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PubMed1997

Stimulus generalization of fear responses: effects of auditory cortex lesions in a computational model and in rats.

The conditioning of fear responses to a simple acoustic stimulus (pure tone) paired with footshock can be mediated by the transmission of auditory information to the lateral nucleus of the amygdala from either the auditory thalamus or the auditory cortex. We examined the processing capacity of the thalamo-amygdala pathway by making lesions of the auditory cortex and testing the extent to which conditioned fear responses generalized to tones other than the one paired with footshock. Two studies were performed, one in an anatomically constrained computational model of the fear conditioning network and the other in rats. Stimulus generalization was unaffected in both. These findings support the validity of the model as an approach to studying the neural basis of conditioned fear learning, and in addition suggest that the thalamo-amygdala pathway, possibly by the use of population coding, is capable of performing at least crude stimulus discriminations.

Acoustic Stimulation↗
PubMedDate not supplied

PMMA-stimulus generalization to the optical isomers of MBDB and 3,4-DMA.

Psychoactive phenylisopropylamines can produce one or more of several different stimulus effects in animals. These effects are typified by the hallucinogen 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), the central stimulant amphetamine, and by N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA), an agent whose actions are not yet well understood. The optical isomers of two phenylisopropylamines known to lack DOM and amphetamine-stimulus character, that is N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminobutane (MBDB) and 1-(3,4-dimethoxyphenyl)-2-aminopropane (3,4-DMA), were examined in rats trained to discriminate 1.25 mg/kg of PMMA from vehicle. The PMMA stimulus (ED(50)=0.4 mg/kg) generalized to all four agents: S(+)-MBDB (ED(50)=0.8 mg/kg), R(-)-MBDB (ED(50)=2.0 mg/kg), S(+)-3,4-DMA (ED(50)=2.6 mg/kg) and R(-)-3,4-DMA (ED(50)=3.9 mg/kg). The results show that these agents produce stimulus effects similar to those produced by PMMA. Both isomers of MBDB have been previously demonstrated to substitute for N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) in rats trained to discriminate MDMA from vehicle, but MBDB-trained animals failed to recognize DOM or amphetamine. Similar results were obtained with the 3,4-DMA optical isomers in the present investigation using rats trained to discriminate MDMA, DOM or (+)-amphetamine from vehicle; both isomers of 3,4-DMA substituted for an MDMA stimulus, but not for a DOM or amphetamine stimulus. Taken together, the evidence suggests that PMMA, S(+)-MBDB, R(-)-MBDB, S(+)-3,4-DMA, R(-)-3,4-DMA, and S(+)-MDMA can produce common stimulus effects in rats. The present findings also better define the PMMA stimulus and the structural requirements necessary to produce this type of stimulus effect.

DOM 2,5-Dimethoxy-4-Methylamphetamine↗
PubMed1969

Stimulus generalization of gravity.

In two experiments, squirrel monkeys were exposed to centrifugally generated, artificial gravity and trained to respond for food reinforcement at selected gravity (g) levels. Experiment I involved a single g value; in Exp. II, subjects were trained to discriminate among two or three g values. After training, generalization tests were administered over a 1.1-g to 2.1-g range. The major findings were as follows: (a) single-stimulus training yielded a linear relationship between percentage of responding and magnitude of gravity. (b) Two-valued discrimination training produced gradient peaks which were shifted from S(D) in a direction away from S(Delta). This effect was cancelled when S(D) was located equidistant between two S(Delta) stimuli. (c) Gradient form was independent of the S(D)-S(Delta) difference, but related to continuum location and/or intensity of discriminative stimuli.

Animals↗
PubMed1967

Stimulus generalization as signal detection in pigeons.

Pigeons that were reinforced for pecking at a single-wavelength responded to nearby wavelengths with lower rates. Response rates indicated the pigeons' certainty that the reinforced stimulus was present. The ratings yielded receiver operating characteristic functions that approximated straight lines on a double probability plot.

Animals↗
PubMed1978

The central tendency effect in stimulus generalization: the effect of sex of subject.

