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only individuals descended from field-collected ␤-males, and then affect only two Ams genotypes: Tfr1Tfr2 females bearing ␤-alleles were assumed to ␣ ␣ mature as ␤-males, and Tfr1Tfr2, Ams Ams males were assumed to mature as Different types of fear- females. This latter effect assumed that the Tfr2 allele interacts with ECF, which ␤ initially occurred only in parental -males, but which was transmitted to F1-2 conditioned behaviour individuals of both sexes and a range of Ams genotypes (Table 3b). Testing the model. In Tables 3a,b, exact probabilities were Bonferroni- mediated by separate adjusted (0.05/k, where k is the number of tests) when multiple crosses with identical Ams and Tfr genotypes, as well as ECF states were tested; similar nuclei within amygdala crosses with nonsignificant exact probabilities were pooled and the exact x2 probability for the pooled frequencies reported; primary sex-determination Simon Killcross*, Trevor W. Robbins & Barry J. Everitt genotypes were unambiguously determined from Pgm genotype frequencies Department of Experimental Psychology, University of Cambridge, within families; the apparent Tfr genotypes among 36 parents (24 crosses) were Downing Street, Cambridge CB2 3EB, UK 15 Tfr1Tfr1,12Tfr1Tfr2 and 9 Tfr2Tfr2; expected genotypes calculated from ...... inferred allele frequencies conform to Hardy–Weinberg expectations, exact x2 The amygdala has long been thought to be involved in emotional probability 0.21. behaviour1,2, and its role in anxiety and conditioned fear has been highlighted3,4. Individual amygdaloid nuclei have been shown to Received 21 January; accepted 24 April 1997. project to various cortical and subcortical regions implicated in 1. Lank, D. B., Smith, C. M., Hanotte, O., Burke, T. & Cooke, F. Genetic polymorphism for alternative 5–7 mating behaviour in lekking male ruff. Nature 378, 59–62 (1995). affective processing . Here we show that some of these nuclei 2. Gross, M. R. Alternative reproductive strategies and tactics; diversity within sexes. Trends Ecol. Evol. have separate roles in distinct mechanisms underlying condi- 11, 92–97 (1996). tioned fear responses. Rats with lesions of the central nucleus 3. Sinervo, B. & Lively, C. M. The rock–paper–scissors game and the evolution of alternative male strategies. Nature 380, 240–243 (1996). exhibited reduction in the suppression of behaviour elicited by a 4. Shuster, S. M. Alternative reproductive behaviors: Three discrete male morphs in Paracerceis sculpta, conditioned fear stimulus, but were simultaneously able to direct an intertidal isopod from the northern Gulf of California. J. Crust. Biol. 7, 318–327 (1987). 5. Shuster, S. M. The reproductive behaviour of ␣-, ␤-, and ␥-males in Paracerceis sculpta, a marine their actions to avoid further presentations of this aversive isopod crustacean. Behaviour 121, 231–258 (1992). stimulus. In contrast, animals with lesions of the basolateral 6. Shuster, S. M. Male alternative reproductive behaviors in a marine isopod crustacean (Paracerceis amygdala were unable to avoid the conditioned aversive stimulus sculpta): The use of genetic markers to measure differences in fertilization success among ␣-, ␤- and ␥- males. Evolution 34, 1683–1698 (1989). by their choice behaviour, but exhibited normal conditioned 7. Shuster, S. M. & Wade, M. J. Equal mating success among male reproductive strategies in a marine suppression to this stimulus. This double dissociation demon- isopod. Nature 350, 606–61 (1991). 8. Sassaman, C. Inbreeding and sex ratio variation in female-biased populations of a clam shrimp, strates that distinct neural systems involving separate amygdaloid Eulimnadia texana. Bull. Mar. Sci. 45, 425–432 (1989). nuclei mediate different types of conditioned fear behaviour. We 9. Hartl, R. & Clark, A. in Principles of Population Genetics 2nd edn, 47–57 (Sinauer, Sunderland, MA, suggest that theories of amygdala function should take into 1989). 10. Heath, D. J. & Ratford, J. R. The inheritance of sex ratio in the isopod, Sphaeroma rugicauda. Heredity account the roles of discrete amygdala subsystems in controlling 64, 419–425 (1990). different components of integrated emotional responses. 11. Bull, J. J. Evolution of Sex Determining Mechanisms (Benjamin-Cummings, Menlo Park, CA, 1983). 12. Legrand, J. J., Legrand-Hamelin, E. & Juchault, P. Sex determination in Crustacea. Biol. Rev. 62, 439– Investigations of the neural basis of pavlovian fear conditioning 470 (1987). and its role in anxiety have suggested that the lateral nucleus of the 13. Juchault, P., Rigaud, T. & Mocquard, J.