Damage to the lateral and central, but not other, amygdaloid nuclei prevents the acquisition of auditory fear conditioning

It is well established that the amygdala plays an essential role in Pavlovian fear conditioning, with the lateral nucleus serving as the interface with sensory systems that transmit the conditioned stimulus and the central nucleus as the link with motor regions that control conditioned fear responses. The lateral nucleus connects with the central nucleus directly and by way of several other amygdala regions, including the basal, accessory basal, and medial nuclei. To determine which of these regions is necessary, and thus whether conditioning requires the direct or one of the indirect intra-amygdala pathways, we made lesions in rats of the lateral, central, basal, accessory basal, and medial nuclei, as well as combined lesions of the basal and accessory basal nuclei and of the entire amygdala. Animals subsequently underwent fear conditioning trials in which an auditory conditioned stimulus was paired with a footshock unconditioned stimulus. Animals that received lesions of the lateral or central nucleus, or of the entire amygdala, were dramatically impaired, whereas the other lesions had little effect. These findings show that only the lateral and central nuclei are necessary for the acquisition of conditioned fear response to an auditory conditioned stimulus.     

Conditioned memory modulation, freezing, and avoidance as measures of amygdala-mediated conditioned fear

Three conditioned aversive responses were used to infer the existence of an unobservable central state of "conditioned fear," and the roles of certain amygdala subregions in producing these responses were investigated. Rats received tone-shock pairings in one compartment of a shuttle box and no tones or shocks in the other, distinctive, compartment. They were then trained to find food in one arm of a Y-maze. After the final training trial they were exposed to different sets of stimuli in the shuttle box with no shock. Twenty-four hours later rats that had received immediate posttraining exposure to the conditioned stimuli (in the shock-paired compartment) made significantly more correct responses on the Y-maze than rats that had been exposed to the neutral stimuli (in the no-shock compartment) or rats that had received delayed posttraining exposure to the conditioned stimuli. This constitutes a demonstration of posttraining memory modulation by conditioned aversive stimuli. Freezing increased during posttraining exposure to the conditioned stimuli compared to the neutral stimuli. When subsequently allowed to move freely between the two compartments, the rats in all groups also showed significant conditioned avoidance of the compartment containing the conditioned stimuli. In a second experiment the effects of lesions confined to specific parts of the amygdala on the three conditioned responses (memory modulation, freezing, avoidance) were tested. Lesions of the central nucleus impaired all three conditioned responses; lesions of the medial nucleus impaired conditioned modulation and avoidance. These lesions had no effect on freezing during the training trials. Lesions of the lateral and basolateral nuclei attenuated freezing during both training and testing. The findings suggest that the central and medial nuclei of the amygdala may be important parts of neural circuits mediating conditioned responses that constitute conditioned aversive states, but that conditioned freezing may be mediated independently.     

Occasional reinforced responses during extinction can slow the rate of reacquisition of an operant response

Three experiments with rats examined reacquisition of an operant response after either extinction or a response-elimination procedure that included occasional reinforced responses during extinction. In each experiment, reacquisition was slower when response elimination had included occasional reinforced responses, although the effect was especially evident when responding was examined immediately following each response-reinforcer pairing during reacquisition (Experiments 2 and 3). An extinction procedure with added noncontingent reinforcers also slowed reacquisition (Experiment 3). The results are consistent with research in classical conditioning (Bouton, M. E., Woods, A. M., & Pineño, O. (2004). Occasional reinforced trials during extinction can slow the rate of rapid reacquisition. Learning & Motivation, 35, 371-390) and suggest that rapid reacquisition after extinction is analogous to a renewal effect that occurs when reinforced responses signal a return to the conditioning context. Clinical implications are also discussed.     

Sequential reacquisition as a function of timeout from avoidance

Rats learned to reacquire four similar three-member response sequences. Each sequence member was associated with a different response lever, and the correct sequence of levers (i.e., 3-1-2, 2-1-3, 1-3-2, and 2-3-1) changed each session. The first two correct responses of each sequence postponed shock for a fixed period of time. The third correct response initiated a signalled timeout from avoidance. Incorrect responses did not affect the shock interval or reset the sequence. The effects of manipulating timeout duration on the sequential reacquisition baseline were investigated. All subjects displayed biphasic reacquisition performances similar to those controlled by food. The phases were characterized by an initial increase in accuracy, which reached a stable level during the latter portion of each session. Timeout duration affected rate of sequence completion and shock density, but not percentage of errors. Rate of sequence completion was fastest with intermediate timeouts (15 to 60 sec), and slowest with extreme durations (1 or 120 sec). Shock densities peaked with extreme durations and were at minimum with intermediate timeout values. The percentage of errors was the same across timeout durations. These data extend the generality of sequential reacquisition as a procedure for studying learning, and demonstrate timeout from avoidance to be a controlling variable.     

