CS-US Delay Research Articles

Timing of conditioned responses utilizing electrical stimulation in the region of the interpositus nucleus as a CS

A large body of evidence indicates that the cerebellum is essential for the acquisition, retention, and expression of the standard delay conditioned eyeblink response and that the basic memory trace appears to be established in the anterior interpositus nucleus (IP). Adaptive timing of the conditioned response (CR) is a prominent feature of classical conditioning-the CR peaks at the time of onset of the unconditioned stimulus (US) over a wide range of CS-US interstimulus intervals (ISI). A key issue is whether this timing is established by the cerebellar circuitry or prior to the cerebellum. In this study timing of conditioned eyeblink responses established via electrical stimulation of the interpositus nucleus as a conditioned stimulus (CS) was analyzed prior to and following modification of the CS-US interval in well-trained rabbits. Consistent with previous results, learning under these conditions is very rapid and robust. The CR peak eyeblink latencies are initially timed to the US onset and adjust accordingly to lengthening or shortening of the CS-US interval, just as with peripheral CSs. The acquisition of conditioned eyeblink responses by direct electrical stimulation of the IP as a CS thus retains temporal flexibility following shifts in the CS-US delay, as found in standard classical eyeblink conditioning procedures.

CS–US delay does not impair appetitive conditioning in Chasmagnathus

Habituation and appetitive conditioning have been already described in the crab Chasmagnathus. The purpose of this work is to study whether associative learning can be obtained despite a long conditioned stimulus–unconditioned stimulus interval. Results of the first experiment show that the weakening of temporal contiguity does not prevent appetitive conditioning to occur while after a long 4-h delay, conditioning wanes completely. A second experiment was conducted, after one and three days of training respectively, confirming the above results. Though initially neutral the context trace may be still available immediately after training and for the period of two but not after 4:00 h, demonstrating a forward limit for the conditioning window. After 3 days of training, a further decrease in the exploratory activity suggested that a longer training could increase the relative weight of habituation. Conditioning and habituation seem to work as opponent processes in the crab Chasmagnathus granulatus: if habituation training in the box is followed by the administration of reinforcement after a short period of time, appetitive conditioning will take place. However, as this interval is increased, habituation prevails. A persistent effect of the exposure to a given environment that may underlie trace conditioning in this crab is discussed in adaptive terms.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents — as well as many other species including man — learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS–US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS–US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras– MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.

What the rat’s nose tells the rat’s mouth: Long delay aversion conditioning with aqueous odors and potentiation of taste by odors

In Experiment 1, olfactory bulbectomized and control rats were trained using operant conditioning to determine the taste threshold of aqueous amyl acetate. Concentrations below gustatory threshold were used in Experiments 2–5 to compare the effectiveness of odors with various concentrations of saccharin as cues for illness. The results showed the following: (1) The effectiveness of odor and taste was directly related to concentration; (2) the strength of an aversion to a concentration of taste could be matched by an appropriate concentration of an odor; (3) odor was as effective as taste with CS-US delays of 4 h; and (4) an effective odor potentiated an aversion to an otherwise ineffective taste. The results challenge the privileged role accorded tastes in food aversion learning and the manner in which tastes are held to interact with odors according to the sensory-and-gate channeling analysis of potentiation (Rusiniak, Hankins, Garcia, & Brett, 1979).

The CS-US interval and taste aversion learning: A brief look.

Temporal parameters of taste aversion learning are known to differ markedly from other learning paradigms in that acquisition occurs despite lengthy delays between exposure to conditioned (CS) and unconditioned stimulus (US). Far less consideration has been paid to very brief CS-US intervals and the possibility that this learning may also be distinguished by an ineffectiveness of close temporal contiguity between CS and US. The effectiveness of a very brief CS-US interval (10 s) was compared with that of 2 lengthier intervals (15 and 30 min). Temporal control of CS delivery (0.15% saccharin solution) into the oral cavity and US delivery (7.5 mg/kg apomorphine hydrochloride) into circulation involved infusion pumps and indwelling catheters. Using a 1-trial learning paradigm, CS-US delays of 15 and 30 min led to significant aversions whereas the 10-s CS-US interval did not, suggesting that close temporal contiguity between CS and US is neither necessary nor sufficient for conditioned taste aversion acquisition.