-P. Evolution of sex-determining mechanisms in a wild amygdala is the site of convergence of neural pathways that carry population of Armadillidium vulgare Latr. (Crustacea: Isopoda): competition between two feminizing parasitic sex factors. Heredity 69, 382–390 (1992). information about conditioned stimuli (CSs) and aversive rein- 14. Rousset, F., Bouchon, D., Pintureau, B., Juchault, P. & Solignac, M. Wolbachia endosymbionts forcers (USs)4,6. The emotional expression of this learned associa- responsible for various alterations of sexuality in Arthropods. Proc. R. Soc. Lond. B 250, 91–98 (1991). 15. Hurst, L. D. The incidences, mechanisms and evolution of cytoplasmic sex ratio distorters in animals. tion may then be mediated by neural connections from the lateral to 4 Biol. Rev. 68, 121–193 (1993). the central nucleus of the amygdala , which, through its projections 16. Rigaud, T. & Juchault, P. Conflict between feminizing sex ratio distorters and an autosomal to hypothalamic and brainstem areas, is thereby able to coordinate masculinizing gene in the terrestrial isopod, Armadillidium vulgare Latr. Genetics 133, 247–252 (1993). the behavioural, endocrine and autonomic responses that form an 17. Juchault, P. & Rigaud, T. Evidence for female heterogamety in two terrestrial crustaceans and the integrated emotional response8. problem of sex chromosome evolution in isopods. Heredity 75, 488–471 (1995). 18. Read, T. R. C. & Cressie, N. A. C. in Goodness-of-fit Statistics for Discrete Multivariate Data 136–139 Serial processing in these regions of the amygdala is known to be (Springer, New York, 1988). involved in the acquisition and expression of conditioned fear 19. Shuster, S. M. & Wade, M. J. Female copying and sexual selection in a marine isopod crustacean. Anim. responses to aversive CSs, such as freezing and fear-potentiated Behav. 42, 1071–1078 (1991). 3–6 20. Shuster, S. M. in Crustacean Sexual Biology (eds Bauer, R. T. & Martin, J. W.) 91–110 (Columbia Univ. startle in animals . However, the role of these regions in alternative Press, New York, 1991). indices of fear conditioning, including the instrumental choice 21. Levins, R. Evolution in Changing Environments (Princeton Univ. Press, Princeton, 1968). 22. Slatkin, M. On the equilibration of fitnesses by natural selection. Am. Nat. 112, 845–859 (1978). responses involved in avoidance or conflict behaviour, is much 9 23. Maynard Smith, J. Evolution and the Theory of Games (Cambridge Univ. Press, New York, 1982). less clear . We therefore wished to investigate whether all forms of 24. Lively, C. M. Canalization versus developmental conversion in a spatially variable environment. Am. fear conditioning are mediated by this serial information flow Nat. 128, 561–572 (1986). 25. Slatkin, M. The evolutionary response to frequency and density dependent interactions. Am. Nat. 114, between the lateral and basal nuclei to the central nucleus of the 384–398 (1979). amygdala. To achieve this we designed a fear-conditioning proce- 26. Wright, S. Evolution and Genetics of Populations Vol. 2 (Univ. of Chicago Press 1969). 27. Wade, M. J., Shuster, S. M. & Stevens, L. Bottlenecks, founder events and inbreeding: Experimental dure in rats in which the development of an aversive CS–US studies of the response to selection with Tribolium. Evolution 50, 723–733 (1996). association could be assessed simultaneously by examination of 28. Shuster, S. M. Changes in female anatomy associated with the reproductive molt in Paracerceis sculpta two dissociable fear responses in the same animal. The aversive CS– (Holmes), a semelparous isopod crustacean. J. Zool. 225, 365–379 (1991). 29. Shuster, S. M. Courtship and female mate selection in a semelparous isopod crustacean (Paracerceis US association created by this procedure would not only produce a sculpta). Anim. Behav. 