Eighty male and 80 female college students were shown a stimulus light of 525 nm, following instructions to remember it, and then were immediately tested for generalization (recognition) with a successively presented set of stimuli including 525 nm and six longer wavelength values. Subjects rated each test stimulus on a 6-point scale ranging from "surely same" as the original stimulus to "surely different." Both men and women showed a central tendency shift. Although the stimulus perceived as most likely to be the original (the mode) shifted to 545 nm in both groups, the men showed a greater mean shift than the women. Shifting was nearly complete in both groups within the first series of test stimuli with the men shifting faster. These findings replicate a 1972 report by Giurintano of greater central tendency shift in men than in women in a line angle generalization task, challenging an assumed analogy between our generalization task and the rod-and-frame test, in which women typically show the greater distortion. The analysis of central tendency shift within the first test series reveals that it occurs much earlier than previously believed.

Discrimination, Psychological↗
PubMed1982

Stimulus generalization of lifted weights in older and younger women.

Older (62 to 86 years) and younger (17 to 30 years) women were essentially similar in acquisition of weight discriminations and in generalized responding to a range of weights during postdiscrimination tests. In both age groups, acquisition was facilitated by increases in the difference between the positive and negative stimuli, and postdiscrimination gradients were displaced away from the value of the negative stimulus, particularly when it was the heavier of the training pair. The finding from personality test scores that the older women were more introverted than the younger ones, together with the absence of age differences in acquisition and generalization, does not support the Eysenck-Gray hypothesis that introversion influences conditioning processes. The results do suggest that basic conditioning processes are maintained with increasing age in healthy older adults.

Adolescent↗
PubMed1994

Electrical stimulation of the dorsal raphe nucleus as a discriminative stimulus: generalization to (+/-)-DOI.

Electrical stimulation of the dorsal raphe nucleus of Sprague-Dawley rats was used as the cue for discrimination using a taste aversion paradigm. Rats were trained to associate saccharin drinking during electrical stimulation of the dorsal raphe nucleus with LiCl injection after the session as the aversive unconditioned stimulus. In sessions without stimulation, rats were allowed to consume saccharin and received a saline injection after the session. Suppression of saccharin consumption during electrical stimulation was learned within 12 trials. Rats trained in the reverse discrimination, i.e., sessions with no electrical stimulation paired with LiCl injection, showed a similar learning curve. Animals injected prior to the session with the hallucinogenic 5-HT2 agonist (+/-)-DOI associated DOI with electrical stimulation of the dorsal raphe nucleus. Thus, animals may be trained to discriminate electrical stimulation of the dorsal raphe nucleus. Furthermore, animals generalize from activation of 5-HT2 receptors to electrical stimulation of the dorsal raphe nucleus.

Amphetamines↗
PubMed1988

Limited stimulus generalization between delta 9-THC and diazepam in pigeons and gerbils.

Pigeons trained to discriminate between the presence and absence of delta 9-THC (0.56 mg/kg, IM, intramuscularly) were tested with diazepam (5.6-30 mg/kg), administered IG (intragastrically), and were found to generalize the response associated with the THC training to the diazepam treatment only to a limited extent. Two groups of gerbils trained to discriminate between the presence and absence of IP (intraperitoneally) injected benzodiazepine agonists (5.6 mg/kg of diazepam and Ro 11-3128, respectively) generally did not generalize the drug response to IP administered THC (5.6 and 17.5 mg/kg). In addition, substitution testing with the dopamine autoreceptor blocker (+)-3PPP yielded non-drug responding in the pigeons. Hence neither proposed structural similarity between 3-PPP and THC, nor purported anxiolytic activity by 3-PPP (see Introduction) matched the THC-induced stimulus effects. The data are discussed with reference to the specificity of the THC cue or stimulus.

Animals↗
PubMed1966

Stimulus generalization: the ordering and spacing of test stimuli.

