Signal clarity: an account of the variability in infant quantity discrimination tasks

Infants have shown variable success in quantity comparison tasks, with infants of a given age sometimes successfully discriminating numerical differences at a 2:3 ratio but requiring 1:2 and even 1:4 ratios of change at other times. The current explanations for these variable results include the two‐systems proposal – a theoretical framework that suggests that there are multiple systems at play and that these systems do not communicate early in infancy, leading to failure in certain numerical comparisons. An alternative proposal is that infants may be attending to continuous extent dimensions in these tasks rather than number per se. However, neither of these two main proposals is independently capable of accounting for the previously published data. Recently the Signal Clarity Hypothesis was proposed to account for and predict the variability (Cantrell & Smith, 2013). According to this hypothesis, infants’ variable success may be understood from a framework of statistical learning taken together with the signal‐to‐noise ratio generated by control procedures in habituation tasks. Here we test specific predictions made by the Signal Clarity Hypothesis. Across four experiments assessing 9‐month old discriminations of small and large sets (2 vs. 4 and 3 vs. 4), we demonstrate that infant success can be predicted by this novel approach and, further, that infants may discriminate smaller ratios of change than previously believed (3:4 numerical change and 2:3 cumulative area change).     

Rule learning by cotton-top tamarins

Previous work suggests that human infants are capable of rapidly generalizing patterns that have been characterized as abstract algebraic rules (Science 283 (1999) 77), a process that may play a pivotal role in language acquisition. Here we explore whether this capacity is uniquely human and evolved specifically for the computational problems associated with language, or whether this mechanism is shared with other species, and therefore evolved for problems other than language. We used the same materials and methods that were originally employed in tests of human infants to assess whether cotton-top tamarin monkeys can extract abstract algebraic rules. Specifically, we habituated subjects to sequences of consonant-vowel syllables that followed one of two patterns, AAB (e.g. wi wi di) or ABB (le we we). Following habituation, we presented subjects with two novel test items, one with the same pattern as that presented during habituation and one with a different pattern. Like human infants, tamarins were more likely to dishabituate to the test item with a different pattern. We conclude that the capacity to generalize rule-like patterns, at least at the level demonstrated, did not evolve specifically for language acquisition, though it remains possible that infants might use such rules during language acquisition.     

A first step in form-based category abstraction by 12-month-old infants

The present experiments investigate how young language learners begin to acquire form-based categories and the relationships between them. We investigated this question by exposing 12-month-olds to auditory structure of the form aX and bY (infants had to learn that a-elements grouped with Xs and not Ys). Infants were then tested on strings from their training language versus strings from the other language using a preferential-listening procedure. Importantly, the X and Y elements were new at test, requiring infants to generalize to novel pairings. We also manipulated the probability of encountering grammatical structures of the training language by mixing strings from two artificial languages according to 83/17 and 67/33 percentage ratios in Experiment 2. Experiment 1 shows that 12-month-olds are capable of forming categories of X- and Y-elements based on a shared feature and, furthermore, form associations between particular a- and b-elements and these categories. Experiment 2 shows that learning was sustained even when 17% of instances from another language were present during training. However, infants failed to generalize when exposed to a larger percentage of strings from another language. The findings demonstrate that the first step of form-based category abstraction (the ability to generalize based on marker-feature pairings) is in place by 12 months of age.     

Perception of the symmetrical patterning of human gait by infants

Two experiments tested 3- and 5-month-old infants' sensitivity to properties of point-light displays of human gait. In Experiment 1, infants were tested for discrimination of point-light displays of a walker and a runner, which, although they differed in many ways, were equivalent with regard to the phasing of limb movements. Results revealed that 3-month-old, but not 5-month-old, infants discriminated these displays. In Experiment 2, the symmetrical phase-patterning of the runner display was perturbed by advancing two of its limbs by 25% of the gait cycle. Both 3- and 5-month-old infants discriminated the walker display from this new phase-shifted runner display. These findings suggest that 3-month-old infants respond to the absolute and relative motions within a single limb, whereas 5-month-old infants respond primarily to the relations between limbs and, in particular, to the bilateral symmetry between the limbs.     

Early information processing among infants with and without spina bifida

This study focuses on the development of early visual information processing among infants with spina bifida (SB) compared to typically developing infants using the habituation–dishabituation paradigm. Analyses were conducted in two stages. First infants were evaluated to determine if 18-month old infants (SB = 47; Control = 40) differed in their ability to shift attention and habituate to two female faces, as well as their responses to composite and novel stimuli. Second, relations between these variables and infant motor and mental functioning were evaluated. The results of the study indicated that difficulties with visual attention skills can be detected as early as 18 months of age among infants with SB. Infants with SB differed significantly from controls on attention getting. Although there were no differences found on habituation and composite tasks, infants with SB differed significantly from controls on their ability to dishabituate. Implications are discussed.     

