PubMed Health⌕ Search

Biomedical subjects

P Praamstra

Publications and source records attributed to P Praamstra.

35 records · Page 2Linked to original sources
PubMed1998

Reliance on external cues for movement initiation in Parkinson's disease. Evidence from movement-related potentials.

The aim of this study was to investigate the neurophysiological mechanisms underlying Parkinson's disease patients' increased reliance on external cues for the initiation of movement. Lateralized movement-related cortical potentials were recorded in a noise-compatibility task with seven patients and seven age-matched control subjects. In this two-choice task, visual stimuli containing incompatible target and distractor elements, which simultaneously instructed for responses from both hands, initially caused activation of the motor cortex controlling the wrong response hand. The incorrect response activation was of higher amplitude in patients than in control subjects, causing a longer response delay relative to response times when target and distractors instructed the same hand. In addition, hand-specific motor cortex activation started earlier in patients than in control subjects. These results indicate that visual stimuli exerted an earlier and stronger influence on movement initiation in patients than in control subjects. We hypothesize that information from sensory stimuli relevant for the generation of a response can have rapid access to motor structures in Parkinson's disease patients, thereby facilitating the initiation of movement. The findings may reflect a compensatory mechanism, but could also be related to excitability changes in the motor cortex intrinsic to the pathophysiology of Parkinson's disease.

Aged↗
PubMed1998

An MEG study of picture naming.

The purpose of this study was to relate a psycholinguistic processing model of picture naming to the dynamics of cortical activation during picture naming. The activation was recorded from eight Dutch subjects with a whole-head neuromagnetometer. The processing model, based on extensive naming latency studies, is a stage model. In preparing a picture"s name, the speaker performs a chain of specific operations. They are, in this order, computing the visual percept, activating an appropriate lexical concept, selecting the target word from the mental lexicon, phonological encoding, phonetic encoding, and initiation of articulation. The time windows for each of these operations are reasonably well known and could be related to the peak activity of dipole sources in the individual magnetic response patterns. The analyses showed a clear progression over these time windows from early occipital activation, via parietal and temporal to frontal activation. The major specific findings were that (1) a region in the left posterior temporal lobe, agreeing with the location of Wernicke"s area, showed prominent activation starting about 200 msec after picture onset and peaking at about 350 msec (i.e., within the stage of phonological encoding), and (2) a consistent activation was found in the right parietal cortex, peaking at about 230 msec after picture onset, thus preceding and partly overlapping with the left temporal response. An interpretation in terms of the management of visual attention is proposed.

Adult↗
PubMed1996

Movement-related potential measures of different modes of movement selection in Parkinson's disease.

Movement-related potentials were recorded preceding self-paced voluntary movements in patients with Parkinson's disease and in healthy subjects of the same age group. We compared the Readiness Potential preceding joystick movements in a fixed direction and preceding joystick movements in freely selected directions. In normal subjects the Readiness Potential amplitude was higher preceding freely selected movements than preceding movements in a fixed direction. The Readiness Potential in Parkinson patients failed to be modified by the different modes of movement selection. The modulation of the Readiness Potential by different ways of preparing for movement might be due to the supplementary motor area (SMA) being more strongly engaged by tasks requiring internal control of movements than by tasks that are externally structured. The results suggest that this task-dependent variation of SMA activity is reduced in Parkinson's disease. A failing capacity to adapt SMA activity to different task demands has previously been suggested by evidence from positron emission tomography studies using similar tasks.

Adult↗
PubMed1996

Dipole source analysis suggests selective modulation of the supplementary motor area contribution to the readiness potential.

The readiness potential preceding voluntary movement is modulated by the mode of movement selection, i.e. it has a higher amplitude preceding freely selected than before prescribed movements (Praamstra, P., Stegeman, D.F., Horstink, M.W.I.M., Brunia, C.H.M. and Cools, A.R. Movement-related potentials preceding voluntary movement are modulated by the mode of movement selection. Exp. Brain Res., 1995, 103: 429-439). One cortical area that is likely to be involved in this modulation is the supplementary motor area (SMA). Recent attempts to elucidate the neural generators of the readiness potential using spatiotemporal dipole source analysis, however, failed to establish a significant SMA contribution to the readiness potential. This might be explained by a failure of the proposed analyses to discriminate between SMA and motor cortex contributions to the readiness potential. We applied a dipole source analysis approach that better separates these overlapping source activities. The resulting source model includes an SMA source generating pre-movement activity consistent with evidence from intracranial recordings in humans. The SMA source accounts almost completely for the modulation of the readiness potential by different modes of movement selection. On the basis of these results, the relation between scalp-recorded movement-related activity, intracranially recorded potentials, and findings from functional imaging studies of voluntary movement, appears more transparent than suggested by previous dipole source analyses of premovement potentials.

