Cardiac arrhythmias in the monkey during classically conditioned fear and excitement

Rhesus monkeys(Macaca mulatto) were tested in classic aversive and appetitive conditioning paradigms following complete coronary artery occlusion (CO) to test the hypothesis that “emotional stress” induces ventricular arrhythmias. Findings were based upon conditioning trials conducted for one or more weeks after occlusion in 13 animals. When all data from each animal for the week following CO were considered, there was no demonstrable tendency for arrhythmias to increase during “fear” conditioned to unavoidable electric shock or during “excitement” in anticipation of food. However, selected trials from six monkeys did reveal instances when changes in the frequency of occurrence of arrhythmias were coupled with behavioral conditioning. While analysis of these trials did not reveal any simple relationship between emotional stress and the development of ventricular arrhythmias after myocardial infarction, certain of the behavioral situations may be more potentially arrhythmogenic than others. For these selected trials, with respect to control, the number of arrhythmias may have increased or decreased upon presentation of the conditional stimulus; the exact response appears to depend upon the immediate physiologic status of the animal as well as on the behavioral condition. “More stressful” situations, such as aversive conditioning, are not necessarily associated with greater numbers of arrhythmias than were “less stressful” situations, such as appetitive conditioning. Arrhythmias appear to occur more frequently when an animal’s heart rate is within a given range; this may reflect underlying cardiac sympathetic and parasympathetic nerve activity.     

Determinants of tonic and phasic reactions in transswitching

Forty-eight college students were assigned randomly to four groups in a 2 × 2 factorial arrangement of phasic conditional stimuli (samevs. different) and tonic conditional stimuli (samevs. different) to receive 2 days of classical conditioning with a transswitching procedure. Tonic stimuli were a 5-minute projected white triangle or circle; phasic stimuli were a 5-second red or green square superimposed over the tonic stimuli. There were six tonic stimulus segments each day, separated by 20-second periods of no stimulus, three containing six trials of the phasic stimulus paired with shock and three containing six trials of the phasic stimulus alone, in the counterbalanced order. Tonic responding at the onset of the tonic stimuli or during brief periods following its onset were recorded, along with phasic responses to the phasic stimuli. Responses included magnitude of skin conductance responses, frequency of unelicited skin conductance responses, and tonic heart rate. Both skin conductance measures of responding to the tonic stimuli differentiated significantly between positive and negative tonic segments during Day 2, but only in the group with two different tonic stimuli and one phasic stimulus (“standard” transswitching). This supported the hypothesis that tonic stimulus differentiation would be absent when two different phasic stimuli were present. The heart rate data did not support this hypothesis, showing tonic differentiation in both groups with two tonic stimuli. Phasic differentiation controlled by the different phasic stimuli was observed on Day 1; on Day 2, phasic differentiation was present only in the group with two tonic and one phasic stimuli and the group with one tonic and two phasic stimuli. The results were interpreted to mean that temporal contiguity between the tonic stimuli and shock or no shock is not sufficient to establish tonic response differentiation in transswitching.     

Cardiovascular dynamics during classical appetitive and aversive conditioning in laboratory primates

This report describes changes in the rate of rise of left and right intraventricular pressures during aversive and appetitive conditioning procedures in chair-restrained rhesus monkeys. The conditioning paradigm consisted of a one-minute tone followed, in the one case, by an electric shock, and in the other, by the delivery of Purina monkey pellets. The conditional cardiovascular response was characterized by short latency, highly significant elevations in the derivatives of both ventricular pressures as well as a marked arterial pressor response and tachycardia. The magnitude of the conditional response to the classical aversive procedure was somewhat larger than that to appetitive conditioning. These alterations in the rate of development of intraventricular pressure can be attributed largely to augmentation in the sympathetic neural input to the heart and contribute to an analysis of selective aspects of the nervous regulation of the heart in intact, behaviorally conditioned animals.     

Differential preparatory cardiovascular responses to aversive and appetitive behavioral conditioning

Dogs were required to press a response panel either to avoid shock or obtain food. During a one-hour interval immediatelypreceding performance on the avoidance task, blood pressure increased while heart rate decreased. In contrast, both blood pressure and heart rate increased during the one-hour interval immediatelypreceding performance on the food task. During both the shock-avoidance and food performancesper se, however, elevations in heart rate and blood pressure were observed, though the avoidance task produced less consistent heart rate changes. These effects were observed (1) with different individual dogs on each of the two training procedures, and (2) with the same dogs exposed successively to both aversive and appetitive behavioral conditioning.     

