328 research outputs found
Verbal instructions and top-down saccade control
Few studies have addressed the interaction between instruction content and saccadic eye movement control. To assess the impact of instructions on top-down control, we instructed 20 healthy volunteers to deliberately delay saccade triggering, to make inaccurate saccades or to redirect saccadesâi.e. to glimpse towards and then immediately opposite to the target. Regular pro- and antisaccade tasks were used for comparison. Bottom-up visual input remained unchanged and was a gap paradigm for all instructions. In the inaccuracy and delay tasks, both latencies and accuracies were detrimentally impaired by either type of instruction and the variability of latency and accuracy was increased. The intersaccadic interval (ISI) required to correct erroneous antisaccades was shorter than the ISI for instructed direction changes in the redirection task. The word-by-word instruction content interferes with top-down saccade control. Top-down control is a time consuming process, which may override bottom-up processing only during a limited time period. It is questionable whether parallel processing is possible in top-down control, since the long ISI for instructed direction changes suggests sequential plannin
In vivo1H-MR spectroscopy of the human heart
4. Conclusions: Combined respiratory and cardiac triggering improves the localization accuracy and spectral quality in cardiac1H-MRS dramatically leading to substantially increased spectral reproducibility. The best practical realization of double triggering turned out to be the use of the ECG amplitude when making use of the fact that it is modulated by respiration. In spite of the spectral quality achieved in most subjects, we still fail to record satisfactory spectra in a minority of subjects. The reasons for this are not understood at present but must be some particulars of either a given subject or the experimental setup. The cardiac1H-MR spectra contain quantifiable contributions from creatine, TMA, lipids, and probably taurine. It is possible that the spectral contributions of creatine are subject to dipolar coupling similar to the observations for skeletal muscl
First attempt to motion corrected flow encoding using free-breathing phase-contrast CINE MRI
International audienceThis study demonstrates the feasibility of free-breathing phase-contrast CINE MRI without averaging. A new version of the CINE GRICS algorithm[1] was used to correct for motion
Quality check of cardiac MRI exams for Fallot patients: Interest of a simple formula to detect invalid exams
PurposeCardiac MRI (CMR) is the key exam for Fallot patients but remains tricky. The CMR report should at least mention left and right ventricle end-diastole volumes (Vl and Vr), ejection fraction (EFl and EFr) and pulmonary regurgitation (PR). Obviously, these variables are linked together by basic physiology rules and indeed VlĂEFl=VrĂEFrĂ(1-PR). We investigated the interest of using such formula as quality check during Fallot CMR exams in our center.Methods98Â consecutive CMR examinations for Fallot (or Fallot-like) cardiopathy between 2010 and 2014 were retrospectively included. The exams failing to pass the formula (with a 10% tolerance) constituted the Invalid-group and a control group of the same size was also constituted. CMR of both groups were randomly submitted to a blinded senior observer. The inter-observer limits of agreements were compared for the different variables within both groups.Results(Fig. 1) 12Â CMR (12%) failed to pass the validation formula. From the 24 reanalyzed CMR, only 4 failed to pass the formula (all from the Invalid-group). Two had persistent defect (VSD or ASD) which were not mentioned to the radiologist and not detected during the CMR. Two had significant artefacts in the aorta or pulmonary trunk due to sterna wires. The inter-observer disagreements for the 8 other CMR of the Invalid-group concerned the Right ventricle end-diastole volume (P<0.05).ConclusionThe use of a simple formula as quality check of CMR examinations for Fallot patients was useful to detect a total of 12% of CMR with issues. 8% of the CMR corresponded to uncertain right ventricle contours, 2% to persistent septal defects that should have been noticed during the examination and 2% to unreliable aortic or pulmonary flow due to artifacts. The formula could have permitted either to detect the anomalies or at least to conclude that the quality of the exams was impaired
Visual exploration behaviour during clock reading in Alzheimer's disease
Eye movement behaviour during visual exploration of 24 patients with probable Alzheimer's disease and 24 ageâmatched controls was compared in a clock reading task. Controls were found to focus exploration on distinct areas at the end of each clock hand. The sum of these two areas of highest fixation density was defined as the informative region of interest (ROI). In Alzheimer's disease patients, visual exploration was less focused, with fewer fixations inside the ROI, and the time until the first fixation was inside the ROI was significantly delayed. Changes of fixation distribution correlated significantly with the ability to read the clock correctly, but did not correlate with dementia severity. In Alzheimer's disease patients, fixations were longer and saccade amplitudes were smaller. The altered visual exploration in Alzheimer's disease might be related to parietal dysfunction or to an imbalance between a degraded occipitoâparietal and relatively preserved occipitoâtemporal visual networ
Hemispheric asymmetry in visuospatial attention assessed with transcranial magnetic stimulation
