Transfer of occasion setting across stimulus and response in operant feature positive discriminations

Occasion setters determine responses of putative DA neurons to discriminative stimuli

Midbrain dopamine (DA) neurons are involved in the processing of rewards and reward-predicting stimuli, possibly analogous to reinforcement learning reward prediction errors. Here we studied the activity of putative DA neurons (n = 37) recorded in the ventral tegmental area of rats (n = 6) performing a behavioural task involving occasion setting. In this task an occasion setter (OS) indicated that the relationship between a discriminative stimulus (DS) and reinforcement is in effect, so that reinforcement of bar pressing occurred only after the OS (tone or houselight) was followed by the DS (houselight or tone). We found that responses of putative DA cells to the DS were enhanced when preceded by the OS, as were behavioural responses to obtain rewards. Surprisingly though, we did not find a homogeneous increase in the mean activity of the population of putative DA neurons to the OS, contrary to predictions of standard temporal-difference models of DA neurons. Instead, putative DA neurons exhibited a heterogeneous response on a single unit level, so that some units increased and others decreased their activity as a response to the OS. Similarly, putative non-DA cells did not show a homogeneous response to the DS on a population level, but also had heterogeneous responses on a single unit level. The heterogeneity in the responses of neurons in the ventral tegmental area may reflect how DA neurons encode context and point to local differences in DA signalling.

The influence of landmark stability on control by occasion setters

In an operant serial feature-positive procedure, an occasion setter (OS_X) signals that a response will be reinforced in the presence of a second stimulus (e.g., a discriminative stimulus, A). During a transfer test, the OS is paired with a different discriminative stimulus. Experiment 1a tested transfer effects in a touchscreen-based spatial occasion setting task with pigeons. During training, four OSs (OS_W, OS_X, OS_Y, and OS_Z) were paired on separate trials with landmark A (LM_A) or B (LM_B) and the opportunity for a reinforced response at one location to the immediate left (R₁) or right (R₂) of the LM (OS_W→LM_A:R₁, OS_X→LM_A:R₂, OS_Y→LM_B:R₁, OS_Z→LM_B:R₂). Training also included non-reinforced trials of LM_A and LM_B alone (LM_A- and LM_B-) and trials of a non-modulated LM with R₁ and R₂ reinforced across separate trials (LM_C:R₁ and LM_C:R₂). After training, the number and spatial location of responses during test trials of a LM paired with the same OS as in training did not differ reliably from transfer tests of an OS paired with a different, modulated LM (OS_W→LM_B and OS_Y→LM_A), but did differ from transfer to the non-modulated LM (OS_X→LM_C). Experiment 1b utilized the same pigeons and training with LM_B to test the degree to which the spatial stability of a LM influenced transfer. Retraining with LM_A was intended to establish it as a non-modulated, stable LM (LM_A:R₂). Subsequent tests with LM_A revealed reduced modulation by the formerly trained OS (OS_W), and complete disruption of modulation of spatial location during transfer with a different OS (OS_Y). These findings further our understanding of the conditions under which OSs may develop and transfer modulation.

Nicotine trained as a negative feature passes the retardation-of-acquisition and summation tests of a conditioned inhibitor

