Journal of Comparative and Physiological Psychology 1975, Vol. 89, No. 10, 1182-1203

Latent Inhibition and Stimulus Generalization of the Classically Conditioned Nictitating Membrane Response in Rabbits (Oryctolagus cuniculus) Following Dorsal Hippocampal Ablation Paul R. Solomon and John W. Moore University of Massachusetts—Amherst Rabbits received 0 or 450 exposures of a tone conditioned stimulus (CS) prior to classical defensive conditioning of the nictitating membrane response based on an infraorbital eye shock unconditioned stimulus. Tone preexposure resulted in retarded conditioning in normal rabbits. This latent inhibition effect was not present in animals with bilateral dorsal hippocumpeotomy produced by aspiration. Control animals with bilateral neocortical and callosal aspiration lesions demonstrated a latent inhibition effect similar to that shown by normal nonoperated animals. The failure of CS preexposure to retard conditioning in hippocampal rabbits was not due to differences in threshold of the conditioned response to the CS or to differences in response mechanisms as determined by tests of habituation and dishabituation of the unconditioned response. A subsequent experiment employed combined-cue summation tests to confirm the fact that preexposure did not endow the tone with conditioned as well as latent inhibitory properties. Finally, tests of stimulus generalization along the auditory frequency dimension indicated flatter relative gradients for hippoeampals than for nonoperated controls, with cortical controls in between. These findings were discussed in terms of Douglas' model of hippocampal function.

that a stimulus has been received and (b) that it has no motivational significance. The first of these functions is accomplished by cortical-hippocampal interaction, while the latter is under the auspices of a midbrain (reticular)-hippocampal arousal system. Once this information has been received, the hippocampus serves as a "nonreinforcement register" whose purpose is to assure that the stimuli associated with nonreinforcement are no longer attended to. The mechanism for behavioral inhibition, then, is for the hippocampus to discharge downward and inhibit the reticular core of the brain, which, in turn, serves to block or attenuate attention to the irrelevant stimulus. Of great importance to the theory is anatomical evidence for the existence of the required cortical-hippocampal (Cragg, 1965; Whitlock & Nauta, This research was supported by National Sci- 1956), limbic-midbrain (Nauta, 1956, 1958), ence Foundation Grant GB-24557 to the second author. We gratefully acknowledge the contribu- and midbrain-limbic (Guillery, 1956; Nauta tion of Donald G. Stein of Clark University, who & Kypers, 1958) pathways. Bchaviorally, it instructed us on the surgical procedures employed would appear that removal of the hippoin this investigation. campus would be disruptive in any situation Requests for reprints should be sent to John W. that requires the organism to tune out and Moore, Department of Psychology, Middlesex House, University of Massachusetts, Amherst, withhold responding to a nonreinforced stimulus. In line with this view are findings Massachusetts 01002. 1192 Several theoretical viewpoints have attempted to explain the complex behavioral changes following hippocampal damage in terms of deficits in inhibition (e.g., Douglas, 1967; Kimble, 1968). A recurrent theme is that the hippocampus is essential in registering stimuli that have no motivational significance (i.e., no rewarding or aversive consequences or associations) and assuring that these stimuli do not maintain excitatory control over behavior. Douglas (1972) has proposed a model for the involvement of the hippocampus in this tuning-out process. Specifically, Douglas suggested that the hippocampus regulates what an organism attends to by modulating midbrain arousal systems. According to the model, the hippocampus is responsible for determining (a)

HIPPOCAMPUS AND LATENT INHIBITION

that hippocampectomized animals exhibit increased resistance to extinction (Isaacson, Douglas, & Moore. 1961; Kimble & Kimble. 1970), impaired acquisition of a successive discrimination (Kimble, 1963), retarded acquisition of complex mazes (Niki, 1966), and impaired reversal learning (Kimble, Note 1). While it can be argued that each of these situations entails inhibition of responding to nonreinforced stimuli, perhaps a more direct approach to examining the role of hippocampus in tuning out a nonreinforced stimulus would be to investigate the effect of hippocampectomy on the phenomenon of latent inhibition. A number of studies have shown that a series of nonreinforced exposures to a conditioned stimulus (CS) prior to acquisition of a conditioned response (CR) retards subsequent conditioning (see Lubow, 1973, for a review of this literature). Lubow and Moore (1959) coined the term "latent inhibition" to describe this phenomenon and likened it to Pavlovian internal inhibition. There have been several experimental investigations of latent inhibition in the rabbit eye blink and nictitating membrane response (NMR) preparation (Reiss & Wagner, 1972; Siegel, 1972; Solomon, Brennan, & Moore, 1974; Solomon, Lohr, & Moore, 1974). These studies support the conclusion from other conditioning preparations (e.g., Rescorla, 1971) that the retardation effect is attributable to the preexposed CS losing salience through some sort of tuning-out process resembling habituation. Given this interpretation, Douglas' (1972) model would predict that the hippocampus is critical to the formation of a latent inhibitor. Specifically, it is predicted that animals with hippocampal damage would have difficulty habituating or tuning out a preexposed CS and therefore would not show retarded conditioning when the preexposed CS is subsequently reinforced by pairing with the unconditioned stimulus (UCS). EXPERIMENT 1 Experiment 1 was designed to test the above prediction for the rabbit NMR. If hippocampectomized rabbits do not habitu-

