Paul Ferrari

Latency of the auditory evoked neuromagnetic field components: stimulus dependence and insights toward perception.

This review will focus on investigations of the auditory evoked neuromagnetic field component, the M100, detectable in the magnetoencephalogram recorded during presentation of auditory stimuli, approximately 100 milliseconds after stimulus onset. In particular, the dependence of M100 latency on attributes of the stimulus, such as intensity, pitch and timbre will be discussed, along with evidence relating M100 latency observations to perceptual features of the stimuli. Comparison with investigation of the analogous electrical potential component, the N1, will be made. Parametric development of stimuli from pure tones through complex tones to speech elements will be made, allowing the influence of spectral pitch, virtual pitch and perceptual categorization to be delineated and suggesting implications for the role of such latency observations in the study of speech processing. The final section will deal with potential clinical applications offered by M100 latency measurements, as objective indices of normal and abnormal cortical processing.

Functional activity within brain tumors: a magnetic source imaging study.

OBJECTIVE To determine whether low-grade gliomas contain functional cortical activity more often than high-grade gliomas within radiologically defined abnormal tissue. METHODS Patients with intra-axial cerebral lesions located in the vicinity of eloquent brain cortex preoperatively underwent magnetic source imaging. A dual 37-channel biomagnetometer was used to perform the imaging. Evoked magnetic fields were analyzed using the single-equivalent dipole representation to ascertain the neuronal source. Stimuli included painless tactile somatosensory stimulation of fingers, toes, and lips and auditory presentation of pure sinusoidal tones. RESULTS A retrospective analysis of 106 nonconsecutively treated patients, who had undergone preoperative magnetic source imaging between February 1996 and December 1999, revealed that 24.5% of the patients had been at risk for neurological deficits, because functionally active tissue was located within or at the border of the tumor. Functional activity was found within the radiologically defined lesion in 18% of Grade 2 tumors, in 17% of Grade 3 tumors, and in 8% of Grade 4 tumors. CONCLUSION The results confirm that, regardless of tumor grade, intra-axial brain tumors may involve or directly border on functional cortex. The degree of involvement of functionally viable cortex appeared greater for low-grade tumors than for high-grade lesions. On the other hand, high-grade lesions were more likely to be associated with functional cortex at their margins or within peritumoral edema. To safely maximize tumor resection, preoperative functional imaging and intraoperative electrophysiological mapping of the cerebral cortex and the white matter tracts are deemed necessary.

Prominence of M50 auditory evoked response over M100 in childhood and autism.

This study investigated the 50 ms (M50) and 100 ms (M100) components of the auditory evoked field to explore their change during development. Using MEG, neuromagnetic fields elicited by a 1 kHz sinusoidal tone were recorded in adults and two groups of children and adolescents with typical development or autism spectrum disorder. M50 amplitude was larger in children than in adults, suggesting a developmental trajectory with M50 amplitude decreasing and M100 increasing with age. Child M50 and M100 latencies were prolonged relative to adults. Children with autism did not differ from control children with respect to these observations. The M50 in relation to the M100 is a robust index of early auditory system maturation suitable for future developmental investigations.

Presurgical mapping with magnetic source imaging: comparisons with intraoperative findings

We compare noninvasive preoperative mapping with magnetic source imaging to intraoperative cortical stimulation mapping. These techniques were directly compared in 17 patients who underwent preoperative and postoperative somatosensory mapping of a total of 22 comparable anatomic sites (digits, face). Our findings are presented in the context of previous studies that used magnetic source imaging and functional magnetic resonance imaging as noninvasive surrogates of intraoperative mapping for the identification of sensorimotor and language-specific brain functional centers in patients with brain tumors. We found that magnetic source imaging results were reasonably concordant with intraoperative mapping findings in over 90% of cases, and that concordance could be defined as “good” in 77% of cases. Magnetic source imaging therefore provides a viable, if coarse, identification of somatosensory areas and, consequently, can guide and reduce the time taken for intraoperative mapping procedures.

Latency of evoked neuromagnetic M100 reflects perceptual and acoustic stimulus attributes

THE latency of components of the auditory evoked neuromagnetic field has been shown to reflect, or encode, stimulus attributes. In particular, the M100 component, occurring ∼100 ms post stimulus onset has a latency that depends on stimulus pitch, spectral complexity and presentation level. This study used magnetoencephalography to record neuromagnetic fields evoked by presentation of two-tone complexes consisting of various proportions of 100 Hz and 1 kHz energy. These are perceived categorically, as evidenced by classification and reaction time measurements. It is found that the M100 latency also varies categorically, that is, characterized by two plateau regions with a sharp interface. Thus, we find that not only does the M100 latency reflect acoustic attributes of a stimulus, but also such perceptual characteristics.

Human neuronal encoding of English syntactic violations as revealed by both L1 and L2 speakers

Our previous study [M. Kubota, P. Ferrari, T.P.L. Roberts, Magnetoencephalography detection of early syntactic processes in humans: comparison between L1 speakers and L2 learners, Neurosci. Lett. 353 (2003) 107-110] showed that an early syntactic response was elicited in first language (L1) speakers for within-phrase, but not across-phrase violations, implying that there may exist a continuum of neuronal error gravity. Such an early component was not elicited by second-language (L2) learners. The current auditory study investigated whether two types of different syntactic violations regarding noun-phrase raising (NP-raising) and case-filter constructions would elicit a prominent early syntactic component in each hemisphere for both L1 and advanced L2 speakers of English. Neuromagnetic fields were recorded, using a dual 37-channel gradiometer system. A prominent component, peaking at approximately 150 ms post-onset, was observed in both hemispheres of two groups in response to NP-raising induced violations, but not case-filter violations. The findings imply that L1 and L2 speakers have similar neuronal mechanisms subserving syntactic processing of such violations.

