Number Versus Contour Length in Infants' Discrimination of Small Visual Sets

This study examined infants' use of contour length in number discrimination tasks. We systematically varied number and contour length in a visual habituation experiment in order to separate these two variables. Sixteen 6- to 8-month-old infants were habituated to displays of either two or three black squares on a page. They were then tested with alternating displays of either a familiar number of squares with a novel contour length or a novel number of squares with a familiar contour length. Infants dishabituated to the display that changed in contour length, but not to the display that changed in number. We conclude that infants base their discriminations on contour length or some other continuous variable that correlates with it, rather than on number.     

Conditional discrimination in mentally retarded adults: the development of generalized skills.

The development of generalized conditional discrimination skills was examined in adults with retardation. Two subjects with histories of failure to acquire arbitrary matching under trial-and-error procedures were successful under procedures that trained one or more prerequisite skills. The successive discrimination between the sample stimuli was established by training the subjects to name the stimuli. The simultaneous discrimination between the comparison stimuli was established using either (a) standard simple discrimination training with reversals or (b) a procedure in which each of the two sample-comparison relations in the conditional discrimination was presented in blocks of trials, with the size of the blocks decreasing gradually until sample presentation was randomized. The amount of prerequisite training required varied across subjects and across successive conditional discriminations. After acquiring either two or three conditional discriminations with component training, both subjects learned new conditional discriminations under trial-and-error procedures. In general, each successive conditional discrimination was acquired more rapidly. Tests showed that conditional responding had become a generalized skill. Symmetry was shown for almost all trained relations. Symmetry trial samples were ultimately named the same as the stimuli to which they were related in training.     

Retrograde amnesia for visual memories after hippocampal damage in rats

It is generally believed that the hippocampus is not required for simple discrimination learning. However, a small number of studies have shown that hippocampus damage impairs retention of a previously learned visual discrimination task. We propose that, although simple discrimination learning may proceed in the absence of the hippocampus, it plays an important role in this type of learning when it is intact. In order to test the role of the hippocampus in simple discrimination learning, we performed a series of experiments utilizing a two-choice picture discrimination task. Our experiments confirm that rats readily learn simple two-choice picture discriminations after hippocampus damage. However, if such discriminations are first learned while the hippocampus is intact, subsequent hippocampus damage causes severe retrograde amnesia for the discriminations. Furthermore, retrograde amnesia for simple picture discriminations was equally severe when the interval between training and damage was 1 d or 60 d; remote picture memories are not spared. Similarly, the rule or schema underlying a recently or remotely acquired picture discrimination learning set was lost after hippocampus damage. The severity of retrograde amnesia for simple picture discriminations is negatively correlated with the volume of spared hippocampus tissue. Thus, the hippocampus plays an essential role in long-term memories supporting simple picture discriminations.     

Conditional discrimination of series containing either two or three rewarded trials

Series of trials differing only in the number of rewarded trials (2R or 3R series) were used to test rats' ability to learn conditional numerical discriminations. Although the series were conditionally related to initially non-numerical situations, the rats successfully learned the discrimination. It is concluded that these discriminations can plausibly be conceptualized as conditional counting. A pair of transfer tests given after the discriminations were well established suggested that, under these conditions, numerical information will overshadow other non-numerical sources of discrimination made relevant by the transfer shifts.     

Number sense in human infants

Four experiments used a preferential looking method to investigate 6-month-old infants' capacity to represent numerosity in visual-spatial displays. Building on previous findings that such infants discriminate between arrays of eight versus 16 discs, but not eight versus 12 discs (Xu & Spelke, 2000), Experiments 1 and 2 investigated whether infants' numerosity discrimination depends on the ratio of the two set sizes with even larger numerosities. Infants successfully discriminated between arrays of 16 versus 32 discs, but not 16 versus 24 discs, providing evidence that their discrimination shows the set-size ratio signature of numerosity discrimination in human adults, children and many non-human animals. Experiments 3 and 4 addressed a controversy concerning infants' ability to discriminate large numerosities (observed under conditions that control for total filled area, array size and density, item size and correlated properties such as brightness: Brannon, 2002; Xu, 2003b; Xu & Spelke, 2000) versus small numerosities (not observed under conditions that control for total contour length: Clearfield & Mix, 1999). To investigate the sources of these differing findings, Experiment 3 tested infants' large-number discrimination with controls for contour length, and Experiment 4 tested small-number discrimination with controls for total filled area. Infants successfully discriminated the large-number displays but showed no evidence of discriminating the small-number displays. These findings provide evidence that infants have robust abilities to represent large numerosities. In contrast, infants may fail to represent small numerosities in visual-spatial arrays with continuous quantity controls, consistent with the thesis that separate systems serve to represent large versus small numerosities.     