40, 390–399 (1990). pavlovian conditioned fear response, but would also provide the 30. Tinturier-Hamelin, E. Sur le polychromatisme de l’isopode Flabellifere Dynamene bidentata (Adams) II. Etude genetique du mutant bimaculata partiellement. Cah. Biol. Mar. 4, 473–591 (1967). necessary information for animals to solve an operant discrimina- tion that would lead to the avoidance of future presentations of the Acknowledgements. This research was supported by the NSF and by organized research and depart- aversive stimulus. mental funding from Northern Arizona University, and was authorized by the Mexican Government. We thank M. Wade and B. Charlesworth for reviewing data and earlier drafts of the manuscript; Y. Toquenaga Rats were trained on a concurrent conditioned-suppression and for statistical advice and for a program for calculating exact x2 tests; D. Dorado, S. Juarez, S. Hag, H. conditioned-punishment task (see Methods). Pressing one lever in Baitoo, S. Bhakta, Y. Bhakta, S. Brekhus; H. Yoon; N. Kim, M. Kim, U. Rao and L. Lynch for assistance in maintaining laboratory animals; and V. Jormalainen, P. Nelson, K. Johnson, G. Davis and M. Pitts for an operant chamber produced an aversive conditioned stimulus discussion. (CS+), pressing the other lever produced control presentations of a

Correspondence and requests for materials should be addressed to S.M.S. (e-mail stephen.shuster @nau.edu). * Present address: Department of Psychology, University of York, Heslington, York YO1 5DD, UK.

NATURE | VOL 388 | 24 JULY 1997 Nature © Macmillan Publishers Ltd 1997 377 letters to nature neutral conditioned stimulus (CS−). In sham-operated control session of training a transitory deficit in suppression that was not animals, the aversive CS+ caused a disruption of ongoing lever apparent in any subsequent sessions, where the level of suppression pressing during each of its presentations, relative to performance did not differ from that of control animals. In contrast, animals with during the control CS− (Fig. 1, left). This conditioned-suppression lesions of the central nucleus of the amygdala or combined lesions effect is a well-established measure of the formation of a pavlovian showed a persistent deficit across all sessions. Hence, although the aversive CS–US association. Behavioural evidence indicates that basolateral amygdala may contribute to the formation of CS–US this is a pavlovian conditioned response deriving from species– associations, the convergence of sensory information mediating species defence responses10. Sham-operated control rats simulta- conditioned fear can clearly also occur independently of this region, neously came to bias their lever-press responses away from the lever for example within the central nucleus of the amygdala or its afferent producing the CS+ and towards the lever producing the neutral CS− circuitry, including direct projections from midline thalamic nuclei (Fig. 1, right). This conditioned-punishment effect also demon- and the sensory posterior thalamus16. strates the presence of an intact aversive CS–US association, as it is Second, our results are not compatible with theories suggesting only on the basis of this association that rats may choose between that all manifestations of aversively motivated conditioned fear the levers and modulate their actions to avoid future aversive behaviour, including conflict and avoidance responses, are events11. The concurrent assessment of conditioned suppression mediated by the various descending brainstem and hypothalamic and conditioned punishment provided simultaneous, alternative projections of the central nucleus3,5,6. This important conclusion measures of the establishment of the aversive CS–US association, follows because rats with complete lesions of the amygdaloid central the former dependent on a conditioned cessation of ongoing nucleus, although impaired in pavlovian conditioned suppression, behaviour, and the latter on a choice between two available actions. Lesions of the lateral and basal amygdala (Figs 2 and 3) did not affect the conditioned suppression of responses elicited by the CS+ (Fig. 1, left), but significantly impaired the ability of the animals to bias their choice of action away from the lever that produced the aversive, punishing stimulus (Fig. 1, right). In contrast, lesions of the central nucleus of the amygdala (Figs 2 and 3) produced severe impairments in the rats’ suppression of baseline responses during presentations of the CS+ (Fig. 1, left), but their ability to direct their responses away from the lever that led to these presentations was unaffected (Fig. 1, right). Animals with lesions to both structures, as expected, showed a blockade of both conditioned-punishment and conditioned-suppression effects (Fig. 1). This double dissociation of the effects of lesions of different amygdaloid nuclei on the behavioural expressions of fear condi- tioning has important implications for our understanding of the role of the amygdala in the formation of aversive CS–US associa- Figure 1 Left, Difference between suppression ratios during presentation of the tions. This is particularly important because the