Elsevier

Neuroscience Research

Volume 29, Issue 4, December 1997, Pages 283-289
Neuroscience Research

Lombard reflex during PAG-induced vocalization in decerebrate cats

https://doi.org/10.1016/S0168-0102(97)00097-7Get rights and content

Abstract

The Lombard reflex occurs when a speaker increases his vocal effort while speaking in the presence of ambient noise. The purpose of this study was to clarify whether the Lombard reflex can be evoked during controlled vocalization in an animal model. In decerebrate cats, repetitive electrical stimulation was applied to the midbrain periaqueductal gray (PAG) to evoke vocalization. Pure tone auditory stimulation was delivered through a loudspeaker. The activities of the laryngeal adductor muscle, diaphragm and external oblique abdominal muscle and the voice intensity were measured during PAG stimulation, in the presence and absence of the auditory stimulation. To clarify the effects of the auditory–laryngeal reflex on the activity of laryngeal adductor motoneurons, the amplitude of the laryngeal reflex evoked by single shock stimulation of the superior laryngeal nerve was also measured during respiration, in the presence and absence of auditory stimulation. The sound made by the cats due to PAG-induced vocalization was augmented by exposure to auditory stimulation, and the activities of the laryngeal adductor muscle and external oblique abdominal muscle were also augmented. During respiration, auditory stimulation also increased the amplitude of the laryngeal reflex evoked in the laryngeal adductor muscle. These results demonstrate that the essential neuronal mechanisms for evoking the Lombard reflex exist within the brainstem.

Introduction

Auditory feedback is suspected to play a significant role in human speech production. The Lombard reflex is one such representative feedback mechanism, which occurs when a speaker increases his voice level in response to an increase in the ambient noise level. This reflex has been employed as a clinical tool for determining simulated hearing loss (Lane and Tranel, 1971). Since it has been analyzed mainly in human subjects (Egan, 1971, Lane and Tranel, 1971), the neural mechanisms underlying this reflex are still unknown.

In decerebrate cats, repetitive electrical stimulation of the midbrain periaqueductal gray (PAG) induces natural-sounding vocalization (Magoun et al., 1937, Kelly et al., 1946, Kanai and Wang, 1962). After the onset of PAG stimulation, the normal respiratory rhythm ceases following which an initial inspiration is induced. Following the inspiration which is accompanied by increased activities of the abductor muscles of the vocal cords and the diaphragm, vocalization accompanied by increased activities of the adductor muscle of the vocal cords and the major expiratory muscles is induced. After that, inspiration and vocalization are induced alternately until the cessation of PAG stimulation. These muscle discharge patterns are consistently observed during vocalization induced by PAG stimulation.

In this study, we induced the Lombard reflex in a decerebrate animal vocalization model in order to clarify the essential brain area for evoking the reflex. Furthermore, we analyzed the effects of auditory stimulation on the activity of the laryngeal adductor muscle, which may contribute to the evocation of the Lombard reflex.

Section snippets

Materials and methods

Data were obtained for 10 adult male and female cats (2.5–4.0 kg). The animals were anesthetized with halothane (Fluothane, Takeda) vaporized in nitrous oxide and oxygen in a plastic box, and then the anesthesia was maintained by way of intranasal intubation. Cats were decerebrated at the precollicular–postmammillary level after bilateral ligation of the carotid arteries. The administration of anesthesic was discontinued following the completion of all surgical procedures and at least 1 h prior

Results

Fig. 1A shows representative recordings of the activities of laryngeal and respiratory muscles in addition to recordings of voice intensity during a control vocalization elicited by PAG stimulation. After the onset of PAG stimulation, inspiration was induced by an increase in DIA activity. Then, vocalization and inspiration were induced alternately. The laryngeal adductor TA muscle discharged strongly in addition to the EA muscle during vocalization. Fig. 1B shows representative recordings of

Discussion

The major findings of this study are that the voice intensity as a result of PAG-induced vocalization was augmented by exposure to auditory stimulation, and the activities of the laryngeal adductor TA and expiratory EA muscles were also augmented. In other words, the Lombard reflex was observed even in the decerebrate cat. Although we employed two instruments, that is, a loudspeaker and a bone oscillator for delivering auditory stimuli, the effects of stimulation on the activities of laryngeal

Acknowledgements

We express our sincere thanks to Associate Professor Alan D. Miller of the Rockefeller University for critically reviewing the final version of this manuscript. Some of these data have been presented previously as part of the First International Conference on Voice Physiology and Biomechanics (ICVPB).

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