Statistical learning by 8-month-old infants.

Learners rely on a combination of experience-independent and experience-dependent mechanisms to extract information from the environment. Language acquisition involves both types of mechanisms, but most theorists emphasize the relative importance of experience-independent mechanisms. The present study shows that a fundamental task of language acquisition, segmentation of words from fluent speech, can be accomplished by 8-month-old infants based solely on the statistical relationships between neighboring speech sounds. Moreover, this word segmentation was based on statistical learning from only 2 minutes of exposure, suggesting that infants have access to a powerful mechanism for the computation of statistical properties of the language input.

Sensitive period for the development of human binocular vision.

Twenty-four subjects with abnormal binocular experience, due to a condition of convergent strabismus that existed during different periods of their lives, were tested. Interocular transfer of the tilt-aftereffect was used to assess binocularity. Individuals between 1 and 3 years of age are most susceptible to abnormal binocular experience.

Stereopsis in human infants.

Stereoscopic depth perception was tested in human infants by a new method based on attracting the infant's attention through movement of a stereoscopic contour formed from a dynamic random-element stereogram. The results reveal that stereopsis emerges at 3 1/2 to 6 months of age, an outcome consistent with evidence for rapid postnatal development of the visual system.

Motor timing learned without motor training.

Improvements due to perceptual training are often specific to the trained task and do not generalize to similar perceptual tasks. Surprisingly, given this history of highly constrained, context-specific perceptual learning, we found that training on a perceptual task showed significant transfer to a motor task. This result provides evidence for a common neural architecture underlying analysis of sensory input and control of motor output, and suggests a potential role for perception in motor development and rehabilitation.

Binocular VEP summation in infants and adults with abnormal binocular histories.

Visual evoked potentials (VEPs) were recorded under binocular and monocular viewing conditions in 19 2- to 10-month-old infants, 19 stereodeficient adults, and 12 normal adults. VEPs were elicited by medium contrast, phase-alternated checkerboards with check sizes ranging from 10-52 min of arc. Binocular VEP summation was defined as the percentage by which the binocular VEP amplitude exceeded the mean of the two monocular VEP amplitudes. In the stereonormal adults, binocular VEP summation was significantly greater than zero and significantly less than 100%. In the stereodeficient adults, binocular VEP summation was not significantly greater than zero. These findings support the hypothesis that the magnitude of binocular VEP summation reflects the activation of binocular cortical neurons. However, binocular VEP summation in the infants averaged approximately 95%. Moreover, infants under 5 months of age, many younger than the reported age of onset for stereopsis, showed binocular VEP summation of nearly 145%. The significantly higher level of binocular VEP summation in the infants was the result of much larger binocular VEP amplitudes, while monocular VEP amplitudes were equivalent to those of stereonormal and stereodeficient adults. These results support the hypothesis that VEP amplitude is mediated by two independent pools of monocular cortical neurons and that binocular VEP amplitude in stereonormal and stereodeficient adults saturates at a lower level than in infants. Thus, it is hypothesized that binocular VEP summation is not representative of the activation of binocular cortical neurons.

Assessment of stereopsis in human infants.

A new method for testing stereopsis in infants is described for use in the early assessment of binocular function. Four 6- and 12-month-old infants were presented with a dynamic random-element stereogram generated on a color television monitor. A stereoscopic form was moved laterally, either left or right, to elicit the visual attention of the infant. An observer, unaware of the direction of form movement, made a forced-choice judgment of the direction of form displacement based on information gleaned from observations of the infant's behavior. The observer's performance for all three age groups exceeded chance levels of responding (p < 0.001). Individual infants within each age group also provided strong evidence of stereopsis. The applicability of the method for the screening of binocular anomalies is discussed.

Pupillary measures of binocular luminance summation in infants and stereoblind adults.

