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.

Figure 1

The dimensions of the animal subjects. Across the six rats, the average head diameter (FG) was 29.6±1.3 mm, the average pinna width (DE) was 11.4±0.5 mm, and the average pinna lengths (AC) and (BC) were 16.8±0.8 and 17.7±1.7 mm, respectively.

Figure 2

Examples of the three primary acoustical cues to sound location, spectral notches, interaural time differences (ITDs), and interaural level differences (ILDs), for five sources in azimuth along the horizontal plane. At each location, the directional transfer functions (DTFs, top panels) and impulse responses (lower panels) are shown for the left (light gray lines) and right (dark lines) ears. Spectral notches are apparent in the DTFs, ITDs are given by the delay between the left- and right-ear impulse responses, and ILDs are given by the difference in the left- and right-ear DTFs.

Figure 3

The DTF gains for the left ear of two animals (R016 and R004) for sources at 15° azimuth and varying in elevation from −45° to +90° (panels A and C) and at 0° elevation varying from −90° (ipsilateral) to +90° (contralateral) azimuth (panels B and D). The two vertical bars in each figure bound the approximate frequency range of the first spectral notches, 16–30 kHz.

Figure 4

A plot of the isofrequency contours of the first notch frequencies for sources in the frontal hemisphere for the left ears of two animals (R016 and R004). First, notch frequencies increase with source elevation and azimuth toward the ipsilateral ear (−90° in this figure).

Figure 5

The DTFs for the left ear of one animal (R004) after the pinna were removed for sources at 15° azimuth and varying in elevation from −45° to +90° (panel A) and at 0° elevation varying from −90° (ipsilateral) to +90° (contralateral) azimuth (panel B). The spectral notches apparent with the pinna (Figs. 3c, 3d) were no longer present after pinna removal. The vertical bars bound the approximate frequency range of the first spectral notches, 16–30 kHz (see Fig. 3).

Figure 6

Spatial distribution of DTF gains for seven frequencies for the right ear (+90° is ipsilateral) of one animal (R002) for three different conditions; intact animal with head and pinna (“head+pinna”), head only after the pinna were removed (“head only”), and the contribution of the pinna (“pinna only”) which was computed as the difference of the “head+pinna” and “head only” measurements. The color bar indicates the gain in decibels.

Figure 7

The ILD spectrum for two animals (R016 and R004). ILD spectrum is the frequency-by-frequency difference between left- and right-ear DTFs at a given location. Positive ILD indicates higher gain at the left ear than the right. ILDs do not change as with source elevation along the midsagittal plane (top row) but do substantially change with source azimuth along the horizontal plane (bottom row). ILDs for some frequencies and some locations may be as large at 40 dB.

Figure 8

(A) The mean ILD cue (◻, +pin) computed across the six intact animals varies as a function of azimuth along the horizontal plane and frequency. Error bars indicated ±1 standard error of the mean (SEM). The mean ILDs are shown for six frequencies from 5 to 30 kHz. The mean ILD cue for three animals after removal of the pinna is also shown (●, −pin). Removal of the pinna reduced the ILDs for frequencies >10 kHz. (B) Rate of change of the ILD cue (i.e., ILD slope) with changes in source azimuth along the horizontal plane between ±30°. The ILD slope was obtained from the mean ILD cues computed across the six animals (panel A). Error bars plot ±1 SEM.

Figure 9

Spatial distribution of ILDs at seven different frequencies for two animals (R016 and R004) for locations in the frontal hemisphere. Positive ILDs indicate higher gain at the left ear (−90°) than the right ear (+90°). Color bar indicates ILD magnitude in dB.

Figure 10

(A) Spatial distribution of envelope ITDs for two animals (R016 and R004) for locations in the frontal hemisphere. Positive ITDs indicate that the signal leads to the left ear. Color bar and countour lines indicate the ITD in microseconds. (B) The envelope ITDs in three animals (R001, R002, and R004) as a function of azimuth along the horizontal plane in two conditions: intact (filled symbols, +pin) and after removal of the pinna (open symbols, −pin). Removal of the pinna reduced the ITDs at all azimuthal locations.