Transcranial magnetic stimulation (TMS) was used to study visuospatial attention processing in ten healthy volunteers. In a forced choice recognition task the subjects were confronted with two symbols simultaneously presented during 120ms at random positions, one in the left and the other in the right visual field. The subject had to identify the presented pattern out of four possible combinations and to press the corresponding response key within 2 s. Double-pulse TMS (dTMS) with a 100-ms interstimulus interval (ISI) and an intensity of 80% of the stimulator output (corresponding to 110-120% of the motor threshold) was applied by a nonfocal coil over the right or left posterior parietal cortex (PPC, corresponding to P3/P4 of the international 10-20 system) at different time intervals after onset of the visual stimulus (starting at 120ms, 270ms and 520ms). Double-pulse TMS over the right PPC starting at 270ms led to a significant increase in percentage of errors in the contralateral, left visual field (median: 23% with TMS vs 13% without TMS, P=0.0025). TMS applied earlier or later showed no effect. Furthermore, no significant increase in contra- or ipsilateral percentage of errors was found when the left parietal cortex was stimulated with the same timing. These data indicate that: (1) parietal influence on visuospatial attention is mainly controlled by the right lobe since the same stimulation over the left parietal cortex had no significant effect, and (2) there is a vulnerable time window to disturb this cortical process, since dTMS had a significant effect on the percentage of errors in the contralateral visual hemifield only when applied 270ms after visual stimulus presentatio
Saccadic eye movement changes in Parkinson's disease dementia and dementia with Lewy bodies
Neurodegeneration in Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) affect cortical and subcortical networks involved in saccade generation. We therefore expected impairments in saccade performance in both disorders. In order to improve the pathophysiological understanding and to investigate the usefulness of saccades for differential diagnosis, saccades were tested in age- and education-matched patients with PDD (n = 20) and DLB (n = 20), Alzheimer's disease (n = 22) and Parkinson's disease (n = 24), and controls (n = 24). Reflexive (gap, overlap) and complex saccades (prediction, decision and antisaccade) were tested with electro-oculography. PDD and DLB patients had similar impairment in all tasks (P > 0.05, not significant). Compared with controls, they were impaired in both reflexive saccade execution (gap and overlap latencies, P 0.05). Patients with Parkinson's disease had, compared with controls, similar complex saccade performance (for all, P > 0.05) and only minimal impairment in reflexive tasks, i.e. hypometric gain in the gap task (P = 0.04). Impaired saccade execution in reflexive tasks allowed discrimination between DLB versus Alzheimer's disease (sensitivity â„60%, specificity â„77%) and between PDD versus Parkinson's disease (sensitivity â„60%, specificity â„88%) when ±1.5 standard deviations was used for group discrimination. We conclude that impairments in reflexive saccades may be helpful for differential diagnosis and are minimal when either cortical (Alzheimer's disease) or nigrostriatal neurodegeneration (Parkinson's disease) exists solely; however, they become prominent with combined cortical and subcortical neurodegeneration in PDD and DLB. The similarities in saccade performance in PDD and DLB underline the overlap between these conditions and underscore differences from Alzheimer's disease and Parkinson's diseas
A multimodal real-time MRI articulatory corpus of French for speech research
In this work we describe the creation of ArtSpeechMRIfr: a real-time as well as static magnetic resonance imaging (rtMRI, 3D MRI) database of the vocal tract. The database contains also processed data: denoised audio, its phonetically aligned annotation, articulatory contours, and vocal tract volume information , which provides a rich resource for speech research. The database is built on data from two male speakers of French. It covers a number of phonetic contexts in the controlled part, as well as spontaneous speech, 3D MRI scans of sustained vocalic articulations, and of the dental casts of the subjects. The corpus for rtMRI consists of 79 synthetic sentences constructed from a phonetized dictionary that makes possible to shorten the duration of acquisitions while keeping a very good coverage of the phonetic contexts which exist in French. The 3D MRI includes acquisitions for 12 French vowels and 10 consonants, each of which was pronounced in several vocalic contexts. Ar-ticulatory contours (tongue, jaw, epiglottis, larynx, velum, lips) as well as 3D volumes were manually drawn for a part of the images
Motion correction of Single Voxel Spectroscopy by Independent Component Analysis applied to spectra from non-anesthetized pediatric subjects
For Single Voxel Spectroscopy (SVS), the acquisition of the spectrum is typically repeated n times and then combined with a factor in order to improve the Signal-to-Noise Ratio (SNR). In practice the acquisitions are not only affected by random noise, but also by physiological motion and subject movements. Since the influence of physiological motion such as cardiac and respiratory motion on the data is limited, it can be compensated for without data-loss. Individual acquisitions hampered by subject movements on the other hand need to be rejected, if no correction or compensation is possible. If the individual acquisitions are stored, it is possible to identify and reject the motion-disturbed acquisitions before averaging.
Several automatic algorithms were investigated using a dataset of spectra from non-anesthetized infants with a gestational age of 40 weeks. Median filtering removed most subject movement artifacts, but at the cost of increased sensitivity to random noise. Neither Independent Component Analysis (ICA) nor outlier identification with multiple comparisons has this problem. These two algorithms are novel in this context. The peak height values of the metabolites were increased compared to the mean of all acquisitions for both methods, although primarily for the ICA method
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