Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. II. Stimulation with amplitude modulated sound

The coding of fine-temporal structure of sound, especially of frequency of amplitude modulation, was investigated on the single-unit level in the auditory midbrain of the grassfrog. As stimuli sinusoidally amplitude modulated sound bursts and continuous sound with low-pass Gaussian noise amplitude modulation have been used. Both tonal and wideband noise carriers have been applied. The response to sinusoidally amplitude modulated sound bursts was studied in two aspects focussing on two types of possible codes: a rate code and a synchrony code. From the iso-intensity rate histogram five basic average response characteristics as function of modulation frequency have been observed: low-pass, band-pass, high-pass, bimodal and non-selective types. The synchronization capability, expressed in a synchronization index, was non-significant for 38% of the units and a low-pass function of modulation frequency for most of the other units. The stimulus-response relation to noise amplitude modulated sound was investigated by a non-linear system theoretical approach. On the basis of first- and second-order Wiener-Volterra kernels possible neural mechanisms accounting for temporal selectivity were obtained. About one quarter of the units had response characteristics that were invariant to changes in sound pressure level and spectral content of the carrier. These units may function as feature detectors of fine-temporal structure of sound. The spectro-temporal sensitivity range of the auditory midbrain of the grassfrog appeared not to be restricted to and showed no preference for the spectro-temporal characteristics of the ensemble of conspecific calls. Comparison of response characteristics to periodic click trains as studied in the companion paper (Epping and Eggermont, 1986) and sinusoidally amplitude modulated sound bursts revealed that the observed temporal sensitivity is due to a combination of sensitivities to sound periodicity and pulse duration. It was found that for most units the first-order kernels for Gaussian amplitude modulated stimuli and Poisson distributed click stimuli were alike. In contrast second-order kernels for the Gaussian amplitude modulated stimuli often represented only static non-linearities, while second-order kernels for Poisson distributed clicks (Epping and Eggermont, 1986) mostly revealed dynamic non-linearities.

[1]  N. Wiener,et al.  Nonlinear Problems in Random Theory , 1964 .

[2]  L. S. Frishkopf,et al.  Neural coding in the bullfrog's auditory system a teleological approach , 1968 .

[3]  R. R. Capranica,et al.  Temporal selectivity in the central auditory system of the leopard frog. , 1983, Science.

[4]  I. A. Vartanian,et al.  TIME DEPENDENT FEATURES OF ADEQUATE SOUND STIMULI AND THE FUNCTIONAL ORGANIZATION OF CENTRAL AUDITORY NEURONS , 1973 .

[5]  P. I. M. Johannesma,et al.  Spectro-temporal characteristics of single units in the auditory midbrain of the lightly anaesthetised grass frog (Rana temporaria L.) Investigated with noise stimuli , 1981, Hearing Research.

[6]  Jos J. Eggermont,et al.  Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. III. Stimulation with natural and synthetic mating calls , 1986, Hearing Research.

[7]  Jos J. Eggermont,et al.  Single-unit characteristics in the auditory midbrain of the immobilized grassfrog , 1985, Hearing Research.

[8]  A. Møller.  Responses of units in the cochlear nucleus to sinusoidally amplitude-modulated tones. , 1974, Experimental neurology.

[9]  J. J. Gelder,et al.  CALLING AND ASSOCIATED BEHAVIOUR OF THE COMMON FROG, RANA TEMPORARIA, DURING BREEDING ACTIVITY , 1978 .

[10]  N. Bibikov,et al.  Coding of Amplitude-Modulated Tones in the Midbrain Auditory Region of the Frog , 1981 .

[11]  M. Schetzen.  The Volterra and Wiener Theories of Nonlinear Systems , 1980 .

[12]  A. Møller,et al.  Statistical evaluation of the dynamic properties of cochlear nucleus units using stimuli modulated with pseudorandom noise. , 1973, Brain research.

[13]  Henry R. Hirsch,et al.  Responses of single units in the cat cochlear nucleus to sinusoidal amplitude modulation of tones and noise: Linearity and relation to speech perception , 1976, Journal of neuroscience research.

[14]  Jos J. Eggermont,et al.  Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. I. Stimulation with acoustic clicks , 1986, Hearing Research.

[15]  G. Rose,et al.  Sensitivity to amplitude modulated sounds in the anuran auditory nervous system. , 1985, Journal of neurophysiology.

[16]  J. Goldberg,et al.  Response of binaural neurons of dog superior olivary complex to dichotic tonal stimuli: some physiological mechanisms of sound localization. , 1969, Journal of neurophysiology.

[17]  A R Moller,et al.  Use of stochastic signals in evaluation of the dynamic properties of a neuronal system. , 1974, Scandinavian journal of rehabilitation medicine. Supplement.

[18]  Adrian Rees,et al.  Responses of neurons in the inferior colliculus of the rat to AM and FM tones , 1983, Hearing Research.