Perception of syllable-final stop consonants by 2-month-old infants

Previous research has shown that infants are capable of perceiving many phonetic distinctions between initial segments of syllables. The present study demonstrates that 2-month-old infants have the ability to distinguish syllables differing only in their final segments. Infants were found to be sensitive to place-of-articulation differences for stop consonants in final segments of both consonant-vowel-consonant (CVC) and vowel-consonant (VC) syllable pairs. Contrary to previous reports for older infants (Shvachkin, 1973), there was no indication that 2-month-olds have any more difficulty with contrasts of final-stop consonant:3 than they do with initial ones.     

Visual speech contributes to phonetic learning in 6-month-old infants

Previous research has shown that infants match vowel sounds to facial displays of vowel articulation [Kuhl, P. K., & Meltzoff, A. N. (1982). The bimodal perception of speech in infancy. Science, 218, 1138–1141; Patterson, M. L., & Werker, J. F. (1999). Matching phonetic information in lips and voice is robust in 4.5-month-old infants. Infant Behaviour & Development, 22, 237–247], and integrate seen and heard speech sounds [Rosenblum, L. D., Schmuckler, M. A., & Johnson, J. A. (1997). The McGurk effect in infants. Perception & Psychophysics, 59, 347–357; Burnham, D., & Dodd, B. (2004). Auditory-visual speech integration by prelinguistic infants: Perception of an emergent consonant in the McGurk effect. Developmental Psychobiology, 45, 204–220]. However, the role of visual speech in language development remains unknown. Our aim was to determine whether seen articulations enhance phoneme discrimination, thereby playing a role in phonetic category learning. We exposed 6-month-old infants to speech sounds from a restricted range of a continuum between /ba/ and /da/, following a unimodal frequency distribution. Synchronously with these speech sounds, one group of infants (the two-category group) saw a visual articulation of a canonical /ba/ or /da/, with the two alternative visual articulations, /ba/ and /da/, being presented according to whether the auditory token was on the /ba/ or /da/ side of the midpoint of the continuum. Infants in a second (one-category) group were presented with the same unimodal distribution of speech sounds, but every token for any particular infant was always paired with the same syllable, either a visual /ba/ or a visual /da/. A stimulus-alternation preference procedure following the exposure revealed that infants in the former, and not in the latter, group discriminated the /ba/–/da/ contrast. These results not only show that visual information about speech articulation enhances phoneme discrimination, but also that it may contribute to the learning of phoneme boundaries in infancy.     

Perception of motion transparency in 5-month-old infants

We investigated the perceptual development of motion transparency in 3- to 5-month-old infants. In two experiments we tested a total of 55 infants and examined their preferential looking behaviour. In experiment 1, we presented transparent motion as a target, and uniform motion as a non-target consisting of random-dot motions. We measured the time during which infants looked at the target and non-target stimuli. In experiment 2, we used paired-dot motions (Qian et al, 1994 Journal of Neuroscience 14 7357-7366) as non-targets and also measured target looking time. We calculated the ratio of the target looking time to the total target and no-target looking time. In both experiments we controlled the dot size, speed, the horizontal travel distance of the dots, and the motion pattern of the dots. The results demonstrated that 5-month-old infants showed a statistically significant preference for motion transparency in almost all stimulus conditions, whereas the preference in 3- and 4-month-old infants depended on stimulus conditions. These results suggest that the sensitivity to motion transparency was robust in 5-month-olds, but not in 3- and 4-month-olds.     

