K.M. Stephan

Quantitative comparison of functional magnetic resonance imaging with positron emission tomography using a force-related paradigm

The intention of our study was to compare functional magnetic resonance imaging (fMRI) with positron emission tomography (PET). We used the same force-related motor paradigm for both techniques, which allows for quantification of stimulus intensity. Regional cerebral blood flow (rCBF) was determined with PET in six male subjects (age 30 +/- 3) using the slow bolus injection technique and oxygen-15-labeled water. Scans were collected during six different conditions: at rest and during repetitive Morse key press at 1 Hz, with the right index finger at a range of different forces. In a second series of experiments fMRI data were acquired under similar conditions in six volunteers in a single slice parallel to and 51 +/- 3 mm dorsal to the anterior and posterior commissure (AC-PC). A conventional 1.5-T clinical magnetic resonance (MR) system and the FLASH technique were used. The data obtained in both series of experiments were subjected to the same statistical analyses. Statistical parametric maps (SPM) were generated by two different approaches: a correlation between peak force and rCBF or fMRI signal and using a categorical comparison of force exerted with rest. SPMs were coregistered with anatomical MR images. PET and fMRI measurements demonstrated activation in the primary motor cortex (M1) and posterior supplementary motor cortex in all subjects. Correlation analysis demonstrated foci in the M1 in four subjects with PET and in only one subject with fMRI. Locations of activation peaks differed by 2 to 8 mm between imaging methods. The relationship between fMRI signal or rCBF and peak force was logarithmic. The maximum increase in fMRI signal was 5.0% +/- 0.9 at 60% of the maximum voluntary contraction while the corresponding increase in rCBF was 13.7% +/- 1.2. The ratio of percentage rCBF change to percentage fMRI signal change was very similar across all force levels. The high degree of correspondence between PET and fMRI data provides good cross-validation for the two techniques.

Conscious and subconscious sensorimotor synchronization--prefrontal cortex and the influence of awareness.

One of the most compelling challenges for modern neuroscience is the influence of awareness on behavior. We studied prefrontal correlates of conscious and subconscious motor adjustments to changing auditory rhythms using regional cerebral blood flow measurements. At a subconscious level, movement adjustments were performed employing bilateral ventral mediofrontal cortex. Awareness of change without explicit knowledge of the nature of change led to additional ventral prefrontal and premotor but not dorsolateral prefrontal activations. Only fully conscious motor adaptations to a changing rhythmic pattern showed prominent involvement of anterior cingulate and dorsolateral prefrontal cortex. These results demonstrate that while ventral prefrontal areas may be engaged in motor adaptations performed subconsciously, only fully conscious motor control which includes motor planning will involve dorsolateral prefrontal cortex.

Reorganization of the Executive Motor System after Stroke

We wished to characterize changes in function of the executive motor system individually and to correlate these with the degree of recovery from motor disability following stroke. Six male stroke pati

A multivariate analysis of evoked responses in EEG and MEG data.

This paper presents a multivariate analysis of evoked responses and their spatiotemporal dynamics as measured with electro- or magnetoencephalography. This analysis uses standard techniques (ManCova) to make possible statistical inference about differential responses, after the data have been transformed using singular value decomposition. The generality of this approach is limited only by the assumptions implicit in the general linear model and can range from simple analyses like Hotellings T2 test (in comparing evoked responses among different conditions) to complex analyses of a multivariate regression type (e.g., characterizing the response components associated with a behavioral or psychophysical parameter). To illustrate the technique we have characterized time-dependent changes (both within and between trials) in magnetic fields, evoked by self-paced movements. Our illustrative analysis showed that movement-evoked components were less prone to adaptation than premovement components, suggesting that functionally distinct (preparatory and early executive) biomagnetic signals show differential adaptation.

Transient phase-locking and dynamic correlations: Are they the same thing?

This work represents an attempt to bring together two important themes in neuronal dynamics. The first is the characterization of dynamic correlations in multiunit recordings of spike activity using joint‐peri‐stimulus time histograms (J‐PSTHs) [Aertsen and Preissl, 1991: Non Linear Dynamics and Neural Networks]. The second is transient phase‐locking at high (gamma) frequencies, either in terms of spiking in separable spike trains [e.g., Eckhorn et al., 1988: Biol Cybern 60:121–130, Gray and Singer, 1989 Proc Natl Acad Sci USA 86:1698–1702], or using continuous electrical or biomagnetic signals [e.g., Desmedt and Tomberg, 1994 Neurosci Lett 168:126–129]. In this paper we suggest that transient phase‐locking is necessary for frequency‐specific, dynamic event‐related correlations. This point is demonstrated using the gamma‐frequency (36 Hz) component of neuromagnetic signals measured in the prefrontal and partial regions of a subject during self‐paced movements. A J‐PSTH analysis revealed dynamic changes in prefronto‐parietal correlations in relation to movement onset. These frequency‐specific dynamic correlations were associated with changes in the degree of phase‐locking, of the sort reported by Desmedt and Tomberg [1994 Neurosci Lett 168:126–129]. Hum. Brain Mapping 5:48–57, 1997.

Prognostic Value of MEP and SSEP in Patients with Chronic UMN Lesions after Stroke

55 patients with chronic upper motor neurone syndrome after stroke were assessed on admission to a neurological therapy center using clinical scores (separate motor scores for hand, arm and leg functi

Prefrontal cortex activation during motor attention, motor imagery and performance of cycling movements

Introduction Bipedal cycling is a symmetrical and highly automated task, which mainly involves primary sensorimotor areas. Abrupt change or other modification of the task lead to additional activity in secondary sensorimotor areas including prefrontal, anterior cingulate, lateral premotor and lateral parietal cortex additionally involved in planning, initiation and adaptations of such movements (1,2). The aim of the present study was to investigate the differential role of prefrontal areas during imagery, performance of and attention towards such highly automated leg movements.