A computer simulation/mathematical model of learning: Extension of DMOD from appetitive to aversive situations

The computer simulation/mathematical model called DMOD, which can simulate over 35 different phenomena in appetitive discrete-trial and simple free-operant situations, has been extended to include aversive discrete-trial situations. Learning (V) is calculated using a three-parameter equation \(\Delta V = \alpha \beta (\lambda - \bar V)\) (see Daly & Daly, 1982; Rescorla & Wagner, 1972). The equation is applied to three possible goal events in the appetitive (e.g., food) case and to three in the aversive (e.g., shock) case. The original goal event can be present, absent, or reintroduced; in the appetitive situation, these events condition approach (Vap), avoidance (Vav), and courage (Vcc), respectively. In the aversive situation, the events condition avoidance (Vav*), approach (Vap*), and cowardice (Vcc*), respectively. The model was developed in simple learning situations and subsequently was applied to complex situations. It can account for such diverse phenomena as contrast effects after reward shifts, greater persistence following partial than following continuous reinforcement, and a preference for predictable appetitive and predictable aversive events. Application of the aversive version of the model to “reward” shifts is described.     

Transfer of conditioned appetitive stimuli to conditioned aversive excitatory and inhibitory stimuli

The transfer of Pavlovian appetitive stimuli to Pavlovian aversive stimuli was examined in three experiments. In Experiment 1, rats received appetitive (Ap) conditioning designed to establish a flashing-light stimulus as either a CS+, CSo, or CS? for food, or to maintain it as a novel stimulus for US-alone subjects. Then, the stimulus was employed as a signal for weak shock in conditioned-emotional-response (CER) training. Both acquisition and extinction results showed that the ApCS+ facilitated and the ApCS? retarded aversive excitatory conditioning relative to the ApCSo and US-alone controls. Experiment 2 replicated the findings of Experiment 1 with both a moderate and a severe shock in CER training. In Experiment 3, different groups received the same appetitive conditioning as before, but to a flashing-light stimulus which was then employed as a signal for no shock in CER training. The ApCS? facilitated and the ApCS+ retarded aversive inhibitory conditioning relative to ApCSo and US-alone controls. Collectively, these findings establish that, in Pavlovian conditioning, transfer of an appetitive CS to an aversive excitor or inhibitor is facilitated by maintaining the initial conditioning contingency.     

Differential conditional emotional and cardiovascular responses--a training technique for monkeys

A training technique has been developed which combines classical differential conditioning and conditional emotional response (CER) procedures. After monkeys were trained to lever-press on a variable-interval schedule, two stimulus lights were presented. One light (CS) was always followed, upon termination, by electrical shock; the other light (DS) was never followed by shock. The response to the CS was a decreased rate of lever-pressing and increased heart rate and blood-flow velocity. None of these responses occurred to the DS. This technique eliminates the possibility of pseudoconditioning and provides measurement of both somatic and autonomic responses in a CER situation.     

Reaction time task as unconditional stimulus

Nonaversive unconditional stimuli (USs) are seldom used in human classic conditioning of autonomic responses. One major objection to their use is that they produce deficits in electrodermal (ED) second- and third-interval response conditioning. However, a nonaversive reaction time (RT) task that includes feedback of success has been shown to be an effective US while avoiding this disadvantage (Lipp and Vaitl 1988). The present study compared this new RT task (RT-new) with a traditional RT task (RT-old) and with a standard aversive US (shock) in differential classic conditioning of ED, heart rate (HR), and digital pulse volume (DPV) responses. Eight-second-delay differential conditioning was applied in three groups of 12 subjects each. Simple geometric features (square, cross) displayed on a television screen served as conditional stimuli (CS⁺ and CS^?). In acquisition, there were no statistically significant differences among the groups; differential conditioning did occur in HR, first- and second-interval ED responses, and first-interval DPV responses. Separate analyses within each group, however, revealed that there was no second-interval ED conditioning in the RT-old group. During extinction, neither DPV nor second-interval ED conditioning could be obtained, whereas HR and first-interval ED conditioning occurred in each group. In third-interval omission ED responses, RT-old and shock groups exhibited extinction, while response differentiation was maintained in the RT-new group throughout extinction. The RT task including feedback proved to be as reliable a US as a standard aversive US, whereas application of a traditional RT task again yielded some weaknesses in second-interval ED conditioning.     