Emerging self-exploration by 2-month-old infants

Two-month-olds and newborns were tested in a situation where they had the opportunity to experience different auditory consequences of their own oral activity on a dummy pacifier. Modulation of oral activity was scored and analyzed relative to two types of contingent auditory feedback, either analog or non-analog to the effort exerted by the infant on the pacifier. The dummy pacifier was connected to an air pressure transducer for recording of oral action. In two different experimental conditions, each time the infant sucked above a certain pressure threshold they heard a perfectly contingent sound of varying pitch. In one condition, the pitch variation was analog to the pressure applied by the infant on the pacifier (analog condition). In another, the pitch variation was random (non-analog condition). As rationale, a differential modulation of oral activity in these two conditions was construed as indexing some voluntary control and the sense of a causal link between sucking and its auditory consequences, beyond mere temporal contingency detection and response–stimulus association. Results indicated that 2-month-olds showed clear signs of modulation of their oral activity on the pacifier as a function of analog versus non-analog condition. In contrast, newborns did not show any signs of such modulation either between experimental conditions (analog versus non-analog contingent sounds) or between baseline (no contingent sounds condition) and experimental conditions. These observations are interpreted as evidence of self-exploration and the emergence of a sense of self-agency by 2 months of age.     

Categorical perception of nonspeech sounds by 2-month-old infants

According to recent investigations, adult listeners perceive rise-time differences in both speech and nonspeech stimuli in a categorical manner (Cutting & Rosner, 1974). Adults labeled sawtooth-wave stimuli as either plucked or bowed. The present study uses the high-amplitude sucking technique to explore the 2-month-old infant’s perception of rise-time differences for sawtooth stimuli. Infants discriminated rise-time differences which marked off the different nonspeech categories, but did not discriminate equal differences within either category. Thus, the present study shows that infants, like adults, can perceive nonspeech stimuli in a categorical manner.     

Positive behavioral contrast in 3-month-old infants on multiple conjugate reinforcement schedules.

Positive behavioral contrast was assessed in two experiments with young infants using multiple conjugate reinforcement schedules. Reinforcement was produced by footkicks which activated the objects of an overhead crib mobile in a manner proportional to the vigor and rate of responding. Distinctive color/pattern cues on the sides of the objects served as discriminative stimuli for components of the multiple schedule. In Experiment 1, infants were trained with one cue (S+) only before insertion of S+ into a multiple schedule with an extinction component. A control group received S+ throughout all sessions. In Experiment 2, a multiple schedule was introduced at the outset, and responses in both components were reinforced before the introduction of extinction in the second component. In a final phase, reinforcement was reintroduced into the second component. Positive behavioral contrast occurred in both experiments. Response reduction in the extinction component was seen only in individual relative response curves. In both experiments, negative emotional behaviors accompanied the extinction component, and in Experiment 1, cooing accompanied presentations of S+.     

Ratio abstraction by 6-month-old infants

Human infants appear to be capable of the rudimentary mathematical operations of addition, subtraction, and ordering. To determine whether infants are capable of extracting ratios, we presented 6-month-old infants with multiple examples of a single ratio. After repeated presentations of this ratio, the infants were presented with new examples of a new ratio, as well as new examples of the previously habituated ratio. Infants were able to successfully discriminate two ratios that differed by a factor of 2, but failed to detect the difference between two numerical ratios that differed by a factor of 1.5. We conclude that infants can extract a common ratio across test scenes and use this information while examining new displays. The results support an approximate magnitude-estimation system, which has also been found in animals and human adults.     

Attribution of beliefs by 13-month-old infants

In two experiments, we investigated whether 13-month-old infants expect agents to behave in a way that is consistent with information to which they have been exposed. Infants watched animations in which an animal was either provided information or prevented from gathering information about the actual location of an object. The animal then searched successfully or failed to retrieve the object. Infants' looking times suggest that they expected searches to be effective when--and only when--the agent had had access to the relevant information. This result supports the view that infants possess an incipient metarepresentational ability that permits them to attribute beliefs to agents. We discuss the viability of more conservative explanations and the relation between this early ability and later forms of theory of mind that appear only after children have become experienced verbal communicators.     

The perception of identity by 6 1/2-month-old infants.

