Total peripheral resistance changes in dogs during aversive classical conditioning

The arterial pressure, heart rate, stroke volume, cardiac output and total peripheral resistance of laboratory dogs were monitored continuously before and during a 30 min period in which aversive classical conditioning procedures were applied. The experimental stimulation generated inhibition of behavioral activity (“freezing”) together with sustained increases in arterial pressure averaging 30%, and increases in heart rate and cardiac output of 60–70%. The calculated total peripheral resistance decreased by an average of 20%. The behavioral and cardiovascular responses were sustained during the aversive conditioning period. The results suggest that significant peripheral vasodilation occurred under these conditions which was probably not due to metabolic byproducts of behavioral activity, but probably reflected sympathetic cholinergic effects upon blood vessels in the skeletal muscles. These data document the significance of the skeletal muscle circulation in the total peripheral resistance to blood flow in unanesthetized animals.     

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.     

Sign-tracking in aversive conditioning

Two experiments were conducted to determine if rats tend to avoid contact with a stimulus that signals the occurrence of shock and contact a stimulus that signals the nonoccurrence of shock. The conditioned stimulus was a 60-sec platform presentation, and the unconditioned stimulus was a 2-sec inescapable shock. In each experiment, the emphasis was on two types of Pavlovian pairings: forward pairing in which each platform presentation was followed by shock, and backward pairing in which each platform presentation was preceded by shock and followed by a lengthy shock-free interval. Experiment 1 showed that in comparison with Random and CS-only procedures, the backward procedure produced a significant increase in approach and contact with the platform, but the forward procedure failed to produce a significant decrease in contact with the platform. Experiment 2, in which all groups were roughly equated for baseline levels of platform preference, demonstrated strong effects of both forward and backward conditioning. The experiments provide evidence for an aversive sign-tracking system in which animals' tendencies to withdraw from or approach and contact a platform CS are determined by the Pavlovian contingencies which render it a reliable signal for the occurrence or nonoccurrence of shock.     

An aversive conditioning unit for ants

An apparatus is described for studying aversive conditioning in ants. The aversive stimulus is mechanically produced vibration. Responses are recorded automatically by an infrared photocell system.     

Classical aversive conditioning of catecholamine and corticosterone responses

Some earlier work, not rigorously controlled, suggested that conditional increases in sympathoadrenal stress hormones could occur. The purpose of this experiment was to test this idea further using an appropriately controlled design. To do this, we subjected rats living in a tether-type apparatus to differential fear conditioning. Chronic catheterization allowed us to sample blood before and after conditional stimulus probes without having to touch the rats. No evidence for conditional changes in norepinephrine and corticosterone was found. In contrast, differential conditioning of epinephrine responses was found. The conditional response, however, was not a simple one in that conditional increases in epinephrine following CS+ probes were not always seen. These data support the idea that learned changes in hormonal stress respondents can occur. But they leave open the question of why clear cut conditional changes in these visceral systems are so difficult to obtain.     

Serotonin neurons and aversive conditioning in the rat

The acute and long-term effects of p -chloroamphetamine (PCA) on one-way and two-way active avoidance (AA), passive avoidance (PA) learning, fear retention (FR) and on central monoamine concentrations were examined in the male rat. Acute PCA administration (0.63–5 mg/kg i.p.), which releases presynaptic 5-HT, produced a dose-related impairment of both one-and two-way AA acquisition, AA retention, PA and fear retention. The selective serotonin (5-HT) uptake inhibitor zimelidine, but not the noradrenaline (NA) uptake inhibitor desipramine, blocked the avoidance deficit induced by acute PCA. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 × 10 mg/kg i.p.) failed to change AA and PA learning but blocked the avoidance deficit induced by acute PCA. Degeneration of locus coeruleus NA neurones with DSP4 (1 × 50 mg/kg), a selective NA neurotoxin, failed to block the acute PCA action. Thus, the acute avoidance learning impairment appears to be specifically related to the acute release of endogenous 5-HT. Both acute and long-term PCA treatment affected 5-HT neurones preferentially in the forebrain while marginal effects were observed in the midbrain and spinal cord. A marked impairment in the retention and retrieval of fear conditioning in the rat was also observed following acute PCA administration. The serotoninergic mechanisms underlying the retrieval deficit were found to be similar but not identical to those involved in AA acquisition. These results suggest an important role for central 5-HT neurones in aversive learning processes. The possible involvement of 5-HT neurones in learning, memorial and/or retrieval processes is discussed.     

