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When a visual scene allows multiple interpretations, the percepts may spontaneously alternate despite the stable retinal image and the invariant sensory input transmitted to the brain. To study the brain basis of such multi-stable percepts, we superimposed rapidly changing dynamic noise as regional tags to the Rubin vase-face figure and followed the corresponding tag-related cortical signals with magnetoencephalography. The activity already in the earliest visual cortical areas, the primary visual cortex included, varied with the perceptual states reported by the observers. These percept-related modulations most likely reflect top-down influences that accentuate the neural representation of the perceived object in the early visual cortex and maintain the segregation of objects from the background.

Ten patients with focal epilepsy were studied with magnetoencephalography (MEG) to determine if a new light-weight magnetically shielded room (lMSR) provides sufficient attenuation of magnetic interference to detect and localize the magnetic correlates of epileptic activity. Interictal MEG epileptic events co-localizing with the presumed location of the epileptogenic zone were found in all patients. MEG measurements performed in the lMSR provide an adequate signal-to-noise ratio for non-invasive localization of epileptic foci.

Processing of global contours requires integration of local visual information. To study the involvement of different cortical areas and the temporal characteristics of their activity in such integration, we recorded neuromagnetic responses to arrays of Gabor patches in which a proportion of the patches was oriented either tangentially or radially with respect to a global circular contour; arrays with random patch orientations served as control stimuli. The first responses at 60-80 ms around the calcarine sulcus were similar to all stimuli. Starting from 130 ms, responses to the tangential contours differed significantly from responses to control stimuli, and the difference reached its maximum at 275 ms. The most pronounced differences emerged around the parieto-occipital sulcus, precuneus, cuneus, and superior and middle occipital gyri. This pattern of cortical activity was similar irrespective of whether the local elements were radial or tangential to the circle; however, the differences were smaller for the radial contours and tended to start 20-30 ms later. Correspondingly, discrimination reaction times were shortest for the contours consisting of tangential elements. These results demonstrate two spatially and temporally distinct stages of visual cortical processing, the first one limited to local features and the second one integrating information at a more global level.

The development of the early component of somatosensory evoked potentials (SEPs) from the neonatal N1 to adult N20 response has previously been described. The main emphasis has been on the change in the response latency during maturation. We used magnetoencephalography (MEG) to characterize the cortical generators of the N1 and the subsequent response in healthy human newborns. Furthermore, we studied the maturation of tactile processing according to responses evoked by tactile stimulation of the index finger in newborns, 6-month-old babies and adults. This study provides evidence of specific differences in the somatosensory processing in neonates compared to that in adults. Although the initial cortical response to electrical median nerve stimulation in the newborns was similar in field distribution to the corresponding N20m in adults, the subsequent major deflection in the response waveform had the opposite polarity. Similar immaturity in cortical processing was seen in the tactile evoked fields in both the newborns and the 6-month-old infants compared with the adults. Our results indicate that although the somatosensory pathway in full-term newborns is sufficiently developed to supply the brain with tactile information, the cortical neuronal networks for processing the input may not function in the same way as in adults.

Studies in fetuses and in prematurely born infants show that auditory discriminative skills are present prior to birth. The magnetic fields generated by the fetal brain activity pass the maternal tissues and, despite their weakness, can be detected externally using MEG. Recent studies on the auditory evoked magnetic responses show that the fetal brain responds to sound onset. In contrast, higher-level auditory skills, such as those involving discriminative and memory functions, were not so far studied in fetuses with MEG. Here we show that fetal responses related to discriminating sounds can be recorded, implicating that the auditory change-detection system is functional. These results open new views to developmental neuroscience by enabling one to determine the sensory capabilities as well as the extent and accuracy of the short-term memory system of the fetus, and, further, to follow the development of these crucial processes.

Magnetic brain responses to speech sounds were measured in 10 healthy neonates. The stimulation consisted of a frequent vowel sound [a:] with a steady pitch contour, which was occasionally replaced by the vowel [i:] with a steady pitch, or the vowel [a:] with a rising pitch, manifesting a change of intonation. The magnetic mismatch-negativity response (MMNm) was obtained and successfully modelled to the speech sound quality change in all infants and to the intonation change in 6 infants. The present results indicate that auditory-cortex speech-sound discrimination may well be studied with magnetic recordings as early as in newborn infants.

We recorded magnetic brain activity from healthy human newborns when they heard frequency changes in an otherwise repetitive sound stream. We were able to record the magnetic counterpart of the mismatch negativity (MMN) previously described only with electric recordings in infants. The results show that these recordings are possible, although still challenging due to the small head size and head movements. The modelling of the neural sources underlying the recorded responses suggests cortical sources in the temporal lobes.

Unilateral stimulation of human peripheral nerves activates the primary somatosensory cortex (SI) contralaterally and the secondary somatosensory cortex (SII) bilaterally. We aimed at characterizing phase locking between SI and SII in response to electric stimuli applied once every 3 s to the right median nerve at the wrist; phase locking between brain regions has been proposed to either reflect joined processing or information exchange. Ongoing neuromagnetic activity of healthy volunteers was recorded with 204 planar gradiometers covering the whole scalp. After selecting a sensor maximally sensitive to activity in the left (contralateral) SI, phase locking between this sensor and the other 203 sensors was examined from single trial data. Statistically significant phase locking was found at approximately 20 Hz, 80-90 ms after the stimuli between the left SI and the right SII in 9 of 10 subjects. Sensors with high phase-locking values over the left SI and right SII were separated by sensors with no phase-locked activity over the scalp midline, indicating that the phase locking was not caused by the sensors seeing activity from the same sources. The observed SI-SII phase locking would not be reflected in the evoked responses because a considerable part of it was not time-locked to the stimuli. Thus, our finding reveals a unique interaction in the sensorimotor system.