Effects of fornix transection on spontaneous and trained non-matching by monkeys

Three monkeys with fornix transection and three normal control monkeys performed a series of tasks which were variations of delayed non-matching. Experiment 1 showed that even at short retention intervals fornix transection impaired the spontaneous tendency to explore novel objects. Experiment 2 provided differential reward for non-matching and showed that the fornix-transected monkeys learned and performed non-matching normally even though the sample-match retention intervals were long throughout the experiment. Experiment 3 showed that non-matching performance was transiently more disrupted in fornix-transected than in normal monkeys when the testing procedure was changed, in a variety of ways, while maintaining the basic non-match rule. Experiment 4 required the monkeys to discriminate objects they had displaced from objects they had seen but not displaced; fornix transection produced in this task a substantial and stable impairment. These four experiments require a revised interpretation of the effects of fornix transection upon recognition memory and exploration. Particularly they contradict the hypothesis, suggested by previous experiments, that fornix transection produces a defect in discrimination of stimulus familiarity in long-term but not in short-term memory. They suggest rather that fornix transection impairs memory of instrumental responses.     

Learning of object-object associations by monkeys

Monkeys were trained in tasks where stimulus objects were presented in pairs, one object of a pair concealing the other, and the monkeys had to displace the top object followed by the bottom object in order to obtain a food reward. In two experiments it was demonstrated that under these conditions the animals formed object-object associations such that the top object recalled in memory the bottom object that had previously been found underneath it. The monkeys were able to discriminate in recall between bottom objects that were identical in secondary reinforcing value and differed only in colour.     

Visual identification following inferotemporal ablation in the monkey

Nine monkeys took part in two experiments on visual learning and the effect of inferotemporal ablation. The first experiment contrasted lesion effects in two tasks given to separate groups of animals: serial visual reversal learning set with only two visual stimuli in use throughout the experiment, and discrimination learning set with a new pair of visual stimuli for each discrimination problem. Following inferotemporal ablation, the within-reversal learning rate recovered to the normal pre-operative level, while within-problem learning in discrimination learning set remained substantially impaired. This result suggests that inferotemporal ablation does not directly impair visual associative learning, but rather produces a defect in visual identification such that only a small set of familiar stimuli can be correctly identified and associated with reward. The second experiment replicated an earlier experiment in showing that monkeys with inferotemporal ablation could learn at a normal rate to choose between stimuli according to their spatial orientation. A relation between the two types of unimpaired visual learning in the operated animals was suggested and analogies with clinical syndromes were discussed. The experimental results were interpreted within a distributed-trace model of learning, and this interpretation was illustrated and supported by a computer simulation presented in the Appendix.     

Delayed matching by fornix-transected monkeys: The sample, the push and the bait

In the acquisition event of ordinary delayed matching to sample the monkey sees a sample, displaces it, and finds a food reward underneath; subsequently the retention test consists of a choice between that sample and a distractor, and the strength of the memory laid down by the acquisition event may be assessed by the correctness of choice at the retention test. The present experiments varied the acquisition events and examined the effect of those variations on normal and fornix-transected monkeys' memory. One variation was to proceed as normally but never to bait the sample at acquisition; this variation allowed assessment of the role of the food reward in ordinary matching. Another was to present the sample, baited and to be displaced, as normally, but to present also the distractor, baited but not requiring to be displaced, in a second acquisition event; this “push-match” variation allowed assessment of memory for the displacement. The main result from normal monkeys was that matching to unbaited samples was learned much faster than matching to baited samples. Following fornix transection, final performance levels in matching to baited and to unbaited samples were unimpaired but push-match showed a permanent deficit. These results support earlier indications that fornix-transected monkeys have normal sensory memory but are deficient in the memory of instrumental responses.     

Acquisition and forgetting in monkeys' memory of informational object-reward associations

The first experiment showed the monkeys could recall whether an object had been rewarded with peanuts or with sultanas, two equally preferred foods. The second investigated the effect of rewarded trials with an object on monkeys' ability to recall a nonrewarded trial with the same object. The third demonstrated that monkeys could use the memory of reward to predict nonreward and the memory of nonreward to predict reward, in a Win-Shift Lose-Stay paradigm. The fourth found differences between Win-Shift Lose-Stay and Win-Stay Lose-Shift in the rate at which associations between objects and reward events were forgotten. These results are discussed in relation to D. L. Medin's (In A. M. Schrier, Ed., Behavioral primatology, Hillsdale, N.J.: Erlbaum, 1977, Vol. I, pp. 33–69) distinction between informational and hedonic effects of reward in monkeys. It is argued that the association between an object and a reward event is represented in memory by many independent traces, different traces recording the object's association with different attributes of the reward event.     

