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S Sunaert

Publications and source records attributed to S Sunaert.

18 recordsLinked to original sources
PubMed2002

Extracting 3D from motion: differences in human and monkey intraparietal cortex.

We compared three-dimensional structure-from-motion (3D-SFM) processing in awake monkeys and humans using functional magnetic resonance imaging. Occipital and midlevel extrastriate visual areas showed similar activation by 3D-SFM stimuli in both species. In contrast, intraparietal areas showed significant 3D-SFM activation in humans but not in monkeys. This suggests that human intraparietal cortex contains visuospatial processing areas that are not present in monkeys.

Animals↗
PubMed2001

Human brain regions involved in heading estimation.

Observer motion in a stationary visual environment results in an optic flow pattern on the retina, which in simple situations can be used to determine the direction of self motion or heading. The present study, using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), investigated the human cerebral activation pattern, elicited when subjects viewing a ground plane optic flow pattern actively judged heading. Several successive experiments controlled for visual input, visuospatial attention, and motor response effects. Results indicate that the network specifically involved in heading consists of only two motion sensitive areas: human MT/V5+, including an inferior satellite, and dorsal intraparietal sulcus area (DIPSM/L), predominantly in the right hemisphere, plus a dorsal premotor region bilaterally. These results suggest possible homologies with the dorsal part of the medial superior temporal area and area 7a in the monkey.

Brain↗
PubMed2001

Brain areas involved in interlimb coordination: a distributed network.

Whereas behavioral studies have made significant contributions toward the identification of the principles governing the coordination of limb movements, little is known about the role of higher brain areas that are involved in interlimb coordination. Functional magnetic resonance imaging (fMRI) was used to reveal the brain areas activated during the cyclical coordination of ipsilateral wrist and foot movements. Six normal subjects performed five different tasks that were presented in a random order, i.e., isolated flexion-extension movements of the right wrist (WRIST) and right foot (FOOT), cyclical coordination of wrist and foot according to the isodirectional (ISODIR) and nonisodirectional (NON-ISODIR) mode, and rest (REST). All movements were auditory paced at 66 beats/min. During the coordination of both limb segments, a distributed network was identified showing activation levels in the supplementary motor area (SMA), cingulate motor cortex (CMC), premotor cortex (PMC), primary sensorimotor cortex (M1/S1), and cerebellum that exceeded the sum of the activations observed during the isolated limb movements. In addition, coordination of the limb movements in different directions was associated with extra activation of the SMA as compared to movements in the same direction. It is therefore concluded that the SMA is substantially involved in the coordination of the nonhomologous limbs as part of a distributed motor network. Accordingly, the long-standing exclusive association that has been made between this medial frontal area and bimanual (homologous) coordination needs to be abandoned and extended towards other forms of interlimb coordination (nonhomologous).

Adult↗
PubMed2001

BOLD contrast fMRI of whole rodent tumour during air or carbogen breathing using echo-planar imaging at 1.5 T.

The aim of this study was to evaluate the feasibility of functional MR imaging (fMRI) at 1.5 T, exploiting blood oxygenation level-dependent (BOLD) contrast, for detecting changes in whole-tumour oxygenation induced by carbogen (5% CO2+95% O2) inhalation of the host. Adult WAG/Rij rats with rhabdomyosarcomas growing subcutaneously in the lower flank were imaged when tumours reached sizes between 1 and 11 cm3 (n=12). Air and carbogen were alternatively supplied at 2 l/min using a snout mask. Imaging was done on a 1.5-T MR scanner using a T2*-weighted gradient-echo, echo-planar imaging (GE-EPI) sequence. Analysis of the whole-tumour EPI images was based on statistical parametric maps. Voxels with and without signal intensity changes (SIC) were recorded. Significance thresholds were set at p<0.05, corrected for multiple comparisons. In continuous air breathing condition, 3 of 12 tumours showed significant negative SIC and 1 tumour had a clear-cut positive SIC. The remaining tumours showed very little or no change. When switching to carbogen breathing, the SIC were significantly positive in 10 of 12 tumours. Negative SIC were present in 4 tumours, of which three were simultaneously characterised by positive SIC. The overall analysis indicated that 6 of the 12 tumours could be considered as strong positive responders to carbogen. Our research demonstrates the applicability of fMRI GE-EPI at 1.5 T to study whole-tumour oxygenation non-invasively. The observed negative SIC during air condition may reflect the presence of transient hypoxia during these measurements. Selection of tumours on the basis of their individual response to carbogen is possible, indicating a role of such non-invasive measurements for using tailor-made treatments.

