抄録
The behavior of adaptation-type neurons to unsteady sounds (tone bursts, amplitude-modulated tones, frequency-modulated tones and combinations of these) was investigated using an electronic model of the auditory nervous system (Fig. 1). When a tone burst is applied to the model, the response of the adaptation-type neurons with CF approximately corresponding to the stimulus frequency gradually reduces after an initial response. However, the response of the neurons distributed at the peripheral part of the responding neurons is phasic, in spite of the fact that the neurons are of the same response type (Fig. 6). That is, a neuron behaves differently according to the relation between the stimulus frequency and the C. F. of the neuron. This phenomenon is observed in the responses of primary and secondary neurons of the sustained type (Fig. 3, 4, 5). Furthermore, for a tone burst with two Frequency components, the response of the adaptation-type neurons varies from adaptive response to on-response depending on the frequency interval and the relative intensity of the two components (Fig. 7). In the adaptation-type neuron model, the coefficient and time constant of the inhibitory connections are greater than those of the excitatory connections, so that the end part of the response of the secondary neuron is suppressed and the duration of the response is shorter than that of the input stimulus (Figs. 8 and 9). For an amplitude-fluctuated tone, therefore, the adaptation-type neuron behaves so as to segment its response at each peak point (Fig. 10). Such a function is one of the important signal processing functions of the adaptation-type neuron and is characterized as temporal sharpening. In the response of the sustained-type secondary neuron to a compound tone that is composed of a pure tone and a frequency-modulated tone the frequency of which changes toward that of the pure tone, the response to the pure tone and to the FM tone are separated spatially in the response pattern. This is the effect of spatial separation. Furthermore, the response to the pure tone is segmented in a temporal response pattern, since the neuron is inhibited when the FM tone is in the inhibitory area. This is the effect of temporal segmentation (Fig. 11, 12). Adaptation-type neurons emphasize the spatial separation and the temporal segmentation. It is concluded that the behavior of the auditory upper nervous system is to funnel the response at the neurons with CF corresponding to the frequency components of the stimulus and to segment temporally the response to the sequential stimulus.