AX+/BX- discrimination learning in the fear-potentiated startle paradigm in monkeys

Individuals with anxiety disorders often do not respond to safety signals and hence continue to be afraid and anxious. Consequently, it is important to develop paradigms in animals that can directly study brain systems involved in learning about, and responding to, safety signals. We previously developed a discrimination procedure in rats of the form AX+/BX-, where cues A and X presented together are paired with an aversive stimulus and cues B and X presented together predict the absence of an aversive stimulus. The present experiment adapted this procedure to the fear-potentiated startle paradigm in rhesus monkeys.     

Emergent conditional discrimination in children: Matching to compound stimuli

This study reports two experiments that first taught preschool children identity-matching to compound sample and compound comparison stimuli. A compound stimulus consisted of a colour and a form superimposed on one another. Test sessions assessed whether children related the form and colour elements of a particular compound stimulus. The test for this was matching to sample in which an arbitrary conditional discrimination was required. A majority of the children selected the correct colour comparison in the presence of each form sample. The children also showed the reverse sample-comparison relations: they matched form comparisons to the corresponding colour samples, respectively. In the context of these arbitrary relations, new colours were paired with the form elements of the samples (Experiment 1), and new form elements were paired with the colour elements of the comparisons (Experiment 2). Subsequent tests assessed whether the new stimulus elements had control over responding when presented as single samples or comparisons. Test results showed that most subjects were able to relate the new stimulus elements to the corresponding colour and form elements, respectively. The study demonstrated that matching to compound stimuli in training and testing conditionsMaygenerate conditional relations between the individual stimulus elements.     

Source monitoring in Alzheimer's Disease

Source monitoring is the process of making judgments about the origin of memories. There are three categories of source monitoring: reality monitoring (discrimination between self- versus other-generated sources), external monitoring (discrimination between several external sources), and internal monitoring (discrimination between two types of self-generated sources). We investigated whether Alzheimer's Disease (AD) patients, when compared with young and older adults, are impaired at the same level on the three source monitoring categories. We designed three tasks, one for each source monitoring category. In the first task, aimed at reality monitoring, participants had to remember whether objects were previously placed in a bag by themselves or by the experimenter. In the second task, assessing external monitoring, participants had to remember whether the experimenter had previously placed objects in the bag with a black or white gloved hand. In the third task, measuring internal monitoring, participants had to remember whether they had previously placed or imagined themselves placing objects in the bag. Participants showed worse performances in the external and internal monitoring tasks, when compared with reality monitoring. The external monitoring deficit was even more pronounced in AD patients. Regression analyses showed that variation in the external monitoring performances was reliably predicted by inhibition. Our results emphasize the role of inhibitory processes in AD patients' source monitoring decline. The close relation between source and inhibitory decline in AD is interpreted in terms of a common neural base for both concepts.     

Selective transfer of responding in conditional discriminations

In each of three experiments rats received discrimination training in which whether or not a 10-sec target stimulus was followed by food was signalled by a 2-min background stimulus. In the first experiment the target was paired with food in the presence but not the absence of the background stimulus. Subsequent tests revealed that the background elevated responding to a target that had taken part in a similar discrimination. However, it had no influence on the responses elicited by a partially reinforced conditioned stimulus. In the remaining experiments the target was paired with food in the absence but not the presence of the background. Test trials then revealed that although the background had an inhibitory influence on the responses elicited by a target from a similar discrimination, it had no influence on the responses elicited by either a partially or a continuously reinforced conditioned stimulus. Various explanations for this selective influence of a background stimulus are considered.     

Transfer of contextual stimulus function via equivalence class development

In a conditional discrimination, 6 college students arranged six Cyrillic letters into groups of three based upon which of two additional Cyrillic letters (contextual stimuli) was present. All subjects demonstrated symmetry and transitivity within each class of equivalent stimuli. In a second conditional discrimination, two more Cyrillic letters were related to each contextual stimulus. Testing of symmetrical and transitive relations between the original contextual stimulus and the two new ones confirmed the development of two three-member classes of contextual stimuli. Subsequent tests demonstrated that the new contextual stimuli controlled the previously trained sample-comparison relations for all subjects.     

