Lauri Parkkonen

Independent component analysis of short-time Fourier transforms for spontaneous EEG/MEG analysis

Analysis of spontaneous EEG/MEG needs unsupervised learning methods. While independent component analysis (ICA) has been successfully applied on spontaneous fMRI, it seems to be too sensitive to technical artifacts in EEG/MEG. We propose to apply ICA on short-time Fourier transforms of EEG/MEG signals, in order to find more interesting sources than with time-domain ICA, and to more meaningfully sort the obtained components. The method is especially useful for finding sources of rhythmic activity. Furthermore, we propose to use a complex mixing matrix to model sources which are spatially extended and have different phases in different EEG/MEG channels. Simulations with artificial data and experiments on resting-state MEG demonstrate the utility of the method.

Sources of Auditory Brainstem Responses Revisited: Contribution by Magnetoencephalography

Auditory brainstem responses provide diagnostic value in pathologies involving the early parts of the auditory pathway. Despite that, the neural generators underlying the various components of these responses have remained unclear. Direct electrical recordings in humans are possible only in limited time periods during surgery and from small regions of the diseased brains. The evidence of the generator sites is therefore fragmented and indirect, based strongly on lesion studies and animal models. Source modeling of EEG has been limited to grand averages across multiple subjects. Here, we employed magnetoencephalography (MEG) to shed more light on the neural origins of the auditory brainstem responses (ABR) and to test whether such deep brain structures are accessible by MEG. We show that the magnetic counterparts of the electric ABRs can be measured in 30 min and that they allow localization of some of the underlying neural sources in individual subjects. Many of the electric ABR components were present in our MEG data; however, the morphologies of the magnetic and electric responses were different, indicating that the MEG signals carry information complementary to the EEG data. The locations of the neural sources corresponding to the magnetic ABR deflections ranged from the auditory nerve to the inferior colliculus. The earliest cortical responses were detectable at the latency of 13 ms. Hum Brain Mapp, 2009.

Phase locking between human primary and secondary somatosensory cortices

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 ≈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.

Early visual brain areas reflect the percept of an ambiguous scene

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.

From local to global: Cortical dynamics of contour integration

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.

Dynamical MEG Source Modeling with Multi-Target Bayesian Filtering

We present a Bayesian filtering approach for automatic estimation of dynamical source models from magnetoencephalographic data. We apply multi‐target Bayesian filtering and the theory of Random Finite Sets in an algorithm that recovers the life times, locations and strengths of a set of dipolar sources. The reconstructed dipoles are clustered in time and space to associate them with sources. We applied this new method to synthetic data sets and show here that it is able to automatically estimate the source structure in most cases more accurately than either traditional multi‐dipole modeling or minimum current estimation performed by uninformed human operators. We also show that from real somatosensory evoked fields the method reconstructs a source constellation comparable to that obtained by multi‐dipole modeling. Hum Brain Mapp, 2009.

Recording epileptic activity with MEG in a light-weight magnetic shield

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.

Auditory cortical responses in humans with congenital unilateral conductive hearing loss.

We recorded auditory evoked magnetic fields from 6 patients with congenital unilateral conductive hearing disorder with a 122-channel whole-head neuromagnetometer. The stimuli were 50-ms 1-kHz tones delivered to the better ear at interstimulus intervals (ISIs) of 2 and 8 s at two different intensities (50 and 70 dB HL). As in normal-hearing subjects, the amplitudes of N100m, the 100-ms response, were larger in 5 patients and the latencies were shorter in 3 patients over the hemisphere contralateral to stimulation. However, in one patient N100m peaked already at 61 ms over the contralateral hemisphere and amplitudes were larger over the ipsilateral hemisphere, possibly reflecting reorganization of the auditory pathways. In 3 patients the latencies were shorter over the ipsilateral hemisphere. The effects of ISI and intensity were similar over both hemispheres and did not differ from those in controls. It seems that congenital unilateral conductive hearing loss does not necessarily lead to any gross disturbances in the human auditory cortex.

Motion sensitivity of human V6: A magnetoencephalography study

Recent studies suggest the presence of a human homologue of monkey V6 in the dorsal posterior bank of the parieto-occipital sulcus. Monkey V6 comprises a retinotopic representation with relative peripheral visual field emphasis and is sensitive to visual motion. We studied sensitivity to visual motion in human parieto-occipital sulcus. Our upper peripheral visual field stimulus enabled us to distinguish V6 from neighbouring areas, whose upper VF representation is located far from V6. We recorded neuromagnetic signals while the subjects (N=10) fixated and a grating first appeared and then started to drift. The most prominent sustained activation for motion was at the posterior bank of the dorsal parieto-occipital sulcus; that is at the known location of the human V6. This finding suggests that human V6 is a motion-sensitive area. The responses in V6 occurred early, with about the same latency as in V1, in line with known connections in the monkey brain. In addition, on the medial surface of the hemisphere we observed a fast sequence of activations following V6: first precuneus and later an area at the dorsal end of the cingulate sulcus. On the lateral side, both temporo-occipital area and intraparietal sulcus were active, but with delayed onset compared to V6. This rapid flow of visual information along the medial dorsal visual pathway supports the view that in humans, as in monkeys, the V6 and the connected areas could be involved in online control of visually guided actions.

Facial expressions of pain modulate observer's long‐latency responses in superior temporal sulcus

The strength of brain responses to others pain has been shown to depend on the intensity of the observed pain. To investigate the temporal profile of such modulation, we recorded neuromagnetic brain responses of healthy subjects to facial expressions of pain. The subjects observed grayscale photos of the faces of genuine chronic pain patients when the patients were suffering from their ordinary pain (Chronic) and when the patients pain was transiently intensified (Provoked). The cortical activation sequence during observation of the facial expressions of pain advanced from occipital to temporo‐occipital areas, and it differed between Provoked and Chronic pain expressions in the right middle superior temporal sulcus (STS) at 300–500 ms: the responses were about a third stronger for Provoked than Chronic pain faces. Furthermore, the responses to Provoked pain faces were about 40% stronger in the right than the left STS, and they decreased from the first to the second measurement session by one‐fourth, whereas no similar decrease in responses was found for Chronic pain faces. Thus, the STS responses to the pain expressions were modulated by the intensity of the observed pain and by stimulus repetition; the location and latency of the responses suggest close similarities between processing of pain and other affective facial expressions. Hum Brain Mapp, 2009.