Temporal location of unsignaled food deliveries: effects on conditioned withdrawal (inhibition) in pigeon signtracking.

Standard models of Pavlovian conditioning neglect local effects of unsignaled unconditioned stimuli (USs) on learning and performance. Using the approach-withdrawal behavior of pigeons toward keylights as conditioned stimuli (CSs), the authors varied the specific times (5-110 s) that USs occurred before or after a CS. Withdrawal from a CS generally increased as the time between a US before and/or after the CS was lengthened. Combinations of 2 distant USs produced more withdrawal from the CS than either US alone, whereas combinations of a distant and a nearby US yielded behavior intermediate between that for either US alone. Postacquisition retardation tests supported similar conclusions. Based on the temporal isolation of CSs and USs, a tentative model was offered to summarize these data. The results and the model suggest that a more molecular, possibly perceptual approach to Pavlovian excitation and inhibition is needed.

A comparison of venlafaxine, trazodone, and placebo in major depression.

A double-blind, placebo-controlled trial was undertaken to compare the safety and efficacy of venlafaxine and trazodone in patients with major depression. Two hundred twenty-five patients entered an initial 6-week treatment phase, and 149 completed it. Ninety-six patients who were responders continued in a 1-year, double-blind, long-term phase during which they received the same medication and doses they had during the short-term phase. Both active treatments were significantly more effective than placebo on some measures during the short-term study, but venlafaxine produced more improvement in the cognitive disturbance and retardation factors on the Hamilton Rating Scale for Depression. Trazodone was more effective against the sleep disturbance factor. Patients on venlafaxine were most likely to enter the long-term phase and to remain in the trial longest. The side effect profiles of the three treatment groups were compared. Venlafaxine was most likely to cause nausea, whereas trazodone was associated with the most dizziness and somnolence.

Two combined, multicenter double-blind studies of paroxetine and doxepin in geriatric patients with major depression.

Depressive illness among the elderly is an important public health concern. However, treatment of the elderly may be complicated by age-related changes in physiology, general medical status, and susceptibility to side effects. There is therefore a need for improved treatment modalities for depressed elderly patients. Paroxetine is an antidepressant that acts through selective inhibition of serotonin reuptake. It lacks the anticholinergic and cardiovascular side effects of most first- and second-generation antidepressants. The authors present the combined data from two similarly designed comparisons of paroxetine and doxepin in outpatients over 60 years of age with major depression. The results show that paroxetine was an effective as doxepin in alleviating depression as measured on the Hamilton Rating Scale for Depression (HAM-D) total score, the Montgomery and Asberg Depression Rating Scale (MADRS), and the Hopkins Symptom Checklist (SCL) depression factor score. Paroxetine was significantly superior to doxepin on the Clinical Global Impressions (CGI) scale for severity of illness, the HAM-D retardation factor, and the HAM-D depressed mood item. Doxepin produced significantly more anticholinergic effects, sedation, and confusion. Paroxetine was associated with more reports of nausea and headache. These results suggest that paroxetine may be a valuable tool for the treatment of major depression in the elderly.

Extinction reveals stimulus control: latent learning of feature-negative discriminations in pigeons.

Various discriminations based on the presence versus absence of a single feature are supposedly learned much better when the feature appears on reinforced rather than nonreinforced trials. However, failures to show discriminative acquisition with the feature on negative trials could reflect a deficiency in control of performance rather than a lack of learning. Five experiments supported this alternative possibility. Pigeons that had yielded little or no evidence of learning (with distinguishing features like a small white square on the response key or a tone located some distance away) revealed clear differences between keypecking to the formerly positive and negative stimuli when all food was removed from the situation. Besides extinction, several other procedures for decreasing the positive predictiveness of the most informative stimulus element also unmasked feature-negative learning, whereas general and specific contextual changes did not. Incompletely mastered feature-positive discriminations improved during extinction, too. The findings of better discrimination performance in extinction were related to analogous effects in previous generalization and discrimination research employing other tasks and arrangements. A sign-tracking analysis could not completely account for the present results.