Ontogenetic differences in the expresion of odor‐aversion learning in 4‐ and 8‐day‐old rats

Two experiments investigated the ability of preweanling rats, 4 or 8 days of age, to form an odor-LiCl association across various CS-US delays. The results of the odor preference test indicated that 8-day-old subjects acquired an aversion to the CS+ odor when trained with either a 0 or 15 min CS-US delay, while 4-day-old subjects did not exhibit a reduction in preference for the CS+ at any of the CS-US delays tested. The absence of a reduction in preference for the CS+ by 4-day-old rats could not be attributed to their failure to acquire the odor aversion, however; they avoided a texture with which the CS+ odor was paired in a second order conditioning paradigm. The results suggest that outcomes appearing to represent age-related differences in associative learning may in some instances be more appropriately viewed as representing ontogenetic differences in the way in which acquired associations are manifested in behavior.

The interstimulus interval and classical conditioning in the marine snail Hermissenda crassicornis

We examined the role of the interstimulus interval for the conditioned association between light and rotation stimuli in the marine snail Hermissenda. This interval between the conditioned stimulus (CS) and the unconditioned stimulus (US) is an important and widespread property of vertebrate associative learning. We demonstrated that with a forward CS-US delay of 1.0 s we were able to produce significant 24-h retention of an associative memory after 50 training trials. Other paired treatments providing intervals of 1.5 s, 0.5 s, simultaneous, and backward arrangements did not support retention at 24 h.

Sterch-based conditioned flavor preferences in rats: Influence of taste, calories and CS-US delay

Six experiments examined the rat's preference for flavors associated with hydrolysed starch (Polycose) solutions. Hungry rats developed a preference for a flavor (CS+) mixed into a saccharin-sweetened 16% Polycose solution over a flavor (CS-) mixed in saccharin-water. The addition of acarbose, a drug which inhibits starch digestion, to the Polycose solution blocked the flavor preference. This finding indicates that it is the postingestive rather than the taste properties of Polycose that condition flavor preferences. Rats also developed a preference for a CS+ flavor that preceded the ingestion of a Polycose solution by 10 min. This flavor conditioning, however, was not obtained in all experiments. Some CS flavors may be more effective than others in producing conditioned flavor preferences over a delay. Preference conditioning over a delay was facilitated when the rats had previously learned that the taste of the US (Polycose) was not associated with rapid caloric repletion; this was accomplished by giving the rats prior exposure to a Polycose + acarbose solution. Rats with such pre-exposure learned to prefer a CS + flavor that preceded by 10 to 60 min the consumption of a Polycose solution (without acarbose). The results demonstrate that calorie-based flavor preferences can be conditioned over a delay, but conditioning is influenced by the nature of the CS and US stimuli used.

Some parameters of conditioned immunosuppression: Species difference and CS-US delay

Three experiments were conducted in which an illness-inducing immunosuppressant, cyclophosphamide (an unconditioned stimulus, US) was associated with a previously presented saccharine solution conditioned stimulus (CS). In each experiment, reexposure to the CS produced a conditioned suppression of the plaque-forming-cell response in the experimental groups. Experiment I demonstrated this result with Fisher 344 rats. Experiment II replicated the effect with Balb/c mice. In Experiment III conditioned immunosuppression was demonstrated when mice received CS-US delays as long as 6 hours. No evidence of a delay gradient was present in either the behavioral or the immunologic data. These parallel findings offer no support for the idea of a dissociation between the taste aversion and conditioned immunosuppression processes.

Effects of CS concentration on long-delay taste aversion learning in preweanling and adult rats

The effect of sucrose solution (CS) concentration on toxiphobia conditioning under both immediate and long-delay CS—US intervals was investigated in preweanlings (18-day-old) and adult (60- to 80-day-old) rats. The main effect of delay interacted with age. With the immediate CS-US interval aversions by preweanlings and adults were equivalent but at the 1-hr delay performance was significantly poorer for the preweanlings. The effect of CS concentration among the immediate and 1-hr CS—US delay groups also depended upon the animal's age. Among adults, the more concentrated the CS the poorer the taste aversion. Floor effect at the short delays, however, may have produced this effect. When a 3-hr CS—US delay was employed, the more concentrated the CS, the better the aversion. Among infants, a U-shaped function best fit the data with the intermediate concentration producing the weakest taste aversion. These findings are discussed in terms of associative and nonassociative processes involved in long-delay learning.

A contingency analysis of taste aversion conditioning

The acquisition of conditioned taste aversion was assessed relative to five control procedures. That is, forward conditioning using multiple-trial, brief-duration taste CSs and weak USs over a 30-min CS-US delay was compared to backward, CS alone, US alone, sham CS and sham US, and random control procedures. The outcome supported an associative conditioning interpretation of the learned aversion. While there were no differences between the various control procedures, all were different from the forward conditioning group. The argument was made that some of the distinctive associative and nonassociative phenomena attributed to taste aversion conditioning (but not seen in the present study) may in part be due to the duration and intensity of both the CS and US events.