Neuronal unit activity in the abducens nucleus during classical conditioning of the nictitating membrane response in the rabbit (Oryctolagus cuniculus).

Neuronal unit activity was recorded from the abducens (6th nerve) nucleus, the "final common path," during classical conditioning of the nictitating membrane (NM) response in the rabbit, with the use of a tone conditioned stimulus, an air puff unconditioned stimulus (UCS), 250-msec interstimulus interval, and 60-sec intertrial interval. Animals were given 2 days of conditioning training (104 trials in eight blocks per day) and 1 day of extinction. Control animals were given comparable periods of stimulus presentations, explicitly unpaired. Activity of small clusters of units--"multiple unit" recording--was compared with the amplitude-time course of the NM response. Between-blocks comparisons of neural and behavioral responses indicated an essentially perfect correlation during acquisition of the conditioned response (Day 1, r = .99; Day 2, r = .98) and a slightly lower correlation during extinction (r = .93) for the conditioning animals. Within-blocks comparisons indicated a close correspondence between the histograms of unit activity and the amplitude-time course of the NM response for the conditioning animals in all phase of training and for the control animals in the UCS trial blocks.     

Occasional reinforced trials during extinction can slow the rate of rapid reacquisition

Two appetitive conditioning experiments with rats examined reacquisition after conditioned responding was eliminated by either extinction or by a partial reinforcement procedure in which reinforced trials were occasionally presented among many nonreinforced trials. In Experiment 1, reacquisition to a conditional stimulus (CS) that had been conditioned and extinguished was more rapid than acquisition in a group that had received no prior conditioning. However, the addition of occasional reinforced trials to extinction slowed this rapid reacquisition effect. Experiment 2 replicated the result and showed that a procedure in which the CS and the unconditional stimulus (US) were unpaired in extinction interfered even further with reacquisition. The results suggest that rapid reacquisition is ordinarily produced when reinforced trials provide a contextual cue that can renew responding by signaling other acquisition trials (Ricker & Bouton, 1996). The effects of partial reinforcement in extinction are surprising from several theoretical perspectives and have useful clinical implications.     

Comparing the effects of two correction procedures on human acquisition of sequential behavior patterns

Thirty-one college undergraduates learned to touch abstract stimuli on a computer screen in arbitrarily designated “correct” sequential orders. Four sets of seven stimuli were used; the stimuli were arrayed horizontally on the screen in random sequences. A correct response (i.e., touching first the stimulus designated as first) resulted in that stimulus appearing near the top of the screen in its correct sequential position (left to right), and remaining there until the end of the trial. Incorrect responses (i.e., touching a stimulus out of sequence) terminated the trial. New trials displayed either the same sequence as the one on which an error had occurred (same-order correction procedure), or a new random sequence (new-order correction procedure). Whenever all responses occurred in the correct sequence, the next trial displayed a new random sequence. Each phase ended when five consecutive correct response sequences occurred. Initially, the same-order correction procedure increased control by the position as well as by the shape of the stimuli; also, it produced more errors, more total trials, more trials to mastery, and more individual patterns of reacquisition than were produced by the new-order procedure.     

Fibroblast growth factor-2 alters the nature of extinction

These experiments examined the effects of the NMDA-receptor (NMDAr) antagonist MK801 on reacquisition and re-extinction of a conditioned fear that had been previously extinguished before injection of fibroblast growth factor-2 (FGF2) or vehicle. Recent findings have shown that relearning and re-extinction, unlike initial learning and extinction, do not depend on NMDAr activation. Three experiments demonstrated that FGF2 prevents the switch from NMDAr-dependent to NMDAr-independent reacquisition and re-extinction, suggesting that FGF2 may lead to the partial erasure of the original fear memory. These findings add to a growing body of work suggesting that FGF2 may be a novel pharmacological enhancer of exposure therapy for humans with anxiety disorders.     

