CUEING PROPERTIES OF THE DECREASE OF WHITE NOISE INTENSITY FOR AVOIDANCE CONDITIONING IN CATS

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1 ACTA NEUROBIOL. EXP. 1979, 39: CUEING PROPERTIES OF THE DECREASE OF WHITE NOISE INTENSITY FOR AVOIDANCE CONDITIONING IN CATS Kazimierz ZIELINSKI Department of Neurophysiology, Nencki Insti'tute of Experimental Biology Warsaw, Poland Abstract. In the main experiment two groups of 6 cats each were trained in active bar-pressing avoidance to a CS consisting of either a 10 db or 20 db decrease of the background white noise of 70 db intensity. The two groups did not differ in rapidity of learning, however cats trained to the greater change.in background noise performed avoidance responses with shorter latencies than did cats trained to smaller change. Within-groups comparisons of cumulative distributions of response latencies for consecutive Vincentized fifths of avoidance acquisition showed the greatest changes in the region of latencies longer than the median latency of instrumental responses. On the other hand, the effects of CS intensity found in between-groups comparisons were located in the region of latencies shorter than the median latency of either group. Comparision with data obtained in a complementary experiment employing additional 17 cats showed that subjects trained to stimuli less intense than the background noise level were marked by an exceptionaly low level of avoidance responding with latencies shorter than 1.1 s, which was lower than expected from the probability of intertrial responses for this period of time. Due to this property of stimuli less intense than the background, the distributions of response latencies were moved to the right, in effect, prefrontal lesions influenced a greater part of latency distributions than in cats trained to stimuli more intense than the background.

2 INTRODUCTION Experiments in which separate groups of cats were trained in active bar-pressing avoidance have shown that the higher the intensity of white noise used as the conditioned stimulus (CS) the larger the proportion of avoidance responses performed with short latencies (28, 30). Further it has been demonstrated that prefrontal lesions in cats (removal of proreus and orbitalis gyri) resulted in a pronounced decrease of short-latency avoidance responses which do not recover during postoperative retraining (27, 30, 31). Since avoidance responding with long latencies did not deteriorate after prefrontal lesions, it has been postulated that different physiological mechanisms are responsible for performance of short- and long-latency avoidance responses (29). This hypothesis was supported by an analysis of the course of avoidance acquisition which showed that short-latency avoidance responses occurred late in training, and the increase in their performance is responsible for the sudden improvement in the ability to avoid shock (12, 25, 33). It was found, however, that the CS consisted in an increase of the background white noise intensity evoked much less proportion of avoidance responses with short latencies than the CS of the same absolute intensity but presented without backgound white noise (31). Thus, not only the CS intensity but also the discriminability of the CS from the background are parameters important for evocation of short-latency responses. Responding with short latencies was especially reduced when the CS consisted of a decrease of the background white noise intensity (28). Using stimuli of different intensities, but lower than the background stimulation intensity, it is possible to place in opposition the effects of two factors: the absolute intensity of the CS and its discriminability from the background. The effects of CSi consisting of different amounts of attenuation of the background white noise intensity were investigated in the present study. Most experiments in which the decrease of physical energy served as CS were design to compare the effectiveness of the same amount of change but in opposite directions. In contrast to the original statements of Pavlov (20) and Hull (9), the "discrimination" interpretation of stimulus intensity effects by Perkins (21) and Logan (16), as well as Champion's (3) "modified Hullian theory", postulated that the effectiveness of the stimulus varies with the degree of physical energy change regardless of its direction. Experiments with eye-lid conditioned reflexes (8, 17, 18), galvanic skin and key-pressing responses (3) showed that the

