Coding of sounds in lower levels of the auditory system

Abstract

The great number of investigations and advanced developments in neurophysiology and psychoacoustics during recent years have extensively increased our knowledge about the frequency analysis of simple sounds in the peripheral auditory system.New methods have facilitated quantitative measurements of the amplitude of the submicroscopic vibration of a narrow segment of the basilar membrane in anaesthetized animals at physiological sound intensities. The results of these studies have quantitatively confirmed the results of past studies by showing that the basilar membrane has a selectivity with regard to tone frequency. In addition to this, the recent studies have increased our knowledge about the finer details of vibration of the basilar membrane. At the lowest levels used in the recent investigations, i.e. about 70 dB SPL, the selectivity in the 7 kHz region of the basilar membrane was found to be greater than expected on the basis of extrapolation of older data. Moreover, the high frequency slope of the tuning curves of the basilar membrane was found to be particularly steep. The results of these recent studies, furthermore, showed that the basilar membrane vibrates in a non-linear way at intensities within the physiological range. This non-linearity results in a broadening of the selectivity curves of a narrow segment of the basilar membrane when the sound intensity is increased.Little is known as to how the motion of the basilar membrane is transformed to excitation of the cochlear sensory cells, i.e. the haircells. The excitation may be related to displacement, spatial differentiation or other transformations of the basilar membrane motion. Recording from the interior of mammalian haircells has so far been unsuccessful, and the neural excitatory process within the haircells in the cochlea is as yet practically unknown. Studies of the haircells in the lateral line organ of fish have provided fundamental knowledge about their excitation; since they in many respects resemble those in the mammalian cochlea, the results very probably can be applied to the excitatory process in the mammalian cochlea.

Responses of Neurons in the Rat's Ventral Nucleus of the Lateral Lemniscus to Monaural and Binaural Tone Bursts

“…Many neurons (37%), however, had multipeaked tuning curves, suggesting the convergence of input from lower structures in shaping frequencyresponse properties. Analysis of Q 10 values showed that most of the neurons in the rat's VNLL were narrowly tuned to sound frequency and that the Q 10 distribution was similar to that reported previously for the rat's inferior colliculus or cochlear nucleus (Kelly et al 1991;Møller 1969Møller , 1972.…”

Syllabic (∼2–5 Hz) and fluctuation (∼1–10 Hz) ranges in speech and auditory processing

“…The TMTF was first introduced into hearing research by Møller (1972a, 1972b), who studied the responses of single-units in the cochlear nucleus to AM and FM stimuli (Section 4). The concept of the TMTF is quite simple: take an input signal and an output signal, related by a system (black box); but instead of relating the raw input/output signals, we instead attempt to relate the envelopes of the input/output signals.…”

The mechanism of acoustic transmission in birds

“…Studies on the peripheral part of the acoustic analyzer (Konishi, 1970;Moller, 1972) demonstrated frequency analysis at the level of the cochlea. Note that the frequency range perceived by the cochlea exceeds the range used for conspecific communication.…”

“…The neuronal system of the auditory analyzer translates the frequency values into temporary flashes of individual neurons (Moller, 1972). Further frequency data in the primary auditory nuclei reflects the location (Konishi, 1970).…”