1193

ate to a preexposed CS, it would be expected that the decrement in acquisition following CS preexposure in normal animals would be reduced or eliminated in animals with hippocampal damage. An earlier investigation (Ackil, Mellgren, Halgren, & Frommer, 1969) examined the consequences of preexposing rats to a tone later used as a cue for active shock avoidance. The results of this study indicated that preexposed neocortical operates and nonoperated controls showed retarded acquisition of the avoidance CR, but hippocampectomized rats did not. However, the failure to find a latent inhibition effect in hippocampectomized animals might have been due to heightened sensitivity to the tone CS or to the shock UCS; that is, it is possible that these stimuli were effectively more intense for hippocampal animals than for controls. Since rate of conditioning and strength of the CR are directly related to CS and UCS intensity (cf. Gormezano & Moore, 1969), the possibility exists that failure to observe differences in conditioning performance between CS preexposed and nonpreexposed hippocampal groups results from a ceiling effect induced by exaggerated reactivity. In order to investigate this possibility, Experiment 1 included (a) postacquisition threshold tests as a check for differential effective intensity of the (tone) CS and (b) an examination of habituation and dishabituation of the unconditioned response (UCR) to eye shock as a check for differential reactivity to the UCS. Method Subjects. The subjects were 30 experimentally naive male and female New Zealand albino rabbits (Oryctolagus cuniculus), weighing between 2.8 and 3.2 kg at the time of surgery. Animals were housed individually and maintained on ad-lib food and water. Surgery. Animals in the hippocampal groups were anesthetized with an intramuscular injection of chlorpromazine (4 mg/kg), followed 1 hr later by sodium pentobarbital (20 mg/kg) injected intravenously through the marginal ear vein. This was followed by subcutaneous injections of a local anesthetic (Lidocaine) across the midline of the scalp. Holes were then opened bilaterally, the dura was incised and retracted, and underlying cortex, corpus callosum, and hippocampus were removed by subpial aspiration. Special care was taken to

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PAUL R. SOLOMON AND JOHN W. MOORE

avoid damage to the caudate nucleus and dorsal thalamus. After bleeding had been well controlled, the wound was packed with Gelfoam which had been soaked in thrombin. Neosporin was applied to the scalp, and the incision closed with wound clips. Finally, each rabbit received an injection of Bicillin (300,000 IT) and was returned to its home cage. Animals receiving cortical lesions underwent the same procedure except that only the cortex and corpus callosum were removed, sparing the underlying hippocampus. Animals were given 21-28 days to recover before the onset of behavioral training and testing. Apparatus. The apparatus and methods used to condition the rabbit's nictitating membrane response (NMR) were basically those described by Gormezano (1966). Four animals were run simultaneously in individual ventilated and soundattenuating file drawers. A panel at the front of each drawer housed two lights (4.5 V dc) mounted behind translucent screens and two speakers, one to deliver the tone CS and the other to deliver a continuous white masking noise (70 dB SPL). Subjects were restrained in Plexiglas boxes, with an adjustable plate and ear clamp securing the head and a second plate over the back restricting general movement. A small nylon loop was sutured through the right NM and attached to the shaft of a Minitorque potentiometer (Conrac No. 85153). Lateral movement of the NM was transduced to a dc signal and recorded on a polygraph located in an adjacent room. A conditioned response was defined as pen deflection of at least 1 mm (corresponding to an NMR of less than 1 mm) occurring within the CS-UCS interval. The CS was a 450-msec tone (1,200 Hz, 76 dB SPL), and the UCS was a 2-mA ac shock of 50 msec duration delivered via two stainless steel wound clips implanted superficially in the skin, one immediately below and the other immediately posterior to the right eye. Procedure. Following suturing of the right NM, all rabbits were habituated to restraint and the experimental chamber for 50 rnin. Half of the animals in the hippocampal, cortical, and nonoperated groups were assigned randomly to the CS preexposure or the nonpreexposure (SIT control) condition. Preexposed groups received 100 CSalone presentations per day, with an intertone interval of 30 sec, for four successive days beginning on the day after suturing. Animals in the corresponding SIT control conditions simply remained in their experimental chamber without stimulus presentations during the CS preexposure phase. On Day 6 only 50 CS preexposures were given (or 5 min of sit tune for controls). This was followed immediately by 50 conditioning trials in which the tone CS was paired with eye shock (with .a CS-UCS interval of 450 msec) for both experimental and control conditions. All groups received .an additional 3 days (Days 7-9) of conditioning (100 trials/day), with an intertrial interval of 30 ;sec for a total of 350 acquisition trials. The complete experimental design consisted of

