Auditory brain stem responses (ABR) in cat. Latency as a function of stimulus polarity, intensity and acoustic waveform.

ABR from cats were measured by use of standard EEG surface electrodes and ERA Madsen-74 equipment. A method for recording the waveform of the stimulus at the ear drum is demonstrated. ABR to both phases of three different stimuli was compared. In four out of five cats the ABR latency for rarefaction clicks was shown to be shorter than for condensation clicks at high stimulus levels. This effect of stimulus polarity was most pronounced for stimuli with few and broad peaks in the acoustic waveform. At lower stimulus intensities no differences in ABR latency could be demonstrated by reversing stimulus polarity. It is suggested that the first negative deflection in the acoustic waveform is mainly responsible for the ABR at high stimulus level. At lower stimulus level not only the onset but a greater part of the acoustic signal is responsible for the response.