Encoding and retrieval are differentially processed by the anterior cingulate and prelimbic cortices: A study based on trace eyeblink conditioning in the rabbit

A growing body of literature suggests that structures along the midline of the prefrontal cortex (mPFC), including Brodmann’s area 32 (prelimbic cortex) and area 24 (anterior cingulate cortex) in the rabbit play a role in retrieval of learned information. The present studies compared the effects of post-training lesions produced either immediately or 1-week following learning, to either prelimbic (area 32) or anterior cingulate (area 24) cortex on trace eyeblink (EB) conditioning. Further, because recent evidence suggests that the mPFC may play an even greater role in learning and memory when emotional arousal is low, these studies compared the effects of lesions in groups conditioned with either a relatively low-arousal corneal airpuff, or a more aversive periorbital eyeshock unconditioned stimulus (US). A total of six groups were tested, which received selective ibotenic acid or “sham” control lesions to either area 32 or 24, immediately or 1-week following asymptotic learning, and conditioned with an eyeshock US or an airpuff US. Results showed that the greatest lesion deficits were found when conditioning with the less aversive airpuff US. Further, lesions produced to area 32 one-week, but not immediately following learning, caused significant deficits in performance, while lesions produced to area 24 immediately, but not 1-week following learning, caused significant deficits in performance. These findings add to the body of evidence which shows that area 32 of the mPFC regulates retrieval, but not acquisition or storage of information, while area 24 mediates a less specific reacquisition process, but not permanent storage or retrieval of information during relearning of memories abolished by mPFC damage. These findings were, however, specific to those experiments in which the relatively non-aversive airpuff was the US.     

Role of the hippocampus in blocking and conditioned inhibition of the rabbit's nictitating membrane response.

Bilateral aspiration of the dorsal hippocampus produced a disrupttion of blocking of the rabbit's nictitating membrane response in Kamin's two-stage paradigm (Experiment 1) but had no effect on the formation of a Pavlovian conditioned inhibitor (Experiment 2). The results of Experiment 1 indicated that normal animals and those with cortical lesions given conditioning to a light-plus-tone conditioned stimulus (CS) gave conditioned responses (CRs) to both the light and the tone during nonreinforced presentations of each (test phase). If, however, compound conditioning was preceded by tone acquisition, only the tone elicited a CR during testing; that is, blocking was observed. In rabbits with hippocampal lesions, however, CRs were given to both the light and the tone during testing whether or not compound conditioning was preceded by tone acquisition. The data from Experiment 2 showed that rabbits with hippocampal lesions could discriminate as well as normal rabbits and those with cortical lesions between a light (CS+) and light plus tone (CS-). In addition, when the inhibitory tone was subsequently paired with the unconditioned stimulus in retardation testing, animals in all three lesion conditions acquired the CR at the same rate. Thus, it appears that hippocampal lesions do not disrupt conditioned inhibition. The results of these experiments were taken as support for the view that the hippocampus is responsible for "tuning out" stimuli that have no adaptive value to the organism.     

Autoshaping the pigeon's gape response: acquisition and topography as a function of reinforcer type and magnitude.

The pigeon's key-pecking response is experimentally dissociable into transport (head movement) and gape (jaw movement) components. During conditioning of the key-pecking response, both components come under the control of the conditioned stimulus. To study the acquisition of gape conditioned responses and to clarify the contribution of unconditioned stimulus (reinforcer) variables to the form of the response, gape and key-contact responses were recorded during an autoshaping procedure and reinforcer properties were systematically varied. One group of 8 pigeons was food deprived and subgroups of 2 birds each were exposed to four different pellet sizes as reinforcers, each reinforcer signaled by a keylight conditioned stimulus. A second group was water deprived and received water reinforcers paired with the conditioned stimulus. Water- or food-deprived control groups received appropriate water or food reinforcers that were randomly delivered with respect to the keylight stimulus. Acquisition of the conditioned gape response frequently preceded key-contact responses, and gape conditioned responses were generally elicited at higher rates than were key contacts. The form of the conditioned gape was similar to, but not identical with, the form of the unconditioned gape. The gape component is a critical topographical feature of the conditioned key peck, a sensitive measure of conditioning during autoshaping, and an important source of the observed similarities in the form of conditioned and consummatory responses.     