six groups of five animals each: hippocampal, cortical, and normal (nonoperated) animals in the CS preexposure condition (H-PRE, C-PRE, and N-PRE, respectively) and hippocampal, cortical, and normal animals in the SIT control condition (H-SIT, C-SIT, and N-SIT, respectively.) On Day 10, the tonal CS was presented 100 times at varying intensity in order to determine threshold for eliciting the CR. All threshold test trials were reinforced. Specifically, five trials were given initially at 76 dB, and then tone intensity was decreased in 5-dB steps until the CR disappeared. Five trials were given at each intensity. When the block of five trials was reached in which no CRs occurred, the intensity was systematically increased in 5-dB steps up to the initial level of 76 dB. Each intensity was presented five times in succession during the increasing series. Auditory threshold was determined to be the first block of 10 trials (ascending plus descending) containing fewer than five CRs. Threshold testing was followed for the next 2 days by stimulus generalization tests described in Experiment 3. The suture in the right NM and the wound-clip shock electrodes were removed after the generalization test, and the animal was returned to its home cage. Ten days after generalization testing, all animals were again sutured, and wound-clip electrodes were reapplied in preparation for tests of UCR habituation. On the following day, animals were placed in the conditioning apparatus and subjected to UCS-alone presentations separated by intervals of 20 sec. For the first 40 trials, shock intensity was .25 mA (50 msec duration). On Trial 41, shock intensity was raised to 2 mA (the level employed during conditioning) and then returned to .25 mA for Trials 42-60. This dishabituation procedure was repeated on Trials 61, 81, and 101, with the 2-mA shock being followed in each case by 19 trials at the .25-mA level. This procedure was followed for two additional daily sessions (120 trials/day), and the amplitude of the UCR was recorded on each trial. Following the tests of UCR habituation and dishabituation, operated animals were sacrificed by sodium pentobarbital overdose and perfused intracardially with .9% saline followed by 10% formalin solution. The brains were removed, stored in formalin, and subsequently embedded in low-viscosity nitrocellulose or gelatin. Coronal sections were taken through the extent of the lesion at 60 jum. At the most anterior and posterior aspects of the lesion, every second section was mounted, while every fourth section was mounted in the central portion of the damaged area. All tissue was then stained with cresyl violet. Results and Discussion

Histology. Examination of brain sections in both Experiments 1 and 2 revealed that all animals in the hippocampal groups had vir-

HIPPOCAMPUS AND LATENT INHIBITION

1195

FIGURE 1. Reconstructions of the lesions sustained by the hippocampal (left) and cortical (right) animals. (The striped and solid areas represent the minimal and maximal damage respectively.)

tually complete bilateral destruction of dor- inhibition for each of the six groups of Exsal hippocampus. Although the postero- periment 1. Both the cortical and normal ventral portion of the structure was spared, groups showed the latent inhibition effect isolation of hippocampus from the fornix was with fewer CRs following 450 preexposures obtained, since the fimbria was bilaterally to the tone CS than following the SIT consectioned in all cases. It should be noted, trol (0 preexposures) treatment. In hippohowever, that this does not preclude the campal animals, however, this pattern was possibility of the ventral hippocampus inter- reversed. Analyses of variance of total CRs acting with other structures via the hippo- during the 350 conditioning trials revealed campal-entorhinal path (Hjorth-Simonsen, the following significant effects: A compari1971). son between hippocampal and cortical aniDamage to structures other than hippo- mals revealed a significant interaction becampus was minimal, with one animal sus- tween lesion type and preexposure level, taining unilateral caudate damage and F(l, 24) = 6.00, p < .01. Similarly, a signifianother slight damage to dorsal thalamus. Neither animal showed noticeable behavioral TABLE 1 differences from others in its treatment con- MEAN NUMBER OF CONDITIONED RESPONSES dition. DURING RETARDATION TESTING AS A FUNCTION Animals in the cortical groups generally OP LESION TYPE AND NUMBER OF CONDITIONED STIMULUS PREEXPOSURES showed more damage to the cortex than did the hippocampal animals, but in no case did Lesion type Preexposure cortically ablated animals show damage to condition Hippocampal Cortical Normal the underlying hippocampus. Reconstructions representing the minimal and maximal SIT 260.6 264.0 293.6 283.2 PRE 201.8 181.8 extent of the lesions are shown in Figure 1. Retardation test for latent inhibition. Table Note. There were 350 possible conditioned re1 depicts the average total number of CRs psonses. Abbreviations: SIT = nonpreexposed: given during retardation testing for latent PRE = preexposed.

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PAUL R. SOLOMON AND JOHN W. MOORE

TABLE 2 MEAN NUMBER OP TRIALS DURING RETARDATION TESTING TO REACH FIVE CONSECUTIVE CONDITIONED RESPONSES AS A FUNCTION OF LESION TYPE AND NUMBER OF CONDITIONED STIMULUS PHEEXPOSUBES (EXPERIMENT 1)

