A discrepancy exists between the dynamic range of peripheral auditory neurones and the range over which the human ear maintains fine intensity discrimination. Psychophysical experiments, using simultaneous high and low pass (bandstop) masking noise, indicate that intensity discrimination at high intensities under these specific conditions is unlikely to be mediated by a spread of activity to neurones innervating neighbouring regions of the basilar membrane.
A conclusion from these psychophysical data is that neurones coming from a limited region of the basilar membrane must, at least under bandstop noise masking conditions, be capable of signalling small intensity differences over a wide dynamic range. It is the validity of this conclusion which the experiments reported in this dissertation have sought to test.
A related but separate problem is the way in which the absolute intensity of a signal is represented in the activity of the cochlear nerve. The encoding of absolute intensity is therefore considered and briefly discussed in relation to the results of the present study.
Microelectrode recordings from the cochlear nerve and nucleus of the anaesthetised cat led to the following conclusions:
1. The range of minimum thresholds of neurones in the cochlear nerve and nucleus of a single ear is limited, at any frequency, to 20-JO dB, which is consistent with the literature and probably excludes the possibility that a high threshold population of neurones had been previously overlooked.
2. The dynamic range of some monotonio cochlear nucleus neurones measured after adaptation is wider than that measured under unadapted conditions. However, the dynamic ranges of cochlear nerve fibres and iii - all monotonic cochlear nucleus neurones, even allowing for the extension afforded by the adaptation process, are still insufficient to account for the wide dynamic range shown psychophysically. Nonmonotonic cochlear nucleus neurones have dynamic ranges of up to 100 dB, which may reflect inhibitory input to such cells from fibres of differing characteristic frequency.
3. Under conditions of bandstop noise masking, analogous to those used in the psychophysical studies, some two-thirds of the neurones in the dorsal cochlear nucleus (with characteristic frequencies from 0.7-29 kHz) have very wide dynamic ranges, some of which were up to 110 dB. In contrast, cochlear fibre dynamic ranges in response to tones are the same, whether or not the bandstop noise masker is present. The results of parametric studies of the responses of cochlear nucleus neurones are consistent with the suggestion that the wide dynamic range, under bandstop noise masking conditions, results from lateral inhibition induced by the masking noise.
4. Under conditions of bandstop noise masking, intensity differences at high intensity levels are not encoded in the cochlear nerve by large differences in the mean discharge rate of single fibres, but could possibly be encoded by small differences in the mean discharge of a minority of fibres. A second possibility for the encoding of intensity differences, under these conditions, is the fine time structure of the cochlear fibre discharge.