Intensity difference thresholds assessed with eighth nerve and auditory cortex potentials: compared values from cochlear and vascular responses.

In normal guinea pigs the eighth nerve compound action potential shows a latency of about 1 ms and the evoked early potential at the auditory cortex occurs after about 10 ms. Determination of auditory sensitivity with both responses using tone-bursts of variable rise/fall time and plateau duration showed they are responses to the onset of the stimulus since the existence of a plateau has no effect. Increases in stimulus rise time diminish the synchrony of both responses, the limits were found to be 2 ms for eighth nerve responses and 10 ms for cortical potentials. Amplitude modulation of continuous tones using these time values evokes clear responses at levels which apparently correspond to intensity difference thresholds measured behaviorally by other authors. Similar measures were performed on saccular acoustic responses, using the model of selective cochlear destruction. Evoked responses could also be observed at levels of amplitude modulation similar to those of normal animals. This fine sensitivity reinforces the idea of a functionality of the saccular acoustic reception.