Days Of Acquisition Training Research Articles

Reinforcing Effects of Fentanyl and a Fentanyl Analog Found in Illicit Drug Markets

HYPOTHESISIllicitly manufactured fentanyl and fentanyl analogs are the main drivers of the worsening opioid epidemic. Although the pharmacology of fentanyl is well characterized, there is little information about the reinforcing effects of newly emerging fentanyl analogs that are contributing to opioid overdose. Here, we compare the effects of fentanyl and the clandestine fentanyl analog, cyclopropylfentanyl, on drug self‐administration behavior in male and female rats.METHODSSprague‐Dawley rats fitted with intravenous (i.v.) jugular catheters were placed in experimental chambers containing two nose poke holes. Active nose poke responses resulted in delivery of a constant volume of drug (0.2 mL) over a period of 2 s on a fixed‐ratio 1 schedule, followed by a 20 s timeout. Acquisition doses for the compounds were 0.01 mg/kg/inj for fentanyl and cyclopropylfentanyl, while 0.1 mg/kg/inj was used for heroin. Following 10 days of acquisition training, dose‐effect testing began with three additional doses, each of which were tested for 3 days. Subjects then returned to the original training dose until responses stabilized and underwent saline extinction.RESULTSAcquisition of fentanyl self‐administration was achieved by both male and female rats, with significant differences between active versus inactive nose pokes for both sexes over the last 4 days of training (p<0.05). A similar profile of acquisition was seen for heroin and cyclopropylfentanyl. Both fentanyl and cyclopropylfentanyl showed a typical inverted‐U dose‐effect function during dose‐effect testing. Maximal response rates were similar among all drugs, with fentanyl showing maximum responding at 0.001 mg/kg/inj, cyclopropylfentanyl at 0.003 mg/kg/inj, and heroin at 0.003 mg/kg/inj.CONCLUSIONBoth male and female rats showed comparable rates of fentanyl acquisition. Further comparison between heroin, fentanyl, and cyclopropylfentanyl show that these compounds support self‐administration, indicating that emerging fentanyl analogs display significant abuse liability and may contribute to compulsive use in humans.

Reinforcing Effects of Phenethylamine Analogs Found in Dietary Supplements

HYPOTHESIS Phenethylamine analogs are often found in dietary supplements, even though they are rarely listed on the ingredient label. Due to their structural similarities to amphetamine, it is anticipated that these compounds display comparable pharmacological effects to amphetamine and other stimulants. Here, we compared the effects of amphetamine and selected phenethylamine analogs on drug self-administration behavior in rats. METHODS Male Sprague-Dawley rats fitted with intravenous (i.v.) jugular catheters were placed in experimental chambers containing two nose poke holes. Active nose poke responses resulted in delivery of a constant volume of drug (0.2 mL) over a period of 2 s on a fixed-ratio 1 schedule, followed by a 20 s timeout. Acquisition doses for the compounds were 0.1 mg/kg/inj amphetamine, 1 mg/kg/inj a-ethylphenethylamine (AEPEA) and N,a-diethylphenethylamine (DEPEA), and 3 mg/kg/inj b-methylphenethylamine (BMPEA). Following 10 days of acquisition training, dose-effect testing began with three additional doses, each of which were tested for 3 days. Subjects then returned to the original training dose until responses stabilized. RESULTS At the end of acquisition training, all four compounds showed a significant difference in active versus inactive nose pokes (p<0.0001), indicating that subjects acquired drug self-administration. During dose-effect testing, all the drugs showed a typical inverted dose-effect function, although there were clear differences in potency. Maximum responding was maintained at 0.03 mg/kg/inj for amphetamine, 0.3 mg/kg/inj for DEPEA and AEPEA and over 1 mg/kg/inj for BMPEA. AEPEA and DEPEA appeared to maintain higher rates of responding than amphetamine, potentially reflecting differences in duration of action. Self-administration of BMPEA occurred despite BMPEA showing minimal locomotor stimulating effects in a previous experiment. CONCLUSION Direct comparisons between the three phenethylamine analogs and amphetamine show that these compounds support self-administration, indicating that dietary supplements containing these compounds could have significant abuse liability.

Impaired cerebellar plasticity and eye-blink conditioning in calpain-1 knock-out mice.