amplitude of each of the succeeding three blocks was expressed as a percentage of the first block. This treatment of amplitude data was also applied to Days 2 and 3 of habituation training. Analysis of variance of these relative amplitude scores was carried Lesion type Preexposure out in a factorial layout with lesion type and condition Hippocampal Cortical Normal preexposure condition as between factors and days and blocks as crossed within fac86.4 SIT 59.6 70.0 tors. The only statistically significant effect PRE 51.2 164.0 151.0 to emerge from this analysis was a blocks Note. Abbreviations: SIT = nonpreexposed; effect, F(3, 73) = 21.70, p < .001, attributaPRE = preexposed. ble to a decrease in relative UCR amplitude of approximately 20% from the first block cant interaction between these two factors to the fourth in all groups. The interaction was found when hippocampal and normals between blocks and lesion type, which would were compared, F(l, 24) = 4.43, p < .05, indicate differences in rate of UCR habituabut not when corticals and normals were 'compared (F < 1). Although hippocampal tion as a function of lesion type, did not approach significance, F(Q, 72) = 1.18, p > animals gave more CRs overall than either 'Cortical or normal groups, this difference .05. In addition to the finding that lesion type was not significant, F(2, 24) = 1.80, p > did not affect habituation to the UCS when .05. Table 2 shows the average number of a relative measure is used, hippocampals, trials to reach a criterion of five consecutive corticals, and normals did not differ with CRs for each treatment condition. Once respect to the absolute magnitude of the again, while the cortical and normal groups UCR to the .25-mA eye shock. The mean showed a pronounced latent inhibition ef- UCR amplitude on Block 1 across all three fect, the hippocampal group did not. Anal- test days was 4.8 mm for hippocampals, 6.8 mm for corticals, and 5.1 mm for normals. yses of variance on this measure yielded Analysis of variance that treated the UCR exactly the same set of statistical decisions amplitude on Block 1 during each of the (p < .05) as reported in the previous parathree test days as a within-groups variable graph for the total CR measure. Therefore, both indexes of latent inhibition support the and treated lesion type and number of preexposures as between-groups variables inhypothesis that hippocampal rabbits do not dicated no significant differences among habituate to a nonreinforced CS. these means. Threshold test. Table 3 shows the mean Dishabituation produced by increasing thresholds of the CR to the tone CS. Analysis of variance indicated no differences as a the habituating stimulus from .25 to 2 mA function of lesion type, number of CS pre- was assessed by comparing the amplitude on the trial before the 2-mA shock with the exposures, or the interaction of these two factors (all .Fs < 1). These findings tend to TABLE 3 rule out the possibility that the failure to MEAN AUDITORY THRESHOLD OF THE CONDITIONED find a latent inhibition effect among hippo- RESPONSE TO THE CONDITIONED STIMULUS As A campectomized rabbits is related to an alFUNCTION OF TREATMENT (EXPERIMENT 1) teration in the threshold of the CS. Lesion type Habituation and dishdbituation of the UCR. Preexposure condition Hippocampal Cortical Normal In order to examine the rate of habituation of the UCR to the eye-shock UCS presented 53 SIT 55 56 .alone, the first 40 trials with shock level of PRE 57 55 56 .25 mA were broken down into blocks of 10. 'The mean UCR amplitude for the first block Note. Entries are in decibels SPL. Abbrevia.served as a reference point, and the UCR tions: SIT = nonpreexposed; PRE = preexposed.

HIPPOCAMPUS AND LATENT INHIBITION

trial immediately following. The UCR amplitude "before" was divided by amplitude "before" plus amplitude "after." A ratio less than .50 indicated dishabituation. Since the dishabituating shock was presented four times (Trials 41, 61, 81, and 101) to each animal on each of 3 days, the analysis of variance of ratio scores included four factors: days, blocks, lesion type, and precxexposure condition. Only the days factor emerged as statistically significant, F(2, 48) = 4.36, p < .025, reflecting an average increase of ratio scores from .45 to .48 from Day 1 to Day 3. More importantly, the interaction between lesion type and days was not significant, F(4, 48) = 1.91, p > .05, suggesting that the progressive decrease in the dishabituating effects of the 2-mA eye shock over days did not differ as a function of lesion type. Insofar as habituation and dishabituation to the UCS can be taken as indexes of reactivity to that stimulus, the above results suggest that failure of hippocampal animals to show a latent inhibition effect cannot be attributed to differences in reactivity of response mechanisms over a series of conditioning trials. In addition, these results tend to support Douglas' (1972) contention that hippocampus has more to do with sensory than response processes. EXPERIMENT 2 Hearst (1972) indicated that repeated nonreinforcement of a CS, before or after conditioning, could endow it with conditioned inhibitory properties. Although the existing literature (Reiss & Wagner, 1972; Solomon, Brennan, & Moore, 1974; Solomon, Lohr, & Moore, 1974) suggests that CS preexposure in the rabbit eye blink and NMR preparation results in a loss of salience rather than an active or conditioned form of inhibition, either mechanism could account for the retarded acquisition following CS preexposurc in the cortical and normal groups of Experiment 1. Since Experiment 1 was predicated on the assumption that latent inhibition represents a conceptually pure form of tuning out an irrelevant stimulus, any conditioned inhibitory properties resulting from CS preexposure would com-