Adult↗
PubMed1996

Linear estimation discriminates midline sources and a motor cortex contribution to the readiness potential.

Spatiotemporal dipole modelling of the generators of the readiness potential (RP) prior to voluntary movements has yielded diverging results concerning the contributions of supplementary motor area (SMA) and primary motor cortex. We applied an alternative approach (i.e. linear estimation theory) to measurements of the RP preceding fixed and freely selected finger movements, measured at 28 electrodes of the extended 10-20 system. The volume conductor properties of the head were modelled by 3 concentric spheres. Current densities were reconstructed on a spherical surface, placed at a depth of 5 mm from the inside of the skull. Lead field normalization was applied. The analysis shows activity on the midline as well as near the primary motor area. Although some features of the reconstructions are not readily interpretable, separate contributions of midline sources (including presumably SMA) and motor cortex to the RP are clearly distinguished.

Adult↗
PubMed1996

Movement preparation in Parkinson's disease. Time course and distribution of movement-related potentials in a movement precueing task.

Investigations of the effects of advance information on movement preparation in Parkinson's disease using reaction time (RT) measures have yielded contradictory results. In order to obtain direct information regarding the time course of movement preparation, we combined RT measurements in a movement precueing task with multi-channel recordings of movement-related potentials in the present study. Movements of the index and middle fingers of the left and right hand were either precued or not by advance information regarding the side (left or right hand) of the required response. Reaction times were slower for patients than for control subjects. Both groups benefited equally from informative precues, indicating that patients utilized the advance information as effectively as control subjects. Lateralization of the movement-preceding cerebral activity [i.e. the lateralized readiness potential (LRP)] confirmed that patients used the available partial information to prepare their responses and started this process no later than controls. In conjunction with EMG onset times, the LRP onset measures allowed for a fractionation of the RTs, which provided clues to the stages where the slowness of Parkinson's disease patients might arise. No definite abnormalities of temporal parameters were found, but differences in the distribution of the lateralized movement-preceding activity between patients and controls suggested differences in the cortical organization of movement preparation. Differences in amplitude of the contingent negative variation (CNV) and differences in the way in which the CNV was modulated by the information given by the precue pointed in the same direction. A difference in amplitude of the P300 between patients and controls suggested that preprogramming a response required more effort from patients than from control subjects.

Adult↗
PubMed1995

Movement-related potentials preceding voluntary movement are modulated by the mode of movement selection.

In two experiments movement-related cortical potentials preceding voluntary movement were recorded. In experiment 1, subjects performed four motor tasks involving joystick movements. The four tasks differed in complexity (single vs sequential movements) and in the mode of movement selection, i.e., whether a movement or movement sequence was made in fixed or in self-determined directions. The choice of these tasks was based, firstly, on previous electrophysiological studies suggesting an effect of task-complexity on the amplitude of the readiness potential (RP) and, secondly, on previous positron emission tomography (PET) studies showing that activity of the supplementary motor area (SMA) is influenced by the mode of movement selection. The results show that, for single movements, RP amplitude is higher preceding freely selected movements than preceding movements in a fixed direction. In experiment 2 this effect was replicated using button presses instead of joystick movements. The results converge with PET evidence obtained in similar tasks and establish that the RP is modulated by the mode of movement selection. This modulation is probably related to differential involvement of the SMA.

Adult↗
PubMed1993

Evoked potential measures of auditory cortical function and auditory comprehension in aphasia.

This study investigated the relation between severity of auditory comprehension impairment in aphasia and the functional integrity of the posterior superior temporal region as evaluated by middle- and long-latency auditory evoked potentials and dipole source analysis. AEPs were studied in 20 stroke patients and in age-matched controls. AEPs and language data were collected 1 year or more post onset, and were compared to performance early after onset. Patients were differentiated in a group with severe and a group with moderate to recovered auditory comprehension impairment. Significant asymmetries of auditory evoked dipole source potentials were more frequent in the severely impaired group. However, a severe auditory comprehension deficit was not incompatible with normal AEPs. The results confirm the importance of the posterior superior temporal region for auditory language comprehension, but the correlation between AEP asymmetry and auditory comprehension seems due to the close spatial relation of AEP generating substrate and posterior language area rather than to partial overlap. Dipole source analysis of AEPs proved to be a valuable method for the assessment of interhemispheric asymmetries, enhancing the sensitivity of AEPs to unilateral damage of auditory cortex.