Calpain-1 and calpain-2 are involved in the regulation of several signaling pathways and neuronal functions in the brain. Our recent studies indicate that calpain-1 is required for hippocampal synaptic plasticity, including long-term depression (LTD) and long-term potentiation (LTP) in field CA1. However, little is known regarding the contributions of calpain-1 to cerebellar synaptic plasticity. Low frequency stimulation (LFS, 5 Hz, 5 min)-induced LTP at parallel fibers to Purkinje cell synapses was markedly impaired in cerebellar slices from calpain-1 knock-out (KO) mice. Application of a selective calpain-2 inhibitor enhanced LFS-induced LTP in both wild-type (WT) and calpain-1 KO mice. Three protocols were used to induce LTD at these synapses: LFS (1 Hz, 15 min), perfusion with high potassium and glutamate (K-Glu) or dihydroxyphenylglycine (DHPG), a mGluR1 agonist. All three forms of LTD were impaired in calpain-1 KO mice. DHPG application stimulated calpain-1 but not calpain-2 in cerebellar slices, and DHPG-induced LTD impairment was reversed by application of a protein phosphatase 2A (PP2A) inhibitor, okadaic acid. As in hippocampus, BDNF induced calpain-1 activation and PH domain and Leucine-rich repeat Protein Phosphatase 1/suprachiasmatic nucleus oscillatory protein (PHLPP1/SCOP) degradation followed by extracellular signal-regulated kinase (ERK) activation, as well as calpain-2 activation leading to degradation of phosphatase and tensin homolog deleted on chromosome ten (PTEN) in cerebellar slices. The role of calpain-1 in associative learning was evaluated in the delay eyeblink conditioning (EBC). Calpain-1 KO mice exhibited significant learning impairment in EBC during the first 2 days of acquisition training. However, after 5 days of training, the percentage of conditioned responses (CRs) between calpain-1 KO and WT mice was identical. Both calpain-1 KO and WT mice exhibited typical extinction patterns. Our results indicate that calpain-1 plays critical roles in multiple forms of synaptic plasticity and associative learning in both hippocampus and cerebellum.

Lasting impact on memory of midlife exposure to exogenous and endogenous estrogens.

We previously demonstrated that 40 days of prior midlife estradiol treatment results in enhanced spatial memory in aging ovariectomized rats long after termination of the estradiol treatment. Our current goal was to determine whether this benefit is due to lasting impacts on memory specifically of previous exogenous estradiol treatment or simply due to delaying cognitive deficits that occur following loss of ovarian hormones. Middle-aged rats were ovariectomized or underwent sham surgery. Ovariectomized rats received estradiol (Previous Estradiol) or vehicle (Previous Vehicle) implants. Rats undergoing sham surgery (Previous Intact) received vehicle implants. Forty days later, Previous Intact rats were ovariectomized, the other 2 groups underwent sham surgeries, and all implants were removed. Thus, no ovarian or exogenously administered hormones were present during behavior testing. Rats underwent 24 days of acquisition training on an 8-arm radial maze. Following acquisition and again 2 months later, rats were tested on delay trials, during which animals had to remember the location of food rewards across time delays inserted between fourth and fifth arm choices. During acquisition, rats that had previous extended exposure to exogenous estradiol (Previous Estradiol) and endogenous ovarian hormones (Previous Intact) significantly outperformed rats that did not experience extended hormone exposure (Previous Vehicle). However, during delays trials the Previous Estradiol group significantly outperformed both the Previous Vehicle and Previous Intact groups. Results demonstrate that whereas extended exposure to endogenous ovarian hormones may provide short-term cognitive benefits, midlife estradiol treatment following ovariectomy provides additional benefits that persist for months following termination of treatment. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