1197

plicate an interpretation of the role of hippocampus in this tuning-out process. It is therefore of some importance to determine whether the parameters of Experiment 1 were such as to prompt the development of conditioned inhibition. A summation test, whereby the preexposed CS is superimposed on a previously established excitatory CS, can be used in conjunction with retardation tests to separate these two interpretations (cf. Rescorla, 1969). If the preexposed CS (tone) has conditioned inhibitory properties, it would be expected to detract from the ability of the excitatory CS (e.g., light) to elicit the CR. On the other hand, if the nonreinforccd tone has undergone a loss of salience, it would have little effect on conditioned responding when compounded with the light. Specifically, a conditioned inhibitor is indicated when (a) conditioned responding to the compound is less than responding to the excitatory component presented alone and (b) this difference exceeds that of a control group that experiences the presumed inhibitor for the first time during the summation test (i.e., Brown & Jenkins' [1967] control for external inhibition). Method Subjects. The subjects were 24 experimentally naive male and female New Zealand white rabbits (Oryctolagus cuniculus), weighing between 2.8 and 3.2 kg at the time of surgery. Each animal was individually housed and maintained on ad-lib food and water. Apparatus and surgery. The apparatus and surgical procedures were the same as in Experiment 1. Procedure. On the day following suturing of the right NM and adaptation to restraint, etc., animals in the hippocampal, cortical, and nonoperated (normal) groups (n = 8) received 5 days (100 trials/day) of acquisition of the conditioned NMR to a light CS (panel lights). Rabbits in like surgical groups were matched on the basis of rate of conditioning to the light: One member of each pair was assigned to a squad that experienced 450 tone-alone presentations (1,200 Hz, 76 dB SPL), given over the succeeding 4.5 days, while the other was assigned to a squad that simply remained in the conditioning chamber (SIT control) with no tone presentations. Midway through the fifth day of tone preexposure, all animals received 25 trials to light and 25 trials to a light plus tone compound. These trials were presented in an unsystematic order and the eye-shock UCS was never presented. Summation testing continued on the

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PAUL R. SOLOMON AND JOHN W. MOORE

next day with 50 light and 50 light-plus-tone trials which followed a warm-up series of 15 reinforced light trials. Test trials were not reinforced.

that inhibitory summation resulted from external inhibition, distraction, or generalization decrement, but not from any conditioned inhibitory property of the tone. Interestingly, these distraction effects appear greater for hippocampal animals than for their cortical or normal counterparts (Table 4). A t test comparing the mean suppression ratios of the hippocampal animals (collapsing across both PRE and SIT conditions) with the pooled ratios of the cortical and normal animals indicated a significant effect, .05, or cortical, tion gradients. Experiments 1 and 2 included postconditioning tests of stimulus .05, groups. The finding that the tone was essentially generalization along the auditory frequency equally effective in distracting from the ex- dimension. citatory potential of the light in both SIT Previous research has been somewhat inand PRE conditions would seem to indicate consistent with regard to whether auditory

TABLE 4 MEAN NUMBER OF CONDITIONED RESPONSES TO LIGHT AND LIGHT PLUS TONE DUBING SUMMATION TEST As A FUNCTION OF LESION TYPE AND NUMBER OF CONDITIONED STIMULUS PBEEXPOSURES (EXPERIMENT 2) Lesion type Preexposure condition

SIT PRE

Cortical

Hippocampal

Normal

L

LT

L

LT

L

LT

38.0 27.3

20.5 16.5

54.6

46.0 26.7

50.8 54.7

46.3 54.7

36.0

Note, Abbreviations: L = light; LT = light plus tone; SIT = nonpreexposed; PRE = preexposed.

1199

HIPPOCAMPUS AND LATENT INHIBITION

generalization gradients of hippocampectomized animals differ from those of normal or sham-operated controls. Freeman, Kramarcy, and Lee (1973) reported no difference between hippocampal rats and sham-operated controls in the shape of excitatory and inhibitory gradients, but Freeman and Kramarcy (1974) reported steeper relative gradients for hippocampal than sham-operated rats. Schwartzbaum, Thompson, and Kellicut (1964) found no difference between hippocampal and normal rats in auditory generalization. Finally, Lockhart and Moore (1975) observed no difference in the slope of auditory gradients between normal rabbits and those with septal lesions given classical NMR conditioning. These studies, which report no difference in auditory generalization following hippocampal lesions, do not agree with the hypothesis that the hippocampus is important to the generalization process. The specific concern of the present research was whether tone preexposure would affect auditory frequency generalization on the rabbit NMR preparation as a function of surgical treatment. Method Subjects. The animals were those employed in Experiments 1 and 2. Apparatus and procedure. The apparatus for testing generalization was as described previously. Generalization testing consisted of 20 nonreinforced presentations of each of five test tones of 400, 800, 1,200, 1,600, and 2,000 Hz in a random sequence and all at an intensity of 76 dB SPL. This procedure was repeated on the next day for a total of 40 presentations of each test frequency. The animals in Experiment 1 were given the gent

,PHATWE

0

oABSOLUTE

eralization test after determination of threshold values of CS intensity and before the UCR habituation test. Experiment 2 animals received 2 days (100 trials/day) of conditioning to the 1,200-Hz tone following summation testing. Generalization testing for these animals began the next day.

