Elsevier

Hearing Research

Volume 172, Issues 1–2, October 2002, Pages 151-159
Hearing Research

Gap detection threshold in the rat before and after auditory cortex ablation

https://doi.org/10.1016/S0378-5955(02)00578-6Get rights and content

Abstract

Gap detection threshold (GDT) was measured in adult female pigmented rats (strain Long–Evans) by an operant conditioning technique with food reinforcement, before and after bilateral ablation of the auditory cortex. GDT was dependent on the frequency spectrum and intensity of the continuously present noise in which the gaps were embedded. The mean values of GDT for gaps embedded in white noise or low-frequency noise (upper cutoff frequency 3 kHz) at 70 dB sound pressure level (SPL) were 1.57±0.07 ms and 2.9±0.34 ms, respectively. Decreasing noise intensity from 80 dB SPL to 20 dB SPL produced a significant increase in GDT. The increase in GDT was relatively small in the range of 80–50 dB SPL for white noise and in the range of 80–60 dB for low-frequency noise. The minimal intensity level of the noise that enabled GDT measurement was 20 dB SPL for white noise and 30 dB SPL for low-frequency noise. Mean GDT values at these intensities were 10.6±3.9 ms and 31.3±4.2 ms, respectively. Bilateral ablation of the primary auditory cortex (complete destruction of the Te1 and partial destruction of the Te2 and Te3 areas) resulted in an increase in GDT values. The fifth day after surgery, the rats were able to detect gaps in the noise. The values of GDT observed at this time were 4.2±1.1 ms for white noise and 7.4±3.1 ms for low-frequency noise at 70 dB SPL. During the first month after cortical ablation, recovery of GDT was observed. However, 1 month after cortical ablation GDT still remained slightly higher than in controls (1.8±0.18 for white noise, 3.22±0.15 for low-frequency noise, P<0.05). A decrease in GDT values during the subsequent months was not observed.

Introduction

The measurement of gap detection threshold (GDT) is an efficient tool for evaluating the time resolution abilities of the auditory system. The estimation of GDT in man has been the subject of many studies, beginning with the paper by Plomp (1964). Numerous experiments have shown that gap detection ability is dependent on the frequency spectrum and intensity of the continuous noise in which the gaps are embedded (Penner, 1977, Giraudi et al., 1980, Tyler et al., 1982, Florentine and Buus, 1984, Shailer and Moore, 1983, Fitzgibbons, 1983, Snell et al., 1994). Minimal GDTs in human were recorded when the noise signal encompassing the gap was presented at 30–40 dB SL and included frequencies above 5–6 kHz. GDT in this case amounted to about 2–3 ms (Plomp, 1964, Fitzgibbons, 1983). Clinical observations have shown that neurological patients with various types of cerebral lesions have difficulties in temporal resolution tasks, such as reduced gap detection abilities (Buchtel and Stewart, 1989, Albert and Bear, 1974, Efron et al., 1985, Phillips and Farmer, 1990).

Neuronal correlates of gap detection have been studied at different levels of the auditory system (for review see Frisina, 2001). The fundamental mechanism of coding sound gaps at the level of the auditory nerve fibres is through a cessation of activity in response to a gap (Relkin and Turner, 1988, Zhang et al., 1990), whereas at the level of cochlear nuclei and the inferior colliculus the mechanism switches to an increase in the spike firing rate at the conclusion of the gap (Boettcher et al., 1990, Kaltenbach et al., 1993, Palombi et al., 1994, Walton et al., 1997, Barsz et al., 1998). Walton et al. (1997) concluded that the gap encoding capabilities of inferior colliculus neurones in the CBA mouse were similar to the behavioural gap functions. Neural correlates of gap detection were studied in the auditory cortex of the cat by Eggermont, 1995, Eggermont, 1999. In these experiments gaps inserted either early (after 5 ms) or late (after 500 ms) in a 1 s duration noise burst were used. The neural detection threshold for the ‘late gap’ was found to be similar to the psychophysical gap threshold (around 5 ms) and was not significantly different from that found in the inferior colliculus (Walton et al., 1997). Significantly larger minimum late-gap durations were found in the anterior auditory field in comparison with the primary and secondary auditory fields.

The measurement of GDT may be used for investigating pathologies of temporal resolution ability and for estimating their severity. For this purpose, studies dealing with the influence of experimentally produced impairments in gap detection in animals may be of importance. The aim of such studies is usually to determine the involvement of individual structures of the auditory system in temporal resolution. Only a few papers on gap detection in animals with a cortical lesion have been published so far (in rat: Ison et al., 1991; in ferret: Kelly et al., 1996). The purpose of this study was to examine the role of the auditory cortex in the ability to detect gaps in noise. GDT was measured in pigmented rats before and after bilateral ablation of the auditory cortex.

Section snippets

Subjects

The ability to detect gaps in a continuously present noise was tested in five adult (8–13 months old) pigmented female rats (strain Long–Evans) with no primary pathology, before and after bilateral lesions of the primary auditory cortex. GDT measurements after a bilateral lesion of the auditory cortex began on the fifth day after surgery and continued for 2 months. In addition, the dependence of GDT on the frequency spectrum and intensity of the carrier noise was studied in three other rats.

Behavioural apparatus and procedures

The

Results

Control experiments revealed that the GDT depends on the spectral characteristics and intensity levels of the continuous noise in which the gaps are embedded. The mean values of GDT measured in eight normal rats when the gaps were embedded in a continuous white noise of 70 dB SPL were significantly smaller (P<0.05, tested with the paired t-test) than when the gaps were embedded in a low-frequency noise of the same intensity; the values were 1.57±0.07 ms and 2.9±0.34 ms, respectively. The

Discussion

The lowest GDTs for rats, measured in our experiments under optimal stimulus conditions (in a broadband noise at an intensity above 40 dB SPL) by an operant conditioning method, fluctuated about 1.6 ms and were slightly smaller than those obtained in previous investigations using a startle amplitude reduction paradigm (Ison, 1982, Ison et al., 1991, Leitner et al., 1993). The examination of Ison (Ison, 1982, Ison et al., 1991) of acoustic startle reflex inhibition by brief gaps in noise showed

Acknowledgements

This work was supported by grants of the Grant Agency of the Czech Republic (309/01/1063) and the Grant Agency of the Ministry of Health (NK/6454-3).

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