Memory codes in pigeon short-term memory: Effects of varying the number of sample and comparison stimuli

In a delayed conditional discrimination task, pigeons can remember either some aspect of the conditional stimulus (i.e., they can code retrospectively) or some aspect of the stimulus to which they will respond at the end of the delay (i.e., they can code prospectively). To determine the nature of the memory code, we varied the number of possible sample stimuli (two or four) and the number of possible comparison stimuli (two or four) factorially across groups. Birds in all four groups were initially trained on a zero-delay, conditional discrimination with lines (vertical and horizontal) and/or shapes (circle and triangle), and were then tested with longer delays between sample offset and comparison onset. Acquisition of the conditional discrimination was affected by both the number of sample and comparison stimuli: birds were slower to reach criterion the greater the number in either stimulus set. During delay testing, however, only the number of comparisons affected performance. Overall, retention was poorer with four comparisons than with two. These data provide evidence for prospective coding in pigeon short-term memory.     

Probability-reward preferences of rhesus monkeys

The relative preferences of rhesus monkeys for reward probability versus amount were investigated with procedures which contrasted general experience with specific instructions, and evaluated the relationship between probability-amount combinations and preference strength. Four stimulus objects, each signifying a different combination of reward frequency and amount (100% with one unit; 50% with two units; 33% with three units; or 25% with four units), were presented in pairs, one pair per daily session, with trial schedules providing the same amount of reward within each set of 12 trials. In Phase A, 4 monkeys (Group 1) were tested on the six choice-pairs with no preliminary training. In Phase B, Group 1 was joined by an additional 4 monkeys (Group 2), and each of the tasks was preceded by a demonstration of the relevant stimuli, one at a time, together with their associated probabilities and amounts. Group 1 animals developed preferences during Phase A for the more frequently rewarded objects, which persisted into Phase B, whereas Group 2 animals showed no preferences. This result indicates that preliminary instructions concerning the reward combinations associated with stimulus objects can prevent the development of a preference for greater probability over greater amount of reward, but cannot extinguish it once it has been formed or reestablished within the context of a particular task.     

Long-term memory for a common object

A series of experiments was done to determine how completely and accurately people remember the visual details of a common object, a United States penny. People were asked to: draw a penny from unaided recall; draw a penny given a list of its visual features; choose from among a list of possible features those which do appear on a penny; indicate what was wrong with an erroneous drawing of a penny; and select the correct representation of a penny from among a set of incorrect drawings. Performance was surprisingly poor on all tasks. On balance, the results were consistent with the idea that the visual details of an object, even a very familiar object, are typically available from memory only to the extent that they are useful in everyday life. It was also suggested that recognition tasks may make much smaller demands on memory than is commonly assumed.     

Stereotyping starlings are more ‘pessimistic’

Negative affect in humans and animals is known to cause individuals to interpret ambiguous stimuli pessimistically, a phenomenon termed ‘cognitive bias’. Here, we used captive European starlings (Sturnus vulgaris) to test the hypothesis that a reduction in environmental conditions, from enriched to non-enriched cages, would engender negative affect, and hence ‘pessimistic’ biases. We also explored whether individual differences in stereotypic behaviour (repetitive somersaulting) predicted ‘pessimism’. Eight birds were trained on a novel conditional discrimination task with differential rewards, in which background shade (light or dark) determined which of two covered dishes contained a food reward. The reward was small when the background was light, but large when the background was dark. We then presented background shades intermediate between those trained to assess the birds’ bias to choose the dish associated with the smaller food reward (a ‘pessimistic’ judgement) when the discriminative stimulus was ambiguous. Contrary to predictions, changes in the level of cage enrichment had no effect on ‘pessimism’. However, changes in the latency to choose and probability of expressing a choice suggested that birds learnt rapidly that trials with ambiguous stimuli were unreinforced. Individual differences in performance of stereotypies did predict ‘pessimism’. Specifically, birds that somersaulted were more likely to choose the dish associated with the smaller food reward in the presence of the most ambiguous discriminative stimulus. We propose that somersaulting is part of a wider suite of behavioural traits indicative of a stress response to captive conditions that is symptomatic of a negative affective state.     

An analysis of the solution of PAN ambiguous-cue problems by rhesus monkeys

Twenty-four monkeys were given 2-choice discrimination problems composed of three planometric plaque stimuli: P, the rewarded stimulus; N, the nonrewarded stimulus; and A, the ambiguous stimulus which was negative when paired with P, but positive when paired with N. When both pair of stimuli, PA and NA, were presented within a given session S was forced either to approach or to avoid the A plaque depending upon the stimulus with which it was paired. The results corroborated previous reports for plaque stimuli by showing PA performance to be superior to NA. The data also revealed that when plaque stimuli with distinctive cues were employed, NA performance exceeded PA as previously reported for stereometric object stimuli. Pretraining Ss with plaque stimuli possessing distinctive cues, then switching to plaques with less distinctive cues, also resulted in superior NA performance. These findings are discussed in the context of an interference-cue theory.     

