A comparative study of the responses of isolated first-order semicircular canal afferents to angular and linear acceleration, analysed in the time and frequency domains

Abstract

The experiments reported here represent a study of the responses of semicircular canals in the isolated surviving labyrinth of elasmobranch fishes, with special reference to their sensitivity to linear acceleration, including positional stimuli, in comparison with their 'classical ’ function as inertial angular accelerometers. Responses to angular velocity steps, to step changes in position (linear acceleration steps), and responses to sinusoidal stimuli were analysed in the time domain or in the frequency domain respectively. In this investigation the existence of peripheral adaptation and the associated time constants were taken into account. It was found that the time constants and phase aspects of the res­ponses to angular and linear acceleration stimuli are consistently com­parable. This is believed to point to similarities in the basic mechanism of both types of canal response. It is suggested, therefore, that the sensitivity of semicircular canals to linear acceleration, referred to in this paper as the ‘Ledoux effect’, originates within the cupula-endolymph system in a manner fundamentally similar to that governing the inertial responses to angular acceleration. Although a considerable proportion of the canal-controlled first-order neurons studied responded to positional stimuli, no predictable and directionally fixed relation was found to exist between head positions and response levels. This suggests that differences in density as between cupula and endolymph, considered to be the cause of the positional responses, must be assumed to be subject to fluctuation both in sign and magnitude. We conclude that it is, therefore, doubtful whether the Ledoux effect falls within the range of ‘normal’ canal function in the control of posture and movement. Nevertheless, its existence cannot be discounted in the qualitative and quantitative analysis of vestibular responses, and, especially, in the differential diagnosis of malfunction in the field of clinical otology. Work along similar lines on higher vertebrates is discussed in the context of the reported results.