Blockade of GABAA receptors in the interpositus nucleus modulates expression of conditioned excitation but not conditioned inhibition of the eyeblink response

The cerebellum and related brainstem structures are essential for excitatory eyeblink conditioning. Recent evidence indicates that the cerebellar interpositus and lateral pontine nuclei may also play critical roles in conditioned inhibition (CI) of the eyeblink response. The current study examined the role of GABAergic inhibition of the interpositus nucleus in retention of CI. Male Long-Evans rats were implanted with a cannula positioned just above or in the anterior interpositus nucleus before training. The rats were trained with two different tones and a light as conditioned stimuli, and a periorbital shock as the unconditioned stimulus. CI training consisted of four phases: 1) excitatory conditioning (8 kHz tone paired with shock); 2) feature-negative discrimination (2 kHz tone paired with shock or 2 kHz tone concurrent with light); 3) summation test (8 kHz tone or 8 kHz tone concurrent with light); and 4) retardation test (light paired with shock) After reaching a criterion level of performance on the feature-negative discrimination (40% discrimination), 0.5 μl picrotoxin (a GABA_A receptor antagonist) was infused at one of four concentrations, each concentration infused during separte test sessions. Picrotoxin transiently impaired conditioned responses during trials with the excitatory stimulus (tone) in a dose-dependent manner, but did not significantly impact responding to the inhibitory compound stimulus (tone-light). The results suggest that expression of conditioned inhibition of the eyeblink conditioned response does not require GABAergic inhibition of neurons in the anterior interpositus nucleus.     

Blockade of GABAA Receptors in the Interpositus Nucleus Modulates Expression of Conditioned Excitation but not Conditioned Inhibition of the Eyeblink Response

The cerebellum and related brainstem structures are essential for excitatory eyeblink conditioning. Recent evidence indicates that the cerebellar interpositus and lateral pontine nuclei may also play critical roles in conditioned inhibition (CI) of the eyeblink response. The current study examined the role of GABAergic inhibition of the interpositus nucleus in retention of CI. Male Long-Evans rats were implanted with a cannula positioned just above or in the anterior interpositus nucleus before training. The rats were trained with two different tones and a light as conditioned stimuli, and a periorbital shock as the unconditioned stimulus. CI training consisted of four phases: 1) excitatory conditioning (8 kHz tone paired with shock); 2) feature-negative discrimination (2 kHz tone paired with shock or 2 kHz tone concurrent with light); 3) summation test (8 kHz tone or 8 kHz tone concurrent with light); and 4) retardation test (light paired with shock). After reaching a criterion level of performance on the feature-negative discrimination (40% discrimination), 0.5 microl picrotoxin (a GABAA receptor antagonist) was infused at one of four concentrations, each concentration infused during separate test sessions. Picrotoxin transiently impaired conditioned responses during trials with the excitatory stimulus (tone) in a dose-dependent manner, but did not significantly impact responding to the inhibitory compound stimulus (tone-light). The results suggest that expression of conditioned inhibition of the eyeblink conditioned response does not require GABAergic inhibition of neurons in the anterior interpositus nucleus.     

Stimulus generalization of conditioned eyelid responses produced without cerebellar cortex: implications for plasticity in the cerebellar nuclei

In Pavlovian eyelid conditioning and adaptation of the vestibulo-ocular reflex, cerebellar cortex lesions fail to completely abolish previously acquired learning, indicating an additional site of plasticity in the deep cerebellar or vestibular nucleus. Three forms of plasticity are known to occur in the deep cerebellar nuclei: formation of new synapses, plasticity at existing synapses, and changes in intrinsic excitability. Only a cell-wide increase in excitability predicts that learning should generalize broadly from a training stimulus to other stimuli capable of supporting learning, whereas the alternatives predict that learning should be relatively specific to the training stimulus. Here we show that deep nucleus plasticity, as assessed by conditioned eyelid responses produced without input from the cerebellar cortex, is relatively specific to the training conditioned stimulus (CS). We trained rabbits to a tone or light CS with periorbital stimulation as the unconditioned stimulus (US), and pharmacologically disconnected the cerebellar cortex during a posttraining generalization test. The short-latency conditioned responses unmasked by this treatment showed strong decrement along the dimension of auditory frequency and did not generalize across stimulus modalities. These results cannot be explained solely by a cell-wide increase in the excitability of deep nucleus neurons, and imply that an input-specific mechanism in the deep cerebellar nucleus operates as well.     

