Activation of human V5 complex and rolandic regions in association with moving visual stimuli

We recorded magnetoencephalographic responses from seven healthy humans during the presentation of stationary and rotating radial gratings. Rotations lasting 1 s evoked movement-specific sustained activity in the parieto-occipitotemporal border area, in agreement with the activation of the V5 complex specialized for the analysis of movement. The source areas of the movement-specific sustained fields were transiently active 100-130 ms after the onsets of both rotating and stationary stimuli, suggesting that movement-related cortical areas respond to any transient changes in the visual environment. Transients were evoked also in other brain areas 60-200 ms after onsets of both stimuli. Four subjects displayed additional motion-related sustained activity in the rolandic region. Sustained activity continued after the stimulus movement in several subjects during perception of the movement aftereffect. The transient activity may evoke visual attention while sustained activity of the V5 complex may be related to the conscious perception of movement.

Modulation of human cortical rolandic rhythms during natural sensorimotor tasks

We studied modulation of cortical neuromagnetic rhythms in association with left and right median nerve stimulation, during rest, finger movements, and passive tactile hand stimulation, in seven healthy, right-handed adults. In the rest condition, the amplitude of the rhythmic sensorimotor activity decreased immediately after the median nerve stimuli and increased above the prestimulus level within 0.4 s afterward, especially in the 7- to 25-Hz band. The rebound occurred 100-300 ms earlier for 20 (7-15)-than for 10 (15-25)-Hz activity. Suppressions and rebounds were strongest in the contralateral sensorimotor hand area for the 20-Hz, but not for the 10-Hz, activity. The maximum rebound was on average 22-34% stronger in the left than in the right hemisphere. Active exploration of objects abolished rebounds of both 10- and 20-Hz signals in the contralateral hemisphere and markedly diminished them ipsilaterally. Finger movements without touching an object and passive tactile stimulation produced a weaker effect. The sensorimotor rhythms thus show a characteristic suppression and subsequent rebound after electrical median nerve stimulation. The rebound is left-hemisphere dominant in right-handed subjects and its suppression reveals bilateral cortical activation during both motor tasks and passive tactile stimulation, especially for explorative finger movements.

Activation trace lifetime of human cortical responses evoked by apparent visual motion

Visually evoked magnetoencephalographic responses were recorded from 11 healthy humans to 1.1 x 1.1 degrees oblique gratings moving quickly 0.2 degree rightwards and back once every 0.2-6.4 s. The aim was to study the duration of sensory memory in the motion-specific visual cortex called V5. Responses from the V5 region peaked at 140-180 ms after stimulus onset. Signal-to-noise ratio allowed source identification in eight subjects: bilaterally in four and unilaterally in four. The response strength as a function of interstimulus interval determined an activation trace lifetime, reflecting how long the preceding stimuli affect the response to the following stimulus, i.e. how long the V5 cortex "remembers' each stimulus. The lifetimes varied interindividually from 0.4 to 1.4 s, but were within 0.1 s in the hemispheres of the four subjects with bilaterally identified sources.

Face-specific responses from the human inferior occipito-temporal cortex

Whole-head neuromagnetic responses were recorded from seven subjects to pictures of faces and to various control stimuli. Four subjects displayed signals specific to faces. The combination of functional information from magnetoencephalography and anatomical data from magnetic resonance images suggests that the face-specific activity was generated in the inferior occipitotemporal cortex. All four subjects showed the face-specific response in the right hemisphere, one of them also in the left. Our results, together with recent position emission tomography and lesion studies, suggest a right-hemisphere preponderance of face processing in the inferior occipitotemporal cortex.

Evidence for reactive magnetic 10-Hz rhythm in the human auditory cortex

We tested the hypothesis that neurons in the human auditory cortex show spontaneous oscillations around 10 Hz, and that this activity ('tau' rhythm) is affected by auditory input. Cortical activity was recorded with a 122-channel whole-scalp neuromagnetometer from healthy adults while they were presented with monaural 500-ms bursts of white noise. The reactivity of spontaneous oscillations was studied over the whole cortex using the Temporal Spectral Evolution method. Oscillatory 6.5-9.5 Hz activity, with sources in the superior temporal lobes, was transiently suppressed by the sounds in eight out of nine subjects. Our results support the existence of a distinct, reactive auditory rhythm in the human temporal cortex.

Human auditory cortex is activated by omissions of auditory stimuli

Cortical signals associated with infrequent tone omissions were recorded from 9 healthy adults with a whole-head 122 channel neuromagnetometer. The stimulus sequence consisted of monaural (left or right) 50-ms 1-kHz tones repeated every 0.2 or 0.5 s, with 7% of the tones randomly omitted. Tones elicited typical responses in the supratemporal auditory cortices. Omissions evoked strong responses over temporal and frontal areas, independently of the side of stimulation, with peak amplitudes at 145-195 ms. Response amplitudes were 60% weaker when the subject was not attending to the stimuli. Omission responses originated in supratemporal auditory cortices bilaterally, indicating that auditory cortex plays an important role in the brain's modelling of temporal characteristics of the auditory environment. Additional activity was observed in the posterolateral frontal cortex and in the superior temporal sulcus, more often in the right than in the left hemisphere.

