Infants' localization of sounds along two spatial dimensions: Horizontal and vertical axes

Abstract

Infants 6, 9, 12, 15, and 18 months of age were tested for their ability to localize sounds in two-dimensional (2-D) space, that is, simultaneously along the horizontal and vertical axes. Infants were seated in a dark room with 10 loudspeakers and light displays to their right or left. Two loudspeakers were each positioned along the horizontal axis at 18 °, 36 °, 54 °, 72 °, and 90 ° off midline, with one 25 ° above and the other 25 ° below the ears along the vertical axis. Infants were presented auditory-alone trials (only eight noise bursts played) and auditory-visual trials (following the first two bursts the light display at the location of the signal loudspeaker was activated). From the videorecords, we scored accuracy, speed, and path of head orientation. Infants made no localization errors on auditoryvisual trials. On auditory-alone trials, younger infants made more and larger localization errors than older infants, particularly for sounds located at the more extreme off-midline positions in space. There was no evidence of any differences in speed to execute a lateral head movement (1.0 °/22 ms) as a function of age, trial type, or signal location, and all infants initiated a horizontal head movement prior to a vertical one. However, on auditory-alone trials, for signals at least 54 ° azimuth, younger infants restricted their head movement primarily to the horizontal plane and only extended this to the vertical plane very late in the response, whereas older infants traversed these two axes simultaneously during much of the response. Localization results are consistent with adult data and a cone-of confusion model for localization of off-midline sounds in 2-D space.