pattern of results aversive CS+ and neutral CS− in sham-operated control rats, and animals with cannot be explained by gross changes in primary motivation, such lesions of the lateral/basolateral nuclei of the amygdala (blA), the nucleus (cnA), 12 as sensitivity to footshock , or by deficits in stimulus discrimina- or with combined lesions of both (cnA þ blAÞ) averaged over 10 sessions. In no tion (as both conditioned suppression and conditioned punishment group did animals show significant suppression of lever pressing during CS− are assessed using measures that contrast behaviour associated with relative to periods without stimulus presentation. A difference score of 0.5 the CS+ with that associated with the CS−). According to the represents complete suppression during CS+ with no suppression during CS− hypothesis that the lateral nucleus of the amygdala is the site of (high fear conditioning); 0.0 represents no suppression during either CS+ or CS− formation of the CS–US association, which then achieves beha- (low fear conditioning). Rats with cnA or cnA þ blA lesions showed significantly vioural output through projections to the central nucleus of the reduced levels of fear conditioning relative to sham-operated controls and rats 4,13 amygdala , lesions of either the central nucleus or the basolateral with lesions of just the blA. Analysis of variance with factor ‘lesion’ (sham, blA, : Ͻ : amygdala should have produced complete and profound deficits in cnA and blA þ cnA) indicated a significant effect of lesion (F3;36 ¼ 8 24, P 0 001). both conditioned suppression and conditioned punishment. Thus Pairwise comparisons showed that performance of cnA rats did not differ from lesions of the basolateral amygdala should have prevented any that of rats with combined lesions, but both groups differed (asterisk, P Ͻ 0:05) formation of CS–US associations, whereas lesions of the central from blA rats and from sham-operated controls. Rats with lesions of the blA did nucleus of the amygdala should have prevented such associations not differ from sham-operated controls. Right, difference in rate of lever pressing from being expressed in the behaviour. The double dissociation on the lever producing the neutral CS− and on the level producing the aversive shown here demonstrates that both the basolateral amygdala and CS+. Lever pressing was measured when the aversive stimulus was not being the central nucleus are not only independently capable of support- presented, and is expressed relative to performance during baseline sessions in ing the formation of CS–US associations, but also independently which no stimuli were presented. In no group did performance on the CS− lever mediate the different forms of behavioural expression of defensive differ significantly from baseline rates. A score of 1.0 represents a complete bias responses that are dependent on these associations. away from the CS+ lever while maintaining responses on the CS− lever at rates Two strong conclusions follow from this dissociation. First, the equivalent to baseline (high fear conditioning); 0.0 represents no difference lateral nucleus cannot be the only site at which pavlovian CS–US between performance on the CS+ and CS− levers, with neither rate differing associations are stored, as rats with extensive lesions of the baso- from baseline performance (low fear conditioning). Sham-operated controls and lateral amygdala, which included the lateral nucleus, were shown to rats with cnA lesions showed a significant bias in responding away from the lever have intact CS–US associations as assessed by conditioned suppres- producing the aversive CS+; animals with blA lesions or combined lesions sion. Although it has been shown that excitotoxic lesions of the showed no such bias. Analysis of variance with factor ‘lesion’ indicated a : Ͻ : basolateral amygdala produce deficits in the acquisition of con- significant effect (F3;36 ¼ 3 42, P 0 05). Pairwise comparisons revealed that 14 ditioned fear responses, as assessed by freezing , fear-potentiated sham controls and rats with cnA lesions did not differ in lever discrimination 13 12 startle and lick suppression , other findings have suggested that (and hence degree of conditioned fear), but both showed significantly greater 15 this deficit is abolished following more extended training . Our rats levels of conditioned fear (asterisk, P Ͻ 0:05) than animals with blA lesions or with lesions in the basolateral amygdala also exhibited in the first combined lesions, which themselves did not differ.

