Sound localization and high-frequency hearing in horses

Abstract

The ability of mammals to hear high-frequency sounds has often been attributed to the need to use binaural intensity differences (ΔI to localize sound [e.g., R. S. Heffner and H. E. Heffner, J. Comp. Physiol. Psychol. 96, 926–944 (1982)]. However, this explanation does not seem to account for the ability of horses to hear high-frequency sounds. Specifically, while horses are able to localize low-frequency tones, they are unable to localize tones above 1 kHz even though they can hear up to 35 kHz. These results suggest that while they can use interaural phase or time differences (Δt) to localize sound, they lack the ability to use the binaural ΔI cue. However, localization tests using filtered noise and sound sources located off to the side indicate that horses use pinna cues to localize sound and rely heavily on the high-frequency content of a sound in order to prevent front-back reversals. It appears, then, that horses have lost the ability to use binaural ΔI and rely primarily on binaural Δt and pinna cues in order to localize sound. Furthermore, since it seems unlikely that horses engage in ultrasonic communication, it apepars that they have evolved high-frequency hearing primarily in response to the need to use pinna cues to localize sound. [Work supported by NSF and NIH.]