Development of the head, pinnae, and acoustical cues to sound location in a precocial species, the guinea pig (Cavia porcellus)
The morphology of the head and pinna shape the spatial and frequency dependence of sound propagation that give rise to the acoustic cues to sound source location. During early development, the physical dimensions of the head and pinna increase rapidly. Thus, the binaural (interaural time and level differences, ITD and ILD) and monaural (spectral shape) cues are also hypothesized to change rapidly. Complex interactions between the size and shape of the head and pinna limit the accuracy of simple acoustical models (e.g. spherical) and necessitate empirical measurements. Here, we measured the cues to location in the developing guinea pig, a precocial species commonly used for studies of the auditory system. We measured directional transfer functions (DTFs) and the dimensions of the head and pinna in guinea pigs from birth (P0) through adulthood. Dimensions of the head and pinna increased by 87% and 48%, respectively, reaching adult values by ˜8 weeks (P56). The monaural acoustic gain produced by the head and pinna increased with frequency and age, with maximum gains at higher frequencies (>8 kHz) reaching values of 10–21 dB for all ages. The center frequency of monaural spectral notches also decreased with age, from higher frequencies (˜17 kHz) at P0 to lower frequencies (˜12 kHz) in adults. In all animals, ILDs and ITDs were dependent on both frequency and spatial location. Over development, the maximum ILD magnitude increased from ˜15 dB at P0 to ˜30 dB in adults (at frequencies >8 kHz), while the maximum low frequency ITDs increased from ˜185 μs at P0 to ˜300 μs in adults. These results demonstrate that the changes in the acoustical cues are directly related to changes in head and pinna morphology.