Lombard reflex during PAG-induced vocalization in decerebrate cats
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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