Narrow-band analysis of compound action potentials for several stimulus conditions in the guinea pig

Compound action potentials (AP) were recorded under various stimulus conditions in 31 guinea pigs. Stimulus attenuation, decrease of inter-stimulus interval, increase of the level of a continuous wide-band noise maker, and lowering the animal's temperature all resulted in a drop of the AP amplitude and an increase in latency. A narrow-band analysis of the compound APs makes it possible to describe these AP changes in terms of the response behaviour of small cochlear regions according to their central frequencies. The results show that intensity-dependent changes in the AP parameters can be explained on the basis of the tuning properties of the auditory nerve fibres when the effect of the rise time of the tone-burst stimulus is taken into account. Shortening of the inter-stimulus interval produces a complex interaction in terms of tone-burst frequency and the region along the cochlear partition that contributes dominantly to the AP. It is concluded that response contributions from the narrow bands with a central frequency near the tone-burst frequency show the most adaptation. The change in amplitude for narrow-band responses under increased masking is similar to that for stimulus attenuation. It seems, however, that the underlying masking mechanism is more comparable to the adaptation mechanism. Cooling of the animal did not affect the sharpness of tuning. In all four recording situations there seems to be a decrease in the amount of synchronization of single-fibre responses as reflected in the width of narrow-band action potentials.