Hedonic and nucleus accumbens neural responses to a natural reward are regulated by aversive conditioning

The nucleus accumbens (NAc) plays a role in hedonic reactivity to taste stimuli. Learning can alter the hedonic valence of a given stimulus, and it remains unclear how the NAc encodes this shift. The present study examined whether the population response of NAc neurons to a taste stimulus is plastic using a conditioned taste aversion (CTA) paradigm. Electrophysiological and electromyographic (EMG) responses to intraoral infusions of a sucrose (0.3 M) solution were made in naïve rats (Day 1). Immediately following the session, half of the rats (n = 6; Paired) received an injection of lithium chloride (0.15 M; i.p.) to induce malaise and establish a CTA while the other half (n = 6; Unpaired) received a saline injection. Days later (Day 5), NAc recordings during infusions of sucrose were again made. Electrophysiological and EMG responses to sucrose did not differ between groups on Day 1. For both groups, the majority of sucrose responsive neurons exhibited a decrease in firing rate (77% and 71% for Paired and Unpaired, respectively). Following conditioning, in Paired rats, EMG responses were indicative of aversion. Moreover, the majority of responsive NAc neurons now exhibited an increase in firing rate (69%). Responses in Unpaired rats were unchanged by the experience. Thus, the NAc differentially encodes the hedonic value of the same stimulus based on learned associations.Our search for sustenance and pleasurable stimuli is often balanced by our desire to avoid punishment and harm. Similarly, neural systems responsible for generating approach behavior must be countered by signals that suppress approach behavior under contexts where approach is dangerous or maladaptive (Hoebel et al. 2007). The nucleus accumbens (NAc) is acutely involved in food intake and goal-directed, approach behavior. Pharmacological manipulations of the NAc promote food intake even in sated rats (Maldonado-Irizarry and Kelley 1995; Stratford and Kelley 1997). Lesions or inactivation of the NAc impair conditioned approach behavior (Cardinal et al. 2002; Blaiss and Janak 2009). Interestingly, drugs that lead to inhibition of select regions of the NAc increase positive hedonic responses to palatable taste solutions (Pecina and Berridge 2005). Recordings from individual NAc neurons have mirrored these findings. We and others have shown that consumption of palatable food stimuli is associated with decreases in the firing rate of the majority of responsive NAc neurons (Nicola et al. 2004b; Roitman et al. 2005; Taha and Fields 2005; Wheeler et al. 2008). In addition, decreases in NAc neural activity are associated with bouts of licking behavior for palatable stimuli (Taha and Fields 2006), and disruption of these decreases halt feeding bouts (Krause et al. 2010). Finally, decreases in NAc neural activity are associated with preferred locations previously paired with drug reward (German and Fields 2007). Thus, decreases in NAc activity appear to be closely linked to positive hedonic stimuli, stimuli that have been explicitly paired with them and behavioral approach.The NAc is also responsive to aversive stimuli (Carlezon and Thomas 2009; Levita et al. 2009). The delivery of aversive taste stimuli to rats is associated with increases in the firing rate of the majority of responsive NAc neurons (Roitman et al. 2005; Wheeler et al. 2008). In addition to responding to primary appetitive and aversive taste stimuli, NAc neurons develop responses to predictors of reward and aversion. Individual NAc neurons selectively encode cues that predict either appetitive (Roitman et al. 2005; Day et al. 2006) or aversive (Roitman et al. 2005) stimuli following purely Pavlovian conditioning or a combination of instrumental and Pavlovian conditioning (Setlow et al. 2003; Nicola et al. 2004a). NAc neurons come to encode departure from locations not associated with reward with the majority response being that of excitation (German and Fields 2007). Thus, NAc neurons appear to encode aversive stimuli and withdrawal behavior with increases in activity. These and other findings have led to the recent postulation that reward and aversion are differentially encoded by the activity of NAc neurons (Carlezon and Thomas 2009).Data supportive of the activity hypothesis (Carlezon and Thomas 2009) have been generated by the use of different stimuli to serve as appetitive or aversive primary or predictive stimuli. Thus, selective encoding could be biased by the sensory properties of each stimulus rather than their hedonic valence. When a novel, palatable taste is paired with visceral malaise, a Pavlovian association is made and a conditioned taste aversion (CTA) is established. Subsequent exposure to the once palatable stimulus is met with avoidance or aversion and rejection (Garcia et al. 1974; Schafe et al. 1995). Thus, the same taste stimulus can either be appetitive or aversive, depending on Pavlovian associations. Here, individual NAc neurons were recorded in rats (Paired) before (Day 1) and after a CTA (Day 5) was established and compared with rats that received equal exposure to the same stimuli but in an unpaired manner (Unpaired), and hence no CTA developed. Simultaneously, oro-motor behavior was characterized to provide an index of the associative strength of the taste stimulus. Using this paradigm, we determined that the population response of the NAc does indeed encode hedonic valence.     

