The acoustical cues to sound location in the rat: Measurements of directional transfer functions

Abstract

The acoustical cues for sound location are generated by spatial- and frequency-dependent filtering of propagating sound waves by the head and external ears. Although rats have been a common model system for anatomy, physiology, and psychophysics of localization, there have been few studies of the acoustical cues available to rats. Here, directional transfer functions (DTFs), the directional components of the head-related transfer functions, were measured in six adult rats. The cues to location were computed from the DTFs. In the frontal hemisphere, spectral notches were present for frequencies from approximately 16 to 30 kHz; in general, the frequency corresponding to the notch increased with increases in source elevation and in azimuth toward the ipsilateral ear. The maximum high-frequency envelope-based interaural time differences (ITDs) were 130 mus, whereas low-frequency (<3.5 kHz) fine-structure ITDs were 160 mus; both types of ITDs were larger than predicted from spherical head models. Interaural level differences (ILDs) strongly depended on location and frequency. Maximum ILDs were <10 dB for frequencies <8 kHz and were as large as 20-40 dB for frequencies >20 kHz. Removal of the pinna eliminated the spectral notches, reduced the acoustic gain and ILDs, altered the acoustical axis, and reduced the ITDs.

Concurrent Development of the Head and Pinnae and the Acoustical Cues to Sound Location in a Precocious Species, the Chinchilla (Chinchilla lanigera)

“…The frequencies at which these transitions occurred shifted from higher frequencies (~18 kHz) for P0-1 animals to lower frequencies (~12 kHz) in mature adults. These general trends in the movement in the acoustic axis in adult animals are similar to those reported in other species [cat (Musicant et al 1990), wallaby (Coles and Guppy 1986), ferret (Carlile 1990), rats (Koka et al 2008), and bats (Firzlaff and Schuller 2003)]. …”

“…The 3-dB bandwidth of filters was held constant across all frequencies at 0.0571 octaves, and the upper and lower slopes of the filters fell off at 105 dB/octave. These filters have properties similar to the bank of triangular band pass filters described in the literature (Xu and Middlebrooks 2000;Schnupp et al 2003;Koka et al 2008).…”

“…However, it has been suggested by Koka et al (2008) and Tollin and Koka (2009a) that the pinnae can also contribute to ITDs. One explanation for the role of the pinnae in ITD production, as suggested by Koka et al (2008), is that a considerable portion of the pinnae and the distal parts of the auditory meatus in mammals such as the cat and rat that have ear canals formed from cartilage remains in the path of the sound. Thus, the pinnae may function to make the acoustically effective diameter of the head larger, thus increasing the magnitude of the ITDs.…”

“…2010)). In the rat, Koka et al (2008) demonstrated that the pinnae contribute to the largerthan-modeled ITDs. We have shown this in the chinchilla and the data are in a concurrent publication.…”

Using Virtual Acoustic Space to Investigate Sound Localisation

“…The lower border for ILD extraction, likewise, seems to be related to the observation that the head of an animal only creates sufficiently large ILDs above a certain frequency. These conclusions are supported by data from animals such as the cat, the ferret, monkey and the barn owl (Koeppl, 1997;Koka et al, 2008;Moiseff and Konishi 1981;Parsons et al, 2009;Spezio et al, 2000;Tollin & Koka, 2009). The use of both ITDs and ILDs in azimuthal sound localisation is known as duplex theory (Blauert, 1997;Macpherson & Middlebrooks, 2002;Rayleigh, 1907).…”