Aged↗
PubMed1993

Phonological effects on the auditory N400 event-related brain potential.

We report 3 experiments exploring the responsiveness of the auditory N400 event-related potential to the phonological relations between word or non-word targets and preceding prime words. When subjects had to decide whether primes and targets rhymed, non-rhyming words produced greater negativity in the N400 time range than rhyming words. The same effect was obtained when these targets were spoken by another voice than the prime words, suggesting that the effect is determined by phonological factors, and not merely by a physical-acoustic mismatch (Experiment 1). In the rhyming task, the differential N400 for non-rhyming vs. rhyming words was equally pronounced for non-rhyming vs. rhyming non-words (Experiment 2). In a lexical decision task on the same stimuli, a difference between non-rhyming and rhyming targets was obtained for words, but not for non-words (Experiment 3). The results show that the auditory N400 is sensitive to phonological variables. It is further proposed that phonological effects on the auditory N400 are not manifestations unique to phonological processes that demand conscious attention, but may also reflect operations that are performed automatically during auditory word recognition.

Acoustic Stimulation↗
PubMed1992

On the possibility of independent activation of bilateral mismatch negativity (MMN) generators.

Event-related potential (ERP) studies on auditory selective attention have suggested that the neuronal traces underlying the mismatch negativity (MMN) develop separately for left and right ear inputs. We investigated whether this feature of the MMN could be exploited to activate the MMN generators in each hemisphere independently. Given the location of the MMN sources, this would allow separate assessment of left and right hemisphere auditory cortical structures. The MMN was elicited in a dichotic oddball paradigm using stimuli and conditions of stimulation that should optimize transmission of left and right ear stimuli to the contralateral auditory cortices. The MMN demonstrated no interhemispheric asymmetries, indicating that the bilateral MMN generators were symmetrically involved. The results of a supplementary experiment provide evidence for the simultaneous existence of multiple traces for MMN generation.

Acoustic Stimulation↗
PubMed1991

Word deafness and auditory cortical function. A case history and hypothesis.

A patient who already had Wernicke's aphasia due to a left temporal lobe lesion suffered a severe deterioration specifically of auditory language comprehension, subsequent to right temporal lobe infarction. A detailed comparison of his new condition with his language status before the second stroke revealed that the newly acquired deficit was limited to tasks related to auditory input. Further investigations demonstrated a speech perceptual disorder, which we analysed as due to deficits both at the level of general auditory processes and at the level of phonetic analysis. We discuss some arguments related to hemisphere specialization of phonetic processing and to the disconnection explanation of word deafness that support the hypothesis of word deafness being generally caused by mixed deficits.

Aphasia, Wernicke↗
PubMed1990

GM2-gangliosidosis. Clinical and biochemical aspects of four cases.

We discuss four cases of GM2-gangliosidosis. In one of them the biochemical diagnostic confirmation was difficult. This case revealed striking discrepancies between the results of different methods of enzyme assay. The hexosaminidase A determination based on pH inactivation is not always reliable; assay with natural substrate may be necessary. However, the results with the newly developed substrate 4-MU-GlcNac-6-SO4 are promising and it seems to be a good alternative to the traditional (pH or heat) inactivation procedures. The deficiency can be shown in leukocytes, plasma and fibroblasts with the 6-sulfated substrate. The carrier state seems better reflected in plasma hexosaminidase A than in leukocyte hexaminidase A levels.

Adult↗
PubMedDate not supplied

Vascular insufficiency of the cervical cord due to hyperextension of the spine.

A patient with acute transverse myelopathy after minor trauma due to hyperextension of the neck is reported. The pattern of neurologic deficit favors the diagnosis of acute central cervical spinal cord syndrome. The proposed causative mechanism is temporary vascular insufficiency of the spinal cord. The architecture of vascular supply of the spinal cord and specific cerebrospinal fluid abnormalities in this patient are compatible with the diagnosis. The differential diagnosis of traumatic vascular spinal cord lesions is discussed.

Child↗