Preverbal infants and children with ocular misalignments do not perform well on standard clinical measures of binocular function which require accurate alignment of the eyes. In 1983, Birch and Held described a pupillary measure of binocular luminance summation that could be used to assess binocular function in strabismic infants. They reported a correlation between the onset age of stereopsis and the onset age of binocular luminance summation, with both emerging at the end of the fourth postnatal month. Using a similar pupillary technique, we measured significant levels of binocular luminance summation in infants as young as 2 mo of age, as well as in stereoblind adults. In addition, the infant's pupillary system, in comparison to the adult's, showed reduced magnitudes and increased latencies to luminance increments. The sensitivity of the pupillary system to luminance increments provided a better predictor of binocular luminance summation (Pearson r = 0.88) than did stereoscopic performance (Pearson r = 0.43). These data suggest that developmental changes in the pupillary system itself, rather than factors intrinsic to binocular vision, may have been the source of the correlation between binocular luminance summation and stereopsis reported by Birch and Held.

Spoken word recognition and lexical representation in very young children.

Although children's knowledge of the sound patterns of words has been a focus of debate for many years, little is known about the lexical representations very young children use in word recognition. In particular, researchers have questioned the degree of specificity encoded in early lexical representations. The current study addressed this issue by presenting 18-23-month-olds with object labels that were either correctly pronounced, or mispronounced. Mispronunciations involved replacement of one segment with a similar segment, as in 'baby-vaby'. Children heard sentences containing these words while viewing two pictures, one of which was the referent of the sentence. Analyses of children's eye movements showed that children recognized the spoken words in both conditions, but that recognition was significantly poorer when words were mispronounced. The effects of mispronunciation on recognition were unrelated to age or to spoken vocabulary size. The results suggest that children's representations of familiar words are phonetically well-specified, and that this specification may not be a consequence of the need to differentiate similar words in production.

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.

Statistical learning of tone sequences by human infants and adults.

Previous research suggests that language learners can detect and use the statistical properties of syllable sequences to discover words in continuous speech (e.g. Aslin, R.N., Saffran, J.R., Newport, E.L., 1998. Computation of conditional probability statistics by 8-month-old infants. Psychological Science 9, 321-324; Saffran, J.R., Aslin, R.N., Newport, E.L., 1996. Statistical learning by 8-month-old infants. Science 274, 1926-1928; Saffran, J., R., Newport, E.L., Aslin, R.N., (1996). Word segmentation: the role of distributional cues. Journal of Memory and Language 35, 606-621; Saffran, J.R., Newport, E.L., Aslin, R.N., Tunick, R.A., Barrueco, S., 1997. Incidental language learning: Listening (and learning) out of the corner of your ear. Psychological Science 8, 101-195). In the present research, we asked whether this statistical learning ability is uniquely tied to linguistic materials. Subjects were exposed to continuous non-linguistic auditory sequences whose elements were organized into 'tone words'. As in our previous studies, statistical information was the only word boundary cue available to learners. Both adults and 8-month-old infants succeeded at segmenting the tone stream, with performance indistinguishable from that obtained with syllable streams. These results suggest that a learning mechanism previously shown to be involved in word segmentation can also be used to segment sequences of non-linguistic stimuli.

Statistical learning in a serial reaction time task: access to separable statistical cues by individual learners.

The ability of adult learners to exploit the joint and conditional probabilities in a serial reaction time task containing both deterministic and probabilistic information was investigated. Learners used the statistical information embedded in a continuous input stream to improve their performance for certain transitions by simultaneously exploiting differences in the predictability of 2 or more underlying statistics. Analysis of individual learners revealed that although most acquired the underlying statistical structure veridically, others used an alternate strategy that was partially predictive of the sequences. The findings show that learners possess a robust learning device well suited to exploiting the relative predictability of more than I source of statistical information at the same time. This work expands on previous studies of statistical learning, as well as studies of artificial grammar learning and implicit sequence learning.

Some effects of laboratory training on identification and discrimination of voicing contrasts in stop consonants.

