Normal short‐latency electrophysiological filtered click responses recorded from vertex and external auditory meatus

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Abstract
We recorded normal electrophysiological responses to third‐octave filtered clicks from external auditory meatus (EAM) and vertex electrodes referred to coupled earlobe electrodes (forehead ground). From both vertex and EAM, polarity‐sensitive responses predominated at low frequencies and exhibited characteristics of both phase‐locked neural responses (frequency‐following response or FFR) and cochlear microphonics (CM). The FFR‐like response predominated at the vertex site and the CM‐like response predominated at EAM. At high frequencies, polarity‐insensitive responses closely resembled rectangular‐pulse click action potentials and brainstem evoked potentials, with clearly defined N1 and V peaks recorded from EAM and vertex, respectively. As frequency was lowered, the N1 and V peak latencies increased, the peaks broadened, and the latency‐intensity curves steepened with greater prolongation occurring at lower click intensities. Lowering click frequency also shortened the N1–V interval and caused the plot of N1–V interval versus click intensity to become steeper. Plots of polarity‐insensitive response amplitudes and thresholds against frequency revealed a high‐frequency bias for both N1 and V, but the V ’’frequency response’’ was flatter. A possible explanation of the shortened N1–V interval at low click frequencies based on this flatter V ’’frequency response’’ is presented.