3 decrease of light or tone intensity is as equally effective as the increase regardless of testing by either within- or between-ss designs. On the contrary, experfments in which avoidance contingencies have been employed showed superior performance to the increase than to the decrease of light or tone intensities both in within- (1) and in between- Ss (10, 11, 14, 15, 19, 22) comparisons. Similarly, it has been shown that an increase in noise intensity evoked more short-latency avoidance responses than a decrease, although the groups of cats compared did not differ in rapidity of avoidance learning (28). In stimulus generalization studies of rats trained in avoidance reflexes it was shown that offset of the tone exerts less stimulus control than onset of the tone (2). Presumably, stimuli less intense than the background were discriminated from the background with greater difficulty in comparison with more intense stimuli. Results of Kamin's (13) study conducted on 1C) groups of rats trained in CER showed that the increase of white noise intensity was significantly superior than the decrease only when the stimulus change was small, whereas no differences were observed when the change was large. The other result found in these studies was independent of both the contingencies employed and the character of the conditioned response. Namely, it was demonstrated that a greater decrease of the background intensity was more effective than a smaller decrease, similar to offset? of stronger and weaker stimuli (3, 5, 13, 18, 22, 26). These data support the notion that amount of change, and not the absolute intensity level, influences the strength of the conditioned response. The design of experiments presented in this paper is based on comparisons of groups of cats, each conditioned to only one intensity of the CS. The main experiment was specially designed and conducted to investigate properties of the avoidance reflex trained to CSi consisting of the decrease in white noise background intensity level. Additional data complementary to the main experiment are also given. MATERIAL AND METHODS The experimental sessions were carried out in a 65X55X40 cm cage with a 10x2 cm bar on the wall opposite to entrance door and located 8 cm above the grid floor. The US was an electric shock 50 Hz a-c from a circuit in which 100 Kohm resistance was connected in series with a cat in order to approximate a constant current source. The

4 voltage of the current was estabilished individually for each cat, based on observations of the animal's behavior during the first 5 sessions. For most of the cats the resulting shock was 3.5 ma intensity. The source of the CS was a permanent magnet speaker, 8 cm in diameter, mounted near the center of the ceiling of the box, through which white noise of a given intensity was delivered from a Grason-Stadler Generator, Model 901A. The cage was placed in a sound-proof CR chamber. The ambient sound level within the chamber, without activation of a speaker, was measured 45 db SPL. During the experiment, before a cat entered the CR chamber, a white noise of 70 db ( dyne/cm2) was on continuously. For one group of cats the CS consisted in the decrease of the background noise intensity to 60 db (Group 70-60), and for the other group the CS was a decrease from the same background to 50 db (Group 70-50). During the first trials of avoidance training a series of platforms of decreasing size were used to shape the bar-press response (32). The responses emitted during the 5 s CS-US interval terminated the CS immediately and prevented shock. Responses made after the onset of shock terminated the CS and the shock simultaneously. Each training session consisted of 10 trials. The intertrial intervals (ITI) in which the 70 db white noise was continuously delivered through the speaker were of 40, 60, and 80 s duration, randomly distributed. At the start of the experiment 16 cats were used, eight subjects in each group. However, for more then 20 sessions two cats from Group and five from the Group showed no progress in avoidance acquisition, since performance of the bar-press escape response was largely limited to shock onset, and they only sporadically emitted avoidance responses. The poor learnes from the Group were repiaced by other sub'jects, so that in each group six cats reached the required criterion of 90 avoidance responses in 10 consecutive sessions. The last 10 training sessions are referred to as the criterion period. After reaching avoidance criterion all cats underwent removal of the prefrontal region (proreal and orbital gyri) by suction under Nembutal anesthesia. Post-operative training began 10 days after surgery and continued until 90 avoidance responses in 10 consecutive sessions were again reached. The first 100 trials after surgery constitute the postoperative period, and the last 100 trials, when the cat performed the avoidance response at the 90 /o level, constitute the post-operative criterion period. During the entire experiment latencies of bar-presses were measured to the nearest 0.2 s, and numbers of intertrial responses (ITRs)

5 during each session were collected. In this paper only data obtained on cats which reached the required critserion of avoidance acquisition are reported. RESULTS Acquisition of the avoidance response. Even among cats that reached the acquisition criterion, substantial individual variability in rate of learning was observed. The criterion of 90 /o avoidance performance was reached after a range of 17 to 64 sessions with a mean of 34.8 sessions in Group and 38.2 sessions in Group This difference was not significant (Mann-Whitney U test). In order to compare groups, corresponding stages of reflex acquisition had to be normalized on a time base. The Vincent method (24) allows calculation of group learning curves when training is carried out tc the same criterion for all subjects. For each animal in the present experiment, the total number of trials, including the criterion sessions, was divided into 5 blocks. The number of trials in each block was the same for a given subject and differed between subjects. For each block Mean median latencies of instrumental avoidancelescape responses and mean ITR rates in consecutive stages (Vincentized fifths) of training Group Stage Median latencies (s) ( Rate of ITR (per min) I 11 I11 IV V Source of variation CS intensity Stage Interaction df 1 1 :l5 < * 4 : 60 < 1 Values of F statistics t 1 < of Vincentized 5th of trials and each subject, the median Latency of instrumental response and the mean ITR rate per min were calculated. Group means of these indices of behavior are presented in Table I. To allow more detailed analysis of the response latency changes for each group and Vincentized 5th, relative cumulative frequency distributions.

