Summating potential and action potential gradients on and in the vicinity of the round window in guinea pig

The purpose of this study was to determine the spatial gradients of summating potential (SP) amplitudes, action potential (AP) amplitudes, and SP/AP ratios for recording loci on the round window (RW) membrane and in its vicinity. Sixteen guinea pigs were tested by means of free-field click and tone burst (2.0 kHz) stimuli. Seven recording regions were specified: five equal-area regions on the RW membrane and two basal promontory regions. Statistically significant differences were found between the promontory regions and the RW membrane regions for the SP, AP, and SP/AP obtained with click stimuli, and for the SP and AP obtained with tone burst stimuli. The SP/AP ratio for tone burst stimuli did not differ significantly across the seven regions. The RW membrane was found to be isoelectric, but there were marked spatial gradients on the basal promontory. These results are of consequence for the interpretation of transtympanic electrocochleography recordings.