Adaptation of differential sensitivity of auditory system neurons to amplitude modulation after abrupt change of signal level

Abstract

Impulse activity of neurons of brainstem auditory nuclei (medulla dorsal nucleus and midbrain torus semicircularis) of the grass frog (Rana temporaria) was recorded under action of long amplitude-modulated tonal signals. After adaptation of neuronal response to acting stimulus (30–60 s after its onset), we performed a sharp change (by 20–40 dB) of the mean signal level with preservation of unchanged frequency and depth of modulation. We also recorded a change of density impulsation and of degree of its synchronization with the modulation period as well as the phase of maximum reaction at the modulation period and phase of the response every 2 or 4 s. In the adapted state, the sharp change of the mean level had been provided, while maintaining frequency and depth unchanged. During the adaptation to long signals with small modulation indexes the firing rate continuously decreased, but synchronization with envelope usually increased considerably. A sharp rise in the mean level resulted in an increase of firing rate, which could be accompanied either by a continuation of synchronization growth (the effect is more typical of the dorsal nucleus) or by a sharp fall in synchrony with its subsequent slow recovery (the effect is more typical of the torus semicircularis). Nature of the changes following the change of the intensity of the reaction could depend on the signal parameters (initial level, magnitude of the jump, frequency and depth of modulation). The connection between the observed physiological data and the psychophysics of differential intensity coding is discussed.