Effect of stimulus intensity level on auditory middle latency response brain maps in human adults.

Auditory middle latency response (AMLR) brain maps were obtained in 11 young adults with normal hearing . AMLR waveforms were elicited with monaural clicks presented at three stimulus intensity levels (50, 70, and 90 dB nHL) . Recordings were made for right and left ear stimulus presentations . All recordings were obtained in an eyes open/awake status for each subject. Peak-to-peak amplitudes and absolute latencies of the AMLR Pa and Pb waveforms were measured at the Cz electrode site. Pa and Pb waveforms were present 100 percent of the time in response to the 90 dB nHL presentation. The prevalence of Pa and Pb to the 70 dB nHL presentation varied from 86 to 95 percent . The prevalence of Pa and Pb to the 50 dB nHL stimulus never reached 100 percent, ranging in prevalence from 77 to 68 percent . No significant ear effect was seen for amplitude or latency measures of Pa or Ph . AMLR brain maps of the voltage field distributions of Pa and Pb waveforms showed different topographic features. Scalp topography of the Pa waveform was altered by a reduction in stimulus intensity level. At 90 dB nHL, the Pa brain map showed a large positivity midline over the frontal and central scalp areas . At lower stimulus intensity levels, frontal positivity was reduced, and scalp negativity over occipital regions was increased . Pb scalp topography was also altered by a reduction in stimulus intensity level. Varying the stimulus intensity significantly altered Pa and Pb distributions of amplitude and latency measures . Pa and Pb distributions were skewed regardless of stimulus intensity.