6 of response latencies were also constructed l. Corresponding curves are given in Fig. 1. Differences between distributions were tested statistically using the Smirnov two-tailed test (4) and results of the analyses are presented in Tables I1 and 111. As seen from Table I shortening of the median latencies of instrumental responses was rapid on early stages and much slower toward the end of training 2. On the other hand, ITR rates do not change statistically in the course of learning. Time in seconds Fig. 1. Cumulative relative frequency distributions of response latencies for Group (left panel) and for Group (right panel) in consecutive Vincentized fifths of training. Filled symbols represent avoidance responses, while open syrnbols represent escape responses emitted during joint CS and US presentations. 1 1 The procedure involved the ordering of frequences of responding by each subjet\ at 0.2 s intervals. The cumulative frequency at each 1 s interval of the CS-US interval and for the first 5 s of join action of the CS and US was then determined and this subtotal was calculated for each subject as per cent value of the total number of trials within the analysed block of trials. Tnen mean percentage for each group and block of trials was estimated and presented in Figures. Accordingly, the various points on the distributions represent the magnitude of the cumulative frequency of responding at successive 1 s period of the CS action, relative to the total opportunity for responding in that group. This technique permits comparisons of the vigor of responding at ear lie^ and later per5ods of CS action so that such influences as CS onset and US onset on responding may be readily observed. Approximate median latency values of total trials for each Vincentized fifth may be derived from cumulative frequency distributions shown in Fig. 1 by examining the response levels at the 50 percentile. It has to be remembered, however, that group mear. from individual median response latencies is not the same value as group median latency value of total trials for a given group.

7 Changes in the distributions of response latencies between consecutive stages of avoidance acquisition in each experimental group Comparisoh I vs. I1 I1 vs. I vs. IV IV vs. V Direction of / I point of maximum difference distance (s) SI < SII Sn < SIII SIII < SIV SIV < sv Group * 0.31 * 0.16* 0.16* I vs. I1 I1 vs. I11 I11 vs. IV IV vs. V sr < SII SII < SIU Sn1 < SIV SIV < s v Group * 0.32* * P < 0.001; Smirnov two-tailed test. ' Inspection of the cumulative frequency distributions of response latencies gives us much more information about changes occurring in the course of training. The lack of a significant difference in median response latencies between two consecutive stages of training in a given experimental group or between two groups at a certain stage of avoidance acquisition does not mean that there were no significant differences in the proportion of responses executed with latencies shorter or longer than the median value. Moreover, the Smirnov test employed for comparisons of cumulative frequency distributions utilizes each CS intensity effect on the distributions of response latencies estimated for 5 consecutive stages of training: Group vs. Group comparisons Stage of training I IV Direction Point of maximum of difference I / distance (s) s60 < s50 s60 < sso Ss0 < s50 Sso < S ** v I S6o < S5o! 0.12* 2.7 I * 0.19** P < 0.01; ** P < 0.001; Smirnov two-ta'iled test. I

