Localization of tones by horses: use of binaural cues and the role of the superior olivary complex.

The research article explores the use of binaural cues by horses to localize sound, focusing particularly on the role of the superior olivary complex in this process.

Objective and Methodology

The researchers conducted a series of tests to understand the ability of horses to use binaural time and intensity difference cues for localizing sounds. Binaural time difference refers to the difference in arrival time of a sound to the two ears, while binaural intensity difference refers to the difference in sound intensity (loudness) perceived by the two ears. The sounds were pure tones of various frequencies, ranging from 250 Hz to 25 kHz, and were emitted from loudspeakers located to the left and right of the horses.

• In a two-choice procedure, localizing sounds were assessed by three horses.
• Two additional horses were assessed using a conditioned avoidance procedure.
• Four more horses were evaluated in an unconditioned orientation test.

Findings

The research revealed that all horses were successful in localizing sounds of frequencies 250 Hz, 500 Hz, and 1 kHz, while failing to localize frequencies of 2 kHz and above. The cutoff frequency for binaural phase cue (delta phi) in horses was calculated to be 1.5 kHz. This points towards the ability of horses to use binaural time differences (delta phi), but their inability to use binaural intensity differences.

Superior Olivary Complex Analysis

Further analysis of the superior olivary complex, a part of the brain which first interfaces binaural interactions, revealed that in horses, the lateral superior olive (LSO) is relatively small and does not exhibit the layered arrangement of bipolar cells typical of most mammals that are adept at using binaural intensity differences. This supports the primary findings of the study and could potentially explain the inability of horses to use binaural intensity differences for localizing sounds.