EFFECTS OF LONG-TERM ADMINISTRATION OF COOKED BEANS (Vigna unguiculata) DIET ON LEARNING AND MEMORY IN MICE

Purpose: The neurotransmitter, serotonin has neurobehavioural actions which include mood, memory, learning and sleep. Beans, the stable diet of Nigerians contains serotonin and its precursor, 5-Hydroxytryptophan in significant amounts. It was therefore the aim of this study to find out whether long term consumption of cooked beans (Vigna unguiculata) diet has effects on some neurobehavioural parameters notably; learning and memory, using Swiss white mice as experimental animals.Method: Thirty (30) CD1 mice were randomly assigned into three groups, viz; (10, control, 20 Test) weighing 15-30g were fed normal rodent chow, 50% cooked beans diet (w/w) respectively and serotonin precursor (5-HTP) diet (0.2mg/50g w/w) for thirty days. All the mice had access to clean drinking water ad libitum. Before the neurobehavioural parameters were assessed, the phytochemical analysis of the beans, LD50 of the beans (Vigna unguiculata) and that of the serotonin precursor (5-HTP) were determined. Serotonin concentration was measured in the beans using gas chromatography analysis. Learning and memory, was investigated alongside food and water intake and body weight change. Involvement of serotonin pathway was investigated using the set of mice administered serotonin precursor for comparison with the beans diet fed mice. Mice were tested in the Morris water maze for 8 days: 3 days of acquisition training, 3 days of reversal training, 1 day of single probe trial and 1 day for visible platform task.Results: Learning on days 1, 2 &amp; 3 of acquisition training was improved in the cooked beans and serotonin precursor diet-fed mice when compared to control (p&lt;0.001, respectively). The trend was similar during the reversal training, memory was improved in the beans and serotonin precursor diet-fed mice when compared to control (p&lt;0.001). During the probe trial, the swim duration in the South-East quadrant was significantly higher for cooked beans and serotonin precursor group compared to control (p&lt;0.05). However, during the visible platform task, the swim latencies for the cooked beans and serotonin precursor group was significantly lower compared to control. Our finding therefore suggests that long term consumption of beans diet enhances learning and memory. Conclusion and Recommendation: In conclusion, long term consumption of cooked beans diet enhances learning and memory. If this research can be extrapolated to humans, we recommend that the serotonergic potential of beans may be harnessed in the prevention, treatment and management of behaviour/ mental disorders as this could potentially open up a vista for improved mental health status in populations globally.

Effects of low level lead exposure on associative learning and memory in the rat: Influences of sex and developmental timing of exposure.

Lead (Pb) exposure during development impairs a variety of cognitive, behavioral and neurochemical processes resulting in deficits in learning, memory, attention, impulsivity and executive function. Numerous studies have attempted to model this effect of Pb in rodents, with the majority of studies focusing on hippocampus-associated spatial learning and memory processes. Using a different paradigm, trace fear conditioning, a process requiring coordinated integration of both the medial prefrontal cortex and the hippocampus, we have assessed the effects of Pb exposure on associative learning and memory. The present study examined both female and male long evans rats exposed to three environmentally relevant levels of Pb (150 ppm, 375 ppm and 750 ppm) during different developmental periods: perinatal (PERI; gestation-postnatal day 21), early postnatal (EPN; postnatal days 1-21) and late postnatal (LPN; postnatal days 1-55). Testing began at postnatal day 55 and consisted of a single day of acquisition training, and three post training time points (1, 2 and 10 days) to assess memory consolidation and recall. All animals, regardless of sex, developmental window or level of Pb-exposure, successfully acquired conditioned-unconditioned stimulus association during training. However, there were significant effects of Pb-exposure on consolidation and memory recall at days 1-10 post training. In females, EPN and LPN exposure to 150 ppm Pb (but not PERI exposure) significantly impaired recall. In contrast, only PERI 150 ppm and 750 ppm-exposed males had significant recall deficits. These data suggest a complex interaction between sex, developmental window of exposure and Pb-exposure level on consolidation and recall of associative memories.

Central cannabinoid receptors modulate acquisition of eyeblink conditioning

Delay eyeblink conditioning is established by paired presentations of a conditioned stimulus (CS) such as a tone or light, and an unconditioned stimulus (US) that elicits the blink reflex. Conditioned stimulus information is projected from the basilar pontine nuclei to the cerebellar interpositus nucleus and cortex. The cerebellar cortex, particularly the molecular layer, contains a high density of cannabinoid receptors (CB1R). The CB1Rs are located on the axon terminals of parallel fibers, stellate cells, and basket cells where they inhibit neurotransmitter release. The present study examined the effects of a CB1R agonist WIN55,212-2 and antagonist SR141716A on the acquisition of delay eyeblink conditioning in rats. Rats were given subcutaneous administration of 1, 2, or 3 mg/kg of WIN55,212-2 or 1, 3, or 5 mg/kg of SR141716A before each day of acquisition training (10 sessions). Dose-dependent impairments in acquisition were found for WIN55,212-2 and SR141716A, with no effects on spontaneous or nonassociative blinking. However, the magnitude of impairment was greater for WIN55,212-2 than SR141716A. Dose-dependent impairments in conditioned blink response (CR) amplitude and timing were found with WIN55,212-2 but not with SR141716A. The findings support the hypothesis that CB1Rs in the cerebellar cortex play an important role in plasticity mechanisms underlying eyeblink conditioning.

