haking things up : Young infants ’ use of sound information for object ndividuation

A search task was used to assess 5to 7-month-olds’ ability to use property-rich sounds to individuate objects. Results suggest that infants interpret an occlusion event involving two distinct rattle sounds as involving two objects but are unsure of how to interpret two identical rattle sounds. Published by Elsevier Inc. Infants use experience with and knowledge about auditory cues in many ways, including those for the purpose of interreting physical events and learning about the physical world. By 4.5 months, infants recognize the visually discernable, hysical components of an auditory stimulus (Spelke & Owsley, 1979; LaGasse, VanVorst, Brunner, & Zucker, 1999). For xample, young infants can identify whether a sound event is produced by a single object, or multiple objects, moving inside container and whether impact sounds are consistent with rigid or compressible objects (Bahrick, 1983, 1987). These results uggest that infants are sensitive to the temporal structure of multimodal events and can identify whether the sounds objects roduce are consistent with the objects’ physical properties. Most important to the present research is that infants may be apable of linking sounds to individual objects. Infant researchers (Walker-Andrews, 1994; Wilcox & Smith, 2010; Wilcox, Woods, Tuggy, & Napoli, 2006) have suggested hat object-related sounds can be classified as belonging to one of the two broad categories. One category, referred to here s property-rich sounds, includes sounds that are causally related to the interactions between objects and/or their parts and re directly linked to the physical properties of the objects (e.g., a compressible object makes a soft, squishy sound when it against a hard surface whereas a rigid object makes a sharp, loud sound). These sounds reveal something about the hysical composition and structure of objects and the nature of their interactions. The other category, referred to here as roperty-poor sounds, includes sounds that are more contrived and not directly or explicitly related to the physical properties f objects. For example, it is difficult to predict the sound a cell phone or an electronic toy will produce based on its physical omposition; likewise, it is difficult to draw inferences about the physical properties of these objects on the basis of the ounds they produce or whether different sounds signal the presence of numerically distinct objects. Wilcox and colleagues Wilcox & Smith, 2010; Wilcox et al., 2006) suggest that young infants, who have limited information processing capacities, Please cite this article in press as: Brower, T. , & wilcox, T. Shaking things up: Young infants’ use of sound information for object individuation. Infant Behavior and Development (2012), doi:10.1016/j.infbeh. 2011.12. 007 re more likely to attend to sounds that are naturally and readily linked to objects. These sounds, which typically result from hysical interactions, reveal something about the physical properties of the object and hence are more reliable predictors f an object’s identity. A similar distinction between these types of sounds has been made in the adult literature, and there ∗ Corresponding author. E-mail address: gooddaytracy@gmail.com (T. Brower). 163-6383/$ – see front matter. Published by Elsevier Inc. oi:10.1016/j.infbeh. 2011.12. 007 ARTICLE IN PRESS G Model INFBEH-721; No. of Pages 5 2 T. Brower, T. wilcox / Infant Behavior & Development xxx (2012) xxx– xxx Fig. 1. Schematic of the initial phase of the test trial for the same-sounds and the different-sounds conditions. A hand (not pictured) manipulated the objects. Immediately following step 5 the platform was pushed forward so that it was directly in front of the infant and the infant was allowed to reach. is evidence that adults find property-poor sounds more difficult to link to individual objects than property-rich sounds (Coward & Stevens, 2004; Gaver, 1986; Petocz et al., 2008). To investigate infants’ capacity to use sound information to individuate objects, Wilcox et al. (2006) presented 4.5-montholds with an auditory event in which two different sounds, separated by a temporal gap, emanated from behind a screen. The screen was then lowered to reveal either one object or two objects on the platform. In the property-rich condition, the sounds were produced by shaking two rattles filled with different substances (dried rice or small bells). In the propertypoor condition, an electronic keyboard was used to produce two tones that differed in pitch and timbre. The infants in the property-rich condition looked reliably longer at the onethan two-object display, suggesting that they interpreted the sound event as involving two objects and found the presence of a single object behind the screen unexpected. In contrast, the infants in the property-poor condition looked about equally at the two displays, as if they failed to draw a conclusion about the number of objects present. Subsequent studies revealed that infants use property-poor sounds to individuate objects by 9 months (Wilcox & Smith, 2010). Finally, when infants are presented with two identical property-rich sounds (e.g., two dried rice rattles), they look equally at a oneand two-object display, a finding that warrants further investigation (Wilcox et al., 2006). The present experiment assessed 5to 7-month-olds’ ability to use property-rich sounds to individuate objects using a two-phase search task similar to that of McCurry, Wilcox, and Woods (2009), In the initial phase (Fig. 1), infants were presented with an occlusion event in which objects seen successively to each side of a screen produced either the same sound when shaken (e.g., both objects were filled with dried rice) or different sounds (e.g., one object was filled with dried rice and the other with small bells). In the final phase of the task, the platform was moved forward so that the screen was directly in front of the infant (the visible object sat at the right end of the platform) and the infant was allowed to search. If infants in the different-sounds condition interpret the event as involving two distinct objects, they should spend more time searching for the object behind the screen than reaching for the object at the end of the platform. If the Please cite this article in press as: Brower, T. , & wilcox, T. Shaking things up: Young infants’ use of sound information for object individuation. Infant Behavior and Development (2012), doi:10.1016/j.infbeh. 2011.12. 007 infants in the same-sounds condition interpret the event as involving a single object that comes to rest at the end of the platform, they should spend more time reaching to the visible object than through the screen (since the screen does not hide an object).