ABR amplitude differences as a function of stimulus types and cell densities. A: The difference in auditory brainstem response amplitudes at 10 dB above threshold and at threshold using click and 0.5-, 1-, 2-, 4-, 8-, 12-, and 16-kHz tone stimuli in middle-aged (solid bars) and old (open bars) monkeys. The larger value observed with click stimuli suggests a steeper amplitude decrease for click stimuli leading down to threshold. This further suggests that clicks and tones are processed distinctly and can indirectly be noted through the ABR. B: Difference in auditory brainstem response amplitudes to clicks at 10 dB SPL above threshold and at threshold as a function of cell density. Correlations were significant for both stains. *P < 0.05.