Visual discrimination learning in rats with lesions of superior colliculus: Door-push and approach errors in modified jumping stand

It has been suggested that, for some species, lesions of the superior colliculus affect visual discrimination learning, but only in certain conditions: (a) when problems are first learnt only after operation, or (b) when discriminanda require detailed scanning, or (c) when “approach” responses to the discriminanda are measured, rather than the response of actually touching them. These suggestions were examined in rats learning visual discriminations in a modified jumping-stand apparatus, after sustaining large lesions of the superior colliculus (and in some cases also of the pretectum). The lesions produced open-field hyperactivity and reduced exploration, indicating effective tectal damage, but the rats learnt a series of difficult discriminations in a door-push task as fast as normal rats, and they did not make more approach errors. Their main abnormality in the discrimination apparatus was that they looked less often between the stimulus doors before stepping across to one of them from the central platform. It is suggested that in rats, as in other animals, lesions of the superior colliculus disrupt the control of scanning head and eye movements; in rats, however, such disruption need not affect discrimination learning (at least in some kinds of apparatus), possibly because the retina of the rat has a relatively poorly developed area centralis.     

A discrimination analysis of training-structure effects on stimulus equivalence outcomes.

Experiments designed to establish stimulus equivalence classes frequently produce differential outcomes that may be attributable to training structure, defined as the order and arrangement of baseline conditional discrimination training trials. Several possible explanations for these differences have been suggested. Here we develop a hypothesis based on an analysis of the simple simultaneous and successive discriminations embedded in conditional discrimination training and testing within each of the training structures that are typically used in stimulus equivalence experiments. Our analysis shows that only the comparison-as-node (many-to-one) structure presents all the simple discriminations in training that are subsequently required for consistently positive outcomes on all tests for the properties of equivalence. The sample-as-node (one-to-many) training structure does not present all the simple discriminations required for positive outcomes on either the symmetry or combined transitivity and symmetry (equivalence) tests. The linear-series training structure presents all the simple discriminations required for consistently positive outcomes on tests for symmetry, but not for symmetry and transitivity combined (equivalence) or transitivity alone. Further, the difference in the number of simple discriminations presented in comparison-as-node training versus the other training structures is larger when the intended class size is greater than three or the number of classes is larger than two. We discuss the relevance of this analysis to interpretations of stimulus equivalence research, as well as some methodological and theoretical implications.     

Stable individual differences in number discrimination in infancy

Previous studies have shown that as a group 6-month-old infants successfully discriminate numerical changes when the values differ by at least a 1:2 ratio but fail at a 2:3 ratio (e.g. 8 vs. 16 but not 8 vs. 12). However, no studies have yet examined individual differences in number discrimination in infancy. Using a novel numerical change detection paradigm, we present more direct evidence that infants' numerical perception is ratio-dependent even within the range of discriminable ratios and thus adheres to Weber's Law. Furthermore, we show that infants' numerical discrimination at 6 months reliably predicts their numerical discrimination abilities but not visual short-term memory at 9 months. Thus, individual differences in numerical discrimination acuity may be stable within the first year of life and provide important avenues for future longitudinal research exploring the relationship between infant numerical discrimination and later developing math achievement.     

Conditional discrimination in mentally retarded subjects: programming acquisition and learning set.

In Experiment 1, 3 subjects with retardation were exposed to two visual-visual arbitrary matching-to-sample problems each day. One conditional discrimination was presented under trial-and-error conditions, and the other was presented under a component training procedure. The latter began by establishing the comparison discrimination and its rapid reversal. The successive discrimination between the sample stimuli was established through differential naming. Then, sample naming was maintained in conditional discrimination sessions in which the same sample was presented in blocks of consecutive trials. Block size was decreased across sessions until sample presentation was randomized as in trial-and-error training (but with naming maintained). Two subjects initially learned only with component training. The performance of the 3rd subject was inconsistent across conditional discriminations. One of the successful subjects ultimately learned rapidly and consistently with trial-and-error procedures. Experiment 2 sought to demonstrate learning set in the other 2 subjects. Elements of the component training procedure were withdrawn over successive conditional discriminations. Ultimately, 1 subject nearly always learned under trial-and-error conditions, and the other learned under trial-and-error conditions combined with differential sample naming.     