Animals↗
PubMed2001

Orientation discrimination of objects and gratings compared: an fMRI study.

We used functional magnetic resonance imaging to compare the human brain regions involved in orientation discrimination of two-dimensional (2D) objects and gratings. The orientation discrimination tasks, identification and successive discrimination, were contrasted to a dimming detection control condition with identical retinal input. Regions involved in orientation discrimination were very similar for the two types of tasks and for the two types of stimuli and both belonged to the dorsal and ventral visual pathways. They included posterior occipital, lingual, posterior fusiform, inferior temporal, dorsal intraparietal and medial parietal regions. The main difference between the two types of stimuli was a larger activation of precuneus when 2D objects were used compared to gratings. The main difference between discrimination tasks was an enhanced activity, at the group level, in superior frontal sulcus in identification compared to successive discrimination, and at least at the single subject level, a larger activity in right fusiform cortex in successive discriminations compared to identification. Thus, in contradiction to generally accepted views, orientation discrimination of gratings and objects involve largely similar networks including both ventral and dorsal visual regions.

Adult↗
PubMed2000

Attention to speed of motion, speed discrimination, and task difficulty: an fMRI study.

We studied the functional neuroanatomy of attention to speed of motion using functional magnetic resonance imaging in eight healthy subjects, who performed a speed discrimination (SID) task using a random textured pattern moving at a reference speed of 6 deg/s. During the control condition (DIM), with retinal stimulation identical to that during SID, subjects detected the dimming of the central fixation point. Attention to speed (SID compared to DIM) activated mainly ventral V3 and V4, dorsal V3 and V3A. Compared to a fixation control condition, speed discrimination recruited a large visuomotor network, including hMT/V5+. However, hMT/V5+ was only marginally more active during speed discrimination than during dimming detection. Thus hMT/V5+ is involved in speed discrimination, in line with the speed discrimination impairments following hMT/V5+ lesions, but our results suggest that this activity simply reflects the processing of motion rather than attention to speed. Manipulating the difficulty of the speed discrimination task over a large range of the psychometric curve revealed that increasing difficulty linearly increases activity in right frontal regions, as well as in lateral occipital and dorsal parietal regions. A weak effect of difficulty was also observed in dorsal V3.

Adult↗
PubMed2000

The representation of shape in the context of visual object categorization tasks.

To investigate the role of human fusiform gyrus in shape processing, we determined the effect of shape degradation on BOLD contrast in this region with fMRI during three tasks requiring subjects to determine either whether two successively presented nonsense shapes had the same global orientation (OR task); whether two successively presented meaningful objects belonged to the same basic level category (CAT task); or whether two successively presented objects represented the same exemplar of a category (EX task). On the behavioral level, shape degradation by locally shifting the pixels constituting the lines of stimuli had no effect on performance in the OR task, while it was detrimental to performance in the CAT and EX tasks. In comparison to the OR task, both the CAT and EX tasks were associated with activations in the occipitotemporal and parietal cortex. When shape degradation was applied, activation in the middle fusiform gyrus was reduced in all tasks. The occurrence of this effect in the OR task indicates that it is independent of memory representations. The persistence of the effect in both tasks that showed a behavioral effect of degradation suggests that it does not reflect the amount of shape processing performed on the stimuli, but rather the specificity of the final perceptual representation that can be built from the shape information that is available. Other studies have shown effects of stimulus familiarity and task requirements in the fusiform gyrus, suggesting that there is no need to assume different modules for perceptual representation and representation in memory.