Detecting and identifying change: additions versus deletions.

The recognition of changes in the features of objects was examined as a function of the nature of the change (additions, deletions, no change). In two experiments we assessed both detection (noticing whether the stimulus had changed) and identification (specifying the exact nature of the change). Both detection and identification were expected to depend upon the subject's awareness of the subsequent recognition tasks while the original stimuli were encoded. In Experiment 1, subjects were not aware of subsequent detection and identification tasks while they initially viewed study slides of the to-be-changed stimuli. During the subsequent presentation of the test stimuli, detection and identification were superior for additions. On the other hand, in Experiment 2 when subjects were aware of the subsequent recognition tasks while viewing each study slide, a detection advantage for deletions obtained. Identification performance depended upon a further factor, whether the features of a stimulus were codable. Only in codable stimuli were deletions easier to identify than additions. The differences between the two experiments in detecting and identifying additions versus deletions are consistent with Tversky's (1977) research that stresses the importance of specifying which representation (the study stimulus or the test stimulus) is the subject of comparison in the comparative judgment.

Bridging temporal gaps between CS and US in autoshaping: insertion of other stimuli before, during, and after CS.

In four experiments pigeons were exposed to key-light illuminations separated from food delivery by 12-60 sec. Approach to the key light did not develop on conventional trace-conditioning arrangements but occurred consistently whenever some auditory or visual stimulus (a) filled the CS-US gap (serial conditioning) or (b) was always present except during the gap. Various comparison groups showed that this enhancement of conditioning cannot be mainly attributed to similarity between the CS and the added stimulus, or to spread of specific responses evoked by that stimulus, or to potentiation of CS's neural aftereffects by the extra stimulus. However, modifications of condition b in the final experiment revealed that CS approach was strong only when the stimulus present during the intertrial interval remained on until the termination of CS; if the stimulus ended at CS onset, conditioning did not occur. Although discriminability of CS-US gaps from intertrial periods seems necessary for conditioning to occur in the absence of close CS-US contiguity, the outcome of the final experiment indicates that such discriminability is not sufficient for conditioning. The results are primarily interpreted in terms of (a) possible second-order conditioning effects and (b) changes in the associative strength of the "local context" existing when CS appears, which may lead to superconditioning of CS.

The feature-positive effect in adult human subjects.

Previous experiments with animals and young children have shown that discriminations based on the presence versus absence of a single feature are learned more easily when the feature appears on reinforced rather than nonreinforced displays. Six experiments demonstrated an analogous effect in college students, across a range of stimulus materials, general procedures, kinds of feedback, pacing of trials, and instructions to the subject. The results were analyzed in terms of the exceptionally strong control of behavior by events that are present on positive trials. These findings have implications for theoretical interpretations of human concept learning and decision making, and offer additional examples of the difficulty organisms experience in using "nonoccurrence" as a cue.

Oddity learning in the pigeon as a function of the number of incorrect alternatives.

Pigeons' rate of learning a two-color oddity task increased as a function of the number of incorrect alternatives from 2 to 24 in Experiments 1, 2, and 3. In general, pigeons that were transferred from many-incorrect-alternative to two-incorrect-alternative oddity performed better than controls, but considerably below baseline (Experiments 2 and 3). In Experiment 4, pigeons showed no unconditioned tendency to peck the odd stimulus among 24 incorect alternatives, when pecks were nondifferentially reinforced, and in Experiment 5, when this procedure was preceded by oddity training, a progressive drop in odd-stimulus pecking was found. In Experiment 6, pigeons exposed to a nine-stimulus array in which the odd stimulus appeared (a) in the center or (b) separate from the array learned faster than when the odd stimulus was at the edge. This outcome suggests ththe figure-ground relation between the odd stimulus and the incorrect alternatives plays a role in the facilitation produced by increasing the number of incorrect alternatives but that poor performance on the standard, three-alternative oddity task appears to be due to center-odd trials which provide a difficult size or number discrimination.