Human cortical oscillations: a neuromagnetic view through the skull

The mammalian cerebral cortex generates a variety of rhythmic oscillations, detectable directly from the cortex or the scalp. Recent non-invasive recordings from intact humans, by means of neuromagnetometers with large sensor arrays, have shown that several regions of the healthy human cortex have their own intrinsic rhythms, typically 8-40 Hz in frequency, with modality- and frequency-specific reactivity. The conventional hypotheses about the functional significance of brain rhythms extend from epiphenomena to perceptual binding and object segmentation. Recent data indicate that some cortical rhythms can be related to periodic activity of peripheral sensor and effector organs.

Visual awareness of objects correlates with activity of right occipital cortex

In the search for human neural correlates of visual awareness, cortical magnetic responses to coherent and meaningful objects were compared with responses to disorganized and meaningless non-objects when observers tried to detect the coherent objects. Three brief stimulus durations were included to vary the detection rate of the objects. Of the multiple brain regions activated, only the right lateral occipital cortex showed signals correlating with the proportion of correct object detections. The results suggest an important role for this area in visual awareness of objects.

Distributions and sources of magnetoencephalographic K-complexes

Whole-head magnetoencephalographic (MEG) and midline electroencephalographic (EEG) signals were simultaneously recorded from 6 subjects during drowsiness and sleep to define the topography and source distribution of K-complexes. In light sleep, K-complexes were also triggered by infrequent tones. Distributions of spontaneous and triggered magnetic K-complexes did not differ systematically, nor did those evoked by right- and left-ear stimuli, but there were large intra- and interindividual differences. Minimum-norm estimates and current dipoles were used to characterize the source currents. Current direction and distribution varied remarkably between the K-complexes appearing in similar situations. In one subject, most K-complexes were adequately modelled with two current dipoles which were situated in the left and right inferior parietal lobes. In other subjects, the current distributions were more complex, suggesting several brain regions to be active during one K-complex; the dominant foci were in frontal and parietal lobes. Our results suggest that the K-complex is not a stereotyped response of the cortex to internal or external stimuli, comparable to evoked responses, but a diffuse and variable cortical reaction during which large areas of cortex may be active.

Magnetic source imaging during a visually guided task

The cerebellum is heavily involved in the control of accurate eye movements. Cerebellar lesions typically results in nystagmus and dysmetria, inability to stop the eyes at the end of a conjugate movement. Up to now, no cerebellar activity has been identified from non-invasive electrophysiological data. Here we report on neuromagnetic signals of eight healthy subjects in association with visually guided horizontal saccades. The signals were averaged with respect to electrically recorded saccade onsets and their topography revealed activation of the cerebellar vermis starting about 30 ms before and peaking about 170 ms after the saccade onset. In darkness, the cerebellar signals, possibly arising from the cerebellum, were suppressed less than the coinciding signals from the posterior parietal lobe.

Cortical sources of human short-latency somatosensory evoked fields to median and ulnar nerve stimuli

;;;;;õry evoked magnetic fields were measured with a 122-channel whole-scalp neuromagnetometer from seven healthy adults. Electric stimuli, with an intensity above the motor threshold, were delivered once every 0.5 s alternately to the median and ulnar nerves at the wrist; both wrists were stimulated successively within one session. In most subjects, two distinct neural sources were identified at the contralateral primary somatosensory cortex SI for both stimuli. The first source (M20) peaked at 21-22 ms and indicated activation of area 3b in the contralateral SI hand region. The same source peaked with opposite current direction at 32 ms. The second source (M40) was slightly medial to M20 and exhibited two peaks with the same current direction, first at 25 ms and most prominently at 42 ms. M20 was on average 7 mm more lateral along the central sulcus for median than ulnar nerve stimuli, in agreement with the somatotopic organization of the SI cortex; similar organization for M40 was less clear. These results suggest that M20 and M40 to upper limb stimulation represent activation of distinct neuronal populations in hand SI cortex, presumably in area 3b.

Time-varying activation of different cytoarchitectonic areas of the human SI cortex after tibial nerve stimulation

We followed cortical activation in eight healthy adults after electric stimulation of the left tibial nerve at the ankle. The recordings were made noninvasively with a whole-scalp neuromagnetometer. The first cortical activation peaked in different subjects at 37-45 ms in the foot area of the right (contralateral) primary somato-sensory (SI) cortex, with mean source current orientation perpendicular to the longitudinal fissure. The current orientation changed within the next 5 ms counterclockwise in all subjects, with a mean rotation of 64 degrees. A two-dipole time-varying model, with two dipoles differing by 28-119 degrees in orientation but less than 1 cm in location in the right SI cortex, explained the signal pattern satisfactorily during the first 100 ms. We suggest that the observed field patterns reflect sequential activation of different cytoarchitectonic areas in the foot SI cortex and imply considerable differences in the structural organization between the foot and the hand SI cortices. The initial activation is considered to take place in area 3b facing the interhemispheric fissure, and the later source, due to the systematic rotations of the field patterns, is assumed to reflect activation of area 5 in the anterior wall of the marginal ramus of the cingulate sulcus.