Results and Discussion An examination of mean absolute and relative generalization gradients from Experiments 1 and 2 revealed similar trends in the shape of generalization gradients as a function of preexposure condition and lesion type. Analyses of variance were carried out on the pooled data from the two experiments. utilizing the basic 2 X 3 factorial design with three levels of lesion type and two levels of tone preexposure. In order to equate cell frequencies at n = 9, the mean of the eight available animals in the C-SIT group was added to that cell. Two measures were analyzed : the absolute percentage of CRs to the CS (1,200 Hz) and the proportion of CRs to the CS out of the total number elicited over the course of generalization testing. The first measure is an index of the overall level of responding (height of the gradient), and the second (relative) measure is an index of the slope of the gradient. The only significant effect to emerge from either analysis was a main effect of lesion type on the relative CR measure, F(2, 48) = 3.72, p < .05. Figure 2 shows the mean absolute and relative generalization gradients as a function of lesion type. The source of the significant main effect mentioned above is revealed in the figure: The mean relative gradient for normal animals is noticeably sharper

u 8. < I— z IU U

4

8

12

8

16

12

16

20

CORTICAL

HIPPOCAM?AL TONE

8

12

16

20

NORMAL

FREQUENCY (Hz x 100)

FIGURE 2. Mean percentage of relative (solid lines) and absolute (dashed lines) conditioned responses (CRs) during generalization testing as a function of lesion type.

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PAUL R. SOLOMON AND JOHN W. MOORE

than that of hippocampal animals, with the within 2 mm of the area containing units cortical animals showing a mean relative showing auditory stimulus control. gradient of intermediate sharpness. PairFinally, the fact that tone preexposure had wise contrasts indicated that the difference no observable effect on stimulus control is between hippocampals and normals in pro- basically consistent with other (unpublished) portion of total CRs to the 1,200-Hz tone results from our laboratory. This observawas statistically significant, .F(l, 48) = 7.43, tion is consistent with the notion that CS p < .01, but the other possible comparisons preexposure results in a loss of salience were not. through an habituation-like process rather Although there were no significant differ- than in a long-lasting, conditioned form of ences in the absolute percentage of condi- inhibition. Any residual conditioned inhibitioned responding to the training frequency tion following conditioning might have been (1,200 Hz) as a function of lesion type, Fig- expected to manifest itself in a lower absoure 2 suggests a trend in relative conditioned lute level of responding to the CS tone than responding to this tone: Normal rabbits ex- observed in 0-preexposcd SIT controls. But hibited a sharp generalization gradient re- this was not the case. Instead, conditioning sembling those previously observed in our following CS preexposure seems to reinstate laboratory (Moore, 1972). Somewhat poorer the salience of the CS to at least its original stimulus control was evident in cortical con- prehabituation level with no carry-over eftrols, and poor stimulus control was demon- fects that could be detected in the generalizastrated by hippocampal rabbits. This trend tion test. The summation data of Experisuggests that the comparative loss of stimu- ment 2 also support this interpretation by lus control along the auditory frequency demonstrating the absence of conditioned dimension was due to damage to pathways inhibitory properties of the preexposed CS. mediating interactions between hippocampus GENERAL DISCUSSION and cortex. Dorsal hippocampal damage alone may not aversely affect auditory While Douglas (1972) equated latent instimulus generalization. The relevant liter- hibition with a unitary process, Pavlovian ature cited in the introduction to Experiment internal inhibition, modern learning the3 would tend to support this view. These ories like Rescorla and Wagner's (1972) studies employed electrolytic lesion tech- continuity theory of conditioning distinguish niques, sparing cortex, etc., whereas the between latent inhibition and conditioned present research employed aspiration. forms of inhibition. Latent inhibition inGabriel, Wheeler, and Thompson (1973) volves a loss of salience of the potential CS, investigated multiple-unit activity from whereas conditioned inhibition involves acthree nonspecific cortical areas of rabbits tive antagonism of the excitatory processes during auditory generalization tests based responsible for the conditioned behavior in on a conditioned avoidance response. All question. The results of Experiment 1 demelectrode placements were 1-2 mm off the onstrated the involvement of the hippocammidline: one cluster at bregma, one at pus in the process responsible for loss of lambda, and one midway in between. Units salience, and in this respect the present inin this third area demonstrated stimulus vestigation supports Douglas' model of control, i.e., gradients of activity corre- Pavlovian inhibition and hippocampal funcsponding to the observed behavioral gradi- tion. Experiment 1 also provided evidence ents. Units in the more anterior and pos- that the essential elimination of latent interior clusters did not show stimulus control. hibition by hippocampectomy was not the It is tempting to speculate as to whether the result of changes in the threshold of the CS reduction of stimulus control produced by or of alterations in response mechanisms as our aspiration of cortex-hippocampus is determined by tests of habituation and disrelated to a distribution in the functioning habituation of the UCR. of "generalization" units investigated by An alternate interpretation of the eliminaGabriel et al. Our aspirations encroached to tion of latent inhibition in hippocampal ani-