8 response latency as raw data instead of single median value for a given subject and Vincentized 5th as is done in analysis of variance. Comparisons of cumulative distributions of response latencies presented in Table I1 indicate that for each consecutive Vincentized 5th of training, response latencies were stochastically shorter than for the preceding one. Similar results for Group were obtained with the exception that cumulative distributions for the third and the forth Vincentized 5ths did not differ statistically. Results in Table I11 showed that at each stage of avoidance acquisition, except for the first one, response latencies for Group were stochastically shorter than for the Group at P < 0.01 or better. Data presented in Tables I1 and I11 also give information at which point on the time scale the maximum distance (Dm,,) between the two compared cumulative distributions occurred. Comparison of the points of Dm,, given in Table I1 with median latency values showed in Table I indicate that in each caste the Dm,, between distributions from conse- cutive Vincentized 5th~ were located to the right on the time scale in regard to the mean median latency for the cumulative distribution more advanced in training. It reflects the finding that in the course of avoidance acquisition the greatest changes in responding occurred within long-latency responses. Similar scrutiny of, the,data presented in Tables I and I11 show that at each stage of training the greatest differences between cumulative distributions of the and groups were located to the left of the mean median latencies of either group compared. The only exception was observed for Group on the fourth Vincentized 5th of training. Some interesting information may be derived from comparisons of response latency distributions (Fig. 1) and ITR rates presented in Table I. If the probability of responding within the CS-US interval is the same as during intertrial intervals, about 10 /o of responses during the first Vincentized 5th of training trials ought to occur during the 5 s period of isolated CS action (1.2 ITRs per min corresponds to 2O/o ITRs per s or 10 /o ITRs per 5s). As may be seen, during the first Vincentized 5th of training, percentages of avoidance responses performed in either group were only slightly higher than values estimated on the basis of ITR rate (14O/o of avoidances in Group and 15O/o of avoidances in Group 70-50). Thus, after such a "clearing" procedure it may be concluded that during the first Vincentized 5th of training only about 5O/o of trials were terminated by real avoidance responses. Thus, during this stage of training escape responses were learned first of all. The shift to more efficient way of responding was rather rapid

9 and already during the second Vincentized 5th the level of avoidance performance exceeded several times that which may be expected on the basis of ITR rates. It should be noted that during all periods of training a very small proportion of avoidance responses was performed with latencies shorter than 1.1 s, and the cumulative distributions of response latencies abruptly changed at this time point. In fact, during the entire training period, the proportion of avoidance responses with latencies shorter or equal to 1.1 s did not exceed levels expected from the ITR rates for a given group and stage of training. Fig. 2. Avoidance reflex performance in consecutive sessions during the pre-operative criterion and post-operative periods. Solid lines refer to Group 70-60, broken lines to Group Effects of prefrontal lesions. As seen from Fig. 2 prefrontal lesions resulted in immediate deterioration of avoidance responding in both groups of cats, and the decrease in avoidance performance was observed in each subject. Five cats (three from Group and two from Group 70-50) showed negative saving scores and required more training sessions to reach the post-operative 90% criterion than during original learning. The ITR rates did not change due to lessions or post-operative re training.

10 Group differences in latencies of the instrumental responses before and after operations. Sso < SS0 denote that during a given period cats from Group emitted a smaller proportion of responses with latencies shorter than point of Dm,, than the Group 7&50 Periods: Pre-operative I ~f~~~ oprration 1 post-o~rative criterion I criterion A. Mean median latencies in s I Group 7MO Group ::: B. Cumulative distributions of response latencies comparisons Direction of difference Point of maximum discrepancy (in s) Dm, sso < sso s60 > sso s60 < SSO * P <0.05; Smirnov two-tailed test. Group means froin individual median latencies of instrumental responses for pre-operative sessions, the first 100 trials after surgery and the post-operative criterion sessions are shown in Table IV. The lesions resulted in marked lengthening of mean median latencies whicn Time in second s Fig. 3. Cumulative relative frequency distributions of response latencies for Group (left panel) and Group (right panel) before prefrontal lesions (solid line), during post-operative retraining (broken line) and post-operative criterion sessions (brofken line separated by dots) periods. Otha denotations as in Fig. 1.