Stimulus-outcome interactions during instrumental discrimination learning by rats and humans

The associative structure mediating goal-directed action was investigated using congruent and incongruent conditional discriminations. The stimulus was the same as the outcome in each component of the congruent discriminations, whereas the stimulus of one component of the incongruent discriminations was the same as the outcome of the other component. Humans, but not rats, learned the congruent discrimination more rapidly than the incongruent discrimination, a difference that the authors attribute to the fact that outcome-response associations caused response conflict in the incongruent discrimination. Moreover, responding was resistant to outcome devaluation following incongruent, but not congruent, training, suggesting that both humans and rats adopted a stimulus-response strategy to resolve the incongruent discrimination.     

Teaching autistic children to respond to simultaneous multiple cues

A number of studies have shown that autistic children tend to learn new discriminations by responding to only a restricted number of available cues and that this may be responsible for some of their abnormal behavior. Therefore, this investigation assessed the feasibility of teaching autistic children to respond to multiple cues. The results showed that four autistic children could learn a conditional discrimination requiring them to discriminate a multiple-cue complex from each of its two component cues. However, the autistic children did not learn this discrimination in the same manner as normal children. In the early trials, the autistics responded at a higher level to one of the two component cues. Only after many trials did the autistics respond equally on the basis of both component cues. The results of an initial attempt to teach a general set to respond to multiple cues showed that, when an autistic child was taught a series of successive conditional discriminations, the child eventually learned a set to approach new discriminations by responding equally on the basis of both component cues. The results are discussed in terms of understanding and treating autistic children's abnormal development.     

Comparing patterning and biconditional discriminations in humans

In two experiments, human participants performed a causal judgment task that simultaneously comprised two reciprocal patterning discriminations and a biconditional discrimination. They learned both patterning discriminations more quickly than the biconditional discrimination. Postdiscrimination tests were used to identify participants who had, or had not, learned to apply the patterning rules, as well as participants who continued to expect summation when presented with two cues that predicted the same outcome. All groups were faster to learn the patterning than the biconditional discriminations. These results are inconsistent with models of stimulus representation that invoke configural representations (e.g., Pearce, 1987, 1994; Rescorla & Wagner, 1972) because these models solve biconditional discriminations more readily than patterning discriminations.     

TEACHING GENERALIZED RECEPTIVE AND PRODUCTIVE BEHIND-FRONT DISCRIMINATIONS TO TWO RETARDED CHILDREN1

Typically, a generalized discrimination for two linguistic forms (e.g., “behind the (noun)” and “in front of the (noun)”) has been developed in a sequence of three treatment conditions, with generative usage developed for each form consecutively in the first two conditions and a generalized discrimination developed for those forms in the third condition. Moreover, generalized discriminations have typically been measured by a set of unreinforced probes, rather than by first-trial performances across a series of new tasks, as in some studies that have taught generative usage of a single form of language. In this study, successive sets of instructions were presented to two children. Each set contained four tasks, the four possible combinations of two new nouns with the linguistic forms “Put the (noun) behind the (noun)” and “Put the (noun) in front of the (noun)”. For each set, receptive usage of nouns was taught, and then the four instructions were presented, once each, to test for generative usage of the prepositional forms. First-trial performances were never prompted. After a baseline of five sets of tasks, correct first-trial performances earned praise and tokens. Also, in each set with three or fewer such performances, the specific receptive discriminations were taught to a criterion before presentation of the next set. Successive sets were presented until all first-trial performances were correct in five consecutive sets, whereupon this generalized discrimination was reversed and subsequently reinstated. Similar procedures were then used to teach the same children a generalized productive-discrimination for the forms “behind the (noun)” and “in front of the (noun)”. This study demonstrated the development of generalized receptive and productive behind-front discriminations without the prior development of generative usage in each linguistic form and with measurement of the generalized discrimination across first-trial performances. The method may offer a less time-consuming procedure for the development and measurement of generalized discriminations in receptive and productive language.     

Contrast discrimination in human infants

The ability to detect differences in spatial contrast is crucial to object recognition and identification. This ability is generally examined by measuring the contrast discrimination function. This function represents, for a variety of conditions, the smallest contrast difference required to discriminate otherwise identical patterns. We examined human infants' ability to discriminate patterns on the basis of differences in spatial contrast. The forced-choice preferential looking procedure was used to estimate contrast increment thresholds at a number of background contrasts. The Weber fractions of 6- and 12-week-old infants were about 1 log unit higher than adult values for background contrasts ranging from 0.14 to 0.55. Furthermore, the slopes of infants' discrimination functions were much shallower than those of adults. These age differences in contrast discrimination imply certain changes in the neural mechanisms that underlie contrast encoding. They also aid our understanding of the anomalies observed in early pattern vision.     

