 50, 541-546. CHUHCH, R. M., BRUSH, F. R., & SOLOMON, R. L. Traumatic avoidance learning: the effects of the CS-US interval with a delayed conditioning procedure in a free responding situation. Journal of Comparative Physiological P,sychoZogy, 1956, 49, 301-808. COLE, XI., & WAHLSTEN, D. Response contingent CS termination as a factor in avoidance conditioning. Psychonomic Science, 1968, 12, 15-16. D’AMATO, M. R., FAZZARO, J., & ETKIN, M. Anticipatory responding and avoidance discrimination as a factor in avoidance conditioning. Journal of Experimental P~~ychology, 1968, 77, 4147. HERHNSTEIN, R. J. Method and theory in the study of avoidance. Psychological Rcview, 1969, 76, 49-69. KAMIX, L. J. Traumatic avoidance learning: the effects of CS-US interval with a trace conditioning procedure. Journal of Comparative Physiological Psychology, 1954, 47, 65-72. KAMIY’, I,. J. Effects of termination of the CS and avoidance of the US on avoidance learning. Jourtral of Comparative Physiological Psychology, 1956, 49, 420424. GAMIN, L. J. CS termination as a factor in the emergence of anticipatory avoidance. Psychological Reports, 1959, 5, 455-456. KAhlIN, I,. J. Temporal and intensity characteristics of the conditioned stimulus. In W. F. Prokasy ( Ed. ), Classical conditioning. New York: Appleton-CenturyCrofts, 1965. MOWHER, 0. H. Learning theory and behavior. New York: Wiley, 1960. RESCOI~LA, R. A., & SOLOMON, R. L. Two process learning theory: relationships between Pavlovian conditioning and instrumental learning. Psychological Review, 1967, 74, 151-182. SCHEFFE, A. The analysis of variance. New York: Wiley, 1959. THEIOS, J., & DUNAWAY, J. E. One-way versus shuttle avoidance conditioning. Psychonomic Science, 1964, 1, 251-252. WAGNER, A. R. Stimulus validity and stimulus selection in associative learning. In N. J. Mackintosh and W. K. Honig (Eds.), Fundamental issues in associative Zearnkg. Halifax: Dalhousie Univ. Press, 1969. WAHLSTEN, D., COLE, M., SHARP, D., & FANTIXO, E. Facilitation of bar-press avoidance by handling during the inter-trial intervals. Journal of Comparative Physiological Psychology, 1968, 65, 170-175. WAHLSTEN, D., & SHARP, D. Facilitation of shuttle avoidance by handling during inter-trial interval. Journal of Comparative Physiological Psychology, 1969, 66, 252-259. WINEH, B. J. Statistical principles in experimental design. New York: McGraw-Hill, 1962. (Received

October

168, 1969)