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External inhibition in a goldfish (Carassius auratus) classical conditioning situation.

Goldfish were classically conditioned with a light as the CS and shock as the US. The UR was a decrease in respiration. After 15 or 60 conditioning trials the fish were tested with novel stimuli (clicks) during the CS-US interval. High and moderate intensity novel stimuli produced a significant decrease in CRs (external inhibition) for fish with 60 conditioning trials (5.5 or 10.5 sec CS-US interval), but not fish with 15 conditioning trials. Low intensity novel stimuli produced no evidence for disinhibition (an increase in CRs). Control groups (e.g., groups with random presentations of the CS and US) showed that the external inhibition for fish with 60 conditioning trials was inhibition of a true CR.

Acoustic Stimulation

Factors affecting the measurement of classically conditioned fear in rats following exposure to escapable versus inescapable signaled shock.

Three experiments are reported in which rats first received 50 escapable or inescapable signaled-shock trials. Experiment 1 (n = 22) employed an acquired-drive paradigm and found inescapable shock subjects learned a hurdle-jump response to escape the signal less rapidly than did escapable-shock subjects. Experiment 2 (n = 24) employed a conditioned emotional response paradigm and found inescapable-shock subjects suppressed more when the signal was introduced in the appetitive bar-pressing task. Both experiments measured spontaneous activity immediately following conditioning and found no group differences. Experiment 3 (n = 39) employed the same activity task and found no difference between escapable- and inescapable-shock groups when the signal was introduced into the activity task. Both groups displayed less activity than a nonshock control group during the signal. The results suggest that lack of control over the shock in the conditioning phase did not result in an increase of conditioned fear. The results are discussed in terms of a learned active-inactive predisposition to respond.

Animals

Classical conditioning in Aplysia californica.

A form of aversive classical conditioning is described in which a chemosensory conditioned stimulus rapidly acquires the ability to modulate a defensive response (escape locomotion). Because Aplysia show both sensitization and classical conditioning, it is now possible to begin to examine the relationship between nonassociative and associative learning on behavioral and cellular levels.

Animals

An analysis of 70 years of morphine classical conditioning: implications for clinical treatment of narcotic addition.

A review of all morphine classical conditioning studies performed since Pavlov reveals significant shifts in methodological and conceptual thinking during the past 7 decades. These shifts have tended to obscure the fact that a great deal of basic data are still needed before morphine classical conditioning can be understood. These data are needed to help clarify the relationship between drug conditioning and drug addiction. Recent evidence, for example, suggests that once morphine autonomic conditional reactions are established, they are highly resistant to extinction. The inclusion of these types of data in any conceptualization of drug dependency would appear to be of help in the formulation of new clinical approaches to this problem.

Animals

Classical conditioning in the sea anemone, Cribrina xanthogrammica.

Earlier attempts to classically condition sea anemones have yielded inconclusive results. Using light as CS and shock as US, conditioning as distinguished from sensitization and pseudoconditioning was demonstrated in anemones. Procedural controls included substitution of light alone, shock alone, and random light and shock in place of paired light-shock trials. Responses measured were electrical output and folding of the oral disc. The conditioned response was distinguished from the unconditioned response to light and the unconditioned response to shock in terms of response latency of both electrical and behavioral measures.

Cnidaria

Effects of LSD on learning as measured by classical conditioning of the rabbit nictitating membrane response.

Acquisition of the classically conditioned, nictitating membrane response was used to assess effects of LSD on learning. Tone and light conditioned stimuli (CS) were presented 800 msec before delivery of the unconditioned stimulus, consisting of a 100 msec electric shock to the skin over the paraorbital region of the head. Extension of the membrane to the CSs in the 800 msec prior to shock onset was recorded as a conditioned response (CR), while extension to shock onset was recorded as an unconditioned response (UCR). LSD (1, 10, 30, 100 or 300 nmol/kg) was injected i.v., 30 min before each daily conditioning session. Dosages of 1 to 100 nmol/kg of LSD produced a dose-dependent enhancement of CR acquisition. Acquisition to a criterion of 10 successive CRs required 184 trials at 30 nmol/kg of LSD as compared to 293 trials with controls. Separate groups of rabbits received explicity unpaired presentations of stimuli (tone alone, light alone and shock alone). The frequency of responding within 800 msec of CS onset or in the 800 msec before shock onset was low (2-3%) and was not affected by any dosage of LSD, indicating that the effects of LSD on acquisition were not due to sensitization, pseudoconditioning or changes in baseline responding. LSD also had no effect on UCR amplitude. Hence, the systematic effects of LSD on acquisition of CRs reflects the action of the drug on learning.

Acoustic Stimulation

Warning signals and alerting: effects on differential classical conditioning.

Three experiments were carried out to investigate the effects of a nondifferential warning signal on differential classical conditioning. In Experiment 1, a 500-msec warning interval was found to result in good differential conditioning performance at an interstimulus interval (ISI) of 500 msec in which differential conditioning did not otherwise occur. The second experiment demonstrated the warning effect with both visual and auditory conditioned stimuli, thus ruling out the possibility that the warning effects were due to eye movements. Experment 3 showed that a warning signal and an increased ISI were functionally equivalent, as demonstrated by conditioning performance and the similar effects obtained when the ISI and warning-interval conditions were switched from optimal to nonoptimal and vice versa.

Acoustic Stimulation

Are there two classes of classically-conditioned responses?

It is proposed that two response classes interact during classical conditioning: (1) a specific effect which directs responding towards or away from the CS, depending upon whether a positive or negative outcome is signaled, and (2) a general, emotional effect which interferes with responding. The interaction between these two response classes can account for the varied results obtained under CER and autoshaping procedures, and must be considered in any analysis which attempts to account for the complex nature of the conditioned response.

Animals