11 Changes in the distributions of response latencies before operation and during post-operative training (group and individual data). S1, S2, and S3 denote cumulative distributions of response latencies during pre-operative criterion, post-operative, and post-operative criterion periods correspondently. S1 > S2 denote that during pre-operative criterion period a greater proportion of responses was emitted with latencies shorter than point of Dm, than during postoperative period. Effect of surgery S1 vs. S2 Effect of post-operative training S2 VS. S3, Change ( Group 7060 C- 1 G 3 G 4 c- 5 GI2 C- 2 bis Group c- 7 G 8 C-14 G15 C-16 C- 1 bis s2 < S3 s2 < s3 St < S3 s2 < S3 sz < S3 S2 < S3 Sz < S3 s2 < S3 S2 < s3 s2 < s3 s2 < s3 s2 < s3 S2 < S3 s2 < s3 P < 0.60; ** P < 0.001: Snirnov two-tailed test..only partially were restored during post-operative retraining. Betweengroups comparisons of cumulative distributions of response latencies indicate that the significant difference between the two groups observed during pre-operative period disappeared after lesioning and was not restored in the course of post-operative retraining. The post-operative lengthening of response latencies, the drop in avoidance performance, the partial shortening of latencies and the full recovery of avoidances after post-operative retraining may be inferred from the gradients presented in Fig. 3. The results of statistical analyses of the cumulative frequency distributions in consecutive periods of testing are presented in Table V. The group data indicate that the prefrontal lobectomy resulted in a significant shift of the cumulative frequency distributions of response latency to the right - i.e., toward longer latencies. Individual data indicate that in 10 out of 12 cats the cumulative distributions of response latencies for the post-

12 Values of the Spearman rank correlation coefficient between numbers of avoidance responses performed during the preoperative criterion period (Xi) and amount of deflection of the cumulative distribution of response latencies due to prefrontal lesions (Yj). Explanation in the text operative period (Sz) were at points of the Dmax lower that the cumulat-- ive distributions of latencies for the pre-operative criterion period (S,). Thus, taking into account only the direction of change in the latency distributions of individual cats, it may be said that the lengthening of response latencies due to prefrontal lesions was significant at the P < confidence level (both groups pooled, sign test, two-tailed, 23), In all 12 cats the cumulative distributions of response latencies for the post-operative criterion period (S3) were at points of Dm,, higher than for the post-operative period (Sz). The direction of change in the latency distributions of individual cats indicates that shortening of response latencies in the course of post-operative retraining was significant at the P < confidence level, using the same ~rocedure. It should be pointed that in 9 cats the Dm,, values between the post-operative reaquisition sessions and post-operative criterion sessions were located to the right on the time scale relative to localization of the Dm,, between pre- and post-operative periods. Thus, the post-operative retraining resulted in greater recovery of long- rather than in shortlatency avoidance responses, and the differences in the localization of the two maximum distances were significant at the P < level (the same procedure). As seen from Fig. 3 the frequency of responding with latencies shorter than 1.1 s was very low after lesions, similar to all stages of avoidance acquisition. Inspection of the same Figure leads to the impression that responding with latencies shorter than 2.1 s recovered only partially during post-operative retraining. This finding is supported by comparisons of cumulative distributions of response latencies for the pre- and post-operative criterion periods. In both groups the

13 maximum distances between cumulative distributions were around 2.5 s.of CS action (Group 70-60: P < 0.025, Dm,, = at the 2.7 s point; Group 70-50: P < 0.001, Dm,, = at the 2.5 s point). A Spearman rank correlation showed that the magnitude of prefrontal lesion effect on response latencies was dependent on the preoperative distribution of response latencies. The data from both groups were pooled for this analysis. The number of avoidance responses performed by a given cat during the pre-operative criterion period with latencie shorter than 1.1 s were denoted as XI, with latencies shorter than 2.1 s as X,, and so on. Then, differences between cumulative distributions of response latencies for the pre-operative criterion and post-operative reacquisition periods (S2-Sl) in consecutive second points were calculated for each cat. The difference between cumulative distributions which occurred at 1.1 s point was denoted as Y1, at 2.1 s Y2, and so on. Table VI presents rank-order correlation coefficients between the Xi and Yj variables. As seen from the Table, the greater the proportions of avoidance responses with latencies shorter than 2.1 s, 3.1 s, and 4.1 s performed before the operation, the more pronounced was the deflection of the cumulative distribution curve at 2.1 s point due to prefrontal lesion. The decrease in avoidance responses with latencies shorter than 1.1 s was similarly correlated with pre-operative levels at 1.1 s and 2.1 s points. The prefrontal lesions exerted a marked effect on escape response latencies which is also illustrated in Fig. 3. During the first 10 postoperative sessions about 25OIo of the shock trials were terminated by escape responses executed with latencies longer than 10 s after CS onset, whereas before operations such long-latency escape responses occurred in less than 5O/0 of the shock trials. Complementary experimental data. Two different explanations may be propssed for the unusually low proportion of avoidance responses (compare: 12, 25, 27-31) executed with latencies shorter than 1.1 s observed in Group and Group of the main experiment. Such distributions of response latencies either are typical for avoidance responses trained with CSi of lower intensity than the background noise level or they are caused by a small difference between the CS and background intensities regardless of the direction of change due to CS onset. The resolution to this issue may be obtained by comparing distributions of response latencies from two groups of cats: one trained with a CS consisting of a small increase, and the other trained with a CS consisting of a small decrease of the background white noise intensity. Material and methods. Detailed description of the methods used