Food delivery as a conditional stimulus: Feature-learning and memory in pigeons.

Three experiments investigated the learning and memory of discriminations based on presence versus absence of a pre-trial food delivery. In Experiment 1 half the illuminations of a response key were followed by food regardless of the subject's behavior. In one group an extra food delivery preceded only reinforced trials (feature-positive condition), whereas in a second group it preceded only nonreinforced trials (feature-negative condition). Key pecks and approaches revealed more rapid and superior discrimination learning in the first group. Experiment 2 replicated the results of Experiment 1 but yielded no evidence that greater "unexpectedness" of pretrial food conditions facilitates discriminative performance. In Experiment 3, individual pigeons trained on a conditional discrimination exhibited a within-subject feature-positive superiority. Delay between pretrial and trial stimuli interacted with feature-positive versus feature-negative training in both the between-group (Experiment 2) and within-subject (Experiment 3) procedures: performance was decremented at both short and long delays in the feature-positive condition but was decremented only at longer delays in the feature-negative condition. The feature-positive superiority obtained here is incompatible with explanations based on either the general concept of "perceptual organization" or on the conditional nature of feature-negative discriminations.

Inhibitory control and errorless discrimination learning.

Pigeons learned to discriminate between a positive stimulus (white key) and a negative stimulus (red or green key, depending on the subject) via Terrace's fading procedure. Generalization tests, conducted with intermittent reinforcement for key pecking at various wavelengths, yielded minima at the value of the negative stimulus in most "errorless" birds. Terrace's contrary finding of flat gradients in errorless subjects probably resulted from a floor-effect (i.e., virtually zero responding) produced by his extinction-test procedure. The present and other findings do not support Terrace's conclusions that the negative stimulus of an errorless discrimination is behaviorally neutral; inhibition apparently develops to the nonreinforced stimulus even during errorless discrimination learning. A negative correlation between stimulus and reinforcer seems the crucial factor in producing an inhibitory stimulus.

Conditioned Approach and Contact Behavior toward Signals for Food or Brain-Stimulation Reinforcement.

When presentation of a retractable lever always preceded food delivery, rats licked or gnawed the lever. They also approached but seldom orally contacted a lever signaling brain-stimulation reinforcement; instead, subjects sniffed, pawed, or "explored" the lever. Therefore, a Pavlovian conditioned stimulus evoked directed skeletal responses whose specific form depended on the forthcoming unconditioned stimulus.

Contrast and stimulus generalization following prolonged discrimination training.

Different groups of pigeons received discrimination training in which the reinforcement-associated and extinction-associated stimuli were respectively either (a) a line tilt vs a blank key, (b) a blank key vs a line tilt, or (c) two different line tilts. The high response rates that developed to the positive stimulus in all groups during discrimination learning were maintained over 64 sessions of training. After these sessions, all subjects were tested for stimulus generalization along the line-tilt dimension. Gradients of relative (per cent) generalization around the stimulus associated with reinforcement (so-called excitatory gradients) and around the stimulus associated with extinction (so-called inhibitory gradients) were as steep as they typically are after much briefer training periods. These results do not support several of Terrace's predictions on the basis of the hypothesis that emotional responses develop to the stimulus associated with extinction during discrimination learning with errors, but eventually dissipate after extended training.

Inhibition and the stimulus control of operant behavior.

A variety of methods, definitions, and theoretical notions that have been used in the study of inhibitory stimulus control were reviewed and evaluated. Preliminary data from several new operant methods were also described. It was proposed that future workers distinguish clearly between two forms of inhibitory control: (a) the learned power of a specific stimulus to reduce behavior, and (b) a dimensional effect, in which responding increases as values progressively more distant from the value of that specific stimulus along some dimension are presented (generalization gradient). Conclusions from several important recent studies were shown to be strongly dependent on the individual experimenter's criterion for deciding when a stimulus is inhibitory. The concept of inhibition seems a very valuable one for the field of operant behavior, and it deserves more attention than it has received in the past.