Comment on paper by Mitchell, Scott, and Mitchell

I have read with interest the paper by Mitchell et al. (1977) and the subsequent correspondence regarding this paper (Mitchell, 1977; Revusky, 1977). are several points missing from the paper and the correspondence which need attention. I submit evidence from our past research for consideration. We conducted several X-ray-induced tasteaversion studies using of rats where saccharin was paired with X-rays, saccharin was paired with sham exposure, water was paired with X-rays, or water was paired with sham exposure (e.g., Morris & Smith, 1964; Smith & Birkle, 1966; Smith & Morris, 1964; Smith, Morris, & Hendricks, 1964; Smith & Schaeffer, 1967). Always using a 24-48 two-bottle preference test, always analyzing preference scores and total fluid consumption, we found aversion in the saccharin-X-ray and no significant aversion in the other groups. Sometimes there was more variability in (Smith, et al. 1964, is an example of this-see Table 1), but I these were not different from the water-sham groups. Later, Scarborough's (e.g., McLaurin, Farley, Scarborough, & Rawlings, 1964; McLaurin & Scarborough, 1963; McLaurin, Scarborough, & Farley, 1964; Scarborough & McLaurin, 1964), began reporting taste aversions in groups. These results were puzzling to us for some time, but we think we subsequently understood his group's findings. In the Scarborough studies, the preference test was started immediately following the pairing of saccharin or water with X-ray. Therefore, we assumed that the aversion consequences of the X-rays were being paired with the saccharin available in the preference test. I think the paper by Schaeffer and me (1967) showed that assumption was correct. Once again, we saw no conditioning in or empty bottle-X-ray groups. Mitchell et al. (1977) correctly state that Roll and I (1967) did .not include noncontingently poisoned controls in our experiment, i.e., rats receiving treatment or no solution X-ray treatment. We did not because there was such a history in our laboratory of that control group's showing no evidence of conditioning. In retrospect, if we had run the noncontingent control I don't believe our conclusions would have differed because we used a two-bottle 48-h preference test and one can see no effects of neophobia or enhanced neophobia with that preference measure (see Carroll, Dine, Levy, & Smith, 1975). I am convinced that some confusion in the taste aversion literature results from the differences between a short-term single-bottle test and the long-term two-bottle test. Where are we? I conclude that the noncontingent controls mean much if the test is a short-term test where neophobia and enhanced neophobia are evident and that the noncontingent controls mean little if the preference test is a long-term one (Carroll et al., 1975). I think that a of the noncontingent controls in the Smith and Roll (1967) study would not change the results of that experiment. That control has not been run under these circumstances, but since one gets no aversion in water-X-ray immediate groups, it seems fruitless to look for aversion in water-X-ray delay groups as long as the aversion test is the long-term twobottle preference test. I cannot, therefore, agree that there has been a general failure to appreciate the importance of nonassociative factors in delayed poisoning taste-aversion experiments. I am convinced that nonassociative factors do not matter when a long-term test is used. A few other points may be worth mentioning. I do not know the evidence for the statement in the Mitchell letter (1977), There is little doubt that animals exposed to X-rays of this magnitude, especially the higher dose (which is approximately one half of the LD5o) , remain 'sick' for several days following treatment (as evidenced by prolonged hypophagia). We have seen no evidence of sickness in a rat 6 h following X-ray exposure. I have seen no prolonged hypophagia, and there is considerable evidence that intakes of saccharin and water are not modified (see Barker & Smith, 1974, Figure 1, page 647). Also, the LD.o for a rat is 750R, three times the exposure Revusky (1968)used. Although Roll and I reported evidence of taste aversion with 12-h CS-US delays, I think the more realistic figure is from the Barker and Smith (1974) paper. In the Barker and Smith paper, we did not see significant evidence of taste aversion beyond the 6-h delay group. A 6-h delay in the Mitchell et al. (1977) study may be too long to clearly demonstrate their point. I would like to see intermediate added to the Mitchell et al. (1977) paper. should be an orderly change in the extinction curves between the P-Soc-I and the P-Soc-D groups. The tests in Revusky's (1968) and Mitchell et al. (1977) papers were short-term tests. I do not understand why the irradiated or poisoned controls did not show any enhanced neophobia. I would suspect the previous experience {not the size of the group as suggested by Mitchell (1977)J made some difer-

CS duration, amount, and concentration effects in conditioning taste aversions

The relative contributions of the duration, amount, and concentration of a taste CS were assessed in pairings with an ionizing radiation US in two experiments. Increasing the duration of the CS (0.1% saccharin) up to 9 min yielded increasingly better conditioning over a 1 2 - hr CS-US delay interval. When the concentration of the sacharin was increased, only a few seconds' exposure to the CS was necessary for near asymptotic conditioning. It was observed that the CS duration effect had not been identified earlier because most investigators utilize 10–20 min CS access periods. The role of CS duration in conditioning over a delay was discussed with reference to the probability of the memory of the CS being present at the time of the pairing with the US event.

