Infants between 26 and 32 weeks of age were presented with sounding objects in the dark at four locations, 30° left and right, within reaching distance (15 cm) and out of reach (60 cm) at the same orientation. Infants' ability to localize the sound was assessed by measuring reaches of either hand into the space occupied by the objects when within reach. In two experiments infants reached more often into the correct location of the sounding object when placed within reach than when placed beyond reach. In the second experiment the objects were presented at midline in the light and off midline in the dark. Infants reached correctly in the dark and showed no inclination to reach toward the midline position where they had handled the object in the light. By 7 months of age infants have at least a dichotomous discrimination of auditory space: within and beyond reach. The implications of these behaviors are discussed in relation to object permanence and cognitive development.Sound localization is the ability to locate the source of a sound in terms of both direction and distance. This ability implies a sense of auditory space, a world in which sounding objects are localized in relation to one's body. When and how the infant comes to differentiate auditory space is unknown. Before exploring infants' responses to sound in depth, the adult literature on this topic will be briefly considered. Adults' perception of auditory distance has received little attention, and much of the work is irrelevant for our purpose because the distances tested were many meters. In reviewing the literature, much of which is in German, Blauert (1983, pp. 117-131) presented data showing that adults could judge closer versus more distant sounds quite accurately at distances under 3 m. At greater lengths, subjects tended to underestimate the distance. Strybel and Perrott (1984) reported distance discrimination of about 10 cm when the reference distance was half a meter, while Ashmead, LeRoy, and Odom (1990) were able to lower adults 1 thresholds to 5.7 cm with a reference distance of 1 m by improvements in methodology. At the least, adults' performance in the close range encourages the possibility that infants would be able to distinguish between a sound close to the body from one placed half a meter or so away. Because the acoustic parameters that determine distance perception are not well understood in
84 3-month-olds were tested in 3 studies of the acquisition and long-term retention of category-specific information. Infants who were trained with a perceptibly different member of an alphanumeric category on each of 3 days generalized responding to a novel instance of the original training category but not a novel member of a novel category during a 24-hour novelty test. 2 weeks later, when infants displayed no evidence of remembering their prior training experience, categorization was reinstated if a novel exemplar from the original training category was used as the retrieval cue in a memory reactivation procedure. A novel exemplar from a novel category was not an effective retrieval cue. The effectiveness of the category-specific retrieval cue was a function of its physical similarity to the individual exemplars encountered during training, not testing. The background against which the alphanumeric exemplars were displayed during training was not an effective retrieval cue in either the 24-hour novelty test or the memory reactivation procedure, indicating that all invariant stimulus attributes do not contribute equally as category cues. These data are the first to document retention of category-specific information after extended intervals. A popular account of categorization holds that infants abstract invariant features from individual exemplars and form a schema or distinctive memory representation of these shared features against which subsequent exemplars are compared. The present data provide support for a more parsimonious account of categorization, based on the retrieval of information about individual exemplars, that does not require an assumption of prototype formation.
The precision of auditory localization in 26-to 30-week-old infants was measured with a test based on the adult minimum audible angle. In this test, the horizontal angle between loudspeakers was varied systematically to determine thresholds for discriminating rightward versus leftward sound displacements. Infants were presented with sounds that shifted from straight ahead to the left or right, and observers judged from the infants' eye and head movements to which side the sound had shifted. From trial to trial, the size of the shift was decreased after correct responding and increased after incorrect responding. Infants discriminated sound displacements of about 19", considerably less accurate than adult values of 1-2". These findings are discussed in terms of their methodological implications and the development of sensitivity to information for sound localization.
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