For many years there has been a consensus that early linguistic experience exerts a profound and often permanent effect on the perceptual abilities underlying the identification and discrimination of stop consonants. It has also been concluded that selective modification of the perception of stop consonants cannot be accomplished easily and quickly in the laboratory with simple discrimination training techniques. In the present article we report the results of three experiments that examined the perception of a three-way voicing contrast by naive monolingual speakers of English. Laboratory training procedures were implemented with a small computer in a real-time environment to examine the perception of voiced, voiceless unaspirated, and voiceless aspirated stops differing in voice onset time. Three perceptual categories were present for most subjects after only a few minutes of exposure to the novel contrast. Subsequent perceptual tests revealed reliable and consistent labeling and categorical-like discrimination functions for all three voicing categories, even though one of the contrasts is not phonologically distinctive in English. The present results demonstrate that the perceptual mechanisms used by adults in categorizing stop consonants can be modified easily with simple laboratory techniques in a short period of time.

The amplitude and angle of saccades to double-step target displacements.

Two experiments examined the magnitude and direction of the initial saccade to a target that underwent two displacements within 200 msec. When the amplitude of the two target displacements was held constant at 10 deg but the angle of the displacements differed by 45 deg, a small but significant number of intermediate-angle saccades occurred. These intermediate-angle saccades were directed to locations between the two targets, thereby generating an angle transition function, and their amplitude was 10-20% less than the amplitude on single-step displacements. These intermediate-angle saccades were not simply the result of programming an oblique saccade because amplitude transition functions virtually identical to those reported by Becker and Jürgens [Vision Res. 19, 967-983 (1979)] for horizontal saccades were obtained for double-step target displacements limited to oblique saccades. Finally, when both target amplitude and target angle were varied in double-step displacements, it became clear that the timing of the amplitude transition function and the angle transition function was not coincident. Across conditions, the angle transition function occurred at a consistent time prior to the initial saccade, whereas the amplitude transition function occurred at a variable time prior to the initial saccade. Because these amplitude and angle transition functions appeared to be dissociated, a modified model of the saccadic programming system for double-step displacements was proposed.

Dark vergence and dark accommodation in human infants.

Vergence angle and accommodative state were assessed photographically in 3-, 6-, and 12-month old infants. In the dark, fluctuations of vergence and accommodation were generally uncorrelated among all groups. The vergence-accommodation functions obtained in the dark had a mean slope of 0.04. These findings provide evidence in infants for an uncoupling of vergence and accommodation in the absence of patterned retinal input.

Oculomotor responses to step-ramp targets by young human infants.

Previous research has shown that the smooth pursuit system in early infancy is quite immature. Infants' tracking of a single, small target moving at velocities greater than 10 deg/sec is almost entirely saccadic until the end of the second postnatal month. The emergence of smooth pursuit is characterized by low gain (less than 0.5) and frequent saccadic intrusions. To provide a quantitative description of pursuit to relatively slow target velocities, 10 infants ranging in age from 7 to 11 weeks viewed a 2 deg target that was stepped 5 to 10 deg from screen center and then ramped back to screen center and 10 deg beyond at a constant velocity of 3, 6 or 12 deg/sec. Smooth pursuit was observed even in the youngest infant whose segments of pursuit between saccades were up to 5 sec in duration. At the slowest target velocity, mean pursuit gain across infants was 0.50, while at 6 and 12 deg/sec mean pursuit gain was 0.25 and 0.11. This systematic decrease in pursuit gain with increasing target velocity implies that pursuit velocity was invariant across the three target velocities. These findings suggest that smooth pursuit can be generated consistently by the end of the second postnatal month, but that it is slow and uncalibrated to the velocity of the target.

Spatiotemporal factors in infant position sensitivity: single bar stimuli.

The ability of 3-month-old human infants to detect the presence of spatial misalignments in single-bar stimuli was investigated in a series of spatiotemporal stimulus manipulations. Both discrete and sinusoidal positional offsets, either stationary or temporally modulated, were presented using the forced-choice preferential looking technique. When discrete offsets were presented in alteration with the absence of offsets, thresholds were a factor of two worse than previous estimates of infant vernier acuity using grating stimuli. However, when offsets (discrete or sinusoidal) were presented in continuous motion, mean threshold was 22-24 arcmin, comparable to previous estimates using gratings with moving discrete offsets. For stimuli containing continuous motion, lowering velocity reduced infants' positional sensitivity. However, when either velocity or temporal frequency was held constant, the most important determinant of positional sensitivity was the sharpness of the offsets. The results from the temporally-modulated stimuli suggest that a simple local flicker mechanism cannot account for sensitivity when continuous motion is used in the stimulus; rather, a local motion mechanism may govern sensitivity in these conditions. We characterize this mechanism as one of "local motion" because it does have a position-sensitive component. The two stationary stimulus conditions in the present study indicate that infants can use a position-sensitive mechanism when no temporal modulation is present.