14 in this experiment has been reported elsewhere (28) and differed from those of the main experiment in two aspects only: CS and US intensities. For one group of cats the CS consisted of an increase of the 60 db background white noise intensity to 70 db (Group 60-70), and for the other in a 10 db decrease from same background (Group 60-50). Except for the first 3 days of training the US intensity was the same for all cats and equal to 4 ma. In each group 10 subjects were used at the beginning of training, however, only 9 cats in Group and 8 cats in Group reached the 90 /o avoidance acquisition criterion. The methods of collecting and analysing data were the same as in the main experiment. Results. The criterion of 90 avoidance responses in 100 trials was reached after 15 to 65 sessions with a mean of 34.0 sessions in Group and 34.6 in Group The mean median latencies and ITR rates for each group during consecutive Vincentized fifths of training are presented in Table VII. Cumulative frequency distributions of TABLE VII Mean median latencies of instrumental avoidancelescape responses and mean ITR rates in consecutive stages (Vincentized fifths) of training Group Median latencies (s) Rate of ITR (per min) Stage / , / ( I I1 I11 IV V * 2.62 Source of variation I df 1 Values of F statistics CS intensity < Stage 55.24* 3.23 Interaction < 1 = ' response latencies are given in Fig. 4 and results of the analyses of differences between consecutive stages of training and the two groups are shown in Tables VIII and IX. As seen from Table VII, the shortening of the median latencies of instrumental responses in the course of training was similar to that observed in cats from the main experiment. Neither groups nor stages of training were differentiated by the values of ITR rates. Data presented in Table VIII indicate that in both groups cumulative response latency distributions were stochastically shorter on each

15 TABLE VIII Changes in the distributions of response latencies between consecutive stages of avoidance acquisition in each experimental group ) Direction I Dm_, Comparison 1 Point of maxiof difference mum distance (s) I vs. I1 11 vs. I vs. IV Iv vs. v I vs. I1 I1 vs. I11 I11 vs. 1% IV vs. V Group Sr < SII SII < SIII SIII< SIV SIV < sv 0.38* 0.30* * Group SI < SII 5.5 SII < SII1 0.40*l 0.31* 4.9 SIII < SIV 2.9 SIV < SV * 0.14* P < 0.001; Sml,rnov mo-tailed test. consecutive Vincentized 5th than on the preceding one. Comparisons of the points of Dmax given in Table VIII with median latency values showed in Table VII indicate that in each case the Dmax between distributions from consecutive Vincentized 5th were located within response latencies longer than the mean median latency for the Vincentized 5th more advanced in training. Thus, similar to Groups from the main experiment, the greatest changes related to training effects occurred within long-latency responses. Comparisons of cumulative distributions between Group and Group indicate that during the first Vincentized 5th response latencies were shorter in Group than CS intensity effect on the distributions of response latencies estimated for 5 consecutive stages of training: Group vs. Group 6&50 comparisons Stage of training Direction of difference Point of maximum distance (s) I I1 III N v S70 < SSo S~O > S5o S70 > Sso S70 > SIO ST0> s * ** 0.14** P < 0.05; ** P < 0.001; Smdrnov two-tailed tea.

