Effect of signaling extra unconditioned stimuli on autoshaping

Robbins (1988) reported data that he viewed as inconsistent with Miller and Schachtman's (1985a) comparator hypothesis of conditioned response generation. Here we explain why we do not find his experiments a compelling test of the comparator hypothesis. We also briefly review other studies that tested the same predictions of the comparator hypothesis that Robbins examined. We conclude that there is considerable evidence that following excitatory or inhibitory conditioning with a target conditioned stimulus (CS) and unconditioned stimulus (US), extinction of other cues that were present during CS training ordinarily increases excitatory responding and decreases inhibitory responding to the CS. However, consistent with Robbins's conclusion, there is scant evidence that after CS-US training, enhancing the associative value of other cues that were present during CS training influences excitatory or inhibitory responding to the CS. The implications of these conclusions for the comparator hypothesis as an explanation of differences in acquired behavior and as a heuristic tool are considered.

Preconditioning experience with the unconditional stimulus (UCS) retards subsequent excitatory conditioning. Three experiments demonstrated that this UCS retardation effect is attenuated by associative manipulations of contextual stimuli of the UCS preexposure environment. The UCS retardation effect was reduced by (a) altering contextual stimuli between preexposure and conditioning (Experiment 1), (b) latently inhibiting contextual stimuli prior to UCS preexposure (Experiment 2), and (c) extinguishing contextual stimuli subsequent to UCS preexposure (Experiment 3). Although UCS preexposure retarded excitatory conditioning, the results of Experiment 4 demonstrated that UCS preexposure facilitated inhibitory conditioning. These results indicate that an association between contextual stimuli and the preexposed UCS contributes to the effects of preconditioning UCS experience on subsequent learning.

Three experiments demonstrated that repeated exposure to an electric-shock unconditioned stimulus (US) resulted in a decrement in retention of conditioned suppression evoked by a previously established excitatory conditioned stimulus (CS) and retarded subsequent acquisition of conditioned suppression to a novel CS paired with shock. Experiment 1 showed that 10 sessions of exposure to shock alone were required to produce a decrement in retention of conditioned suppression, whereas retardation in the acquisition of conditioned suppression was obtained following either 5 or 10 sessions of exposure to shock alone. Experiment 2 demonstrated that the magnitude of both the decrement in retention of conditioned suppression and the retardation in the acquisition of conditioned suppression produced by exposure to shock alone was directly related to the intensity of those shocks. Experiment 3 demonstrated that the decrement in retention of conditioned suppression produced by 10 sessions of exposure to shock alone was inversely related to the interval between the last exposure to shock and the test of the target CS. These findings are discussed in terms of associative and nonassociative accounts of the effects of US-alone procedures.

Effect of amount of context extinction on revaluation of a target CS.

Effect of intertrial unconditioned stimulus (US) presentations upon responding to a conditioned stimulus predictive of either the same or a different appetitive US

Two theories of timing, scalar expectancy theory (SET) and learning-to-time (LeT), make substantially different assumptions about what animals learn in temporal tasks. In a test of these assumptions, pigeons learned two temporal discriminations. On Type 1 trials, they learned to choose a red key after a 1-sec signal and a green key after a 4-sec signal; on Type 2 trials, they learned to choose a blue key after a 4-sec signal and a yellow key after either an 8-sec signal (Group 8) or a 16-sec signal (Group 16). Then, the birds were exposed to signals 1 sec, 4 sec, and 16 sec in length and given a choice between novel key combinations (red or green vs. blue or yellow). The choice between the green key and the blue key was of particular significance because both keys were associated with the same 4-sec signal. Whereas SET predicted no effect of the test signal duration on choice, LeT predicted that preference for green would increase monotonically with the length of the signal but would do so faster for Group 8 than for Group 16. The results were consistent with LeT, but not with SET.

Twenty-four-month-old and 4-month-old rats were trained on a peak-interval procedure, where the time of reinforcement was varied twice between 20 and 40 sec. Peak times from the old rats were consistently longer than the reinforcement time, whereas those from younger animals tracked the 20- and 40-sec durations more closely. Different measures of performance suggested that the old rats were either (1) systematically misremembering the time of reinforcement or (2) using an internal clock with a substantially greater latency to start and stop timing than the younger animals. Old rats also adjusted more slowly to the first transition from 20 to 40 sec than did the younger ones, but not to later transitions. Correlations between measures derived from within-trial patterns of responding conformed in general to detailed predictions derived from scalar expectancy theory. However, some correlation values more closely resembled those derived from a study of peak-interval performance in humans and a theoretical model developed by Cheng and Westwood (1993), than those obtained in previous work with animals, for reasons that are at present unclear.