Adult↗
PubMed2000

Attention mechanisms in visual search -- an fMRI study.

The human visual system is usually confronted with many different objects at a time, with only some of them reaching consciousness. Reaction-time studies have revealed two different strategies by which objects are selected for further processing: an automatic, efficient search process, and a conscious, so-called inefficient search [Treisman, A. (1991). Search, similarity, and integration of features between and within dimensions. Journal of Experimental Psychology: Human Perception and Performance, 17, 652--676; Treisman, A., & Gelade, G. (1980). A feature integration theory of attention. Cognitive Psychology, 12, 97--136; Wolfe, J. M. (1996). Visual search. In H. Pashler (Ed.), Attention. London: University College London Press]. Two different theories have been proposed to account for these search processes. Parallel theories presume that both types of search are treated by a single mechanism that is modulated by attentional and computational demands. Serial theories, in contrast, propose that parallel processing may underlie efficient search, but inefficient searching requires an additional serial mechanism, an attentional "spotlight" (Treisman, A., 1991) that successively shifts attention to different locations in the visual field. Using functional magnetic resonance imaging (fMRI), we show that the cerebral networks involved in efficient and inefficient search overlap almost completely. Only the superior frontal region, known to be involved in working memory [Courtney, S. M., Petit, L., Maisog, J. M., Ungerleider, L. G., & Haxby, J. V. (1998). An area specialized for spatial working memory in human frontal cortex. Science, 279, 1347--1351], and distinct from the frontal eye fields, that control spatial shifts of attention, was specifically involved in inefficient search. Activity modulations correlated with subjects' behavior best in the extrastriate cortical areas, where the amount of activity depended on the number of distracting elements in the display. Such a correlation was not observed in the parietal and frontal regions, usually assumed as being involved in spatial attention processing. These results can be interpreted in two ways: the most likely is that visual search does not require serial processing, otherwise we must assume the existence of a serial searchlight that operates in the extrastriate cortex but differs from the visuospatial shifts of attention involving the parietal and frontal regions.

Adult↗
PubMed1999

Motion-responsive regions of the human brain.

Functional magnetic resonance imaging was used to map motion responsive regions of the human brain by contrasting passive viewing of moving and stationary randomly textured patterns. Regions were retained as motion responsive if they reached significance either in the group analysis or in the majority of hemispheres in single-subject analysis. They include well-known regions, such as V1, hMT/V5+, and hV3A, but also several occipito-temporal, occipito-parietal, parietal, and frontal regions. The time course of the activation was similar in most of these regions. Motion responses were nearly identical for binocular and monocular presentations. Flicker-induced-activation introduced a dichotomy amongst these motion responsive regions. Early occipital and occipito-temporal regions responded well to flicker, while flicker responses gradually vanished as one moved to occipito-parietal and then parietal regions. Finally, over a more than four-fold range, stimulus diameter had little effect on the motion activations, except in V1.

Adult↗
PubMed1999

Human cortical regions involved in extracting depth from motion.

We used functional magnetic resonance imaging (fMRI) to investigate brain regions involved in extracting three-dimensional structure from motion. A factorial design included two-dimensional and three-dimensional structures undergoing rigid and nonrigid motions. As predicted from monkey data, the human homolog of MT/V5 was significantly more active when subjects viewed three-dimensional (as opposed to two-dimensional) displays, irrespective of their rigidity. Human MT/V5+ (hMT/V5+) is part of a network with right hemisphere dominance involved in extracting depth from motion, including a lateral occipital region, five sites along the intraparietal sulcus (IPS), and two ventral occipital regions. Control experiments confirmed that this pattern of activation is most strongly correlated with perceived three-dimensional structure, in as much as it arises from motion and cannot be attributed to numerous two-dimensional image properties or to saliency.

Adolescent↗
PubMed1998

Functional MRI of the brain: localisation of eloquent cortex in focal brain lesion therapy.