Nature © Macmillan Publishers Ltd 1997 378 NATURE | VOL 388 | 24 JULY 1997 letters to nature showed normal formation of an aversive CS–US association as cerebellum and midbrain, including the interpositus and red assessed by instrumental conditioned punishment behaviour. Thus nuclei17). Serial information transfer from the basolateral to the the neural basis of any aversive CS–US association and its beha- central nuclei of the amygdala seems to be critical only for certain vioural expression is distributed more widely in the brain. This classes of conditioned fear responses, assessed using specific finding is in line with previous data showing that, although behavioural output systems (indeed, the basolateral amygdala has important in freezing and startle behaviour, the central nucleus of been shown to be important in the unconditioned, as well as the the amygdala is unlikely to be important in active avoidance, conditioned, modulation of startle responding18), and does not conflict or punishment behaviours, or in aversive eyelid condition- seem to provide a general mechanism underlying fear conditioning. ing in the rabbit (which seems to depend on the deep nuclei of the Similarly, just as different forms of conditioned fear responses seem

Figure 2 Representation of largest (pale shading) and smallest (dark shading) lesions of: a, lateral/baso- lateral nuclei; b, central nucleus; and c, both basolateral and central nuclei of the amygdala. Outlines are repro- duced from ref. 30 and represent sections ranging from 1.33 to 3.9 mm posterior to Bregma.

Figure 3 Photomicrographs of sections from three representative brains showing an intact brain (a, b) and excitotoxic lesions of the basolateral (c, d) and central (e, f) amygdala. a, b, A section (right side) through the amygdala of a sham-operated control at low (a) and higher (b) magnification. The appearance of the lateral (la) and basal magnocellular (bm) nuclei of the amygdala, which are together referred to as the basolateral amygdala, are visible, along with the central nucleus (c). The lateral and medial boundaries of the amygdala are formed by the external capsule (ec) and optic tract (ot), respectively. c, d, A section (left side) through the brain of a subject with a lesion of the basolateral amygdala at the same magnifications as a, and b. The distinctive magnocellular neurons of the basal magnocellular nucleus have been destroyed, so only glial nuclei are visible; the central nucleus has been spared (compare c with a and d with b). e, f,A section (left side) through the brain of a subject with a lesion of the central nucleus, at the same magnifica- tions as a and b. The distinctive neurons of the central nucleus can no longer be seen; only glial nuclei remain stained in this area). The basolateral parts of the amygdala have been spared (compare e with a and f with b).

NATURE | VOL 388 | 24 JULY 1997 Nature © Macmillan Publishers Ltd 1997 379 letters to nature to depend on dissociable systems in the amygdala, assessments of low response rate, the number of stimulus presentations on such schedules is pavlovian appetitive responding have failed to demonstrate a effectively independent of response rate, so all animals received equivalent unitary role for amygdala nuclei in the development of simple numbers of CS–US pairings and control CS presentations, regardless of their appetitive CS–US associations. Rather, they implicate the basolat- actual level of responding. In each of 10 30-min sessions the rats received an eral amygdala in the control of behaviour by secondary reinforcers9, average of about 12 presentations of the CS+ and CS−. Rates of pressing on the and they implicate the central nucleus of the amygdala in behaviour two levers were recorded during baseline sessions in which no stimuli were dependent on the predictive status of the CS itself, regardless of its presented, and in test sessions during the CS+ and CS− and in the absence of association with primary reinforcement8. stimulus presentation, during the inter-trial interval. Our results support the hypothesis3–7 that projections from the Surgical procedures. The rats were anaesthetized using Avertin and received central nucleus of the amygdala are involved in conditioned either bilateral excitotoxic lesions of the central nucleus of the amygdala with responses elicited by aversive CSs. However, they also suggest that 0.063 M ibotenic acid (AP −2.2, −2.7, L Ϯ4.0, V −7.8 (from Bregma), 0.2 ␮l per different projections from the basolateral amygdala are important infusion site), or bilateral excitotoxic lesions of the basolateral amygdala with in the production and direction of instrumental actions in response 0.09 M quinolinic acid (AP −2.3, −3.0, L Ϯ4.6, V −7.3 (from Bregma), 0.3 ␮l per to fearful stimuli9. The demonstration that these dissociable systems infusion site). Sham-lesioned animals received identical surgical treatment can support different forms of fear-related behaviour, each depen- except for the infusion of excitotoxin. The choice of neurotoxins