Contextual conditional discriminations

Three experiments used a discriminated operant procedure to study conditional discrimination learning in rats. The first experiment showed that rats were capable of learning a biconditional discrimination in which two contexts served as conditional cues signalling the reinforcement contingencies associated with two discriminative stimuli. The discrimination was learned equally well when one discriminative stimulus signalled food, the other its absence, and when one stimulus signalled food, the other extinction plus mild footshock.

In Experiment 2 it was shown that prior training on such a conditional discrimination enhanced the subsequent context specificity of simple conditioning relative to control groups of animals for whom the prior training had not been conditional. Experiment 3 showed that a reversal of the significance of one pair of discriminative stimuli produced no spontaneous reversal in performance to a second, target, pair.

The pattern of results is best accounted for by an analysis of contextual conditional discrimination learning in terms of stimulus configurations and offers no support for the notion that rats may learn a general conditional rule or set.     

Mediated generalization in discrimination learning by Rhesus monkeys

Monkeys received discrimination training in which the choice of either a simultaneous or a serial compound of two visual images was rewarded before we assessed the monkeys' conditioned preference for one of the images from the compound. This preference was reduced or blocked if the other image had been associated with reward rather than nonreward prior to compound training. By contrast, the preference was enhanced if the other image was associated with reward rather than nonreward after compound training. The magnitudes of the blocking and enhancement were unaffected by the temporal structure, simultaneous or serial, of the stimulus compound. These results are discussed in terms of the representation of stimulus compounds, the role of within-compound associations, and the similarities between serial and simultaneous visual compounds as processed by monkeys.     

Mediated generalization in discrimination learning by Rhesus monkeys

Monkeys received discrimination training in which the choice of either a simultaneous or a serial compound of two visual images was rewarded before we assessed the monkeys' conditioned preference for one of the images from the compound. This preference was reduced or blocked if the other image had been associated with reward rather than nonreward prior to compound training. By contrast, the preference was enhanced if the other image was associated with reward rather than nonreward after compound training. The magnitudes of the blocking and enhancement were unaffected by the temporal structure, simultaneous or serial, of the stimulus compound. These results are discussed in terms of the representation of stimulus compounds, the role of within-compound associations, and the similarities between serial and simultaneous visual compounds as processed by monkeys.     

Some characteristics of concurrent discrimination and retention by monkeys

Before and after learning-set training, 12 rhesus monkeys were tested on the acquisition and retention of tasks consisting of eight concurrent object discrimination problems. Training on the concurrent discrimination was administered unitil a fairly stringent acquisition criterion was met. Under these procedures, retention, unlike acquisition, was little influenced by initial object preferences. Excellent retention was observed both before and after learning-set training. In a second experiment, these same monkeys were tested on a series of concurrent tasks which provided different numbers of objects as the sets of correct and incorrect discriminanda. Task solutions depended largely upon acquiring and retaining a list of correct objects despite designation of the large or small sets as the correct one. The animals seemed not to use “exclusion” strategies even when this might have provided an efficient task solution. It was considered that the monkeys' performances were based on stimulus sampling characteristics like those seen in other discrimination testing situations.     

Incidental learning,attention, and curiosity in squirrel monkeys calorie deprived as infants

The long-term effects of infantile malnutrition on information processing were studied in squirrel monkeys. From 2 through 8 weeks of age, four low-calorie infants were fed reduced amounts of a high protein diet, limiting their mean body weight gain to 23% of that of four well-fed controls, with all of this gain occurring during the first 2 weeks on the low-calorie diet. Thereafter, both groups received the standard diet ad libitum. At 68 weeks of age, incidental learning was tested by introducing shape cues that were redundant to the solution of a previously learned, two-choice, color discrimination. Removal of the color cues showed that the low-calorie monkeys had learned significantly less about the shape cues than had the controls. The basis for this incidental learning failure was then examined with an embedded-figures dicrimination, to measure attentional ability, and a novel stimulus substitution task, to measure curiosity. The groups did not differ in detecting embedded figures, but the low-calorie monkeys were significantly less likely than controls to select or handle novel stimuli that had been substituted for previously learned discriminative cues. It was concluded that undernutrition in infancy may induce a long-term strategy of only learning information which leads to an immediate reward, while suppressing the acquisition of information due to intrinsic curiosity.