16 in Group 60-70, whereas on the next stages the opposite direction d differences was observed. However, only during the fourth and fifth Vincentized 5ths were response latencies for Group significantly shorter than for Group At each stage of training the greatest differences between cumulative distributions of the and groups were located to the left regards of the mean median latencies of either group compared. Collectively, comparisons of the localization of the maximum distance with mean median latencies yielded identical results as far as effects of the course of training and stimulus intensity are concerned in both experiments reported in this paper. Comparisons of response latency distributions (Fig. 4) and ITR rates presented in Table VII indicate that during the first Vincentized lime in seconds 'Fig. 4. Cumulative relative frequency distributions of respxue latencies for Group (left panel) and Group 6050 (right panel) in consecutive Vincentized fifths of training. Other denotations as in Fig. 1. 5th of training the percentages of avoidance responses performed by Group were iower than estimated on the basis of the ITR rate (8Vo avoidances compared with 11.5O/o responses expected during the 5 s CS-US interval from ITR rate). On the contrary, during the same stage of training group performed markedly more avoidance responses (17OIo avoidances compared with 6.5OIo expected from ITR rate). -4s far 2s the proportion of avoidance responses with latencies shorter than or equal to 1.1 s are concerned, in the course of training differences between the two groups emerged. In Group 60-50, during all sta-

17 ges of trainins, the proportion of such resporses was lower than expected from the ITR rate. In Group 60-70, the proportion of avoidance responses executed with latencies shorter than or equal to 1.1 s was lower during the first and second Vincentized 5ths, and on the next stages of training two to three times higher than expected from the ITR rate. DISCUSSION The main aim of the present study was to experimentally test some of the controversies between two opposite theoretical approaches to the effectiveness of conditioned stimuli consisting of the decrease of the background stimulation intensity level. According to classical Pavlovian and Hullian statements, the higher the absolute intensity of the CS the greater strength of the evoked conditioned response. On the other hand interpretations of stimulus intensity effects by Perkins (21), Logan (16), and Champion (3) predicted that a CS more remote from the background ought to evoke a conditioned response of greater strength regardless ~f the direction of change from the backgrund intensity. There is general agreement among authors studying the effects of stimuli more intense than the background that latencies of the conditioned response are shorter when the eliciting stimulus is of greater intensity (3, 6, 7, 12, 29-31). Iq the present experiment it was shown that except at the very early stage of avoidance acquisition, the CS that is less intense but more remote from the background intensity evoked responses having latencies stochastically shorter than those to CS of more intensity but closer to the background noise intensity. Thus, the shorter latencies of responses performed by Group than by Group indicate that a greater decrease of the background noise intensity is a more salient stimulus than the smaller decrease, independent of the absolute intensity of the CS. Thus, differences betwen median latencies of instrumental responses emitted by Group and supported the change interpretation. However, in opposition to the stimulus change interpretation, the supplementary experiment showed that a CS consisting of a 10 db increase of noise intensity evokes responses with shorter latencies than a CS consisting of a 10 db decrease of the background noise level. Similar asymmetry in effects on response strength of stimuli more and less intense than the background were observed in other studies in which the avoidance contingency has been employed (1, 14, 19, 22), whereas data obtained by other methods of conditioning rather seem to support the stimulus change interpretation. It may be supposed that the complexity of the avoidance conditioning make this variety of conditioned 2 - Acta Neurobiologiae Exp. 5/79

18 reflexes a more suitable model for studying stimulus intensity effects on learning and performance. The data presented in this paper taken collectively indicate that neither classical nor more modern interpretations of stimulus dynamism are able to explain the obtained results. A similar conclusion was drawn by Kish (14) after analysis of his experimental data. The results of the preseiit experiments support the hypothesis suggesting different mechariisms for short- and long-latency avoidance responses (29). Differences between cumulative distributions of response latencies emitted by groups of cats trained with a CS of greater or less decrease in background noise intensity (main experiment) or trained to the increase or decrease of background noise (supplementary experiment) were always located to the left relative to the mean latencies of either group compared. This finding indicates that the intesity of CSi and their relations to the background influence short-latency avoidance responses first of all, similar to experiments in which no whlte noise was presented during intertrial intervals (12). On the contrary, comparisons of cumulative latency distributions for consecutive Vincentized fiths of training indicate that the greatest changes of responding occurred within responses emitted with longer latencies: initially within escape and then within long-latency avoidance responses. And again, this principle has also been observed in experiments with accoustic stimuli presented without background white noise (12). Only at the last stage of avoidance consolidation, changes related to continued training may occur within responses emitted with latencies slightly shorter than the mean median latency of the two distributions compared. Thus, a number of independent experiments clearly indicate that two parameters: stimulus intensity characteristics and the length of training exert their effects mainly on two different subclasses of avoidance responses. Changes in responding due to prefrontal leslons are also in agreement with previous reports (27, 29-31), showing that post-operative recovery of avoidance performance is related to the increase in the probability of responding with longer latencies than those most strongly affected by lesions. Further, the main effect of prefrontal lesions on short-latency avoidance is additionally supported by the fact that in the present experiment differences between Group and Group 70-60, manifested in cumulative response latency distributions, disappeared after surgery. These differences were related to short-latency avoidance responses which strongly deteriorate after prefrontal lesions. In previous experiments, differences between distributions of response latencies in groups of cats trained to a CS of different intensity

