Individual differences in external-ear transfer functions of cats.

Knowledge of the direction-dependent filter characteristics of the external ears is useful for the study of spatial hearing in experimental animals. The present study examined individual differences in the directional components of external-ear transfer functions (directional transfer functions, DTFs) among 24 anesthetized cats. Ears were fixed in a frontal position. Inter-cat differences in DTFs were quantified across a mid-frequency range from 8 to 16 kHz and across 30 locations in the horizontal plane and vertical midline. Across cats, DTFs showed similar direction dependence, but tended to differ in regard to the center frequencies of spectral features, such as spectral peaks and notches. Certain mid-frequency notches, for instance, varied in frequency across cats by nearly a factor of 2. Scaling of DTFs in frequency could reduce the overall differences between pairs of cats. Scale factors that minimized inter-cat differences ranged as high as 1.57 and correlated moderately with cats' body weights. Nevertheless, appreciable individual differences remained after frequency scaling. Inter-cat differences in DTFs were substantially larger than differences that resulted from variability in positioning the ears. The results suggest some guidelines regarding the conditions under which it is acceptable to apply DTF measurements from one cat to another.