Newborn habituation to visual stimuli: a sex difference in novelty detection

Thirty-six newborn human infants were repeatedly exposed to one of two visual stimuli, a four-square or 144-square black and white checkerboard target, until a set criterion of habituation was demonstrated, as measured by a decrement in visual fixation time. When the habituation criterion was reached, independent groups of Ss were either presented with the same target or with a target of either moderate or large discrepancy from the standard habituation stimulus. Results indicated that (1) there was habituation of visual attention, which replicated previous findings, and suggests that some infants soon after birth are capable of storing simple visual information, and (2) following habituation female infants displayed greater recovery of attention than male infants when the moderate stimulus change was introduced.     

Stimulus effects on cancellation task performance in children with and without dyslexia

Both the sensitivity and administration time of a test are important in evaluating visuospatial attention in clinical settings, especially with respect to external validity. The purpose of the present study was to propose an adaptive model that provides a reference for test modification by manipulating target-to-distractor (T/D) ratios and the number of stimuli on the computerized cancellation test system. Tasks with different T/D ratios and numbers of stimuli were presented to two groups—children with and without dyslexia (n=41 and 65, respectively)—to determine whether their visuospatial attention performance differed on different test forms. In general, there were significant differences between the two groups in hit rates, completion times, and performance quality (PQ) scores. The PQ score of visual attention was affected by the T/D ratios rather than by the number of stimuli. The findings suggested that the T/D ratio has a strong effect on PQ scores, and that it should be taken into consideration in test and task design.     

Binocular vision and spatial perception in 4- and 5-month-old infants

Four experiments investigated the relation between the development of binocular vision and infant spatial perception. Experiments 1 and 2 compared monocular and binocular depth perception in 4- and 5-month-old infants. Infants in both age groups reached more consistently for the nearer of two objects under binocular viewing conditions than under monocular viewing conditions. Experiments 3 and 4 investigated whether the superiority of binocular depth perception in 4-month-olds is related to the development of sensitivity to binocular disparity. Under binocular viewing conditions in Experiment 3, infants identified as disparity-sensitive reached more consistently for the nearer object than did infants identified as disparity-insensitive. The two groups' performances did not differ under monocular viewing conditions. These results suggest that, binocularly, the disparity-sensitive infants perceived the objects' distances more accurately than did the disparity-insensitive infants. In Experiment 4, infants were habituated to an object, then presented with the same object and a novel object that differed only in size. Disparity-sensitive infants showed size constancy by recovering from habituation when viewing the novel object. Disparity-insensitive infants did not show clear evidence of size constancy. These findings suggest that the development of sensitivity to binocular disparity is accompanied by a substantial increase in the accuracy of infant spatial perception.     

Development of the perception of invariants: substance and shape

Three experiments investigated the perception of substance and shape as invariant properties of objects by three-month-old infants. In experiment 1, infants were habituated to two differently shaped objects undergoing a rigid motion. After habituation of the infants, the objects were presented undergoing a different rigid motion, or undergoing a deforming motion, or undergoing the same rigid motion. Habituation was maintained to the new rigid motion, indicating that the two rigid motions were perceived as sharing an invariant property. Dishabituation, on the other hand, occurred when a deforming motion followed a rigid one. In experiment 2, infants were habituated to one shape undergoing two different rigid motions. After habituation, the shape was changed but the same two motions continued. Dishabituation occurred, compared to a group with no shape change, indicating that shape is distinguished as an invariant property over two rigid motions. In experiment 3, habituation to a shape undergoing two rigid motions was followed by a new shape presented motionless, or the same shape presented motionless. Cessation of motion did not prevent recognition of shape as invariant. Two properties of an object, substance and shape, thus appear to be detectable as invariant in an event sequence, an instance of "phenomenal doubling" at an early age.     

Fourteen‐month‐old infants learn similar‐sounding words

Can infants, in the very first stages of word learning, use their perceptual sensitivity to the phonetics of speech while learning words? Research to date suggests that infants of 14 months cannot learn two similar‐sounding words unless there is substantial contextual support. The current experiment advances our understanding of this failure by testing whether the source of infants’ difficulty lies in the learning or testing phase. Infants were taught to associate two similar‐sounding words with two different objects, and tested using a visual choice method rather than the standard Switch task. The results reveal that 14‐month‐olds are capable of learning and mapping two similar‐sounding labels; they can apply phonetic detail in new words. The findings are discussed in relation to infants’ concurrent failure, and the developmental transition to success, in the Switch task.     

