Katharina Nebel

Movement preparation in self-initiated versus externally triggered movements: an event-related fMRI-study

In the present study, we used fMRI to investigate whether event-related preparatory processes of self-initiated and externally triggered movements differ. Twenty subjects were examined with 1000 T2*-weighted images in two consecutive sessions. During the first session subjects performed self-initiated abductions of the right index finger. For the second session subjects were instructed to perform the movements in response to visual cues. Number and timing of movements were matched between conditions. For statistical inference on multisubject level, random effects analyses were performed. Significantly enhanced activity during self-initiated compared to externally triggered movements was found within the left SMA, the left pre- and sensorimotor cortex, the right putamen, the left anterior cingulate gyrus, and the left inferior parietal lobe. The significantly increased activity during self-initiated in comparison to externally triggered movements might represent differential demands of the two conditions on the neuronal motor net during movement preparation, reflecting utilization of precise knowledge when to move in self-initiated movements. Our results emphasize a possible role of the primary motor cortex for movement preparation as observed in electrophysiological studies, but do not support a specific involvement of the dorsolateral prefrontal cortex as suggested by former block design studies.

Sparse imaging and continuous event-related fMRI in the visual domain: A systematic comparison

Continuous image acquisition as used in most functional magnetic resonance imaging (fMRI) designs may conflict with specific experimental settings due to attendant, noisy gradient switching. In sparse fMRI, single images are recorded with a delay that allows the registration of the predicted peak of an evoked hemodynamic response (HDR). The aim of this study was to assess validity and sensitivity of single‐trial sparse imaging within the visual domain. Thirteen subjects were scanned twice. Either continuous or sparse image acquisition was applied while participants viewed single trains of flashlights. Sparse fMRI results were compared to continuous event‐related fMRI results on single‐ and multisubject level regarding spatial extent, overlap, and intensity of activation. In continuously recorded data, the variability of the HDR peak latency was examined because this measure determined the timing of sparse image acquisition. In sparse fMRI, the sensitivity was analyzed considering different numbers of averaged trials. Sparse imaging detected the core activity revealed using continuous fMRI. The intensity of signal changes detected by continuous or sparse fMRI was comparable. The HDR peak latency was stable across sessions, but intersubject and regional variability might have affected the power of sparse fMRI. In sparse imaging, adding trials resulted in extension of activation and improvement in statistical power. The comparison with established continuous fMRI confirms the validity of sparse imaging. Conventional event‐related data acquisition and analysis provided more comprehensive results. However, only sparse fMRI offers the opportunity to apply stimuli and record further biosignals free of scanner‐related artifacts during intervals without image acquisition. Hum. Brain Mapping 24:130–143, 2005.

Reorganization of Motor Execution Rather Than Preparation in Poststroke Hemiparesis

Background and Purpose— The aim of the present study was to examine movement-related potentials (MRPs) in patients in the “chronic” stage after cortical stroke with recovered hemiparesis compared with healthy control subjects. Methods— Right index finger MRPs were derived from 12 patients ≥1 year after infarction in the territory of the left middle cerebral artery as well as from 12 control subjects. MRP components were compared between groups. Results— In the patient group, the component directly preceding movement onset (negative slope [NS]) was significantly reduced over the lesioned hemisphere contralateral to the movement. Furthermore, increased motor potentials (MPs) were observed over the contralesional hemisphere during movement execution. No changes in the early MRP (Bereitschaftspotential) reflecting movement preparation were found. Conclusions— Because the NS is supposed to be generated by the primary motor cortex, the decreased component over the lesioned hemisphere is interpreted to represent impaired contralateral M1 functioning in stroke patients. Contralesional activity has been reported as a probable sign of brain plasticity by functional imaging studies. Our results broaden these findings, giving new insights into the temporal course of movement-related brain activity in recovered cortical stroke patients. The data point to a functional reorganization of motor execution rather than preparation in poststroke hemiparesis.

Prefrontal cortex activity in self-initiated movements is condition-specific, but not movement-related.

Activity of the prefrontal cortex (PFC) has been observed in previous block-design brain imaging studies of self-initiated movements. However, the meaning of these activations remained unclear. A functional MRI experiment was carried out, which utilized an epoch and an event-related analysis approach to the data. We hypothesized that event-related activity of the PFC would argue for a contribution to movement preparation. In contrast, epoch-, but not event-related activity pointed to tonic activations, probably reflecting enhanced attentional states or working memory processing. Twenty-one subjects were examined with 845 T2*-weighted images. During active phases, subjects were instructed to perform self-initiated movements of the right index finger with intertrial intervals of about 8 s. On single subject level, epoch- and event-related regressors were entered into a combined model, estimating the exclusive contribution of either regressor. For statistical inference on multisubject level, random effects analyses were performed. For the epoch regressor, activity within the right dorso- and ventrolateral prefrontal cortex, the bilateral insula, and the right inferior parietal lobe was observed. The event-related regressor detected activity within the right inferior parietal lobe, ventral from the activity found with the epoch regressor. The present results indicate a condition-, but not a movement-related function of the PFC in self-initiated movements. Furthermore, anatomically distinct regions within the inferior parietal cortex seem to be involved in condition-specific and movement-related processes. The observed condition-specific activations are suggested to reflect attentional or working memory processes, supervising task performance, rather than movement preparation or initiation.

Self-initiated movements in chronic prefrontal traumatic brain injury: An event-related functional MRI study

Electrophysiological studies found traumatic lesions of the prefrontal cortex (PFC) to affect the preparation of self-initiated movements. However, a precise localization of the observed effects has not been reported yet. Moreover, previous reports did not account for effects of lesion location. Therefore, the present study utilized functional MRI to investigate the influence of predominantly left or right hemispheric PFC lesions on movement preparation in chronic traumatic brain injury (TBI) patients. 18 TBI patients with MRI-defined contusions of the PFC as well as 18 healthy control subjects matched for age and gender were examined. Patients were divided into subgroups of predominantly left or right hemispheric lesions. During functional image acquisition, subjects performed self-initiated abductions of their right index finger. Neuropsychological tests of attention and working memory, which are supposed to modulate preparatory processes, were conducted. Patients with predominantly left contusions demonstrated enhanced activity of the dorsal lateral premotor cortex in comparison to healthy control subjects. In predominantly right lesioned patients, reduced activity within the right caudate head was observed. Compared to predominantly left lesioned patients, neuropsychological tests yielded reduced task performances in the right lesion subgroup. In line with previous electrophysiological research, imaging results of the present study are interpreted to represent altered preparatory networks in chronic prefrontal TBI patients. Since attentional and working memory functions are supposed to modulate preparatory processes, differences between the patient subgroups are suggested to result from the more pronounced cognitive impairments in the right-lesioned group.