Infants' pitch perception: masking by low- and high-frequency noises.

The present research employed an operant conditioning procedure typically used with infants to test noise masking of pure tones and tonal complexes in adults and in 7-month-old infants. Adults and infants were presented with either pure tones of 160 and 200 Hz or harmonic tonal complexes with pitches equivalent to 160 and 200 Hz. The tonal complexes did not contain energy at the fundamental frequency. After learning these tasks, subjects in the tonal complex group categorized spectrally varying tonal complexes according to the pitch of the missing fundamental. Stimuli were subsequently presented in combination with either a low- or a high-frequency noise. Both age groups successfully discriminated pure tones when combined with a high-frequency noise but not when combined with a low-frequency noise in the same frequency range as the pure tone. Infants, like adults, successfully categorized harmonic tonal complexes based on the pitch of the missing fundamental when those stimuli were combined with a low-frequency noise in the range of the missing fundamental but not when combined with a high-frequency noise which covered the frequency range of the harmonics themselves. These results suggest that infants rely primarily on a central process and not peripherally generated combination tones to hear the pitch of the missing fundamental.

Infants' pitch perception: inharmonic tonal complexes.

Two experiments assessed the effects of inharmonicity on 7- to 8-month-old infants' perception of the pitch of tonal complexes. A number of harmonic and inharmonic complexes were presented in a visually reinforced operant head turn procedure. In both experiments, infants demonstrated the ability to discriminate two harmonic complexes based on missing fundamental frequencies of 160 and 200 Hz. After learning this basic task, infants learned to discriminate inharmonic complexes, which were created by shifting the partials of the harmonic complexes upward by 30 Hz (experiment 1) or 20 Hz (experiment 2). Finally, three spectrally different inharmonic complexes represented each pitch, and infants attempted to categorize those complexes according to their pitches. In both experiments, infants successfully discriminated the pitches of the spectrally varying tonal complexes, but their performance deteriorated for the more strongly inharmonic complexes of experiment 1. These results suggest that, as for adults, the salience of pitch for inharmonic sounds decreases with increasing inharmonicity.

Infants require low-frequency energy to hear the pitch of the missing fundamental.

The present experiment assessed 7- to 8-month-old infants' perception of pitch for harmonic tonal complexes containing either low- or high-frequency energy. In a visually reinforced, operant head turn procedure, infants first learned to discriminate two harmonic tonal complexes based on missing fundamental frequencies of 160 and 200 Hz. After learning this basic task, infants were presented with spectrally varying harmonic tonal complexes that contained either resolvable, low-frequency energy, or unresolvable, high-frequency energy. Infants learned to categorize the low-frequency signals according to the pitches of their missing fundamental frequencies but failed to categorize the high-frequency signals. These results suggest that infants require low-frequency, resolvable energy to hear the pitch of the missing fundamental.

Is visually guided reaching in early infancy a myth?

The issue examined was whether infants require sight of their hand when first beginning to reach for, contact, and grasp objects. 7 infants were repeatedly tested between 6 and 25 weeks of age. Each session consisted of 8 trials of objects presented in the light and 8 trials of glowing or sounding objects in complete darkness. Infants first contacted the object in both conditions at comparable ages (mean age for light, 12.3 weeks, and for dark, 11.9 weeks). Infants first grasped the object in the light at 16.0 weeks and in the dark at 14.7 weeks, a nonsignificant difference. Once contact was observed, infants continued to touch and grasp the objects in both light and dark throughout all sessions. Because infants could not see their hand or arm in the dark, their early success in contacting the glowing and sounding objects indicates that proprioceptive cues, not sight of the limb, guided their early reaching. Reaching in the light developed in parallel with reaching in the dark, suggesting that visual guidance of the hand is not necessary to achieve object contact either at the onset of successful reaching or in the succeeding weeks.

Newborns' head orientation toward trains of brief sounds.

Four experiments assessed the importance of stimulus number, repetition rate, and duration for newborns' head orientation toward brief sounds and related those parameters to the critical ones found for adults. Infants' responses to various trains of repeated 14-ms rattle sounds were compared with those to a 10-s rattle sound, known to elicit head orientation. Directional responses did not differ from the standard when rattle bursts were repeated at a rate of 20 per second for 1 s (experiment 1). Responding did differ from the standard and deteriorated to chance levels when either the number of moderately paced (6/s) stimulus bursts was decreased to six or fewer (experiments 2 and 3) or the duration of rapidly repeated (24/s) bursts was shortened to 500 ms (experiments 4A and 4B). These results suggest that newborns' head orientation depends, in part, upon the number of stimulus bursts and stimulus duration.

Stimulus duration and repetition rate influence newborns' head orientation toward sound.

Three experiments evaluated the effects of stimulus duration and repetition rate on newborns' head orientation responses. In Experiment 1, 28 infants turned toward a 20-sec continuous rattle sound but not toward 14- and 500-msec rattle sounds. Signal energy as a possible explanation for the infants' difficulty orienting toward brief sounds was explored in Experiment 2. Twenty neonates did not turn toward a single 90 dB, 14-msec rattle sound, although a longer duration (10 sec) sound containing less energy (70 dB) did elicit reliable head orientation. In Experiment 3, 16 neonates heard trains of repeated 14-msec rattle sounds (2/sec, 1.3/sec, and 1/sec) lasting 10 sec as well as a 10-sec continuous rattle sound. They turned toward the most rapidly repeating brief sound and the continuous one, while the slowly repeating sounds elicited little head movement in any direction. These results suggest that newborns' head orientation is selectively deficient for brief sounds, that the difficulty does not result from lessened energy in the brief sounds, and that the efficacy of repeated brief sounds depends upon their repetition rates.

