The use of 64-channel electroencephalography and positron emission tomography to study vestibular evoked myogenic potentials
McNerney, Kathleen Szalda
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The vestibular evoked myogenic potential (VEMP) is a response that can be recorded from the sternocleidomastoid (SCM) muscle in response to either clicks or low-frequency tonebursts that are presented at a high stimulus level. The VEMP is most likely generated by the saccule. The saccule is part of our vestibular system, and normally contributes to our sense of balance. However, recent studies have shown that it is also responsive to sound. The exact neural pathway of the VEMP is not yet fully understood, limiting its clinical application. The present study has identified the areas of the brain that are activated by stimuli used clinically to evoke the VEMP, by utilizing both EEG (brain electrical activity) recordings, as well as functional imaging, which were collected separately. Stimuli consisted of 500 Hz tonebursts. Analysis of event related potentials (ERPs) derived from EEG recordings revealed that the responses to the 90 dB pSPL stimuli were absent or smaller than the responses that were recorded in response to the 120 dB pSPL stimuli. We recorded an initial negative-going peak which occurred at ∼13.5 ms, with the frontal electrodes producing the largest response amplitude, as well as a later positive-going peak which occurred at ∼20.5 ms, with the occipital electrodes and the electrodes which were placed over the right temporal lobe producing the largest amplitude. Source localization techniques (CURRY) were used to analyze the EEG data, while both PET and EEG data were analyzed using SPM5. When we compared the areas activated by the high vs. low-level stimuli, we found that the VEMP stimuli activated vestibular regions of the brain, specifically the premotor cortex, the inferior and medial temporal gyri, and Brodmann area 40, in addition to the auditory areas typically activated by sound, such as the primary auditory cortex. In addition, the brain activity for the initial negative-going peak was localized in the primary visual cortex, as well as the precuneus. In conclusion, several brain regions were activated in response to stimuli that are used clinically to evoke the VEMP. We believe that these areas are representative of the neural correlates of the VEMP.