Use of 64-channel electroencephalography to study neural otolith-evoked responses.

BACKGROUND: The vestibular evoked myogenic potential (VEMP) is a myogenic response that can be used clinically to evaluate the function of the saccule. However, to date, little is known about the thalamo-cortical representation of saccular activation. It is important to understand all aspects of the VEMP, as this test is currently used clinically in the evaluation of saccular function. PURPOSE: To identify the areas of the brain that are activated in response to stimuli used clinically to evoke the VEMP. RESEARCH DESIGN: Electroencephalography (EEG) recordings combined with current density analyses were used to identify the areas of the brain that are activated in response to stimuli presented above VEMP threshold (500 Hz, 120 dB peak SPL [pSPL] tone bursts), as compared to stimuli presented below VEMP threshold (90 dB pSPL, 500 Hz tone bursts). Ten subjects without any history of balance or hearing impairment participated in the study. RESULTS: The neural otolith-evoked responses (NOERs) recorded in response to stimuli presented below VEMP threshold were absent or smaller than NOERs that were recorded in response to stimuli presented above VEMP threshold. Subsequent analyses with source localization techniques, followed by statistical analysis with SPM5 (Statistical Parametric Mapping), revealed several areas that were activated in response to the 120 dB pSPL tone bursts. These areas included the primary visual cortex, the precuneus, the precentral gyrus, the medial temporal gyrus, and the superior temporal gyrus. CONCLUSIONS: The present study found a number of specific brain areas that may be activated by otolith stimulation. Given the findings and source localization techniques (which required limited input from the investigator as to where the sources are believed to be located in the brain) used in the present study as well as the similarity in findings between studies employing galvanic stimuli, fMRI (functional magnetic resonance imaging), and scalp-recorded potentials in response to VEMP-eliciting stimuli, our study provides additional evidence that these brain regions are activated in response to stimuli that can be used clinically to evoke the VEMP.

Multi-site phasic neural activity mediates the execution of an auditory continuous performance task: a PET and electrophysiological study.

BACKGROUND AND PURPOSE: We studied an auditory continuous performance task with positron emission tomography (PET) and EEG-derived current density reconstructions (CDRs) to define the spatial and temporal aspects of auditory attention. METHODS: The CDRs were employed to segregate responses to targets and non-targets at sites identified by PET. We then studied the time course of brain activity using statistical parametric mapping (SPM) of the CDR data. RESULTS: In contrast to target EEG activity, non-targets did not produce significant peaks after 300 ms. Pre-300 ms biphasic activation of auditory, left posterior frontal, left supplemental, and primary motor cortices and the anterior cingulate (AC) and biphasic suppression of posterior cingulate and occipital cortex were identical for targets and non-targets and may mediate the target non-target decision. SPM analysis of post-300 ms CDRs showed cingulate cortices were the first to be reactivated, remained active through 672 ms, and were accompanied by reactivation and deactivation of the same sites observed in the pre-P300 responses. CONCLUSIONS: The cingulate may play an important role in post-decisional activity and control activity at other sites involved in post-decisional cognitive processing.

Mapping the 40-Hz auditory steady-state response using current density reconstructions.

We mapped the 40-Hz aSSR from nine normal subjects using PET-independent low-resolution electroencephalographic tomography (LORETA) as well as PET-weighted LORETA and minimum norm (MinNorm) current density reconstructions. In grand mean data, PET-independent LORETA identified seven sites with peaks in current density in right temporal lobe, right brainstem/cerebellum, right parietal lobe, left cerebellum/temporal lobe, and right frontal lobe. PET-weighted LORETA found six of the same sites as the PET-independent LORETA: the right brainstem source was eliminated and two right-frontal sources were added. Both LORETA analyses revealed considerable phase dispersion across identified sources. In both LORETA analyses, the relative time course of activation measured from an arbitrary starting phase progressed from right temporal lobe to right mid-frontal lobe to right parietal-frontal to right inferior parietal and finally to left cerebellum and left temporal lobe. MinNorm analysis incorporating PET information identified sources in the same locations as specified in the PET data. These sources were synchronized, with their amplitudes peaking almost simultaneously. Both PET-independent and PET-weighted LORETA results suggest that the aSSR is: (1) the result of a reverberating network with two or more groups of sources that recurrently excite each other or (2) the result of sequential auditory processing through various levels of a hierarchical network. In contrast, the PET-weighted MinNorm results suggest that the 40-Hz response represents simultaneous activation over widely spaced areas of the brain, perhaps due to synchronization of gamma-band activity to a common neural clock.