Personality and conditioning with appetitive and aversive stimuli

Two studies examining relationships of extraversion (E), neuroticism (N) and psychoticism (P) to conditioning performance are reported. In the first, 31 male volunteers developed electrodermal CRs to appetitive or aversive stimuli of either weak or strong rated intensity. Factor analysis yielded general factors of classical CR acquisition and extinction across reinforcement type. Stability and E loaded the acquisition factor, these Ss, from post-hoc analysis, revealing superior conditioning in the strong appetitive condition. In the second study, 20 of these 31 Ss participated in two series of discrimination motor reaction time trials, in which attainment of criterion RT was reinforced by appetitive slides, and slower-than-criterion responding by aversive slides. Introversion was correlated with greater response acquisition under both conditions, although post-hoc analysis suggested that improvement under positive reinforcement did not differ from simple practice. Results are congruent with a model combining reinforcer preference with response potential to describe personality-conditioning relationships.     

Brain Indices of Nonconscious Associative Learning

Using a classical conditioning technique, this study investigated whether nonconscious associative learning could be indexed by event-related brain activity (ERP). There were three phases. In a preconditioning baseline phase, pleasant and unpleasant facial schematics were presented in awareness (suprathreshold). A conditioning phase followed, in which stimuli were presented outside awareness (subthreshold, via energy masking), with an unpleasant face (CS+) linked to an aversive shock and a pleasant face (CS−) not linked to a shock. The third, postconditioning phase, involved stimulus presentations in awareness (suprathreshold). Evidence for acquisition of a conditional response was sought by comparing suprathreshold pre- and postconditioning phases, as well as in the subthreshold conditioning phase itself. For the pre-postconditioning phase analyses, significant ERP component differences differentiating CS+ and CS− were observed for N1, P2, and especially P3. For the conditioning phase, significant differences were observed in the 100–400 ms. post-stimulus region reflecting a CS+ processing negativity. Brain activity does indeed index the acquisition of a conditional response to subthreshold stimuli. Associative learning can occur outside awareness.     

Pharmacological analysis of conditioning in sensorimotor cortex neurons in a model

In awake mobile rabbits, with electrodes implanted in the medial lemniscus, midbrain tegmental reticular nucleus, and pyramidal tract, combined stimulation of two brain structures resulted in elaboration of conditional connections in sensorimotor cortex neuronal populations. The main criterion of the conditioning was the appearance of changes in the neuronal activity on omission of the second stimulus. These changes represented a complex of electrical events, some of which were similar to and others different from the evoked responses to the second stimulus. Application of atropine, sulfate, chlorpromazine hydrochloride, serotonin creatinine sulfate, andγ-aminobutyric acid (GABA) to the cortex at the site of the recording exerted a modulating effect on the conditional neuronal activity patterns. Of the above substances, GABA and atropine had the most pronounced effect. The GABA removed the short-latency components of the conditional changes which were similar to evoked responses. The atropine abolished the long-latency changes which differed from evoked responses.     

Reinforcement strength in classical conditioning of leg flexion,freezing, and heart rate in cats

Stable leg-flexion CRs were successfully elaborated in cats receiving tone-strong shock pairings, but not in cats receiving tone-weak shock pairings. Both shock USs elicited reliable flexion URs in the presence of the CS, thus satisfying the contiguity requirement basic to the Pavlovian paradigm. Elaboration of the flexion CRs required a large number of trials relative to the conditioned freezing and decelerative heart rate responses which appeared after only a very few trials in the strong-US cats. As with flexion CRs, freezing and heart-rate responses never developed with the weak-shock US. When the weak-US cats were later switched to the strong US, freezing and heart-rate CRs quickly appeared and flexion CRs appeared after fewer strong-US trials than in cats receiving the strong shock originally. The results were interpreted as supporting a reinforcement conception of classical defense conditioning and as indicating the importance of using a US capable of eliciting emotional responses.     