Assessment of vernier acuity development using the "equivalent intrinsic blur" paradigm.

Vernier acuity in human infants is more than two orders of magnitude poorer than in adults and does not appear to reach adult levels until well beyond the first postnatal year. One source of these developmental differences in vernier acuity may be the presence of levels of intrinsic blur that are higher in infants than in adults. We investigated this hypothesis by measuring vernier acuity in 3-month-olds, 5-month-olds, and adults using stimuli blurred by two-dimensional Gaussian filters. Experiment 1 showed that more stimulus blur is required to degrade vernier acuity in infants than in adults. From these data we estimated that the level of equivalent intrinsic blur for this vernier acuity task decreased by approx. 1.5 log units between 3 months of age and adulthood. These results also suggested that this reduction of equivalent intrinsic blur can account entirely for the improvement in vernier acuity between 3 and 5 months postnatal. However, the large further improvement which occurs between 5 months of age and adulthood cannot be explained solely by equivalent intrinsic blur. In Expt 2, we measured vernier acuity in 3-month-olds and adults using a non-blurred stimulus with the same luminance contrast as the most-blurred stimulus used in Expt 1. Infants' and adults' thresholds were degraded slightly, relative to the non-blurred stimulus from Expt 1, but still were significantly better than the most blurred condition of Expt 1. This suggests that the results from the first experiment were not due simply to the reduction in overall luminance contrast which occurs when stimuli are blurred.

Infants' perception of transparency.

Four- and 7-month-old infants' perception of transparency was investigated with computer-generated achromatic or color displays depicting a semitransparent box occluding the center of a rod. Following habituation, infants viewed test displays consisting of either a two-color rod (corresponding to the habituation display's proximal characteristics) or a solid rod (corresponding to the distal characteristics of the event depicted by the habituation display). Looking-time results from 4-month-olds suggested perception of transparency in color displays but not in an achromatic display. An additional condition indicated that transparency perception may rely on the visibility of background texture through the transparent surface. Seven-month-olds, in contrast, provided some evidence of transparency perception in the achromatic display. Implications for the development of infants' responses to object properties and perceptual segregation are discussed.

Discrimination of frequency transitions by human infants.

Difference limens (DLs) for linear frequency transitions using a 1.0-kHz pulsed-tone standard were obtained from 6- to 9-month-old human infants in a series of three experiments. A repeating standard "yes-no" operant headturning technique and an adaptive staircase (tracking) procedure were used to obtain difference limens from a total of 71 infants. The DLs for 300-ms upward and downward linear frequency sweeps were approximately 3%-4% when the repeating standard was an unmodulated 1.0-kHz pulsed tone of 300-ms duration. These DLs for frequency sweeps were not significantly different from DLs for frequency increments and decrements using 330-ms pulsed tones [J. M. Sinnott and R. N. Aslin, J. Acoust. Soc. Am. 78, 1986-1992 (1985)]. The DLs for frequency sweeps of 50 ms appended to the beginning or the end of a 250-ms unmodulated 1.0-kHz tone were approximately 6%-7%. This greater DL for brief frequency sweeps was confirmed by varying the duration but not the extent of the sweep. Finally, DLs were greater than 50% when the repeating standard was a 50-ms rising or falling frequency sweep appended to the beginning of a 250-ms unmodulated 1.0-kHz tone. These results suggest that rapid frequency transitions are much more difficult to discriminate from frequency transitions of the same category (rising or falling) than from either a frequency transition of the opposite category (falling or rising) or an unmodulated tone.(ABSTRACT TRUNCATED AT 250 WORDS)