In Study 1, sixteen 6 1/2-month-olds were habituated to a Reversible stimulus (an upright face that could be perceived as an entirely different upright face when it was rotated 180 degrees) and to a Nonreversible stimulus (a face that could be perceived as upright in only one orientation). Following habituation for each type of stimulus, test trials paired the habituated face with a novel stimulus (an inversion of the same face). For both Reversible and Nonreversible stimuli, the physical difference between the old and new test stimuli was the same (a 180 degrees rotation); however, infants devoted more visual attention to the 180 degrees rotation only when it was a Reversible face, suggesting that the identity change was detected. Experiment 2 ruled out the explanation that infants might have failed to dishabituate to the inversion of the Nonreversible stimulus because they could not remember it. Results are interpreted as evidence that 6 1/2-month-old infants are not limited to face recognition based on similarity in pattern arrangement alone, but are capable of processing faces at a representational level.     

Compound-stimulus information processing by 3-month-old infants

According to Cohen's (1973) component-compound developmental hypothesis, before 20 weeks of age, infants do not process compound-stimulus information when presented with simple colored forms. This hypothesis was examined in the present study. Eighty 3-month-old infants were presented with novel and familiar simple colored forms (e.g., red square vs. green square). Conditions were controlled so that novelty discriminations based on component- or compound-stimulus information could be separated. Contrary to the component-compound developmental hypothesis, the results indicate that by 12 weeks of age infants encode compound-stimulus information when presented with simple colored forms. These results are discussed in terms of the property-set model of stimulus recognition (Hayes-Roth & Hayes-Roth, 1977).     

Perception of tones by infants learning a non-tone language

This article examines the perception of tones by non-tone-language-learning (non-tone-learning) infants between 5 and 18 months in a study that reveals infants’ initial sensitivity to tonal contrasts, deterioration yet plasticity of tonal sensitivity at the end of the first year, and a perceptual rebound in the second year. Dutch infants in five age groups were tested on their ability to discriminate a tonal contrast of Mandarin Chinese as well as a contracted tonal contrast. Infants are able to discriminate tonal contrasts at 5–6 months, and their tonal sensitivity deteriorates at around 9 months. However, the sensitivity rebound sat 17–18 months. Non-tone-learning infants’ tonal perception is elastic, as is shown by the influence of acoustic salience and distributional learning: (1) a salient contrast may remain discriminable throughout infancy whereas a less salient one does not; (2) a bimodal distribution in tonal exposure increases non-tone-learning infants’ discrimination ability during the trough in sensitivity to tonal contrasts at 11–12 months. These novel findings reveal non-tone-learning infants’ U-shaped pattern in tone perception, and display their perceptual flexibility.     

Detection of gender-related categories by 10-month-old infants

Thirty-two 10-month-olds completed an habituation procedure where a male or female face was paired with a specific object. Results reveal 10-month-olds can form categories based on correlations among attributes of social information. Basic processes involved in gender role formation are present in 10-month-olds.     

Recognition of color-form compounds by 4-month-old infants

In two studies, a paired-comparison procedure was used to investigate whether 4-month-old infants can perceive and remember correlations of color and form. In Study 1, infants were shown simultaneous presentations of two different colored shapes until two criteria for familiarization had been met. They were then offered a simultaneous choice between a colored form that they had already seen and one that was a new combination of a familiar color and a familiar shape. The infants looked significantly longer at the new combination than at the familiar combination on the test trial. In Study 2, the same procedure was used, except that the familiarization criteria were less stringent and additional analyses of the infants' test trial behavior were conducted. Infants again exhibited a significant preference for the new combination, both when looking time accumulated during the entire test-trial was analyzed and when looking just up to the “first look away” was considered. These findings that 4-months-olds can perceive and remember compounds of visual attributes conflict with the results of several studies in which the habituation and test stimuli were presented sequentially rather than simultaneously. Differences in the nature of the information processing that may be evoked by the two methods are discussed.     