Early methylphenidate exposure enhances cocaine self-administration but not cocaine-induced conditioned place preference in young adult rats

Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration. The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms. Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal days (PDs) 11 to 20, and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included 8 days of acquisition training, 8 days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10, or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) one schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for 5 days. Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats. These data suggest that in males, methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.

The effects of methamphetamine exposure during preadolescence on male and female rats in the water maze

Exposure to methamphetamine early in life can have lasting effects on cognitive processes. The maturation of neurotransmitter systems targeted by methamphetamine differs by gender during childhood and preadolescence, which could lead to differential long-term effects of early drug exposure. Therefore, the current study assessed whether preadolescent exposure to methamphetamine has gender specific long-term effects on adult spatial memory in rodents. Male and female rats were given 1 daily injection of 0 or 2 mg/kg methamphetamine or not handled from PD21–35 and then tested as adults (PD95) in the Morris water maze. In general, male rats performed better than female rats in the water maze task regardless of treatment group. Female rats exposed to methamphetamine from PD21–35 had shorter latencies and took more direct paths to the hidden platform compared to control females during the 4 days of acquisition training and when the hidden platform was moved each day on matching to place trials. Male rats exposed to methamphetamine swam a shorter distance to the hidden platform on the first day of acquisition training, similar to the methamphetamine exposed females. However, the methamphetamine exposed males performed more poorly compared to control males in the matching to place trials. Overall, the current study found that methamphetamine exposure during preadolescence has long-term effects on spatial memory in a gender specific manner. These findings may contribute to our general understanding of the long-term effects of psychostimulant exposure at early developmental stages.

A meta-analysis of animal studies on disruption of spatial navigation by prenatal cocaine exposure

Water-maze testing has been used to assess prenatal cocaine (PCOC)-induced deficits in behavioral studies of spatial navigation and memory abilities. Effects of PCOC in acquisition or in probe trials over water-maze testing days were rarely detected. Despite an absence of effects of PCOC when data were collapsed over multiple days, there was a potential difference when examined during the first day of acquisition training, characterized by a PCOC-associated decrease in learning efficiency but not capacity. Here, we review studies of PCOC-related changes in day-1 water-maze acquisition training and examine the relationship between experimental methodologies and PCOC-treatment procedures and the variability in effect size estimates across studies. The results revealed a significant increase in latencies to goal platform on acquisition training day-1 in PCOC-exposed offspring vs. controls (effect size: r = 0.44). Significant effects attributable to variations in the PCOC-treatment procedures across studies were also identified. The moderating variable of PCOC “dose” was significant as lower doses of PCOC exposure yielded larger treatment effects. “Duration” of PCOC exposure was not significant, although a trend for greater effects was observed in studies that employed longer daily treatment schedules or schedules administered in later gestational periods. This analysis identified a consistent difference in acquisition training day-1 of water-maze testing in PCOC-exposed offspring indicating a PCOC-induced deficiency in spatial learning. These findings of impaired spatial learning efficiency are of particular interest given clinical scenarios involving acutely impaired spatial memory and related learning in PCOC-exposed children that highlight the potential consequences in classroom learning.

Effects of Subanesthetic Concentrations of Isoflurane and Their Interactions with Epinephrine on Acquisition and Retention of the Rabbit Nictitating Membrane Response

