FUNDAMENTAL STUDY OF THE NEURAL MECHANISM IN CATS SUBSERVING THE FEATURE EXTRACTION PROCESS OF COMPLEX SOUNDS

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The responses of single auditory neurons of the cat to relatively complex, time-varying stimuli, that is, FM, AM, and FM-AM sounds, along the auditory pathway were studied.<BR>1) Directional sensitivity of neurons in response to either ascending or descending FM sound was found at the superior olive and it became more distinct at the higher auditory centers.<BR>2) Some medial geniculate units responded only to a particular direction of FM sound and did not respond to pure tone at all.<BR>3) Neurons in the upper levels of the auditory pathway can be characterized by the rate of frequency modulation. Three kinds of collicular neurons can be classified according to their FM directionalities and the rate of frequency modulation.<BR>4) Directional sensitivity of neurons in response to either increasing or decreasing AM sound was found at the cochlear nerve, with increasing AM predominating. In general, at the higher auditory centers, the slow-adapting type neurons are predominantly sensitive to increasing AM sound, while the fast-adapting type neurons, in particular “on” type units, are predominantly sensitive to decreasing AM sound.<BR>5) Neuronal excitability did not vary significantly with the rate of amplitude modulation for “on” type units, but it was lower for the slowadapting neurons when the rate of amplitude modulation was slowed down.<BR>6) The neuronal responsiveness to each of four kinds of FM-AM sounds was investigated. In the lower levels of the auditory pathway, there were no significant differences in response to each of the FM-AM sounds, while in the higher levels the neurons responded mainly to a particular combination of FM-AM sound which can be determined by the response characteristics by examining the FM and AM sounds independently.<BR>7) The possible mechanisms of neuronal FM and AM directionalities were considered on the basis of synaptic excitation and inhibition.

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