The development of a human auditory localization response: a U-shaped function.

Research during the past 10 years on the neonatal head-turn response to off-centred rattle sounds is reviewed, and various procedural and stimulus conditions that influence the probability of eliciting a correct response are identified. Also, the existence of a U-shaped developmental function is confirmed in a cross-sectional study of 104 infants between 3 days and 7 months of age. Neonates responded reliably, but slowly; the response decreased in frequency and magnitude between 1-3 months of age and increased again by 4-5 months of age. Speculation that this U-shaped function reflects a maturational shift in locus of control from subcortical to cortical structures was supported by the infants' responses to the presence effect (PE), which is thought to be cortically mediated. The PE was produced by playing the rattle sound through two loudspeakers with the output of one delayed by 5 ms, relative to the other; adults perceive only one sound at the leading loudspeaker. As predicted, neonates failed to respond to the PE, and the onset of correct PE responses corresponded closely to the upswing in the U-shaped function for SS responses. Other explanations for the temporary decline in orientation responses to sound are also discussed.

Infant pitch perception: evidence for responding to pitch categories and the missing fundamental.

While numerous studies on infant perception have demonstrated the infant's ability to discriminate sounds having different frequencies, little research has evaluated more sophisticated pitch perception abilities such as perceptual constancy and perception of the missing fundamental. In the present study 7-8-month-old infants demonstrated the ability to discriminate harmonic complexes from two pitch categories that differed in pitch by approximately 20% (e.g., 160 vs 200 Hz). Using a visually reinforced conditioned head-turning paradigm, a number of spectrally different tonal complexes that contained varying harmonic components but signaled the same two pitch categories were presented. After learning the basic pitch discrimination, the same infants learned to categorize spectrally different tonal complexes according to the pitches signaled by their fundamental frequencies. That is, the infants showed evidence of perceptual constancy for the pitch of harmonic complexes. Finally, infants heard tonal complexes that signaled the same pitch categories but for which the fundamental frequency was removed. Infants were still able to categorize the harmonic complexes according to their pitch categories. These results suggest that by 7 months of age infants show fairly sophisticated pitch perception abilities similar to those demonstrated by adults.

The effects of sound duration on newborns' head orientation.

Two experiments assessed the importance of sound duration for eliciting head orientation responses from newborn infants. In Experiment 1, thirty infants turned with equal frequency toward 20-s continuous rattle sounds and 20-s trains of rattle segments. The duration of the rattle segments--14 and 100 ms (2/s), or 500 ms (1/s)--did not influence the likelihood of turning. Response latencies and durations proved quite similar for all stimuli. In Experiment 2, twenty-four infants heard continuous rattle sounds of four different durations: 1, 5, 10, and 20 s. They turned reliably to all stimulus durations; furthermore, the magnitude and temporal characteristics of head orientation responses did not differ for the four stimulus durations. These results suggest that the newborn's head orientation response may reflect a motor program that is initiated by auditory input and then executed in a similar fashion regardless of further stimulation.

Cardiac orienting and vowel discrimination in newborns: crucial stimulus parameters.

To delineate the acoustic parameters crucial for eliciting a cardiac deceleratory response in newborns, 2 experiments investigated stimulus dimensions implicated as important by previous research. In experiment 1, the temporal pattern and spectral complexity of moderately intense auditory stimuli influenced the cardiac responses of 24 alert newborns. Temporal pattern determined response direction: pulsed stimuli elicited cardiac deceleration, whereas continuous stimuli elicited acceleration. Stimulus complexity influenced response magnitude: the most complex stimulus, a synthetically produced vowel, produced both the largest deceleration for pulsed stimuli and the largest acceleration for continuous ones. Experiment 2 (N = 16) extended the temporal pattern effect to other vowel stimuli in a no-delay discrimination paradigm. These findings indicate that the cardiac deceleratory response to auditory stimuli is a predictable component of the newborn infant's behavior. Furthermore, they suggest that stimulus parameters that influence cardiac deceleration may also play a critical role in stimulus discrimination.

Stimulus-presentation probability influences newborns' head orientation to sound.

Although newborn infants are capable of turning their heads toward laterally-presented sounds, the incidence of such responding varies across studies. Previous work suggests that as the probability of a laterally-presented sound increases head-turning performance improves. To test this "lateral-stimulus-presentation probability" hypothesis, we presented 30 alert newborns with a rattle sound from a lateral source on either 1/4, 2/4, or 3/4 of all sound trials. On remaining sound trials the rattle came from a loudspeaker located above the infant's head. For 12 infants all lateral trials were presented consecutively; these trials were spaced across the session for the other infants. The likelihood of correct head turning increased linearly as the lateral-stimulus-presentation probability increased. The distribution of lateral trials did not influence the incidence of correct head turning.