Nicotine functions as a negative feature in a Pavlovian discriminated goal-tracking task. Whether withholding of responding to the conditional stimulus (CS) reflects nicotine functioning as a conditioned inhibitor is unknown. Accordingly, the present research sought to determine whether nicotine trained as a negative feature passed the retardation-of-acquisition and summation tests, thus characterizing it as a pharmacological (interoceptive) conditioned inhibitor. In the retardation test, rats received either nicotine (0.4 mg/kg) or chlordiazepoxide (5 mg/kg) negative feature training in which the drug state signaled when a 15-sec light CS would not be paired with sucrose; light was paired with sucrose on intermixed saline sessions. Following acquisition of the discrimination, both groups received nicotine CS training in which sucrose was intermittently available on nicotine but not intermixed saline sessions. Acquisition of conditioned responding to the nicotine CS was slower in the nicotine negative feature group than in the chlordiazepoxide negative feature group. In the summation test, rats were assigned to either the nicotine negative feature group or a pseudoconditioning control. In this control, the light CS was paired with sucrose on half the nicotine and half the saline sessions. Both groups also received excitatory training in which a white noise CS was paired with sucrose. The summation test consisted of presenting the white noise in conjunction with nicotine. Conditioned responding evoked by the white noise was decreased in the negative feature but not the pseudoconditioning group. Combined, the results provide the first evidence that an interoceptive stimulus (nicotine) can become a conditioned inhibitor.

Operant serial feature-positive conditional discrimination with composite features and different topography responses

In a serial feature-positive conditional discrimination procedure the properties of a target stimulus A are defined by the presence or not of a feature stimulus X preceding it. In the present experiment, composite features preceded targets associated with two different topography operant responses (right and left bar pressing); matching and non-matching-to-sample arrangements were also used. Five water-deprived Wistar rats were trained in 6 different trials: X-R-->Ar and X-L-->Al, in which X and A were same modality visual stimuli and the reinforcement was contingent to pressing either the right (r) or left (l) bar that had the light on during the feature (matching-to-sample); Y-R-->Bl and Y-L-->Br, in which Y and B were same modality auditory stimuli and the reinforcement was contingent to pressing the bar that had the light off during the feature (non-matching-to-sample); A- and B- alone. After 100 training sessions, the animals were submitted to transfer tests with the targets used plus a new one (auditory click). Average percentages of stimuli with a response were measured. Acquisition occurred completely only for Y-L-->Br+; however, complex associations were established along training. Transfer was not complete during the tests since concurrent effects of extinction and response generalization also occurred. Results suggest the use of both simple conditioning and configurational strategies, favoring the most recent theories of conditional discrimination learning. The implications of the use of complex arrangements for discussing these theories are considered.

Protection from extinction

The effect of the presence of a conditioned inhibitor on extinction of excitatory conditioning was studied in one magazine approach and three autoshaping experiments using rats and pigeons. In each case, the presence of an inhibitor reduced responding to an exciter during extinction but allowed substantial recovery of responding to that exciter when subsequently tested separately. Control stimuli with a history of being irrelevant to reinforcement or being nonreinforced had less of a protective effect. This constitutes a clear demonstration of protection from extinction, a phenomenon of substantial theoretical and applied importance.

Effects of noise on the performance of rats in an operant discrimination task

Two experiments examined the effect of the presentation of an irregular, moderate intensity auditory stimulus ('noise') on the performance of rats in an operant discrimination task. In Experiment 1, rats first learned to press a lever in the presence of a visual stimulus but not in its absence. Discrimination performance was impaired during subsequent exposure to noise. In Experiment 2, different groups of rats learned the discrimination task under a noise or a no-noise condition. Thereafter, all rats were tested under each noise condition. Discrimination performance was best when the noise condition at test was identical to the noise condition at training. These results were discussed in the framework of arousal, distraction, generalization decrement, and contextual occasion setting. They point to the necessity of using a 2x2 factorial design in human and animal research on noise effects, with noise condition at training (noise present or absent) and noise condition at test (noise present or absent) as factors.

Serial and simultaneous feature positive discriminations in a human Skinner box

In the present experiment participants were presented either a serial (F-->T+/T-) or a simultaneous (FT+/T-) feature positive discrimination using a Skinner box for humans. After the participants mastered the discrimination, the associative properties of F were examined using a transfer test and a counterconditioning manipulation. F affected responding to a transfer target less in the Serial than in the Simultaneous condition. However, counterconditioning of F did not affect initial discrimination performance in either condition. These results were discussed in the framework of occasion setting, elemental learning, configural learning, and a neural network model.