Masking Of Action Potentials In The Guinea Pig Cochlea, Its Relation To Adaptation

Abstract

The amplitude, A_N1, and the latency, τ_N1, of the N₁ component of round-window-recorded APs in the guinea pig are modified by simultaneous background noise and also for some time afterward by a preceding noise burst ('forward masking') White-noise masking acts differently below and above 60 dB with respect to its influence on A_N1 and τ_N1.

Forward masking is more effective if the delay time is short. On the basis of different slopes of the curves relating A_N1 to noise intensity, the time during which forward masking has an effect can be subdivided into three periods. The period of shor delay is dominated by refractory mechanisms; during the period with long delays only adaptive mechanisms cause the masking; in the middle period both mechanisms are at work.

Adaptation experiments performed in the presence of masking noise indicate a competitive action between masking and adaptation. At equal stimulus intensities there is a definite relation between S/N value and ISI value.

The results of masking of the compound action potential are compatible with masking experiments in single nerve fibers as reported in the literature.

In genera, masking results form simultaneous action of both refractory and adaptive mechanisms, but the ISI-dependent properties for higher ISI values are due to adaptation only. This explains the different degrees of synchronization during masking and adaptation which are reflected in the different behaviour of the τ_N1 in the two experimental situations.

Key Words:

• Forward masking
• Compound action potential
• Cochlear electrophysiology