Relative salience of saccharin and quinine in long-delay taste aversion learning

In Expt I, 82 rats were subjected to toxicosis (cyclophosphamide injection), or saline injection, immediately or six hr after exposure to either 1 g/liter sodium saccharin, 10 −4 M quinine hydrochloride or water. As measured by resistance to extinction, both saccharin and quinine were discriminatively salient following immediate toxicosis, but only the quinine cue was salient following a six hr delay. In Expt II, 56 rats were used to repeat the previous delay experiment using a higher concentration of saccharin (1.5 g/liter) and a lower concentration of quinine (5 × 10 −5 ), and both cues were discriminatively salient. These results provide context for previous studies showing a loss of salience for 1 g/liter saccharin and for 5 × 10 −5 M quinine following immediate toxicosis in rats lacking gustatory neocortex. It is suggested that the salience dimension disrupted by both procedures, gustatory neocortex ablation and long CS-US delay, is either stimulus intensity or novelty.

Functional decortication in the CS-US interval decrease efficiency of taste aversive learning

Conditioned taste aversion was studied in 160 rats with the cortical spreading depression (CSD) technique. Water-deprived animals had 15-min access to water on the familiarization Days 1 and 2. On Day 3 they were offered saccharin (0.1%, CS) followed after 30–300 min by intraperitoneal LiCI injection (0.14 M, 2% body wt, US). Saccharin intake on Day 4 increased proportionally to the duration of CS-US interval from 40% (30 min) to 86% (240 min) and 103% (300 min) of Day 3 consumption. Bilateral CSD evoked 15 min after CS termination and maintained during the rest of the CS-US interval impaired the taste aversion conditioning with 120- and 180-min delays (85% and 89%) but not with 30- and 60-min delays (32% and 50%). Gustatory experience acquired during unilateral CSD on Day 3 yielded strong conditioned aversion with 20- and 60-min CS-US delays (40% and 43%). When bilateral CSD was induced 10 min after hemidecorticate exposure to saccharin, significant aversion was found with the 20- and 40-min delays (38% and 53%) but not with 50- and 60-min delays (95% and 87%). It is concluded that CSD interferes with the short-term storage of the gustatory information acquired with the intact brain.

Learned safety and the CS-US delay gradient in taste-aversion learning

Decrements in taste-aversion learning produced by extensions of the interval between the CS flavor and poisoning (the US) may be attributed to decay of the CS trace during long CS-US intervals. Kalat and Rozin (1973) alternatively proposed that such decrements occur because during extended CS-US intervals subjects learn that the CS is safe. They sought to show that trace decay is not responsible for the CS-US delay gradient by demonstrating that learning is disrupted even if the CS is reintroduced during extended CS-US intervals. The present experiments show that such a second CS presentation during conditioning may (1) enhance subsequent intakes of the CS solution whether or not subjects are poisoned, and/or (2) facilitate aversion learning, the facilitation being greater the closer the second CS exposure is to poisoning. These results question the adequacy of previous evidence for the contribution of learned safety to the CS-US delay gradient and suggest that some other process is also involved.

Odor aversions produced over long CS-US delays

Rats were allowed to drink water while a stream of amyl acetate vapor was directed towards the end of the drinking spout. Toxicosis was then induced via the injection of lithium chloride after delays of 0, 0.5, 1, 4, and 12 hr for different groups. When compared with no-toxicosis controls, it was found that significant aversions were obtained for all groups except the 12 hr group.

"Learned safety" as a mechanism in long-delay taste-aversion learning in rats.

Rats learn taste aversions with unusually long CS-US delays. This has previously been explained as slow decay of a CS trace or as relative lack of interference. We propose, however, that the CS-US delay gradient is a learning curve: During the delay, a rat gradually learns that a taste is safe. A solution which a rat drinks only once becomes safe and resistant to learned aversions for at least 3 wk., suggesting a learned safety mechanism. If a rat drinks a solution twice (within the effective CS-US interval) before a single poisoning, it learns less aversion than if it received only the second presentation. The learned-safety theory explains this result; a trace-decay or interference model cannot.