Purkinje cell loss by OX7-saporin impairs acquisition and extinction of eyeblink conditioning

The current study examined the effects of globally depleting Purkinje cells in the cerebellar cortex with the immunotoxin OX7-saporin on acquisition and extinction of delay eyeblink conditioning in rats. Rats were given OX7-saporin or saline 2 wk before the start of eyeblink conditioning. The rats that reached a performance criterion of two consecutive days with 80% or greater conditioned responses were given 5 d of extinction training followed by 2 d of reacquisition training. Rats that received infusions of OX7-saporin had 77.2%-97.9% Purkinje cell loss and exhibited impaired acquisition and extinction. The amount of Purkinje cell loss was correlated with the magnitude of the acquisition and extinction impairments. The highest correlations between Purkinje cell number and the rate of acquisition were in lobule HVI and the anterior lobe. The highest negative correlation between Purkinje cell number and the percentage of conditioned responses during extinction was in the anterior lobe. The results indicate that cerebellar Purkinje cells, particularly in the anterior lobe and lobule HVI, play significant roles in acquisition and extinction of eyeblink conditioning.     

Evidence for the thalamic targets of the medial hypothalamic defensive system mediating emotional memory to predatory threats

Previous studies from our laboratory have documented that the medial hypothalamic defensive system is critically involved in processing actual and contextual predatory threats, and that the dorsal premammillary nucleus (PMd) represents the hypothalamic site most responsive to predatory threats. Anatomical findings suggest that the PMd is in a position to modulate memory processing through a projecting branch to specific thalamic nuclei, i.e., the nucleus reuniens (RE) and the ventral part of the anteromedial nucleus (AMv). In the present study, we investigated the role of these thalamic targets in both unconditioned (i.e., fear responses to predatory threat) and conditioned (i.e., contextual responses to predator-related cues) defensive behaviors. During cat exposure, all experimental groups exhibited intense defensive responses with the animals spending most of the time in the home cage displaying freezing behavior. However, during exposure to the environment previously associated with a cat, the animals with combined RE + AMv lesions, and to a lesser degree, animals with single AMv unilateral lesions, but not animals with single RE lesions, presented a reduction of contextual conditioned defensive responses. Overall, the present results provide clear evidence suggesting that the PMd’s main thalamic targets (i.e., the nucleus reuniens and the AMv) seem to be critically involved in the emotional memory processing related to predator cues.     

Reacquisition of heroin and cocaine place preference involves a memory consolidation process sensitive to systemic and intra-ventral tegmental area naloxone

To investigate the effect of naloxone on a putative memory consolidation process underlying reacquisition of heroin and cocaine conditioned place preference, four studies were conducted in male Sprague–Dawley rats using a common procedure involving: place conditioning (0.3 or 1 mg/kg heroin or 20 mg/kg cocaine; ×4 sessions), extinction (vehicle × 4 sessions), and reconditioning (0 or 1 mg/kg heroin or 20 mg/kg cocaine; ×1 session). Systemic naloxone injections (0, 1 and 3 mg/kg) or bilateral intra-ventral tegmental area (VTA) naloxone methiodide infusions (2 nmol in 0.5 μl × side) were administered at different times following reconditioning. Post-reconditioning administration of naloxone dose-dependently blocked, attenuated and had no effect on reacquisition of heroin CPP when administered immediately, 1 h and 6 h after reconditioning, respectively. The highest dose of naloxone also blocked reacquisition of cocaine CPP, and did not produce a conditioned place aversion in heroin-naïve and heroin pre-treated animals. Post-reconditioning infusions in the VTA, but not in adjacent structures, blocked reacquisition of heroin CPP when administered immediately, but not 6 h, after reconditioning. These data suggest that reacquisition of drug-cues associations involves a memory consolidation process sensitive to manipulations of the endogenous opioid system, and indicate that opioid receptors in the VTA may be critically involved in the re-emergence of drug seeking behavior.     

