Neurophysiological aspects of hearing in the cod (Gadus morhua)
The spontaneous spike activity of the primary afferents was statistically analysed in order to gain insight into its origin and to provide a basis for examining its modulation by sound. The responses to pure tone stimulation were investigated by examining the change in spike rate, degree of synchrony and the phase locking angles, to pressure and velocity stimuli at different stimulus intensities, over a range of frequencies. The 3-dimensional directional characteristics of the units were studied by presenting vibrational stimuli on different axes around the animal. It was found that most spontaneous spike activity showed serial dependence and thus the ISI histogram is an incomplete description of the activity. There is evidence of resonance of the hair cell/primary afferent complex, both from certain patterns of spontaneous spike activity and from the relationship between the driven spike activity and the phase of the stimulus. The phase angle of response to pure tone stimulation increased or decreased as the stimulus intensity increased depending on the whether the stimulus frequency was above or below the best frequency of the unit. It is shown for the first time that cod primary afferent units respond to sound in 3 dimensions.