Pitch change of a continuous tone activates two distinct processes in human auditory cortex: a study with whole-head magnetometer.

Previous studies have shown that a frequency change in a continuous tone elicits an NI type of ERP (event-related potential) component. It remained unclear, however, whether this response is a "genuine" N1 (onset detector response) or the mismatch negativity (MMN), a change-detector type of ERP response, elicited in previous studies by an infrequent change in a sequence of homogeneous stimuli. A further possibility is a nearly perfect overlap of the two types of ERP components.

Auditory stimuli activate parietal brain regions: a whole-head MEG study.

Previous studies using magnetoencephalographic (MEG) recordings have revealed neuronal populations responding to discrete auditory stimuli in the supratemporal cortex of the human brain. We used the novel whole-head magnetometer (Neuromag-122) to determine whether regions outside the auditory cortex are activated by auditory stimulation as well. In the present study we report evidence for activation of the parietal cortex of the human brain in response to auditory stimuli.

Magnetoencephalography in studies of human cognitive brain function.

Magnetoencephalography provides a new dimension to the functional imaging of the brain. The cerebral magnetic fields recorded noninvasively enable the accurate determination of locations of cerebral activity with an uncompromized time resolution. The first whole-scalp sensor arrays have just recently come into operation, and significant advances are to be expected in both neurophysiological and cognitive studies, as well as in clinical practice.

Minimum-norm estimation in a boundary-element torso model.

The paper deals with the bioelectric and biomagnetic inverse problems. The authors present a method to estimate primary-current distributions in a homogeneous, realistically shaped boundary-element torso model. The reconstruction surface is triangulated to keep the procedure computationally feasible.

Interpreting magnetic fields of the brain: minimum norm estimates.

The authors have applied estimation theory to the problem of determining primary current distributions from measured neuromagnetic fields. In this procedure, essentially nothing is assumed about the source currents, except that they are spatially restricted to a certain region. Simulation experiments show that the results can describe the structure of the current flow fairly well.

Interaction between representations of different features of auditory sensory memory.

The neurophysiological basis of sensory memory was studied by measuring the magnetic counterpart (MMNm) of the mismatch negativity (MMN) with a whole-head 122-channel magnetometer. The MMNm is a response to a difference in the presented stimulus and a neuronal memory trace formed by repeated standard stimuli. This trace must contain information about the feature differing in the deviant.

Tonotopic auditory cortex and the magnetoencephalographic (MEG) equivalent of the mismatch negativity.

Two tone stimuli, one frequent (standard) and the other infrequent (a slightly higher, deviant tone), were presented in random order and at short intervals to subjects reading texts they had selected. In different blocks, standards were either 250, 1,000, or 4,000 Hz, with the deviants always being 10% higher in frequency than the standards of the same blocks. Magnetic responses elicited by the standard and deviant tones included N1m, the magnetoencephalographic equivalent of the electrical N1 (its supratemporal component).

Sampling theory for neuromagnetic detector arrays.

The sampling theorem for wave-number-limited multivariable functions is applied to the problem of neuromagnetic field mapping. The wave-number spectrum and other relevant properties of these fields are estimated. A theory is derived for reconstructing neuromagnetic fields from measurements using sensor arrays which sample either the field component Bz perpendicular to the planar grid of measurement points, or the two components delta Bz/delta x and delta Bz/delta y of its gradient in the xy plane.

The effect of stimulation rate on the signal-to-noise ratio of evoked responses.

The signal-to-noise ratio (SNR) in averaged evoked responses is proportional to the signal amplitude and to the square root of the stimulation frequency. If the SNR-stimulation-rate dependence is known for some specified component or feature of the response it is possible to select a rate that maximizes the SNR of that component within a given measurement time. The same stimulation rate also minimizes the acquisition time for a given SNR.

Memory-related processing of complex sound patterns in human auditory cortex: a MEG study.