Objective spatial coding by 6.5-month-old infants in a visual dishabituation task

Forty 6.5-month-old infants were tested in a visual dishabituation variation of Bremner and Bryant’s reaching task used to evaluate the spatial representations of infants. In the visual dishabituation version of this task, infants were habituated to a display in which an object held a constant position at a corner of the table. Following habituation, the object was either moved to the opposite table-corner or nothing on the table was changed. Also, the infant either remained in her starting position or she moved to the opposite side of the table. The results show that, following habituation to an object, infants dishabituated to a change in the actual location of an object and not to a change in the egocentric relationship between the infant and the object. We conclude that even in a landmark-free environment (1) 6.5-month-old infants are capable of representing space allocentrically, and (2) they have the ability to update their location during passive movement.     

Enumeration of collective entities by 5-month-old infants

Recent findings suggest that infants are capable of distinguishing between different numbers of objects, and of performing simple arithmetical operations. But there is debate over whether these abilities result from capacities dedicated to numerical cognition, or whether infants succeed in such experiments through more general, non-numerical capacities, such as sensitivity to perceptual features or mechanisms of object tracking. We report here a study showing that 5-month-olds can determine the number of collective entities -- moving groups of items -- when non-numerical perceptual factors such as contour length, area, density, and others are strictly controlled. This suggests both that infants can represent number per se, and that their grasp of number is not limited to the domain of objects.     

Syllabic pitch perception in 2- to 3-month-old infants

The pitch patterns present in speech addressed to infants may play an important role in perceptual processing by infants. In this study, the high-amplitude sucking procedure was used to assess discrimination by 2- to 3-month-old infants of rising versus falling pitch patterns in 400-msec synthetic [ra] and [la] tokens. The syllables’ intonation contour was modeled on infant-directed speech, and covered a range characteristic of an adult female speaker (180–300 Hz). Group data indicated that the 2- to 3-month-old infants discriminated the pitch contour for both stimuli. Results are discussed with reference to previous studies of syllabic pitch perception.     

Some consequences of stimulus variability on speech processing by 2-month-old infants.

The present study explores how stimulus variability in speech production influences the 2-month-old infant's perception and memory for speech sounds. Experiment 1 focuses on the consequences of talker variability for the infant's ability to detect differences between speech sounds. When tested with high-amplitude sucking (HAS) procedure, infants who listened to versions of a syllable, such as [symbol: see text], produced by 6 male and 6 female talkers, detected a change to another syllable, such as [symbol: see text], uttered by the same group of talkers. In fact, infants exposed to multiple talkers performed as well as other infants who heard utterances produced by only a single talker. Moreover, other results showed that infants discriminate the voices of the individual talkers, although discriminating one mixed group of talkers (3 males and 3 females) from another is too difficult for them. Experiment 2 explored the consequences of talker variability on infants' memory for speech sounds. The HAS procedure was modified by introducing a 2-min delay period between the familiarization and test phases of the experiment. Talker variability impeded infants' encoding of speech sounds. Infants who heard versions of the same syllable produced by 12 different talkers did not detect a change to a new syllable produced by the same talkers after the delay period. However, infants who heard the same syllable produced by a single talker were able to detect the phonetic change after the delay. Finally, although infants who heard productions from a single talker retained information about the phonetic structure of the syllable during the delay, they apparently did not retain information about the identity of the talker. Experiment 3 reduced the range of variability across talkers and investigated whether variability interferes with retention of all speech information. Although reducing the range of variability did not lead to retention of phonetic details, infants did recognize a change in the gender of the talkers' voices (from male to female or vice versa) after a 2-min delay. Two additional experiments explored the consequences of limiting the variability to a single talker. In Experiment 4, with an immediate testing procedure, infants exposed to 12 different tokens of one syllable produced by the same talker discriminated these from 12 tokens of another syllable.(ABSTRACT TRUNCATED AT 400 WORDS)