Derived control by compound and single stimuli in a matching-to-sample task in children

A matching-to-sample procedure was used to investigate whether 9-year-old children would demonstrate the emergence of a derived compound-sample conditional discrimination following training in four interrelated single-sample conditional discriminations and vice versa, as adults did in previous studies. In Experiment 1, three out of three children demonstrated the emergence of a compound-sample conditional discrimination following training in four single-sample conditional discriminations. In Experiment 2, two out of three children acquired a compound-sample conditional discrimination and they demonstrated the emergence of four single-sample conditional discriminations; one of them did so only after being exposed to a remediation training and testing procedure. Training variables that facilitated discrimination emergence in both directions are discussed. In general, results showed that the sophisticated learning skills that are supposedly possessed by adults are not required to demonstrate the two types of derived control under study.     

Perception of acoustic correlates of major phrasal units by young infants.

How might young learners parse speech into linguistically relevant units? Sensitivity to prosodic markers of these segments is one possibility. Seven experiments examined infants' sensitivity to acoustic correlates of phrasal units in English. The results suggest that: (a) 9 month olds, but not 6 month olds, are attuned to cues that differentially mark speech that is artificially segmented at linguistically COINCIDENT as opposed to NONCOINCIDENT boundaries (Experiments 1 and 2); (b) the pattern holds across both subject phrases and predicate phrases and across samples of both Child- and Adult-directed speech (Experiments 3, 4, and 7); and (c) both 9 month olds and adults show the sensitivity even when most phonetic information is removed by low-pass filtering (Experiments 5 and (6). Acoustic analyses suggest that pitch changes and in some cases durational changes are potential cues that infants might be using to make their discriminations. These findings are discussed with respect to their implications for theories of language acquisition.     

Comparison of two discrimination methods in teaching Chinese children with autism

In teaching conditional discriminations to children with autism spectrum disorder (ASD), practitioners may progress from simple to conditional discriminations or may teach conditional discriminations from the onset of instruction. Some research indicates that teaching simple discriminations first may be unnecessary and that teaching may more efficiently focus on conditional discriminations exclusively. This study replicated comparisons of simple-to-conditional and conditional-only discrimination training methods in teaching audio-visual conditional discriminations to Chinese preschoolers with ASD. Results indicated the conditional-only training method appeared to be more efficient in teaching these skills.     

Elemental and Configural Processes in Patterning Discrimination Learning

Three experiments used appetitive conditioning with rats to examine the involvement of elemental and configural processes in positive and negative patterning discriminations. The first experiment demonstrated that negative and, to some extent, positive patterning discriminations were learned more rapidly when these discriminations consisted of stimulus elements that had previously been the reinforced as opposed to the non-reinforced elements of a simple discrimination. Experiment 2 revealed an excitatory summation effect during the early phase of negative patterning learning that depended upon discrimination pretraining. The final experiment demonstrated faster discrimination learning between the compound and the less salient, rather than the more salient, element of an instrumental patterning task. The present set of results were interpreted as reflecting the possibility, consistent with connectionist theory, that internal representations of the conditioned stimuli change over the course of a patterning discrimination.     

Apparent motion enhances visual rhythm discrimination in infancy

Many studies have demonstrated that infants exhibit robust auditory rhythm discrimination, but research on infants' perception of visual rhythm is limited. In particular, the role of motion in infants' perception of visual rhythm remains unknown, despite the prevalence of motion cues in naturally occurring visual rhythms. In the present study, we examined the role of motion in 7-month-old infants' discrimination of visual rhythms by comparing experimental conditions with apparent motion in the stimuli versus stationary rhythmic stimuli. Infants succeeded at discriminating visual rhythms only when the visual rhythm occurred with an apparent motion component. These results support the view that motion plays a role in infants' perception of visual temporal information, consistent with the manner in which natural rhythms appear in the visual world.     

Intersensory redundancy guides attentional selectivity and perceptual learning in infancy

This study assessed an intersensory redundancy hypothesis, which holds that in early infancy information presented redundantly and in temporal synchrony across two sense modalities selectively recruits attention and facilitates perceptual differentiation more effectively than does the same information presented unimodally. Five-month-old infants' sensitivity to the amodal property of rhythm was examined in 3 experiments. Results revealed that habituation to a bimodal (auditory and visual) rhythm resulted in discrimination of a novel rhythm, whereas habituation to the same rhythm presented unimodally (auditory or visual) resulted in no evidence of discrimination. Also, temporal synchrony between the bimodal auditory and visual information was necessary for rhythm discrimination. These findings support an intersensory redundancy hypothesis and provide further evidence for the importance of redundancy for guiding and constraining early perceptual learning.