Post-Conditioning Devaluation of an Instrumental Reinforcer has no Effect on Extinction Performance

The extent to which a representation of the reinforcer controls an instrumental response can be assessed by studying the effect of post-conditioning changes in the reinforcer value. In the first experiment rats were trained to press a lever for sucrose pellets on a variable-interval (VI) schedule. The sucrose was subsequently devalued by pairing with Lithium Chloride (LiCl). This had no effect on lever pressing in extinction, although it profoundly reduced reacquisition responding and consumption. In Experiment II rats were trained to shuttle between the two distinctive chambers of a choice-box, in which lever pressing was reinforced in one chamber by sucrose and in the other chamber by food pellets programmed on independent VI schedules. A LiCl-induced taste-aversion was conditioned to the sucrose, and although this markedly affected reacquisition, extinction responding in the sucrose chamber and chamber preference were unaffected. These results indicate that instrumental performance can be independent of the current value of the reinforcer, and are discussed with reference to stimulus-response theory and second-order Pavlovian conditioning.     

Purkinje cell activity in the cerebellar anterior lobe after rabbit eyeblink conditioning

The cerebellar anterior lobe may play a critical role in the execution and proper timing of learned responses. The current study was designed to monitor Purkinje cell activity in the rabbit cerebellar anterior lobe after eyeblink conditioning, and to assess whether Purkinje cells in recording locations may project to the interpositus nucleus. Rabbits were trained in an interstimulus interval discrimination procedure in which one tone signaled a 250-msec conditioned stimulus-unconditioned stimulus (CS-US) interval and a second tone signaled a 750-msec CS-US interval. All rabbits showed conditioned responses to each CS with mean onset and peak latencies that coincided with the CS-US interval. Many anterior lobe Purkinje cells showed significant learning-related activity after eyeblink conditioning to one or both of the CSs. More Purkinje cells responded with inhibition than with excitation to CS presentation. In addition, when the firing patterns of all conditioning-related Purkinje cells were pooled, it appeared that the population showed a pattern of excitation followed by inhibition during the CS-US interval. Using cholera toxin-conjugated horseradish peroxidase, Purkinje cells in recording areas were found to project to the interpositus nucleus. These data support previous studies that have suggested a role for the anterior cerebellar cortex in eyeblink conditioning as well as models of cerebellar-mediated CR timing that postulate that Purkinje cell activity inhibits conditioned response (CR) generation during the early portion of a trial by inhibiting the deep cerebellar nuclei and permits CR generation during the later portion of a trial through disinhibition of the cerebellar nuclei.     

Gabaergic mechanisms of hypothalamic nuclei in the expression of conditioned fear

The amygdala, the dorsal periaqueductal gray (dPAG), and the medial hypothalamus have long been recognized to be a neural system responsible for the generation and elaboration of unconditioned fear in the brain. It is also well known that this neural substrate is under a tonic inhibitory control exerted by GABA mechanisms. However, whereas there is a growing body of evidence to suggest that the amygdala and dPAG are also able to integrate conditioned fear, it is still unclear, however, how the distinct hypothalamic nuclei participate in fear conditioning. In this work we aimed to examine the extent to which the gabaergic mechanisms of this brain region are involved in conditioned fear using the fear-potentiated startle (FPS). Muscimol, a GABA-A receptor agonist, and semicarbazide, an inhibitor of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD), were used as an enhancer and inhibitor of the GABA mechanisms, respectively. Muscimol and semicarbazide were injected into the anterior hypothalamus (AHN), the dorsomedial part of the ventromedial nucleus (VMHDM), the dorsomedial (DMH) or the dorsal premammillary (PMD) nuclei of male Wistar rats before test sessions of the fear conditioning paradigm. The injections into the DMH and PMD did not produce any significant effects on FPS. On the other hand, muscimol injections into the AHN and VMHDM caused significant reduction in FPS. These results indicate that injections of muscimol and semicarbazide into the DMH and PMD fail to change the FPS, whereas the enhancement of the GABA transmission in the AHN and VMHDM produces a reduction of the conditioned fear responses. On the other hand, the inhibition of this transmission led to an increase of this conditioned response in the AHN. Thus, whereas DMH and PMD are known to be part of the caudal-most region of the medial hypothalamic defensive system, which integrates unconditioned fear, systems mediating conditioned fear select the AHN and VMHDM nuclei that belong to the rostral-most portion of the hypothalamic defense area. Thus, distinct subsets of neurons in the hypothalamus could mediate different aspects of the defensive responses.