Auditory-visual intermodal matching of small numerosities in 6-month-old infants

Recent studies have reported that preverbal infants are able to discriminate between numerosities of sets presented within a particular modality. There is still debate, however, over whether they are able to perform intermodal numerosity matching, i.e. to relate numerosities of sets presented with different sensory modalities. The present study investigated auditory-visual intermodal matching of small numerosities in infancy by using a violation-of-expectation paradigm. After being familiarized with events of a few objects impacting a surface successively, 6-month-old infants were alternatively presented with two and three tones while the movement of each object remained hidden behind an opaque screen. The screen was then removed to reveal either two or three objects. Results showed that the infants looked significantly longer at the numerically nonequivalent events (the three-tone/two-object and the two-tone/three-object events) than at the numerically equivalent events (the two-tone/two-object and the three-tone/three-object events) irrespective of the rate or duration of auditory tones presented. These findings suggest that infants are capable of performing intermodal matching of small numerosities and that they might possess abstract representations of numerosity beyond sensory modalities.     

Individuation of pairs of objects in infancy

Looking-time studies examined whether 11-month-old infants can individuate two pairs of objects using only shape information. In order to test individuation, the object pairs were presented sequentially. Infants were familiarized either with the sequential pairs, disk-triangle/disk-triangle (XY/XY), whose shapes differed within but not across pairs, or with the sequential pairs, disk-disk/triangle-triangle (XX/YY), whose shapes differed across but not within pairs. The XY/XY presentation looked to adults like a single pair of objects presented repeatedly, whereas the XX/YY presentation looked like different pairs of objects. Following familiarization to these displays, infants were given a series of test trials in which the screen was removed, revealing two pairs of objects in one of two outcomes, XYXY or XXYY. On the first test trial, infants familiarized with the identical pairs (XY/XY) apparently expected a single pair to be revealed because they looked longer than infants familiarized with the distinct pairs (XX/YY). Infants who had seen the distinct pairs apparently expected a double pair outcome. A second experiment showed outcomes of a single XY pair. This outcome is unexpected for XX/YY-familiarized infants but expected for XY/XY-familiarized infants, the reverse of Experiment 1. This time looking times were longer for XX/YY infants. Eleven-month-olds appear to be able to represent not just individual objects but also pairs of objects. These results suggest that if they can group the objects into sets, infants may be able to track more objects than their numerosity limit or available working memory slots would normally allow. We suggest possible small exact numerosity representations that would allow tracking of such sets.     

A comparison of two types of extinction following fixed-ratio training

Five groups of pigeons were food reinforced on various schedules. Half of each group were extinguished in the normal manner; the others were presented with a stimulus change, previously paired with reinforcement, each time they completed their respective fixed ratios. Response rate in training was an increasing negatively accelerated function of the FR. Increasing the FR produced transitory rate changes, the amount of which yielded a quantitative index of ratio strain. Cumulative records of extinction performance revealed that the stimulus change exerted discriminative control by maintaining the cohesiveness of FR response units. Nevertheless, neither the absolute number of extinction responses nor extinction response units differed appreciably for the two extinction procedures.     

Speed discrimination in 6- and 10-month-old infants follows Weber's law

The speed of a moving object is a critical variable that factors into actions such as crossing a street and catching a ball. However, it is not clear when the ability to discriminate between different speeds develops. Here, we investigated speed discrimination in 6- and 10-month-old infants using a habituation paradigm showing infants events of a ball rolling at different speeds. The 6-month-olds looked longer at novel speeds that differed by a 1:2 ratio than at the familiar ones but showed no difference in looking time to speeds that differed by a 2:3 ratio. In contrast, the 10-month-olds succeeded at discriminating a 2:3 ratio. For both age groups, discrimination was modulated by the ratio between novel and familiar speeds, suggesting that speed discrimination is subject to Weber's law. These findings show striking parallels to previous results in infants' discrimination of duration, size, and number and suggest a shared system for processing different magnitudes.     

Baby arithmetic: one object plus one tone

Recent studies using a violation-of-expectation task suggest that preverbal infants are capable of recognizing basic arithmetical operations involving visual objects. There is still debate, however, over whether their performance is based on any expectation of the arithmetical operations, or on a general perceptual tendency to prefer visually familiar and complex displays. Here we provide new evidence that 5-month-old infants recognize basic arithmetic operations across sensory modalities. Using a violation-of-expectation task that eliminated the possibility of the familiarity and complexity preference, 5-month-old infants were presented alternatively with two types of arithmetical events: the expected, correct outcomes of operations (1 object+1 tone=2 objects and 1 object+2 tones=3 objects) and the unexpected, incorrect ones (1 object+2 tones=2 objects and 1 object+1 tone=3 objects). Results showed that subjects looked significantly longer at the unexpected events than at the expected events, suggesting that infants are able to recognize basic arithmetic operations across sensory modalities.     

