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A covariance matrix-based subtraction method has recently been proposed to remove interference using two MEG measurements: The first has both target and interfering activities and the second only has the interference. This paper compared covariance matrix-based subtraction with conventional waveform-based subtraction, which requires that the waveforms of interference be equal at every time point between the two measurements. Our analysis showed that covariance-subtraction only requires that the time-average of the squared intensity of interference be equal between the two measurements. As a result, the method is still effective when the onset of interference differs or even their measured waveforms differ between the two measurements. The covariance- and waveform-subtraction methods were both applied to remove the interference caused by response-button-pressing finger movements in auditory-evoked MEG measurements. The results of this application demonstrated the superiority of the covariance-subtraction method over the conventional waveform-subtraction method.

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 approximately 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.

Current brain imaging techniques, such as computed tomography (CT) and magnetic resonance imaging (MRI), provide noninvasive, high-resolution images that depict fine anatomic structure and delineate pathology by control of image contrast and sensitivity to the physicochemical microenvironment. These methods, although invaluable for the identification, characterization, and localization of lesions, do not provide any assessment of the functional viability of brain tissues, nor of the spatial organization of sensory, motor, and cognitive systems. However, such additional functional information is of great significance to the clinician in the determination of treatment strategies and patient management.

Recent work has suggested that, in addition to spatial tonotopy, pitch and timbre information may be encoded in the temporal activity of the auditory cortex. Specifically, the post-stimulus latency of the maximal cortical evoked neuromagnetic field (M100 or N1m) is a function of stimulus frequency. We investigated the additional effect of varying the stimulus intensity on the M100 response. A 37-channel biomagnetometer recorded neuromagnetic fields over the temporal lobe of healthy volunteers in response to monaurally presented tones. The frequency dependence of the M100 latency remained remarkably invariant even at low stimulus intensity. Thus, for peri-threshold stimuli, frequency information appears encoded in the temporal form of the evoked response.

It has been proposed that the auditory cortex of deaf subjects may provide an example of cross-modal compensatory plasticity. We investigated whether sensory stimulation could elicit responses from auditory areas of a congenitally deaf subject. Neuromagnetic fields were recorded using a 37-channel biomagnetometer under conditions of: 1) visual stimulation; 2) somatosensory stimulation; and 3) a simple motor task. Visual items were reversing checkerboards and single light spots, presented in various portions of the visual field; somatosensory stimuli were pneumatic taps delivered to individual digit-segments and the lip; the motor task was self-paced finger tapping. In addition, functional magnetic resonance imaging was used to observe the activation elicited by full-field checkerboard and sign language stimuli. No responses to passively presented visual or somatosensory stimuli were observed in the auditory cortex. In contrast, somatosensory, motor, and visual cortices revealed evoked magnetic responses comparable to those from control subjects, indicating canonical anatomic and physiological organization in these areas. These data suggest that primary projection areas do not reveal obvious plastic effects. We suggest that in the human auditory cortex compensatory plasticity emerges primarily as a property of non-primary areas and is best observed under attentionally demanding conditions., (Copyright (c) 1997 Wiley-Liss, Inc.)

The use of positron emission tomography to identify sensory and motor systems in humans in vivo has been very successful. In contrast, studies of cognitive processes have not always generated results that can be reliably interpreted. A metaanalysis of five positron emission tomography studies designed to engage phonological processing (Petersen, Fox, Posner, Mintun, & Raichle, 1989; Zatorre, Evans, Meyer, & Gjedde 1992; Sergent, Zuck, Levesque, & MacDonald, 1992; Demonet, Chollet, Ramsay, Cardebat, Nespoulous, Wise, & Frackowiak, 1992; and Paulesu, Frith, & Frakowiak, 1993) reveals that the results do not converge as expected: Very similar experiments designed to isolate the same language processes show activation in nonoverlapping cortical areas. Although these PET confirm the importance of left perisylvian cortex, the experiments implicate distinct, nonoverlapping perisylvian areas. Because of the divergence of results, it is premature to attribute certain language processes or the elementary computations underlying the construction of the relevant linguistic representations to specific cerebral regions on the basis of positron emission tomographic results. It is argued that this sparse-overlap result is due (1) to insufficiently detailed task decomposition and task-control matching, (2) to insufficient contact with cognitive psychology, psycholinguistics, and linguistic theory, and (3) to some inherent problems in using substractive PET methodology to study the neural representation and processing of language.

The auditory evoked neuromagnetic fields elicited by synthesized speech sounds (consonant-vowel syllables) were recorded in six subjects over the left and right temporal cortices using a 37-channel SQUID-based magnetometer. The latencies and amplitudes of the peaks of the M100 evoked responses were bilaterally symmetric for passively presented stimuli. In contrast, when subjects were asked to discriminate among the same syllabic stimuli, the amplitude of the M100 increased in the left and decreased in the right temporal cortices. Single equivalent current dipole modeling of the activity elicited by all stimulus-types localized to a well-circumscribed area in supratemporal auditory cortex. The results suggest that attentional modulation affects the two supratemporal cortices in a differential manner. Task-conditioned attention to speech sounds is reflected in lateralized supratemporal cortical responses possibly concordant with hemispheric language dominance.

This study investigated post-stimulus latency of the M100 component of auditory evoked fields as a function of tone frequency. A 37-channel biomagnetometer was used to record neuromagnetic fields over the temporal lobe in response to monaurally presented tones. M100 peaks in evoked field amplitude were found for each subject. The post-stimulus latency was observed to vary parabolically with tone frequency. Latencies as short as 99 ms were found in response to mid-audio range frequencies (1000-2000 Hz), whereas lower (100-500 Hz) and higher (3000-5000 Hz) frequencies were associated with longer latencies (up to 153 ms). All fields gave M100 dipole localizations in auditory cortex. Although it was not possible to resolve spatial tonotopy, it appears that frequency information is encoded in the temporal evoked response.