Variable effects of click polarity on auditory brain-stem response latencies: analyses of narrow-band ABRs suggest possible explanations.

The auditory brain-stem responses (ABRs) to rarefaction and condensation clicks were obtained for 12 normal-hearing subjects in quiet, and high-pass masking at 8, 4, 2, 1, and 0.5 kHz. Derived narrow-band wave V latency differences were analyzed with respect to (1) stimulus polarity, (2) absolute differences irrespective of polarity. The analyses revealed no significant stimulus polarity effects on latency for the derived bands. Absolute latency differences regardless of polarity tended to be greater for those derived bands having lower characteristic frequencies (CFs). However, these differences were smaller than the expected half-period of the theoretical CF. Further analyses in three additional subjects using repeated runs of the same polarity indicate that this increase in absolute latency difference with lower derived band CF does not reflect a simple half-period change owing to polarity, but rather to the increase variability in measuring the peak latency of the lower CF derived bands. The variability is consistent with variability of eighth nerve PST histograms behavior observed in animal work [Kiang et al., "Discharge patterns of single fibers in the cat's auditory nerve," Research Monograph No. 35 (MIT, Cambridge, MA, 1965)]. Thus claimed polarity effects observed in other ABR work using absolute values may have been affected by this variability. It appears from these current data that half-period latency shifts of wave V owing to stimulus polarity differences are not observed in derived bands responses initiated from frequency specific regions of the cochlea.