Evidence concerning the concentrations of volatile anesthetics that prevent learning and recall is limited. Epinephrine is believed to enable learning during anesthesia. We investigated the effects of isoflurane and its interaction with epinephrine on learning and subsequent retention of the rabbit's classically conditioned nictitating membrane response. In experiment 1, a tone (conditioned stimulus, CS) preceded paraorbital shock (unconditioned stimulus, US) during 60-min daily sessions of 60 presentations of these paired stimuli for 6 days of acquisition training under 0, 0.4%, or 0.8% isoflurane (n = 8, 13, and 9, respectively). Responses were recorded as conditioned responses (CRs) if they occurred during the CS and before the onset of the US. After 1 day of rest, the animals were given 3 days of extinction consisting of 60 presentations of CS-alone and without isoflurane to assess the retention of CRs from acquisition training. In experiment 2, epinephrine in a dose of 0, 0.01, or 0.1 mg/kg was injected subcutaneously in rabbits receiving 0.4% isoflurane. Two types of epinephrine were used, a sustained release form and epinephrine hydrochloride. Acquisition and retention were tested in the same way as in experiment 1. No isoflurane or epinephrine was used during retention testing. Learning was significantly suppressed during 0.4% isoflurane (approximately equal to 0.2 MAC) treatment and eliminated during 0.8% (approximately equal to 0.4 MAC). Information learned during administration of 0.4% isoflurane was not retained (P < 0.05). Although the low doses of epinephrine improved learning during the last day of the acquisition phase (P < 0.05), there were no differences between the treatment groups on any of the remaining acquisition or extinction days. There was no learning during treatment with 0.8% concentration. Even a 0.4% concentration, which allowed some learning, abolished CRs in extinction, perhaps because of state-dependent retrieval. Epinephrine did not alter substantially the rates of CR acquisition or resistance to extinction.

Sympathoadrenal effects on extinction of rewarded running in the rat

Two groups of rats underwent adrenal demedullation or sham surgery before being trained to run in a straight runway for food reward at a short intertrial interval (90 s). Half of each group of animals were then injected IP with the sympathetic neurotoxin, 6-hydroxydopamine HBr (6-OHDA; 30 mg/kg); the remainder received saline vehicle. After 3 more days of acquisition training, running was extinguished over 12 trials in all rats. Running times were similar in all groups at the start of extinction. In saline-injected animals, extinction proceeded more slowly in the demedullated than in the sham-operated group, but this effect was largely abolished by 6-OHDA. Consistent with previous results, there was evidence that 6-OHDA facilitated extinction, irrespective of adrenal demedullation, but this effect was most apparent in the time taken to emerge from the start-box. The rats were killed two days after the end of the experiment; cardiac noradrenaline levels were depleted by 83% of control values in 6-OHDA-treated animals. The results suggest that the adrenal medulla and sympathetic neurones have opposing and interacting effects on the response to nonreward.

Haloperidol and nonreinforcement produce different patterns of response slowing in a food reinforced runway task

Rats were trained to traverse a runway for food reward and speeds were measured in the start, run and goal segments of the alley. After eight days of acquisition training, subjects were tested for four days under conditions of either extinction or haloperidol pretreatment. Although both haloperidol and extinction produced a suppression of the running response, the pattern of this suppression in the three alley segments was quite different for the two conditions. Haloperidol tended to be more effective than extinction in slowing start speeds but less effective than extinction in reducing run and goal speeds. This differential effect of haloperidol and extinction on speeds in different alley segments provides further evidence that haloperidol-induced impairments in performance cannot result entirely from a blunting of primary reinforcement.

Further evidence for a dissociation of peripheral and central effects of vasopressin

The influence of AVP on open field behavior was studied at 15 and 60 min after s.c. injection. AVP injected 15 min before the session modified all patterns of behavior in the open field. Locomotion and rearing along the wall were significantly decreased, while locomotion in the center was increased. AVP also reduced grooming and defecation. Injection 60 min prior to the session did not affect the open field behavior. AVP increased resistance to extinction of pole-jumping avoidance behavior independent of whether the first acquisition session was given at 20 min after injection at the height of the behavioral depression or at 65 min when these effects had disappeared. However, the rats trained at 20 min after AVP administration made significantly more avoidances on the second day of acquisition training than did the controls. The vasopressin fragment DGAVP, which exhibits almost no peripheral effects, did not cause gross behavioral changes in the open field as did AVP, but it did increase resistance to extinction of the avoidance response comparable to that of AVP. These findings indicate that, although peripheral effects may contribute to the effects of AVP on modulation of memory processes, such effects are not essential.

