Abstract
The behaviour of 90 clinically normal 6-week-old babies previously studied as neonates was video-recorded while they were presented with sequences of sound stimuli. The stimuli varied in sound pressure level, bandwidth and rise time, and included two voice signals and a no-sound (control) trial. Video records were made both for the whole body aspect and for a 3 1/2 X 'close-up' of the head. Later, the video records were shown to 6 observers who were allowed to see the babies for 10 s at each trial. The first 5 s was a prestimulus observation period, and the second 5 s usually contained a sound stimulus. Between the trials, the observers were given 20 s in which to record (i) prestimulus activity, (ii) confidence in response, and (iii) movement details (data for [iii] not reported here). Different segments of the baby's body were observed on different occasions as detailed in our neonatal report. The response confidence ratings were analysed, using aspects of signal detection theory, to show differences amongst various body segments (p less than 0.001), sound pressure levels (SPL) (p less than 0.001), band widths (BW) (p less than 0.001), and rise times (RT) (p less than 0.001). There were significant interactions between SPL and BW (p less than 0.05) and SPL X RT (p less than 0.001). The 3 factor interaction (SPL X BW X RT) was also significant (p less than 0.001). A 90-dB broad-spectrum noise band was by far the most effective stimulus. The response to different sound stimuli was affected by the baby's posture (lying or sitting) and by prestimulus activity state. The results are discussed in relation to our previous neonatal study.
Published Version
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