Twenty-four pigeons learned a successive discrimination between 500 mmu (S+) and 574 mmu (S-). When tested in extinction, some birds received stimuli around S+, with no S- presentations. These birds showed a positive peak shift, with maximum responding not at 550 mmu, but displaced to 538 mmu and 544 mmu. Other birds were tested with stimuli around S-, with no S+ presentations. These birds showed a negative shift, with least responding not at 574 mmu, but at 586 mmu. Though the first group was tested around S+ and the second around S-, total responding between groups did not differ. When retested on the other half of the continuum, however, birds that had gone from the S+ half to the S- half responded fewer times than those that had gone from the S- half to the S+ half. In a second experiment, reducing stimulus spacing from 6 mmu to 2 mmu produced flatter gradients and decreased the amount of positive shift. In a third experiment, birds were tested across the whole continuum with stimuli presented in serial order. A sequence from 538 mmu to 586 mmu produced no responding after the first part of the session; a sequence from 586 mmu to 538 mmu produced responding throughout the session.

Animals↗
PubMed1975

Stimulus generalization of suppression in rats following aversively motivated instrumental or Pavlovian training.

Following 100 or 300 avoidance training trials, instrumental subjects and their yoked Pavlovian counterparts were tested for generalization of lick suppression along the frequency dimension of the avoidance conditioned stimulus. Gradients of stimulus control were evident after 300 instrumental avoidance training trials, and additional intradimensional Pavlovian discrimination training further sharpened the gradients. After 100 trials, the yoked Pavlovian subjects suppressed more than their instrumental counterparts. However, with increased Pavlovian training, flatter gradients with decreased suppression were obtained. Results from a second experiment revealed that, whereas Pavlovian experience decreased suppression to the tone, subjects suppressed drinking in the presence of static, environmental cues. Data from both experiments supported interpretations that stress the role of response control over environmental events.

Acoustic Stimulation↗
PubMed1962

Instructions and stimulus categorizing in a measure of stimulus generalization.

In Experiment I, three groups of 20 Ss each were exposed to a light of 550 mmu (yellowish-green) for 60 sec and then viewed a random sequence of wavelengths with instructions to respond only to the original color. The instructions given the three groups were worded differently in an attempt to vary the strength of a set-to-discriminate assumed to be created by this procedure. The three groups produced similar gradients, each with a peak of responding at 540 mmu, in agreement with Kalish's (1958) published gradient for the 550 mmu standard stimulus value. It was suggested that the nature of the task is such that a strong discriminatory set is produced regardless of the wording of the instructions.A temporal analysis of the gradient as it develops during the testing revealed that initially the peak of responding occurs at 550 mmu; but as testing progresses, it shifts gradually in the direction of the shorter wavelengths (purer greens). Experiment II was performed to test the generality of the phenomenon of regression to the primary color. Two groups of 20 Ss each were tested for generalization following exposure to 510 mmu (bluish-green) and 525 mmu (pure green), respectively. We predicted that the 510 mmu gradient would reveal a progressive shift toward the longer wavelengths (purer greens), whereas the 525 mmu gradient would show no tendency to shift. The results were strikingly in accord with these predictions. We concluded that although a physiological process could not be ruled out, the verbal labeling of the standard stimulus value may well be responsible for the regression of the gradient toward the primary color.

Color↗
PubMed1982

Discriminative stimulus effects of pentobarbital in rhesus monkeys: tests of stimulus generalization and duration of action.

Rhesus monkeys were trained to emit 20 or 30 consecutive responses on one lever following an IM injection of pentobarbital (10 or 18 mg/kg) and the same number of consecutive responses on another lever following an injection of saline. The required number of correct consecutive responses in both cases resulted in food delivery. When responding was reliably under the control of the presession injection, the ability of a variety of other compounds to produce pentobarbital-appropriate responding was examined. Diazepam, clobazam, methohexital, pentobarbital, and phenobarbital, given 10 or 20 min before the session, produced dose-related pentobarbital-appropriate responding in each monkey. Ethylketazocine and dextromethorphan produced responding primarily on the saline-appropriate lever, whereas codeine, cyclazocine, dextrorphan, and ketamine resulted in responding that was, on the average, intermediate between that appropriate for pentobarbital and that appropriate for saline. When tested at various times after their injection, methohexital (3.2 mg/kg) and pentobarbital (10 mg/kg) produced pentobarbital-appropriate responding within 10 min. Barbital (56 mg/kg) resulted in pentobarbital-appropriate responding only if at least 1 h intervened between the injection and the experimental session. The discriminative effects of methohexital, pentobarbital, and barbital lasted approximately 20-60, 120-240, and 480-720 min, respectively. The time-course of the discriminative stimulus effects of barbiturates in the rhesus monkey appears to parallel closely other pharmacological actions of these compounds.

