Neuromagnetic signatures of the spatiotemporal transformation for manual pointing.

Movement planning involves transforming the sensory signals into a command in motor coordinates. Surprisingly, the real-time dynamics of sensorimotor transformations at the whole brain level remain unknown, in part due to the spatiotemporal limitations of fMRI and neurophysiological recordings. Here, we used magnetoencephalography (MEG) during pro-/anti-wrist pointing to determine (1) the cortical areas involved in transforming visual signals into appropriate hand motor commands, and (2) how this transformation occurs in real time, both within and across the regions involved. We computed sensory, motor, and sensorimotor indices in 16 bilateral brain regions for direction coding based on hemispherically lateralized de/synchronization in the α (7-15 Hz) and ß (15-35 Hz) bands. We found a visuomotor progression, from pure sensory codes in 'early' occipital-parietal areas, to a temporal transition from sensory to motor coding in the majority of parietal-frontal sensorimotor areas, to a pure motor code, in both the α and ß bands. Further, the timing of these transformations revealed a top-down pro/anti cue influence that propagated 'backwards' from frontal through posterior cortical areas. These data directly demonstrate a progressive, real-time transformation both within and across the entire occipital-parietal-frontal network that follows specific rules of spatial distribution and temporal order.

Neuromagnetic responses associated with perceptual segregation of pitch.

In recent EEG investigations [Johnson, 2003] [Hautus, 2005], we described a novel late negative ERP component associated with binaural processing of auditory pitch based solely on interaural timing differences ("dichotic pitch"), an acoustic phenomenon that is closely analogous to visual perception of stereoscopic depth based on retinal disparities. The present study extends this research with neuromagnetic recordings of auditory evoked fields (AEFs) elicited by dichotically-embedded pitches. Eight healthy adult subjects listened to control stimuli consisting of 500 ms segments of broadband acoustic noise presented identically to both ears via earphones, and dichotic pitch stimuli created by introducing a dichotic delay to a narrow frequency region of the same noise segments and resulting in a perception of a pitch lateralized to the left or right of auditory space. Auditory-evoked fields (AEFs) were recorded using a 151 channel whole-head MEG system. Comparison of control and dichotic-pitch AEFs showed reliable amplitude differences during a time window of 150-350 ms. AEFs over the left hemisphere showed larger effects for contralateral than ipsilateral pitches, while the right hemisphere showed no differences for differently lateralized sources. The results indicate a relatively late stage of neural processing of binaurally-derived cues for the perceptual segregation of concurrent sound sources and support a right-hemisphere dominance for the processing of sound-source localization.

Modulation of neuromagnetic oscillatory activity during the observation of oro-facial movements.

A number of MEG/EEG studies have shown modulation of endogenous sensorimotor (mu and beta) rhythms during the observation of hand movements. These modulations are similar to patterns that occur during execution of movement and it has been hypothesised that the neural substrates of these rhythms may play a role in action representation and understanding the actions of others. In this experiment we wished to determine whether similar responses would be obtained during the observation of oro-facial movements. Neuromagnetic recordings (151 channels, CTF Systems) were obtained from six healthy subjects while they (1) observed a video of an experimenter making oro-facial movements (2) imitated the same movements and (3) observed hand movements. Source scanning using synthetic aperture magnetometry (SAM) was used to find changes in source power between these active conditions compared to pre-stimulus control conditions where no movement occurred. SAM images were created with 5 mm resolution in the beta (15-35 Hz) and mu (8-15 Hz) bands and showed source power decreases over parietal, occipital and sensorimotor areas. Time-frequency analysis of virtual SAM sensors from sensorimotor areas showed event-related desynchronisation of mu and beta bands following the onset of movement in all three conditions. These data demonstrate comparable activations of visuomotor mechanisms during observation or imitation of mouth movements and during observation of hand movements. These results support the notion that sensorimotor mechanisms play a role in achieving a representation of the oro-facial gestures of others.

Topographic distribution of the 40 Hz auditory evoked-related potential in normal and aged subjects.

Galambos, Makeig and Talmachoff (1981) described what they called the 40 Hz event-related potential (ERP). This steady-state response is an EEG following response to repetitive auditory stimulation which becomes sinusoidal in form and maximal in amplitude at rates between 35 and 45 Hz. The present study was designed to examine the scalp topography of the 40 Hz ERP in order to complement previous magnetoencephalographic studies which implicate auditory cortex in the generation of the response. In addition, this study was designed to collect normative data on an aged sample in order to assess the effects of aging on the response. 40 Hz ERP's were recorded from a group of seven audiometrically and neurologically normal elderly subjects (mean age = 69.6 years) and a younger group of five normal adults (mean age = 38.0 years), using 1000 Hz tones presented binaurally at 40 per second. A 21 channel recording system was used to obtain a comprehensive picture of the scalp distribution of the response. Recorded ERP's were Fourier transformed to enhance the signal-to-noise ratio. No significant differences were found in phase or amplitude of the 40 Hz ERP between the two age groups, indicating that the normal aging process does not have an effect on this response. Topographic maps of the 40 Hz ERP showed reversals of electrode potential in temporal regions, supporting an interpretation of bilateral sources in temporal cortex. The data presented in this study complement previous studies of the 40 Hz event-related magnetic field and support the position that temporal cortex is involved in the generation of the response.

The magnetoencephalographic localisation of source-systems in the brain: early and late components of event related potentials.

The application of magnetoencephalography (MEG) to the analysis of sources in the brain responsible for early and late components of evoked potentials is discussed. Representative data are presented and discussed which demonstrate localisation of sources assuming single equivalent dipoles. Distributed systems as sources for some steady-state responses are discussed in relation to the broader issue of the usefulness of equivalent single dipole models. These issues are related to the use of MEG for the analysis of source systems influenced by alcohol and other drugs.