Serotonin neurons and aversive conditioning in the rat

The acute and long-term effects of p-chloroamphetamine (PCA) on one-way and two-way active avoidance (AA), passive avoidance (PA) learning, fear retention (FR) and on central monoamine concentrations were examined in the male rat. Acute PCA administration (0.63–5 mg/kg i.p.), which releases presynaptic 5-HT, produced a dose-related impairment of both one-and two-way AA acquisition, AA retention, PA and fear retention. The selective serotonin (5-HT) uptake inhibitor zimelidine, but not the noradrenaline (NA) uptake inhibitor desipramine, blocked the avoidance deficit induced by acute PCA. Degeneration of brain 5-HT neurons by a high neurotoxic dose of PCA (2 × 10 mg/kg i.p.) failed to change AA and PA learning but blocked the avoidance deficit induced by acute PCA. Degeneration of locus coeruleus NA neurones with DSP4 (1 × 50 mg/kg), a selective NA neurotoxin, failed to block the acute PCA action. Thus, the acute avoidance learning impairment appears to be specifically related to the acute release of endogenous 5-HT. Both acute and long-term PCA treatment affected 5-HT neurones preferentially in the forebrain while marginal effects were observed in the midbrain and spinal cord. A marked impairment in the retention and retrieval of fear conditioning in the rat was also observed following acute PCA administration. The serotoninergic mechanisms underlying the retrieval deficit were found to be similar but not identical to those involved in AA acquisition. These results suggest an important role for central 5-HT neurones in aversive learning processes. The possible involvement of 5-HT neurones in learning, memorial and/or retrieval processes is discussed.     

Avoidance conditioning with shock contingent upon the avoidance response

Rats learned either a lever-press response, a shuttle response or a one-way crossing response, which produced one immediate shock but was instrumental in avoiding five identical shocks scheduled to occur later. These responses were acquired both with and without support of an escape contingency. These results support shock-frequency reduction as a sufficient condition for the acquisition and maintenance of avoidance.     

Classical aversive conditioning of catecholamine and corticosterone responses.

Some earlier work, not rigorously controlled, suggested that conditional increases in sympathoadrenal stress hormones could occur. The purpose of this experiment was to test this idea further using an appropriately controlled design. To do this, we subjected rats living in a tether-type apparatus to differential fear conditioning. Chronic catheterization allowed us to sample blood before and after conditional stimulus probes without having to touch the rats. No evidence for conditional changes in norepinephrine and corticosterone was found. In contrast, differential conditioning of epinephrine responses was found. The conditional response, however, was not a simple one in that conditional increases in epinephrine following CS+ probes were not always seen. These data support the idea that learned changes in hormonal stress respondents can occur. But they leave open the question of why clear cut conditional changes in these visceral systems are so difficult to obtain.     

Reinstatement of fear responses in human aversive conditioning

The treatment of choice for a number of anxiety disorders is exposure therapy. However, successful reduction of fear through exposure is sometimes followed by a (partial) return of symptoms of fear (return of fear, ROF; Clin. Psychol. Rev. 9 (1989) 147). Several possible learning mechanisms have been suggested to explain ROF (e.g. mechanisms related to spontaneous recovery, renewal, reacquisition and reinstatement). The present study focuses on reinstatement, which refers to the observation that mere US-only presentations can 'reinstate' previously extinguished fear responses. Although animal research has repeatedly demonstrated this phenomenon, little is known about fear reinstatement in humans. The present study employed a differential aversive conditioning procedure: after acquisition and a subsequent extinction procedure, a series of four unpredicted US-only trials was scheduled in the reinstatement group. The control group did not receive additional US presentations. A significant reinstatement effect was observed for US-expectancy ratings and fear ratings in the reinstatement group, but not in the control group. No differences were observed in a reaction time measure of resource allocation to the conditioned stimuli. These findings constitute a first demonstration of reinstatement of conditioned fear responses in humans. Implications for exposure treatment and suggestions for future research are discussed.