Negative reinforcement as shock-frequency reduction

Is a conditioned aversive stimulus necessary in avoidance conditioning? Or is a reduction in the rate of aversive stimulation alone sufficient to generate and maintain an avoidance response? Rats were subjected to an avoidance procedure in which shocks occurred randomly in time, but a response could reduce the overall rate of shock. Fifteen acquisition curves, obtained from 16 animals, showed both immediate and delayed, rapid and gradual increases in response rate; there was no representative acquisition curve. Response rates were directly related to the amount by which the response reduced shock frequency. In extinction, when shock rates were not affected by responding, the response total was inversely related to the amount by which the response had reduced shock frequency during prior conditioning, with as many as 20,000 extinction responses when the shock frequency reduction had been relatively small. Responding on this procedure shows that avoidance conditioning can occur without benefit of either classical exteroceptive stimuli or covert stimuli inferred from the temporal constancies of a procedure. It also shows that reduction in shock rate is alone sufficient to maintain avoidance.     

Aversive and appetitive electrodermal classical conditioning using pictures as stimuli

The principal goal of this study was to verify whether it was possible to obtain both aversive and appetitive electrodermal classical conditioning, using pictures as conditioned stimuli (CS), and unconditioned stimuli (US). Additionally, we tried to verify whether, as a consequence of such conditioning, diminution of the unconditioned response (UR) was observed. With this aim, IAPS (?International Affective Picture System?) pictures were selected as stimuli. A picture showing a burnt face was used as the aversive US (USav), and a picture showing a scene with erotic content was used as the appetitive US (USap). As the aversive CS (CSav), and appetitive CS (CSap), two images with intermediate values of valence and arousal showing male faces were selected. In the experimental group, 10 CSav/USav and 10 CSap/USap trials were presented. In the control group 10 CSav, CSap, USav, and USap trials were presented in pseudorandom order. Skin conductance response (SCR) elicited by both the CSs and the USs was scored. Results showed aversive conditioning, but neither appetitive conditioning nor UR diminution. Problems to obtain conditioning using pictures as stimuli and possible options to overcome them in future research are discussed.     

Constraint, alcoholism, and electrodermal response in aversive classical conditioning and mismatch novelty paradigms

This study investigated whether low levels of the personality trait of constraint and early-onset alcoholism would be associated with deficits in aversive conditioning and smaller responses to novelty in a stimulus mismatch protocol. Personality traits (constraint and socialization) and skin conductance responses (SCRs) during conditioning and novelty paradigms were assessed in alcoholics (n=41) and non-alcoholics (n=32). The conditioning protocol involved measuring SCRs after conditioned stimuli (CS+: tones) paired with shock, CS− tones unpaired with shock, and CS+ probes unpaired with shock. The mismatch protocol involved measuring SCRs to auditory stimuli consisting of a series of 5 pure tones of the same pitch followed a shorter white noise stimulus (the novel stimulus). Contrary to the hypothesis, alcoholics did not differ from non-alcoholics in SCRs to CS+ probes or on the mismatch measure (SCR novel tone—SCR to 5^th tone). Higher levels of constraint and self-reports of fear during conditioning were associated with smaller responses to both the CS+ probes and the CS− tones as well as the mismatch measure within non-alcoholics, but not within alcoholics. In alcoholics, low constraint was associated with greater habituation to CS+ probes, and poor differential conditioning on measures of change across trials in SCR to CS+ probes and CS− stimuli. The results suggest that different processes influence levels of constraint in non-alcoholics and alcoholics. The data indicate that low constraint in non-alcoholics is associated with allocating fewer processing resources to potentially significant stimuli, rather than being associated with a specific deficit in aversive conditioning per se.     