Segmentation of the speech stream in a non-human primate: statistical learning in cotton-top tamarins

Previous work has shown that human adults, children, and infants can rapidly compute sequential statistics from a stream of speech and then use these statistics to determine which syllable sequences form potential words. In the present paper we ask whether this ability reflects a mechanism unique to humans, or might be used by other species as well, to acquire serially organized patterns. In a series of four experimental conditions, we exposed a New World monkey, the cotton-top tamarin (Saguinus oedipus), to the same speech streams used by Saffran, Aslin, and Newport (Science 274 (1996) 1926) with human infants, and then tested their learning using similar methods to those used with infants. Like humans, tamarins showed clear evidence of discriminating between sequences of syllables that differed only in the frequency or probability with which they occurred in the input streams. These results suggest that both humans and non-human primates possess mechanisms capable of computing these particular aspects of serial order. Future work must now show where humans' (adults and infants) and non-human primates' abilities in these tasks diverge.     

Preference for rotating objects in 5-month-old infants

Five-month-olds were habituated to several objects undergoing a variety of rotations or nonrotations. The test trials presented a new object undergoing examples of the old and new motions. Without exception, infants preferred the rotating object on the test trials, regardless of their previous experience.     

Neural and behavioral correlates of visual recognition memory in 4- and 8-month-old infants.

The neural and behavioral correlates of the 4- and 8-month-old infant's ability to distinguish between frequently and infrequently presented familiar and novel events was examined. Cortical event-related potentials (ERPs) were recorded as infants were familiarized to two faces. During the test trials that followed, one of these faces was presented frequently, and the other infrequently; on each of the remaining 20% of the trials, a previously unseen, novel face was presented. Following the ERP phase, infants' looking times were recorded to pairs of faces, some of which had been seen during the ERP testing, and some of which had not. At 4 months the ERP activity invoked by the three classes of events was similar, suggesting that infants were unable to distinguish among them. At 8 months the ERP activity differed only between the Infrequent Novel events and the two classes of familiar events, but did not differ between the frequently and infrequently presented familiar events. The ERP findings complement previously reported data from 6-month-old infants in describing a trend whereby infants become increasingly able to respond to stimuli on the basis of whether they have been seen before, and not on the basis of how often they had been seen. The behavioral data at both 4 and 8 months were less clear cut than the ERP data. These findings are discussed in the context of the neural and cognitive processes involved in dissociating probability information from novelty detection.     

The first steps in word learning are easier when the shoes fit: comparing monolingual and bilingual infants

English, French, and bilingual English-French 17-month-old infants were compared for their performance on a word learning task using the Switch task. Object names presented a /b/ vs. /g/ contrast that is phonemic in both English and French, and auditory strings comprised English and French pronunciations by an adult bilingual. Infants were habituated to two novel objects labeled 'bowce' or 'gowce' and were then presented with a switch trial where a familiar word and familiar object were paired in a novel combination, and a same trial with a familiar word–object pairing. Bilingual infants looked significantly longer to switch vs. same trials, but English and French monolinguals did not, suggesting that bilingual infants can learn word–object associations when the phonetic conditions favor their input. Monolingual infants likely failed because the bilingual mode of presentation increased phonetic variability and did not match their real-world input. Experiment 2 tested this hypothesis by presenting monolingual infants with nonce word tokens restricted to native language pronunciations. Monolinguals succeeded in this case. Experiment 3 revealed that the presence of unfamiliar pronunciations in Experiment 2, rather than a reduction in overall phonetic variability was the key factor to success, as French infants failed when tested with English pronunciations of the nonce words. Thus phonetic variability impacts how infants perform in the switch task in ways that contribute to differences in monolingual and bilingual performance. Moreover, both monolinguals and bilinguals are developing adaptive speech processing skills that are specific to the language(s) they are learning.     

The relative contribution of relative height, linear perspective, and texture gradients to pictorial depth perception in 7-month-old infants

In the current preferential-reaching experiments, 7-month-olds were tested for their ability to respond to a combination of relative height and texture gradients. The infants were presented with a display in which these pictorial depth cues specified that two toys were at different distances. The experimental displays differed from the textured surfaces employed in earlier studies in that linear perspective of the contours of the texture elements was omitted. Experiment A shows that the infants still preferred to reach for the apparently nearer toy under monocular, but not binocular, viewing conditions, indicating that they responded to the pictorial depth cues. In experiment B, relative height and texture provided the infants with conflicting information for depth. Here, relative height outperformed texture information. A statistical comparison between the experiments as well as systematic comparisons with experimental conditions from an earlier study (Hemker et al, 2010 Infancy 15 6-27) revealed that texture gradients, unlike linear perspective, neither enhanced nor weakened the effect exerted by relative height. In sum, 7-month-old infants are obviously more sensitive to relative height and to the linear perspective of the surface contours than to the texture gradients of compression, perspective, and density.