HIPPOCAMPUS AND LATENT INHIBITION

mals, albeit speculative, is that hippocampectomy selectively impairs retention of inhibitory training. Schmaltz and Theios (1972) found that hippocampectomy disrupted retention of extinction of the classically conditioned NMR. In their study, hippocampal animals attained an initial extinction criterion just as rapidly as did normal and cortical controls. However, whereas control animals required progressively fewer trials to reach the extinction criterion in subsequent daily acquisition-extinction tests, hippocampal animals required the same number of trials to extinguish in the later tests as in the initial extinction test. This absence of savings across repeated extinction tests suggests that hippocampal animals were unable to retain the previous extinction experiences. Similarly, while hippocampectomized rabbits in the present study might have been able to tune out the preexposed CS over the short term, say within a single daily session of 100 CS presentations, they might not have been able to cumulate this experience over the daily preexposure sessions for 4.5 days. This sort of memory deficit could account for the failure of these animals to demonstrate a latent inhibition effect in Experiment 1. Experiments 1 and 2 demonstrated that latent inhibition of the rabbit NMR involves an habituation-like process and that hippocampectomy disrupts this process. Two questions that merit consideration are (a) whether the elimination of latent inhibition by hippocampectomy necessitates neocortical or callosal damage and (b) whether hippocampectomy would also disrupt active forms of inhibition like that demonstrated in our laboratory (e.g., Mis, Norman, Hurley, Lohr, & Moore, 1974), utilizing Pavlov's conditioned inhibition paradigm. The first question is prompted by reports in the literature that hippocampectomy does not alter the rate of habituation of rats' exploratory behavior unless overlying neocortex is damaged (e.g., Douglas & Isaacson, 1964). Future research on the role of hippocampus in latent inhibition of the rabbit NMR preparation should employ more discrete electrolytic lesions sparing neocortex as one means of probing the question of

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neocortical-hippocampal interaction in elimination of latent inhibition. Whether hippocampectomy disrupts active or conditioned as well as latent inhibition remains essentially an open empirical question. Successful differential conditioning in Pavlov's conditioned inhibition paradigm, where one CS is always reinforced when presented alone but never reinforced in the presence of another CS, requires that the animal not tune out the inhibitory cue. Consequently, just as appears to be the case with conditioned excitation, ablation-induced disruption of the tuning-out process might not affect the development of conditioned inhibition in the rabbit NMR preparation. Relevant to this hypothesis is the finding of Micco and Schwartz (1971) that hippocampectomy disrupts the ability of a Pavlovian conditioned fear inhibitor to suppress Sidman avoidance responding in rats. Since this result is inconsistent with a simple stimulus-gating interpretation of hippocampal function, it may be that hippocampectomy adversely affects both latent and conditioned forms of inhibition. Douglas' model does not address questions of stimulus generalization, but the possible relationship between the hippocampus and the nonspecific cortical "generalization" units of Gabriel et al. (1973) is reminiscent of the sort of interaction envisioned by Douglas in which complimentary action of the amygdala and hippocampus alerts the cortex as to the motivational significance of stimuli through modulation of the reticular core. The amygdala has responsibility for increasing cortical arousal to excitatory stimuli (i.e., stimuli with motivational associations), while the hippocampus is responsible for tuning out inhibitory, i.e., inconsequential stimuli. It seems reasonable to interpret the reduction of stimulus control along the auditory frequency dimension by hippocampectomized rabbits (Experiment 3) as resulting from a disruption of the capacity of these animals to encode the physical parameters of the CS to the exclusion of other stimulus values. REFERENCE NOTE 1. Kimble, D. P. The behavioral effects of hippocampal lesions in the rat. Paper presented at the

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meeting of the American Psychological Association, Washington, D.C., September 1967. REFERENCES Ackil, J. E., Mellgren, R. L., Halgren, C., & Frommer, G. P. Effects of OS preexposures on avoidance learning in rats with hippocampal lesions. Journal of Comparative and Physiological Psychology, 1969, 69, 739-747. Brown, P. L., & Jenkins, H. M. Conditioned inhibition and excitation in operant discrimination learning. Journal of Experimental Psychology, 1967, 76, 255-266. Cragg, B. G. Afferent connections of the allocortex. Journal of Anatomy, 1965, 135, 460-485. Douglas, R. J. The hippocampus and behavior. Psychological Bulletin, 1967, 67, 416-422. Douglas, R. J. Pavlovian conditioned inhibition and the brain. In R. A. Boakes & M. S. Halliday (Eds.), Inhibition and learning. New York: Academic Press, 1972. Douglas, R. J., & Isaacson, R. L. Hippocampal lesions and activity. Psychonomic Science, 1964, 1, 187-188. Freeman, F. G., & Kramarcy, N. R. Stimulus control of behavior and limbic lesions in rats. Physiology and Behavior, 1974, IS, 609-615. Freeman, F. G., Kramarcy, N. R., & Lee, J. Discrimination learning and stimulus generalization in rats with hippocampal lesions. Physiology and Behavior, 1973, 11, 273-275. Gabriel, M., Wheeler, W., & Thompson, R. F. Multiple-unit activity of the rabbit cerebral cortex during stimulus generalization of avoidance behavior. Physiological Psychology, 1973, ;, 313-320. Gormezano, I. Classical conditioning. In J. B. Sidowski (Ed.), Experimental methods and instrumentation in psychology. New York: McGraw-Hill, 1966. Gormezano, I., & Moore, J. W. Classical conditioning. In M. H. Marx (Ed.), Learning: Processes. London: Macmillan, 1969. Guillery, R. W. Degeneration in the postcommisural fornix and mamillary peduncle of the rat. Journal of Anatomy, 1956, 91, 91-115. Hearst, E. Some persistent problems in the analysis of inhibition. In R. A. Boakes & M. S. Halliday (Eds.), Inhibition and learning. New York: Academic Press, 1972. Hendrickson, C. W., Kimble, R. J., & Kimble, D. P. Hippocampal lesions and the orienting response. Journal of Comparative and Physiological Psychology, 1969, 67, 220-227. Hjorth-Simonsen, A. Hippocampal efferents to the ipsilateral entorhinal area: An experimental study in the rat. Journal of Comparative Neurology, 1971, 148, 219-232. Isaacson, R. L., Douglas, R. J., & Moore, R. Y. The effect of radical hippocampal ablation on acquisition of an avoidance response. Journal of Comparative and Physiological Psychology, 1961, 54, 625-628.