Pavlovian conditioning with proximal stimuli

This experiment was conducted with the objective of demonstrating that the effective stimuli in Pavlovian Conditioning are not environmental stimuli but internal physiological processes elicited by environmental input (proximal stimuli). In order to achieve the objective, afterimages in color vision were used: looking at a diffuse lightened circle after seeing a red circle yields an image of a green circle. A differential conditioning paradigm with two sequential compounds was run. In one group (G⁺B^?: n₁=10), a red circle followed by a green circle was paired with shock, whereas a red circle followed by a blue circle remained unpaired. A second group (G^?B⁺: n₂=10) received red-blue paired trials and unpaired red-green trials. Immediately after that training, subjects were tested with a new, never trained sequential compound: a red circle followed by a diffuse lightened circle. Furthermore, they were tested with the already trained compounds. Taking the environmental point of view, the never trained stimulus should elicit an orienting response lying inbetween the excitatory reaction to the paired stimulus and the inhibitory reaction to the unpaired stimulus. From the proximal point of view, the diffuse light should elicit an excitatory reaction in group G⁺B^? and an inhibitory reaction in group G^?B⁺. Electrodermal conditioned anticipatory and omission responses were measured. The results supported the proximal hypothesis. Hence, defining input in environmental terms may be the wrong way. Instead, in conceptualizing the stimulus in conditioning, the following should be considered: the processing organism itself is creating the effective stimuli.     

Appetitive and aversive olfactory learning induce similar generalization rates in the honey bee

Appetitive and aversive learning drive an animal toward or away from stimuli predicting reinforcement, respectively. The specificity of these memories may vary due to differences in cost–benefit relationships associated with appetitive and aversive contexts. As a consequence, generalization performances may differ after appetitive and aversive training. Here, we determined whether honey bees show different rates of olfactory generalization following appetitive olfactory conditioning of the proboscis extension response, or aversive olfactory conditioning of the sting extension response. In both cases, we performed differential conditioning, which improves discrimination learning between a reinforced odor (CS+) and a non-reinforced odor (CS?) and evaluated generalization to two novel odors whose similarity to the CS+ and the CS? was different. We show, given the same level of discriminatory performance, that rates of generalization are similar between the two conditioning protocols and discuss the possible causes for this phenomenon.     

COMPARING PLEASURE AND PAIN: THE FUNDAMENTAL MATHEMATICAL EQUIVALENCE OF REWARD GAIN AND SHOCK REDUCTION UNDER VARIABLE INTERVAL SCHEDULES

The relationship between positive and negative reinforcement and the symmetry of Thorndike's law of effect are unresolved issues in operant psychology. Here we show that, for a given pattern of responding on variable interval (VI) schedules with the same programmed rate of food rewards (positive reinforcement VI) or electric shocks (negative reinforcement VI), there is a fundamental mathematical equivalence between reward gain and shock reduction. We also provide the first normative account of how animals should respond on a negative VI schedule, showing that it is better to space responses evenly than to respond with a variable interresponse time (IRT). Published data from rats, however, indicate that these animals respond irregularly, often with a burst of activity immediately following a shock. While this is irrational in the experimental setting, it may represent an appropriate response to the heterogeneity of stimuli commonly encountered in natural environments. We discuss the broader implications of our analysis for understanding how animals evaluate appetitive and aversive stimuli.     

The development of an attentional bias for angry faces following Pavlovian fear conditioning

Although it is well documented that fear responses develop following aversive Pavlovian conditioning, it is unclear whether fear learning also manifests in the form of attentional biases for fear-related stimuli. Boschen, Parker, and Neumann (Boschen, M. J., Parker, I., & Neumann, D. L. (2007). Changes in implicit associations do not occur simultaneously to Pavlovian conditioning of physiological anxiety responses. Journal of Anxiety Disorders, 21, 788-803.) showed that despite the acquisition of differential skin conductance conditioned responses to angry faces paired (CS+) and unpaired (CS−) with an aversive shock, development of implicit associations was not subsequently observed on the Implicit Association Test. In the present study, participants (N = 76) were assigned either to a Shock or NoShock group and completed a similar aversive Pavlovian conditioning procedure with angry face CS+ and CS− stimuli. Participants next completed a visual probe task in which the angry face CS+ and CS− stimuli were paired with angry face control stimuli and neutral faces. Results confirmed that differential fear conditioning was observed in the Shock group but not in the NoShock group, and that the Shock group subsequently showed a selective attentional bias for the angry face CS+ compared with the CS− and control stimuli during the visual probe task. The findings confirm the interplay between learning-based mechanisms and cognitive processes, such as attentional biases, in models of fear acquisition and have implications for treatment of the anxiety disorders.