Pacemaker-accumulator (PA) systems have been the most popular kind of timing model in the half-century since their introduction by Treisman (1963). Many alternative timing models have been designed predicated on different assumptions, though the dominant PA model during this period — Gibbon and Church’s Scalar Expectancy Theory (SET) — invokes most of them. As in Treisman, SET’s implementation assumes a fixed-rate clock-pulse generator and encodes durations by storing average pulse counts; unlike Treisman’s model, SET’s decision process invokes Weber’s law of magnitude-comparison to account for timescale-invariant temporal precision in animal behavior. This is one way to deal with the ‘Poisson timing’ issue, in which relative temporal precision increases for longer durations, contrafactually, in a simplified version of Treisman’s model. First, we review the fact that this problem does not afflict Treisman’s model itself due to a key assumption not shared by SET. Second, we develop a contrasting PA model, an extension of Killeen and Fetterman’s Behavioral Theory of Timing that accumulates Poisson pulses up to a fixed criterion level, with pulse rates adapting to time different intervals. Like Treisman’s model, this time-adaptive, opponent Poisson, drift–diffusion model accounts for timescale invariance without first assuming Weber’s law. It also makes new predictions about response times and learning speed and connects interval timing to the popular drift–diffusion model of perceptual decision making. With at least three different routes to timescale invariance, the PA model family can provide a more compelling account of timed behavior than may be generally appreciated.

Two theories of timing, scalar expectancy theory (SET) and learning to time (LeT), make substantially different assumptions about what animals learn in temporal tasks. In a test of these assumptions, pigeons learned two discriminations: On Type 1 trials, they learned to choose a red key after a 1-s signal and a green key after a 4-s signal; on Type 2 trials, they learned to choose a blue key after a 4-s signal and a yellow key after a 16-s signal. Then, two psychometric functions were obtained by presenting them with intermediate durations (1 to 4 s and 4 to 16 s). The two functions did not superpose, and most bisection points were not at the geometric mean of the training stimuli (contra SET); for most birds, the function for Type 2 trials was to the left of the function for Type 1 trials (contra LeT). Finally, the birds were exposed to signals ranging from 1 to 16 s and given a choice between novel key combinations (e.g., red vs. blue). The results with the novel key combinations were always closer to LeT's than to SET's predictions. Observations of the birds' behavior also suggest that, more than being a mere expression of an internal clock, behavior constitutes the clock.

Five appetitive conditioning experiments with rats examined the ability of extinction cues (ECs) to reduce spontaneous recovery after extinction procedures that varied the temporal relation between the ECs and the conditioned stimulus (CS) and included presentations of additional events (e.g. other stimuli correlated with extinction, CSs, and the US). In extinction, two different ECs were presented either closely in time before (i.e. recent to), or more distant (i.e. remote) from a nonreinforced CS. Both recent and remote ECs reduced spontaneous recovery to the CS when present during testing (Experiments 1-4). Each EC reduced recovery despite the addition during extinction of a second EC and CS (Experiments 1, 3, and 4), and the US (Experiment 3). Experiments 4 and 5 investigated the recent and remote ECs' tendency to control a serial occasion setting discrimination involving the target CS under explicit training conditions. Neither EC gained such discriminative control. Possible explanations of the results are discussed, including configural learning, occasion setting, and contextual cue control.

BackgroundConditioned pain modulation (CPM) evaluates the effect of a painful conditioning stimulus (CS) on a painful test stimulus (TS). Using painful cutaneous electrical stimulation (PCES) as TS and painful cold water as CS, the pain relief was paralleled by a decrease in evoked potentials (PCES-EPs). We now aimed to compare the effect of CPM with cognitive distraction on PCES-induced pain and PCES-EP amplitudes.MethodsPCES was performed using surface electrodes inducing a painful sensation of 60 (NRS 0–100) on one hand. In a crossover design healthy subjects (included: n = 38, analyzed: n = 23) immersed the contralateral hand into 10 °C cold water (CS) for CPM evaluation and performed the 1-back task for cognitive distraction. Before and during the CS and 1-back task, respectively, subjects rated the pain intensity of PCES and simultaneously cortical evoked potentials were recorded.ResultsBoth CPM and cognitive distraction significantly reduced PCES-EP amplitudes (CPM: 27.6 ± 12.0 μV to 20.2 ± 9.5 μV, cognitive distraction: 30.3 ± 14.2 µV to 13.6 ± 5.2 μV, p < 0.001) and PCES-induced pain (on a 0–100 numerical rating scale: CPM: 58 ± 4 to 41.1 ± 12.3, cognitive distraction: 58.3 ± 4.4 to 38.0 ± 13.0, p < 0.001), though the changes in pain intensity and PCES-amplitude did not correlate. The changes of the PCES-EP amplitudes during cognitive distraction were more pronounced than during CPM (p = 0.001).ConclusionsCPM and cognitive distraction reduced the PCES-induced pain to a similar extent. The more pronounced decrease of PCES-EP amplitudes after distraction by a cognitive task implies that both conditions might not represent the general pain modulatory capacity of individuals, but may underlie different neuronal mechanisms with the final common pathway of perceived pain reduction.