Magnetic mismatch fields elicited by vowel duration and pitch changes in Japanese words in humans: comparison between native- and non-speakers of Japanese

Previous event-related brain potential research showed that mismatch negativity was elicited by phoneme contrasts in fluent second language (L2) learners, but not in non-speakers of L2. The present study tested whether the magnetic mismatch field (MMF) would be elicited in response to temporal and spectral changes in three Japanese synthesized words for both native- and non-speakers of Japanese. Magnetoencephalography responses were recorded with a dual 37-channel gradiometer. Unlike short-to-long vowel duration and falling-to-level pitch changes, long-to-short duration and level-to-falling pitch changes elicited a prominent MMF bilaterally for both groups, peaking at around 100 ms after change onset for duration and 200 ms for pitch. The MMF component is sensitive to vowel shortening rather than lengthening and to pitch falling rather than leveling. Automatic detection of changes in vowel shortening and pitch falling is a useful index of language-non-specific auditory memory traces.

Preoperative magnetic source imaging for brain tumor surgery: a quantitative comparison with intraoperative sensory and motor mapping.

OBJECT The aim of this study was to compare quantitatively the methods of preoperative magnetic source (MS) imaging and intraoperative electrophysiological cortical mapping (ECM) in the localization of sensorimotor cortex in patients with intraaxial brain tumors. METHODS Preoperative magnetoencephalography (MEG) was performed while patients received painless tactile somatosensory stimulation of the lip, hand, and foot. The early somatosensory evoked field was modeled using a single equivalent current dipole approach to estimate the spatial source of the response. Three-dimensional magnetic resonance image volume data sets with fiducials were coregistered with the MEG recordings to form the MS image. These individualized functional brain maps were integrated into a neuronavigation system. Intraoperative mapping of somatosensory and/or motor cortex was performed and sites were compared. In two subgroups of patients we compared intraoperative somatosensory and motor stimulation sites with MS imaging-based somatosensory localizations. Mediolateral projection of the MS imaging source localizations to the cortical surface reduced systematic intermodality discrepancies. The distance between two corresponding points determined using MS imaging and ECM was 12.5 +/- 1.3 mm for somatosensory-somatosensory and 19 +/- 1.3 mm for somatosensory-motor comparisons. The observed 6.5 mm increase in site separation was systematically demonstrated in the anteroposterior direction, as expected from actual anatomy. In fact, intraoperative sites at which stimulation evoked the same patient response exhibited a spatial variation of 10.7 +/- 0.7 mm. CONCLUSIONS Preoperative MS imaging and intraoperative ECM show a favorable degree of quantitative correlation. Thus, MS imaging can be considered a valuable and accurate planning adjunct in the treatment of patients with intraaxial brain tumors.

Neuromagnetic auditory cortex responses to duration and pitch changes in tones: cross-linguistic comparisons of human subjects in directions of acoustic changes

Our recent magnetic mismatch field (MMF) study found that shortened-vowel duration changes and level-to-falling pitch changes in Japanese words elicited a prominent MMF in two hemispheres for both native and nonnative speakers (Inouchi, M., Kubota, M., Ferrari, P. and Roberts, T.P.L., Magnetic mismatch fields elicited by Japanese words: vowel duration and pitch by native and nonnative speakers, Poster presented at the 31st Annual Meeting of Society for Neuroscience, November 10-15, San Diego, CA, 2001). The current study investigated whether shortened duration changes and level-to-falling pitch changes in non-speech (tones) would elicit a more prominent MMF component than lengthened duration changes and falling-to-level pitch changes, respectively. Stimuli included three computer-synthesized tones with varying duration or frequency modulation: (1). short duration and level pitch; (2). long duration and level pitch; (3). long duration and falling pitch. Magnetoencephalography responses were recorded with a dual 37-channel gradiometer system. The results showed that the prominent MMF component was generated in long-to-short duration changes and level-to-falling pitch changes in each hemisphere for both Japanese and American subjects. The component peaked at around 100 ms after change onset for duration changes and 170 ms for pitch changes. The MMF component in tones, like in words, was particularly sensitive to duration shortening and pitch falling. In summary, changes in duration shortening and pitch falling are particularly salient cues for pre-attentive auditory change detection in each hemisphere.

Increased somatosensory neuromagnetic fields ipsilateral to lesions in neurosurgical patients.

&NA; This study is aimed to determine whether the presence of a tumor in close proximity to the somatosensory cortex of the post‐central gyrus affects the response evoked from such an area by painless tactile stimulation, recorded by MEG. The main finding of this study is that somatosensory evoked field strengths from cortex ipsilateral to and abutting an intracranial tumor are significantly elevated compared with either contralateral control fields, or with somatosensory evoked fields elicited from cortex ipsilateral to, but distant from tumors (typically involving language areas, such as inferior frontal lobe). Biophysical and biochemical explanations implicate hyperactivity of cortical neurons close to the tumor, suggesting a possible role of MEG as a measure of tumor infiltration.