Activation of human mesial cortex during somatosensory target detection task

We recorded somatosensory evoked fields (SEFs) from 10 healthy subjects to ulnar and median nerve stimuli presented at random intervals of 2.4-21.6 s. The subjects either counted the stimuli or ignored them by reading a book. The stimuli activated in both conditions the contralateral SI cortex, the ipsi- and contralateral SII cortices, and the posterior parietal cortex (PPC), in line with earlier observations. In addition, a novel response was observed in nine subjects at 120-160 ms. It was clearly enhanced by attention and was generated in the mesial cortex of the paracentral lobule, close to the end of the central sulcus.

Cortical magnification, scale invariance and visual ecology

The visual world of an organism can be idealized as a sphere. Locomotion towards the pole causes translation of retinal images that is proportional to the sine of eccentricity of each object. In order to estimate the human striate cortical magnification factor M, we assumed that the cortical translations, caused by retinal translations due to the locomotion, were independent of eccentricity. This estimate of M agrees with previous data on magnifications, visual thresholds and acuities across the visual field. It also results in scale invariance in which the resolution of objects anywhere in the visual field outside the fixated point is about the same for any viewing distance. Locomotion seems to be a possible determinant in the evolution of the visual system and the brain.

Movement-related slow cortical magnetic fields and changes of spontaneous MEG- and EEG-brain rhythms

Cortical activity was recorded from 5 healthy adults with a 122-channel whole-head magnetometer while the subjects performed during unilateral finger movements at self-paced intervals exceeding 6 s. The readiness field (RF) started over the contralateral somatomotor area 0.3-1 s prior to the movement onset in subjects (Ss) 1, 2, and 4, and culminated in the motor field (MF) 30 ms after it (Ss 1-4). These signals were followed by movement evoked fields MEFI (Ss 1-5) and MEFII (Ss 1-4) at 100-150 ms and 200-250 ms after the movement onset, respectively. One subject showed clear RF over the ipsilateral hemisphere as well. The contralateral dominance of the RF contrasted the more symmetric distribution of the simultaneously recorded electric Bereitschaftspotential (BP). The RF onset never preceded the BP onset. We suggest that BP receives contribution from the early bilateral activation of the crown of the precentral gyrus, whereas RF reflects later activity of the fissural motor cortex. Spontaneous oscillations in the background activity (spontaneous activity) of approximately 10 Hz started to dampen 2-3 s prior to the movement onset in the somatomotor areas of both hemispheres with contralateral predominance (S1 and S3), and returned to a steady level 0.8-2 s after the movement onset in all subjects. Higher frequency bands in the same area displayed a prominent rebound about 1 s after the movement onset in 4 subjects. Execution of self-paced movements is evidently expressed differently in the slow movement-related fields and in the cortical spontaneous activity.

Information processing in the human brain: magnetoencephalographic approach

Rapid progress in effective methods to image brain functions has revolutionized neuroscience. It is now possible to study noninvasively in humans neural processes that were previously only accessible in experimental animals and in brain-injured patients. In this endeavor, positron emission tomography has been the leader, but the superconducting quantum interference device-based magnetoencephalography (MEG) is gaining a firm role, too. With the advent of instruments covering the whole scalp, MEG, typically with 5-mm spatial and 1-ms temporal resolution, allows neuroscientists to track cortical functions accurately in time and space. We present five representative examples of recent MEG studies in our laboratory that demonstrate the usefulness of whole-head magnetoencephalography in investigations of spatiotemporal dynamics of cortical signal processing.

Human cortical 40 Hz rhythm is closely related to EMG rhythmicity

We recorded cortical neuromagnetic rhythms during self-paced index-finger movements from a subject previously reported to show prominent 40 Hz electroencephalographic activity during motor behavior. The 10 and 20 Hz components of the rolandic mu rhythm were bilaterally suppressed, whereas the contralateral 40 Hz (35-41 Hz) activity was slightly enhanced before both fast and slow movements and strongly enhanced during slow movements. The 40 Hz rhythm originated mainly in the hand motor cortex and was clearly correlated with the rhythmicity of the electromyogram from the extensor muscles, with a systematic time lag. In this subject motor preparation, and especially control of finger movements, may thus be associated with enhanced cortical rhythms near 40 Hz. The coherence of these rhythms with muscular firing patterns likely reflects communication between the sensorimotor cortex and the motor units.