Electrocochleography for different electrode positions in guinea pig.

In 20 guinea pigs the relation between the electrode position on the cochlear surface and the recorded potentials was studied. We found the greatest AP amplitudes at the free lateral part of the cochlea; AP amplitudes decreased when the electrode was moved into the direction of the attachment of the cochlea to the petrous bone. Moving the electrode from the basal turn to the apex the maximal AP amplitude was found at the apex. The most negative SP was always recorded from the apex. This negative SP became smaller when the electrode was moved to the basal turn. Near the round window a small negative SP was recorded for 1 kHz stimuli and a positive SP for most 4 kHz stimuli. Moving the electrode from the free lateral part of the cochlea to the place of attachment to the petrous bone the magnitude of the SP decreased in a fashion similar to the AP amplitude. For 4 kHz stimuli we recorded a range of SP-AP ratios from -0.21 to +0.50 merely by varying the electrode position. Thus the SP-AP ratio highly depends on the electrode position.