Kimble, D. P. The effects of bilateral hippocampal lesions in rats. Journal of Comparative and Physiological Psychology, 1963, 56, 273-283. Kimble, D. P. Hippocampus and internal inhibition. Psychological Bulletin, 1968, 70, 285-295. Kimble, D. P., & Kimble, R. The effects of hippocampal lesions on extinction and "hypothesis" behavior in rats. Physiology and Behavior, 1970, 5, 735-738. Lockhart, M., & Moore, J. W. Classical differential and operant conditioning in rabbits (Oryctolagus cuniculus) with septal lesions. Journal of Comparative and Physiological Psychology, 1975, 88, 147-154. Lubow, R. E. Latent Inhibition. Psychological Bulletin, 1973, 79, 398-407. Lubow, R. E., & Moore, A. N. Latent inhibition: The effect of non-reinforced preexposure to the conditioned stimulus. Journal of Comparative and Physiological Psychology, 1959, 52, 416-419. Micco, D. J., & Schwartz, M. Effects of hippocampal lesions upon the development of Pavlovian internal inhibition in rats. Journal of Comparative and Physiological Psychology, 1971, 76, 371-377. Mis, F. W., Norman, J. B., Hurley, J. W., Lohr, A. C., & Moore, J. W. Electrical brain stimulation as the reinforced CS in Pavlov's conditioned inhibition paradigm. Physiology and Behavior, 1974, 12, 689-692. Moore, J. W. Stimulus control: Studies on auditory generalization in rabbits. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning II: Current research and theory. New York: Appleton-Century-Crofts, 1972. Nauta, W. J. H. An experimental study of the fornix in the rat. Journal of Comparative Neurology, 1956, 104, 247-272. Nauta, W. J. H. Hippocampal projections and related neural pathways to the midbrain in the cat. Brain Research, 1958, 81, 319-340. Nauta, W. J. H., & Kypers, H. J. G. M. Some ascending pathways in the brain stem reticular formation. In H. H. Jasper, L. D. Proctor, R. S. Knighton, W. C. Noshay, & R. T. Costello (Eds.), Reticular formation of the Brain. Boston: Little, Brown, 1958. Niki, H. Response perseveration following hippocampal ablation in the rat. Japanese Psychological Research, 1966, 8, 1-9. Raphaelson, A. C., Isaacson, R. L., & Douglas, R. J. The effect of distracting stimuli on the runway performance of limbic damaged rats. Psychonomic Science, 1965, 3, 483-484. Reiss, S., & Wagner, A. R. CS habituation produces a "latent inhibition effect" but no active conditioned inhibition. Learning and Motivation, 1972, 3, 237-245. Rescorla, R. A. Pavlovian conditioned inhibition. Psychological Bulletin, 1969, 70, 77-94. Rescorla, R. A. Summation and retardation tests of latent inhibition. Journal of Comparative and Physiological Psychology, 1971, 75, 77-81.

HIPPOCAMPUS AND LATENT INHIBITION Rescorla, R. A., & Wagner, A. R. A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning II: Current research and theory. New York: Appleton-Century-Crofts, 1972. Schmaltz, L. W., & Theios, J. Acquisition and extinction of a classically conditioned response in hippocampectomized rabbits (Oryctolagus cuniculus). Journal of Comparative and Physiological Psychology, 1972, 79, 328-333. Schwartzbaum, J. S., Thompson, J. B., & Kellicut, M. H. Auditory frequency discrimination and generalization following lesions of the amygdaloid area in rats. Journal of Comparative and Physiological Psychology, 1964, 57, 257-266. Siegel, S. Latent inhibition and eyelid conditioning. In A. H. Black & W. F. Prokasy (Eds.),

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Classical conditioning II: Current research and theory. New York: Appleton-Century-Crofts, 1972. Solomon, P. R., Brennan, G., & Moore, J. W. Latent inhibition of the rabbit's nictitating membrane response as a function of CS intensity. Bulletin of the Psychonomic Society, 1974, 4, 445-449. Solomon, P. R., Lohr, A. C. & Moore, J. W. Latent inhibition of the rabbit's nictitating membrane response: Summation tests for active inhibition as a function of number of CS preexposures. Bulletin of the Psychonomic Society, 1974, 6, 557-559. Whitlock, D. G., & Nauta, W. J. H. Subcortical projections from the temporal neocortex in Macaco, mulatta. Journal of Comparative Neuy, 1956, 108, 183-212. (Received January 21, 1975)