19 characteristics did not disappeared after prefrontal lesions (30, 31). Mowever, in the present experiment the maximum distance between the pre- and post-operative cumulative frequency distributions of response latencies were observed much further to the right on the time scale than in previous studies. The are two possible explanations for these differences. In previous experiments the cats after reaching the 90 /o criterion were given an additional 10 sessions (control period) before lesions (27, 30, 31), whereas such overtraining was not given in the main experiment reported in this paper. It seems, however, that the real explanation must take into account the shape of the response latency distributions to stimuli consisting of the decrease of background noise intensity. In all three groups trained with such a CS avoidance responses were emitted only sporadically with latencies shorter than 1.1 s. The cumulative distributions of response latencies at this point showed a drastic change in slope, in contrast to groups trained to stimuli more intense than the background. In other experiments, before prefrontal lesions the cats performed from 40 /0 (27) to 8O/o (31) avoidances with latencies shorter than 1.1 s, depending on CS intensity. Due to this property of CSi at lower intensity than the background, both experimental variables influencing short-latency responses, CS intensity characteristic and prefrontal lesions, affected a greater part of the cumulative distribution curves 'than in the case of stimuli more intense than the background. Similarly, correlation coefficient analyses showed that post-operative deflection of the response latency distributions was influenced by a greater part of the pre-operative distribution curve than was observed in cats trained tq the intense CS (31). The extremely low probability of avoidances with latencies shorter than 1.1 s observed in cats trained to stimuli consisting of a decrease in background white noise intensity provides further support for the thesis that intensity characteristics of the CS exert effects on short-latency avoidance responses. Performance of long-latency avoidances is controlled by other factors. This investigation was supported by Project of the Polish Academy of Sciences. REFERENCES 1. BIRKIMER, J. C. and DRANE, D. L Stimulus intensity dynamism with stimuli equal decibel distances above and below background. Psychon. Sci. la:

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21 24. VINCENT, S. B The function of the vibrissae in the behavior of the white rat. Behav. Monogr. 1: WERKA, T. and ZIELINSKI, K Effects of lessions in nucleus centralis of emygdala on acquisition and retention of avoidance reflex in cats. Acta Neurobiol. Exp. 38: ZELONYI, G. P A new conditioned reflex (to sound termination) (in Russian). Kha~kovskii Meditsinskii Zhurnal 5: ZIELINSKI, K Retention of the escape reflex after prefrontal lobectomy in cats. Acta Neurobiol. Exp. 30: ZIELINSKI, K Increase versus decrease in noise intensity as a cue in avoidance conditioning. Acta Neurobiol. Exp. 31: ZIELIRSKI, K Effects of prefrontal lesions on avoidance and escape reflexes. Acta Neurobiol. Exp. 32: ZIELINSKI, K Stimulus intensity and prefrontal lesians effects on latencies of the bar-pressing avoidance response in cats. Bull. Acad. Pol. Sci. S&. Sci. Biol. 20: ZIELIfiSKI, K Changes in avoidance latencies after prefrontal lesions in cats: group versus individual data. Acta Neurobiol. Exp. 34: ZIELIRSKI, K. and CZARKOWSKA, J Go-no go avoidance reflex differentiation and its retention after prefrontal lesion in cats. Acta Neurobiol. Exp. 33: zieli~~ski, K. and JAKUBOWSKA, E Temporal and stimulus intensity factors in avoidance reflex acquisition in cats. Activ. Nerv. Sup. 19: Accepted 27 April 1979 Kavimien ZIELIRSKI, NmcM Institute of Experimental Bdology, Pasteusa 3, Warsaw,. Poland.