We have attempted to develop a behavioral and neuronal model for classical conditioning in the corticorubrospinal system. A conditioned stimulus (CS) was applied to the cerebral peduncle (CP) in cats which had lesions that interrupted the corticofugal fibers caudal to the red nucleus. The unconditioned stimulus (US) was an electric shock to the skin of the forelimb that produced flexion of the limb. After pairing of the CS and US in close temporal association, an initially ineffective stimulus to the cerebral peduncle was found to give rise to the flexion of the elbow. Extinction of the conditioned response was achieved by applying the CS alone or by reversing the sequence of the stimuli (US-CS: backward pairing). Furthermore, the US alone did not produce an increase in the effectiveness of the CS stimulus. Finally, pairing the fixed CS stimuli with the US at random intervals did not produce any increase in performance in response to the CS. In these respects, the observed behavioral modification has the features of associative conditioning. Because the thresholds for and the strength of elbow flexion induced by stimulation of the nucleus interpositus of the cerebellum were identical in the experimental and control animals, the interpositorubrospinal system cannot be the site of the plastic change. Since the conditioned response is most probably mediated by the corticorubrospinal system, it is likely that a modification of the corticorubral synapses underlies this behavioral change.

Conditioned lick suppression by water-deprived rats was used to elaborate on recent evidence that the attenuated conditioned response elicited by an overshadowed stimulus may be enhanced by extinction of the overshadowing stimulus with which it had been trained in simultaneous compound. Using a modified serial stimulus arrangement in which a light coexisted with the last half of a tone that terminated with footshock, it was found in Experiment 1 that the tone overshadowed the light. Extinction of the tone-shock association resulted in a virtually complete recovery of the response to the overshadowed light. Using this serial overshadowing procedure, the possibility that the strength of a conditioned response to an element trained in compound covaries as a function of the strength of the response to the other element was tested in Experiment 2. Following overshadowing training similar to that of Experiment 1, independent reinforcement of the overshadowed light, that is, associative inflation, was found to have no deleterious effect on the response to the overshadowing tone. This suggests that the effects of postconditioning extinction and inflation of one element do not have symmetrical effects upon responding to the other element. The results of Experiment 2 were replicated in Experiment 3 using a simultaneous compound stimulus as opposed to the serial compound of the previous studies. These results are discussed in terms of various associative and cognitive models of learning and performance.

Trial frequency effects in human temporal bisection: Implications for theories of timing

Changes in the excitability of the hamstring flexor withdrawal reflex produced by conditioning stimuli applied to C-afferent fibers of different origins have been examined in the decerebrate spinal rat. In the absence of conditioning stimuli, the flexor reflex elicited by a standard suprathreshold mechanical stimulus to the toes is stable when tested repeatedly for hours. Three categories of conditioning stimuli have been used in an attempt to modify the excitability of the flexor reflex; electrical stimulation of a cutaneous (sural) nerve or a muscle (gastrocnemius-soleus) nerve at C-fiber strength; the application of mustard oil, a chemical irritant that activates chemosensitive C-afferents, to the skin or injected intramuscularly and intraarticularly; and the indirect activation of high-threshold muscle afferents by fused tetanic contractions of the tibial muscles. Conditioning stimuli of an intensity sufficient to activate C-afferent fibers result in a heterosynaptic facilitation of the flexor motoneuronal response to the standard test input, which lasts from 3 min to more than 3 hr, depending on the stimulus and the C-afferents activated. Pretreatment of the sciatic nerve with the C-fiber neurotoxin capsaicin abolishes all the postconditioning facilitations, which is an indication that it is likely that it is C-afferents that are primarily responsible for the facilitatory effects of the conditioning stimuli, although some A delta afferents may contribute. Capsaicin pretreatment does not modify the reflex response to the test stimulus. The most prolonged increase in the excitability of the flexor reflex resulted from intraarticular injections of 5 microliter mustard oil. Using the subsequent injection of lignocaine intraarticulary, it was found that the prolonged facilitation of the reflex is triggered by the afferent input generated by the conditioning stimulus and does not require an ongoing input for its maintenance. These results indicate that there is a spectrum of central changes in the stimulus response relations of the spinal cord resulting from the activation of C-fibers of different origins. The prolonged duration of some of these changes means that the peripheral activation of C-afferents will modify the functional response of the spinal cord to other inputs applied long after the conditioning input, and this may be responsible for some of the sensory and motor alterations found after peripheral tissue injury.