Responses of the human brain to a complex sound pattern were recorded with a 24 channel magnetometer. The sound pattern consisted of 9 successive 50 ms segments, each with a different frequency. An infrequent change in the frequency of one of the segments elicited a magnetic mismatch response (MMNm) which peaked at about 200 ms after the deviant segment onset and resembled the electrical mismatch negativity (MMN).

Models of source currents in the brain.

Electroencephalography (EEG) and magnetoencephalography (MEG) provide signals that are weighted integrals of source currents in the brain. In addition to technical aspects, the two methods differ in their sensitivities to various cerebral sources. Moreover, it is more difficult to determine the lead fields of EEG than of MEG.

Estimates of visually evoked cortical currents.

We have measured magnetic fields evoked by the onset of checkerboard-like sectorial patterns presented at 16 locations near the center of the visual field. Small stimuli (less than 2 degrees), which, nevertheless, gave sufficiently strong responses to enable source localization, were used to limit cortical activation to a small area, thus simplifying the analysis of the magnetic field data. We focused on optimizing the experimental design: cortical sources could be located from measurements at just one position of our 24-channel magnetometer and with as few as 15-20 repetitions of the stimulus.

Seeing faces activates three separate areas outside the occipital visual cortex in man.

We have examined magnetic cortical responses of 15 healthy humans to 46 different pictures of faces. At least three areas outside the occipital visual cortex appeared to be involved in processing this input, 105-560 ms after the stimulus onset. The first active area was near the occipitotemporal junction, the second in the inferior parietal lobe, and the third in the middle temporal lobe.

Estimates of neuronal current distributions.

The main purpose of measurements of the magnetic field produced by cerebral electrical activity is to locate this activity or to determine its distribution in the brain. In this paper, methods to locate or otherwise characterize source currents in the brain are briefly discussed. It is emphasized that the optimal source estimation method depends crucially on the availability of prior information and on the questions one wants to resolve with the measurements.

Modulation of auditory evoked magnetic fields by benzodiazepines.

The N1 and P2 components of the auditory evoked magnetic field were shown to be modified by the benzodiazepines diazepam and triazolam. Previous studies indicate that the electrical sources of these components are located in the auditory cortex, implying that benzodiazepines have a direct or indirect effect on neuronal activity at this level. The recorded changes were comparable to those previously reported using auditory evoked potential measurements.

Cerebral magnetic fields.

A critical presentation is given about the state-of-the-art of neuromagnetism, the study of neural functions by the measurement of magnetic fields. First, an introduction is given about the neural origin of electromagnetic fields. The forward and inverse problems are then studied.

[Physical basis of the generation of neuromagnetic fields].

Electrical processes at the neuronal level are considered, which cause the neuromagnetic fields. Those fields presently measured are due to the synchronous activity of about 10(5) neurons. Short magnetic signals can be caused by afferent and efferent strains of action potentials, the longer ones being due to the synaptic activity at the ends of apical dendrites in the fissural cortex.

Selective localization of alpha brain activity with neuromagnetic measurements.

A method of localizing brain activity by a new combination of magnetic and electrical recording, relative covariance, is described. The successful application of this method to alpha EEG is reported. Spontaneous human brain activity was recorded simultaneously with fixed scalp electrodes and a movable magnetometer.

A four-channel SQUID magnetometer for brain research.

A 4-channel differential SQUID magnetometer has been built. Its design principles and construction are described. Proper matching of the detection coil to the SQUID input is discussed.

Responses of the primary auditory cortex to pitch changes in a sequence of tone pips: neuromagnetic recordings in man.

Auditory evoked magnetic fields of the human brain were recorded with a four-channel 1st order gradiometer. Pitch deviance in a sequence of repetitive tone pips elicited magnetic evoked-response changes with a topography suggesting that a neuronal mismatch process to the deviant tones activates the primary auditory cortex.

Somatosensory evoked cerebral magnetic fields from SI and SII in man.

We have recorded cerebral magnetic fields elicited by electrical stimulation of median and peroneal nerves. Field mapping indicates that the deflections at 30-80 and 150-180 msec are due to activity at SI. Additional activity at 90-125 msec is generated at SII, on the superior bank of the sylvian fissure.