MEG and EEG auditory responses to tone, click and white noise stimuli

Abstract

We report the results of two experiments comparing the amplitude of MEG auditory evoked fields (AEFs) and EEG auditory evoked potentials (AEPs) to tone, click and white noise stimuli. In the first experiment, AEFs and AEPs, bandpassed between 2.0 and 40 Hz, were averaged for 500 msec following 128 irregularly spaced alternating 92 dB clicks and 88 msec long 2 kHz tones, in 16 adult human subjects (8 male, 8 female). Recordings were made from both hemispheres, one side at a time, at a position 1 4 of the way up from T 3 to C 3 and T 4 to C 4. The magnetic sensor was a figure-8 SQUID gradiometer with a 4 cm baseline oriented for maximum sensitivity to a current dipole(s) perpendicular to the sylvian fissure. The dependent variable was the maximum amplitude of the responses occurring within 250 msec of stimulus delivery. A 4-way (2b × 2w × 2w × 2w) ANOVA was used for statistical analysis of amplitude data examining differences between sex, laterality (contralateral vs. ipsilateral), auditory stimuli (click vs. tone) and hemisphere (left vs. right). AEFs in response to tone stimuli were of higher amplitide than those for click stimuli, for both left and right hemispheres, and for both contralateral and ipsilateral ear stimulation ( F = 65.2, df = 1, 14, P < 0.001). The opposite relationships (click responses of greater amplitude than tone responses) were found with EEG AEPs ( F = 10.4, df = 1, 14, P < 0.01). AEFs and EEG AEPs were both of higher amplitude in response to contralateral compared to ipsilateral ear stimulation, when averaged across both left and right hemispheres, both clicks and tones, and both sexes. The second experiment, using 8 subjects, was identical except white noise bursts were used as stimuli instead of clicks. We found significantly higher amplitude AEFs to tones compared to equal length and SPL white noise bursts, with a non-significant trend for the reverse to be true for AEPs. Overall, tones appeared to produce the highest amplitude AEFs, and clicks the lowest amplitude, with white noise stimuli being intermediate. AEF amplitude as a function of stimulus type parallels previously reported auditory cortex unit activation by similar stimuli, and thus AEFs may more closely reflect cortical unit activity patterns, whereas other influences appear operative in influencing AEP amplitude.