Animals↗
PubMed2002

Stimulus generalization by fenfluramine in a quipazine-ketanserin drug discrimination is not dependent on indirect serotonin release.

The purpose of this study was to determine if animals trained to discriminate a serotonin2A (5-HT2A) receptor agonist from a 5-HT2A receptor antagonist would also be sensitive to alterations in serotonin neurotransmission brought about by 5-HT reuptake inhibitors and releasers. Previous work from our laboratory has shown that the quipazine-ketanserin discrimination is mediated solely by the 5-HT2A receptor, thus providing a behavioral continuum of 5-HT2A receptor function. Rats were trained to discriminate quipazine (0.35 mg/kg) from ketanserin (1.0 mg/kg) on a variable interval-30 schedule of reinforcement. Following acquisition, substitution tests were conducted with the training drug, quipazine, and agents that have been shown to alter the synaptic levels of 5-HT, including fenfluramine, norfenfluramine, 5-methoxy-6-methyl-2-aminoindan (MMAI) and fluoxetine. All compounds substituted, except fluoxetine. Antagonist tests with mianserin and MDL 100,907 indicated that fenfluramine's and MMAI's substitution for quipazine was mediated by the 5-HT2A receptor. Animals were pretreated with PCPA to determine whether 5-HT release or direct agonism mediated the discriminative stimulus effects of fenfluramine and MMAI. PCPA blocked the substitution of MMAI but not of fenfluramine for quipazine. Analysis of 3H-IP formation in cells showed that norfenfluramine dose-dependently stimulated phosphoinositide hydrolysis to levels similar to that of serotonin and quipazine. These results indicate that fenfluramine's substitution for quipazine in rats trained on a quipazine-ketanserin discrimination are due to direct agonism at the 5-HT2A receptor likely mediated by norfenfluramine, an active metabolite.

Animals↗
PubMed1999

Stimulus generalization, context change, and forgetting.

Forgetting is often attributed to retrieval failure caused by background contextual cues changing over time. However, generalization between stimuli may increase over time and make them increasingly interchangeable. If this effect occurs with contextual cues, it might cancel any effect of a changing context. The authors review the evidence and suggest a resolution of this paradox. Although generalization gradients can change over time, the effect is not always strong. Increased responding to nontarget stimuli is not often shown, and few studies have demonstrated such changes with contextual cues in a way that rules out other interpretations. Even this example of forgetting may be caused by retrieval failure. The physical contexts manipulated in learning and memory experiments themselves occur within a superordinate temporal context and can thus be forgotten with no inherent challenge to a context-change account of forgetting.

Animals↗
PubMed1993

Incomplete stimulus generalization from a mixture of d-amphetamine and morphine to different doses of the component drugs.

Pigeons were trained to discriminate a mixture of 1.8mg/kg morphine plus 1.0mg/kg d-amphetamine from saline, and then tested for generalization to various doses of d-amphetamine and morphine, alone and in combination. The birds discriminated the drug mixture from saline with more than 90% responding on the drug key. The training doses of morphine (1.8mg/kg) and d-amphetamine (1.0mg/kg), as well as lower doses of these drugs, did not reliably generate responding on the drug key when given in combination with saline. Higher doses of morphine and d-amphetamine did generate responding on the drug key, but not as reliably as the training dose combination. Combination of the training dose of morphine with doses of d-amphetamine higher than those in the training dose combination resulted in somewhat less responding on the drug key than that seen with the training dose combination in one bird. These data and those from other experiments where animals were trained to discriminate drug mixtures can be characterized as the net effect of stimuli produced by the component drugs and interactions between the component drugs.

Journal Article↗