Auditory-nerve correlates of loudness summation with stimulus bandwidth, in normal and pathological cochleae

The firing of guinea pig auditory nerve fibres was measured in response to bands of noise of many different bandwidths, centre frequencies and intensities. The total amount of activity in the auditory nerve fibre array was calculated as a function of the bandwidth and intensity of the stimulus. As the bandwidth of the stimulus increased, while its net intensity was kept constant, the total firing rate increased steadily. There was no sign of a breakpoint corresponding to the critical bandwidth, seen in psychophysical judgements of loudness with similar stimuli. Moreover, the slope of the relation between total activity and stimulus bandwidth was particularly shallow in fibres with shallow tuning curves. The results suggest that (i) while loudness may in broad terms be related to the total amount of activity in the auditory nerve, this is not true in detail, (ii) the critical bandwidth in loudness summation does not relate to the resolution bandwidth of the cochlea, and (iii) the slope of the psychophysical loudness summation function may be able to give information about the sharpness of neural tuning.