PET imaging of the 40 Hz auditory steady state response.

The auditory steady state response (aSSR) is an oscillatory electrical potential recorded from the scalp induced by amplitude-modulated (AM) or click/tone burst stimuli. Its clinical utility has been limited by uncertainty regarding the specific areas of the brain involved in its generation. To identify the generators of the aSSR, 15O-water PET imaging was used to locate the regions of the brain activated by a steady 1 kHz pure tone, the same tone amplitude modulated (AM) at 40 Hz and the specific regions of the brain responsive to the AM component of the stimulus relative to the continuous tone. The continuous tone produced four clusters of activation. The boundaries of these activated clusters extended to include regions in left primary auditory cortex, right non-primary auditory cortex, left thalamus, and left cingulate. The AM tone produced three clusters of activation. The boundaries of these activated clusters extended to include primary auditory cortex bilaterally, left medial geniculate and right middle frontal gyrus. Two regions were specifically responsive to the AM component of the stimulus. These activated clusters extended to include the right anterior cingulate near frontal cortex and right auditory cortex. We conclude that cortical sites, including areas outside primary auditory cortex, are involved in generating the aSSR. There was an unexpected difference between morning and afternoon session scans that may reflect a pre- versus post-prandial state. These results support the hypothesis that a distributed resonating circuit mediates the generation of the aSSR.

Functional imaging during covert auditory attention in multiple sclerosis.

Recent literature suggests that the brain in multiple sclerosis (MS) undergoes reorganization that subserves the performance of visual and motor tasks. We identified sites of cerebral activity in 16 MS patients while performing a covert attention (CA) task, presented in the auditory modality. Positron emission tomography (PET) revealed activation of rostral/dorsal anterior cingulate cortex (ACC) in normal subjects studied previously. Activity in this region was not significant in MS patients, but there was a large region of activity in superior temporal cortex. Decreased activation of frontal attentional networks and greater activity in sensory/perceptual cortical areas (auditory association cortex) suggests a reduction of transmission along white matter tracts connecting these regions. This study demonstrates cingulate hypoactivity and cerebral reorganization during auditory attention in MS.

Mapping the neural systems that mediate the Paced Auditory Serial Addition Task (PASAT).

The paced auditory serial addition task (PASAT), in which subjects hear a number-string and add the two most-recently heard numbers, is a neuropsychological test sensitive to cerebral dysfunction. We mapped the brain regions activated by the PASAT using positron emission tomography (PET) and 15O-water to measure cerebral blood flow. We parsed the PASAT by mapping sites activated by immediate repetition of numbers and by repetition of the prior number after the presentation of the next number in the series. The PASAT activated dispersed non-contiguous foci in the superior temporal gyri, bifrontal and biparietal sites, the anterior cingulate and bilateral cerebellar sites. These sites are consistent with the elements of the task that include auditory perception and processing, speech production, working memory, and attention. Sites mediating addition were not identified. The extent of the sites activated during the performance of the PASAT accounts for the sensitivity of this test and justifies its use in a variety of seemingly disparate conditions.

Brain imaging of the effects of lidocaine on tinnitus.

Using a single-blind placebo-controlled design, we mapped lidocaine related changes in neural activity, measured by regional cerebral blood flow (rCBF) with (15)O-H(2)O positron emission tomography. Intravenous lidocaine produced both increases and decreases in the loudness of tinnitus. The change in tinnitus loudness was associated with a statistically significant change in neural activity in the right temporal lobe in auditory association cortex. Decreases in tinnitus loudness resulted in larger changes in rCBF than increases. The unilateral activation pattern associated with tinnitus, in contrast with the bilateral activation produced by a real sound, suggests that tinnitus originates in the central auditory system rather than the cochlea. In addition, generalized lidocaine effects were found in the basal ganglia, thalamus, and a region spanning the Rolandic fissure.