Effects of US parameters on classical conditioning of cat hindlimb flexion

Unconditioned stimulus (US) intensity and duration were manipulated to determine their effects on cat hindlimb flexion conditioning. Seven consecutive days of acquisition training of a hindlimb flexor response to a tone conditioned stimulus (CS) were followed by 2 days of extinction. Eight animals in each of 12 groups received a 1-, 2-, 3-, or 4-mA, 60-Hz shock delivered to the right hindleg for 25, 50, or 100 msec as the US. Analysis of conditioned-response (CR) frequency indicated that conditioned responding was a positive function of both the intensity and duration of shock, although these variables did not interact with one another. CR latency and amplitude were decreased and increased, respectively, by increases in US intensity. The pattern of results reported here may support a contiguity notion of conditioning, and are discussed in the context of other conditioning preparations.

Wild caught vs inbred mouse strains: Differences in wheel-turn avoidance learning

Three Bar Harbor inbred mouse strains; C57BL/6J, C3H/HeJ, and DBA/2J, and two wild caught but laboratory bred Peromyscus strains; P. leucopus and P. maniculatus bairdi, were compared over 8 days of acquisition training on a wheel-turn avoidance task. In the acquisition of the conditioned avoidance response, the Bar Harbor inbreds did not differ but were, as a group, significantly better than the wild caught strains. Differences in activity rate were also apparent, but do not account for the differences in avoidance performance.

The effect of motivational states on ECS-induced perseveration

Forty-eight naive male albino rats were given 6 days of acquisition training in a straight runway. Following training on the first 5 days of acquisition, all Ss received 10 g of Lab Chow approximately 10 min after the experimental session. Upon completion of acquisition, Ss received either ECS or no treatment and were given 0, 10, or 20 g of food. All Ss were tested in extinction 24 h after treatment, and results showed that as motivational level decreased, the running speed of no-treatment Ss declined at a greater rate than did the running speed of Ss receiving ECS. This was interpreted in terms of the perseveration effect of ECS on the runway response.

Negative contrast as a function of the interval between preshift and postshift training

Rats were trained to lick either 32% or 4% sucrose solutions for 5 min per day. Following 10 days of acquisition training, the 32% rats were shifted to the 4% solution. A negative contrast effect was found with a 1-day but not with a 32-day retention interval interposed between the shift phases. An attempt to reinstate the memory of the 32% solution in the 32-day retention condition prior to the first postshift test failed to affect postshift licking behavior.

Modification of reinforcement aftereffects and level of training.

In the first of 2 experiments, 3 groups of 8 rats each were given 1 day of acquisition training in a runway under the same reinforcement schedule, RNNRNNRNNR. The groups were differentiated only on the basis of NT-length, which was regulated by placing Ss by hand in the baited goalbox during the intertrial interval following Trials 1, 4, 7 (Group N2-length), Trials 2, 5, 8 (Group N1-length), or Trials 3, 6, 9 (Group N0-length). on the day subsequent to acquisition training 10 N trials were administered. The second experiment was essentially a replication of the first, except it only included Groups N1 and N2, each of which contained 11 Ss. The finding that resistance to extinction was an increasing function of N-length was interpreted as supporting the modified aftereffects hypothesis and as presenting particular difficulties for some alternative views.

Latent inhibition: effects of frequency of nonreinforced preexposure of the CS.

It has been found that nonreinforeed preexposure to the to-be-conditioned stimulus results in a decrement in performance during the acquisition phase of a classical defensive leg response conditioning procedure. The present study shows that: (a) this decrement is a function of the frequency of preexposure, (b) the size of the decrement remains constant over 4 days of acquisition training, and (c) the decrement as a function of preexposure is not significant during extinction. In a previous experiment Lubow and Moore (1959) demonstrated a phenomenon which they labeled latent inhibition. This term describes the decrement in conditioning performance 'which results from the nonreinforeed preexposure of the to-be-conditioned stimulus. The purpose of the following study was to extend the earlier findings. In that study, experimental and control observations were made from the same &. That is, S was preexposed for 10 trials to either a flashing light or a moving rotor; following this, S was conditioned on alternate trials to the light and to the rotor. Reinforced acquisition training was started immediately after the preexposure trials, and continued until a predetermined criterion of 10 CRs was reached in that session. In the present study we were interested in the effect of frequency of nonreinforeed preexposure of the to-be-conditioned stimulus on acquisition and extinction performance; and in the duration of latent inhibition, both within an acquisition session and between sessions. Finally, we were interested in demonstrating the latent inhibition phenomenon within the context of a different experimental design in which different Ss were used for different groups.