*Currently at Disney’s Animal Kingdom, Lake Buena Vista, FL

1. 2pPP10. Laboratory rats do not use binaural time cues to localize sound.Christina M. Wesolek*, Gimseong. Koay, and Henry E. Heffner, Dept. of Psychology, University of Toledo, Toledo, OH. 43606 Introduction Because the results were so unexpected, we replicated them using various rise-fall times and signal intensities. We also determined the ability of the rats to localize a 1-kHz tone amplitude-modulated at either 250 or 500 Hz, which provided a phase difference in the envelope of the sound. None of these manipulations changed character of the rats’ performances or our conclusion that they cannot use the binaural time cue. Acoustic stimuli 100-ms tone pips were generated digitally, randomly attenuated over a 3.5-dB range, gated with a rise-fall gate (5, 20, or 50-ms rise-fall), band-pass filtered, amplified, and sent to one of a pair of matched loud-speakers (Motorola piezoelectric speakers or Infinity 2000 woofers). Additional stimuli included a 1-kHz tone amplitude-modulated at 250 and 500 Hz (100% modulation depth). The speaker pairs were centered on midline and separated by 60º. Separate tests were conducted with the tones at 15, 30, and 50 dB sensation level (SL). The output of the speakers was measured using Brüel & Kjaer sound measuring equipment and the acoustic spectra of the tones was carefully checked for distortion with a Zonic spectrum analyzer. Behavioral procedure Unlike the previous two studies, the animals were tested in a conditioned suppression task. In this test, they drank from a water spout while tone pips were presented from their right side and stopped drinking whenever a tone pip was presented from their left side in order to avoid a brief shock. Breaking contact with the spout when a tone came on the left side was counted as a “hit” whereas breaking contact when a tone came on the right side was counted as a “false alarm” (FA). The measure of performance was the hit rate corrected for the false alarm rate. There are two binaural cues for localizing sound: the difference in the time of arrival of a sound at the two ears and the difference in the intensity of the sound at the two ears. The use of these two cues can be demonstrated by determining an animal’s ability to localize pure tones: the ability to localize high-frequency tones indicates that an animal is able to use the binaural intensity cue whereas the ability to localize low-frequency tones indicates that it can use the binaural time cue (in the form of a binaural phase difference). Interestingly, the highest frequency that can be localized using the binaural phase cue varies between species from the 500-Hz upper limit of cattle to the 6.3-kHz upper limit of Jamaican fruit bats (H. Heffner & Heffner, 2003). In analyzing this variation, we noticed that the upper limit for the laboratory rat is not well established. A 1975 study by Bruce Masterton and colleagues indicated the rat’s upper limit was between 4 and 8 kHz (Fig. 1 Left), whereas a 1986 study by Jack Kelly and Gerard Kavanagh indicated that it was between 2 and 4 kHz (Fig. 1 Right). Thus, we sought to determine which of the two studies was correct. Conclusion It is unlikely that our results are due to the use of the conditioned suppression procedure because it has been used to demonstrate the use of binaural phase in other species (H. Heffner & Heffner, 2003) and has been shown to give the same results as the two-choice procedure in sound localization testing (e.g., R. Heffner & Heffner, 1988). We also have no reason to think that the difference is due to the use of hooded, as opposed to albino, laboratory rats. One possible explanation is that the sound systems used by the previous studies might have contained some additional cues, such as high-frequency artifacts or intensity differences between the left and right loudspeakers. The study by Masterton et al. (1975) was conducted before the widespread availability of spectrum analyzers and it is possible that their low-frequency tones were not pure. The study by Kelly & Kavanagh (1986), however, did carefully analyze the spectra of their signals and we see no obvious way to reconcile our findings with theirs. Nevertheless, no matter what we tried, we were unable to demonstrate that our rats could use the binaural time cue. Results To our surprise, our results failed to confirm either of the two previous studies. Instead, we found that although rats can localize high frequencies, indicating that they use the binaural intensity cue, they cannot localize low frequencies, indicating that they cannot use the binaural time cue (Fig. 2). Figure 1. Left: Ability of two albino rats to localize single tone pips in a two-choice test. Tone pips were 40-ms rise, 60 ms on full, 40-ms decay; angle of separation was 60º. This study concluded that rats use the binaural phase cue to localize tones below 8 kHz and the binaural intensity cue to localize tones above 8 kHz. Right: Ability of seven albino rats to localize single tone pips in a two-choice test. Angle of separation was 60º. Tone pips were 20-ms rise, 25 ms on full, 20-ms decay. Animals C through G had bilateral auditory cortex lesions whereas animals A and B were normal. This study indicated that rats use the binaural phase cue to localize tones below 4 kHz and the binaural intensity cue to localize tones above 4 kHz. References Heffner, H. E. & Heffner, R. S. (2003). Audition. In S. F. Davis (Ed.) Handbook of Research Methods in Experimental Psychology, pp. 13- 440. Blackwell: Malden, MA. Heffner, R. S., & Heffner, H. E. (1988). Sound localization acuity in the cat: Effect of azimuth, signal duration, and test procedure. Hearing Research, 36, 221-232. Kelly, J. B. & Kavanagh (1986). Effects of auditory cortical lesions on pure- tone sound localization by the albino rat. Behavioral Neuroscience, 100, 569-575. Masterton, B., Thompson, G. C., Bechtold, J. K., & RoBards, M. J. (1975). Neuroanatomical basis of binaural phase-difference analysis for sound localization: A comparative study. Journal of Comparative and Physiological Psychology, 89, 379-386. Methods Subjects Four Harlan Sprague-Dawley, male hooded rats (Rattus norvegicus) were used in this study. Figure 2. Ability of four hooded rats to localize single tone pips in a conditioned suppression test. Angle of separation was 60º. Tone pips were 50-ms rise, 50 ms on full, 50-ms decay. The ability of the rats to localize high-frequency tones indicates that they can use the binaural intensity-difference cue. However, their inability to localize tones below 2.8 kHz indicates that they are unable to use the binaural phase cue. *Currently at Disney’s Animal Kingdom, Lake Buena Vista, FL

2. Abstract: The use of binaural time and intensity cues to localize sound can be investigated by determining the ability of a subject to localize pure tones in a free-field. Specifically, the ability to localize low-frequency tones, which do not produce an intensity difference at the two ears, demonstrates the use of the binaural phase (time) cue whereas the ability to localize high-frequency tones, to which the auditory system cannot phase lock, demonstrates the use of the binaural intensity-difference cue. Because previous studies of the laboratory rat (Rattus norvegicus) disagreed on the highest frequency that could be localized using binaural phase difference, the ability of rats to localize pure tones was reexamined. The results indicated that, contrary to previous studies, laboratory rats are not able to localize low-frequency tones even when they are amplitude modulated. Thus, it appears that laboratory rats are unable to use the binaural time-difference cue to localize sound. Because the previous studies were conducted before the widespread availability of spectrum analyzers, it is possible that their results were due to the presence of high-frequency harmonics in their low-frequency tones. These results have relevance for the anatomical and physiological study of binaural processing in laboratory rats.