was based on dent on the formation of CS–US associations, suggests that aversive extensive pilot experiments achieving discrete lesions of the two areas. Sub- responding in general may comprise aspects of behaviour derived sequent histological analysis revealed selective lesions of the central nucleus from both systems. It is well established that projections from the (n ¼ 6), the lateral and basal magnocellular nuclei (n ¼ 9), along with lesions central nucleus are important in reflexive emotional responding3–6. of both structures (n ¼ 9) and sham controls (n ¼ 18). However, the role of direct projections from the basolateral amygdala Received 7 March; accepted 2 May 1997 19 to areas such as the ventral striatum and medial prefrontal cortex is 1. Weiskrantz, L. Behavioural changes associated with ablations of the amygdaloid complex in monkeys. not fully understood. These regions are important elements in the J. Comp. Physiol. Psychol. 49, 381–391 (1956). limbic cortico-ventral striatopallidal circuitry that provides an 2. Kluver, H. & Bucy, P. C. Preliminary analysis of the temporal lobes in monkeys. Arch. Neurol. 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Selden, N. R. W., Everitt, B. J., Jarrard, L. E. & Robbins, T. W. Complementary roles for the amygdala actions may underlie the effects of aversive conditioned punishers and hippocampus in aversive conditioning to explicit and contextual cues. Neuroscience 42, 335–350 (1991). on aversive instrumental discrimination. Indeed, one way in which 13. Sananes, C. B. & Davis, M. N-Methyl-D-Aspartate lesions of the lateral and basolateral nuclei of the the present results may be interpreted is to regard the basolateral amygdala block fear-potentiated startle and shock-sensitization of startle. Behav. Neurosci. 106, 72–80 amygdala as part of a system responsible for voluntary or inten- (1992). 14. Maren, S., Aharonov, G. & Fanselow, M. S. Retrograde abolition of conditional fear after excitotoxic tional instrumental choice behaviour based on emotional events, lesions in the basolateral amygdala of rats: Absence of a temporal gradient. Behav. Neurosci. 110, 718– whereas the central nucleus of the amygdala is involved more closely 726 (1996). 15. Parent, M. B., Tomaz, C. & McGaugh, J. L. Increased training in an aversively motivated task in the reflexive, automatic pavlovian conditioned responses evoked attenuates the memory-impairing effects of posttraining N-Methyl-D-Aspartate-induced amygdala by motivationally salient stimuli. lesions. Behav. Neurosci. 106, 789–797 (1992). These results also have more direct implications for clinical 16. Amaral, D. G., Price, J. L., Pitkanen, A. & Carmichael, S. T. in The Amygdala: Neurobiological Aspects of Emotion, Memory and Mental Dysfunction (ed. Aggleton, J. P.) 1–66 (Wiley-Liss, New York, 1992). research into the function of the amygdala in fear, anxiety and 17. Thompson, R. F. & Krupa, D. J. Organization of memory traces in the mammalian brain. Annu. Rev. emotional behaviour in general. The amygdala in humans has been Neurosci. 17, 519–549 (1994). 23 18. Wan, F. J. & Swerdlow, N. 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Amygdala activity at encoding correlated with long-term, free-recall of emotional they are part. Ⅺ information. Proc. Natl Acad. Sci. USA 93, 8016–8021 (1996)...... 26. Adolphs, R., Tranel, D., Damasio, H. & Damasio, A. Impaired recognition of emotion in facial expressions following bilateral damage to the human amygdala. Nature 372, 669–672 (1994). Methods 27. Calder, A. J. et al. Facial emotion recognition after bilateral amygdala damage: Differentially severe Behavioural procedures. We trained 42 rats in operant chambers to maintain impairment of fear. Cogn Neuropsychol. 13, 699–645 (1996). 28. Scott, S. K. et al. Impaired auditory recognition of fear and anger following bilateral amygdala lesions. pressing on two levers for food on independent variable interval 60-s schedules. Nature 385, 254–257 (1997). After lesions were made the rats were trained on a concurrent conditioned 29. Mowrer, O. H. On the dual nature of learning—A re-interpretation of ‘‘conditioning’’ and ‘‘problem- punishment and suppression task. Superimposed on the schedules of food solving’’. Harvard Educat. Rev. 17, 102–148 (1947). 30. Swanson, L. W. Brain Maps: Structure of the rat brain (Elsevier, Amsterdam, 1992). reinforcement were two further independent variable interval 120-s schedules Acknowledgements. We thank C. Morrison and H. Sweet for help with histology. This work was of a response-contingent 10-s auditory CS+ (3-kHz tone or 10-Hz clicks at supported by a grant from the Human Frontiers Science Program, and a research fellowship to S.K. from 80 dB, counterbalanced) terminated by mild footshock (0.2 mA for 0.5 s) on Magdalene College, Cambridge. one lever, and a matched neutral CS− (clicks or tone) on the other. Even with a Correspondence and requests for materials should be addressed to S.K. (e-mail: [email protected]).

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