The aim of this study was to assess the feasibility of functional MRI (fMRI) in a clinical environment on a large patient group, and to evaluate the pretherapeutic value of localisation of eloquent cortex. Forty patients with focal brain lesions of different origin were studied using fMRI. Functional information was obtained using motor, somatosensory, auditory and phonological stimuli depending on the localisation of the lesions. To obtain information about the spatial accuracy of fMRI, the results were compared with postoperative electrocortical stimulation. Two patients with secondary trigeminal neuralgia were scanned using a motor protocol and were implanted with an extradural plate electrode. Imaging was successful in 40 of 42 patients (including the 2 with trigeminal neuralgia). These patients were analysed for strength of activation, the relation of the lesion to activation sites and the presence of mass effect. The correlation between these data and surgical findings provided significant additional clinical information. Functional MRI can be accurately performed in patients with focal brain lesions using a dedicated approach. Functional MRI offers important clinical information as a contribution to a decrease in posttherapeutic morbidity. The accuracy of the technique can be confirmed by other modalities, including invasive cortical electrostimulation.

Adolescent↗
PubMed1998

Human cerebral activity evoked by motion reversal and motion onset. A PET study.

In this PET study, we have investigated the human brain activity evoked by a visual motion paradigm commonly used to measure motion-related visual evoked potentials (VEPs). Because standard PET activation studies have been performed with motion along four axes, we first determined the pattern of brain activation when motion was restricted to a single axis. Motion back and forward along a single horizontal axis compared with a static condition revealed weak differential activations in the cuneus and the parietal cortex. Human area MT/V5 (middle temporal area) was hardly activated at all in this subtraction. Additional functional MRI experiments proved that MT/V5 activity is significantly higher for motion along four axes than for motion along a single axis. Secondly, we attempted to isolate the pattern of brain activity related to the reversal of motion direction and to the onset of motion, i.e. two transient motion components commonly used in measuring motion-related VEPs. To that end, we added a continuous linear contrast modulation, that reached maximum contrast at reversal or onset of motion, and compared both conditions with a contrast-modulated static or continuous motion condition. Subtraction of the static random dot pattern condition from the single-axis motion reversal condition, both contrast-modulated, revealed three significant activations: the anterior parieto-occipital sulcus, the lateral sulcus and the anterior claustrum. Additional analysis showed that these activations were not due to motion appearance or disappearance, but were due to the combination of motion reversal and contrast modulation. Hence, these activations do not reflect the motion reversal transient per se. In order to isolate a metabolic response to the reversal transient per se, we used a conjunction analysis, which suggests that activity in human MT/V5, the cuneus and a parietal insular region could underlie the motion reversal VEP in our experiments. Subtraction of the static random dot pattern condition from the single-axis motion onset condition, both contrast-modulated, revealed a single significant activation in the posterior cingulate cortex. Although the significance of this activation is unclear, it adds further evidence for the visual function of this region.

Adult↗
PubMed1998

Human brain regions involved in direction discrimination.

To obtain further evidence for the functional specialization and task-dependent processing in the human visual system, we used positron emission tomography to compare regional cerebral blood flow in two direction discrimination tasks and four control tasks. The stimulus configuration, which was identical in all tasks, included the motion of a random dot pattern, dimming of a fixation point, and a tone burst. The discrimination tasks comprised the identification of motion direction and successive direction discrimination. The control tasks were motion detection, dimming detection, tone detection, and passive viewing. There was little difference in the activation patterns evoked by the three detection tasks except for decreased activity in the parietal cortex during the detection of a tone. Thus attention to a nonvisual stimulus modulated different visual cortical regions nonuniformly. Comparison of successive discrimination with motion detection yielded significant activation in the right fusiform gyrus, right lingual gyrus, right frontal operculum, left inferior frontal gyrus, and right thalamus. The fusiform and opercular activation sites persisted even after subtracting direction identification from successive discrimination, indicating their involvement in temporal comparison. Functional magnetic resonance imaging (fMRI) experiments confirmed the weak nature of the activation of human MT/V5 by successive direction discrimination but also indicated the involvement of an inferior satellite of human MT/V5. The fMRI experiments moreover confirmed the involvement of human V3A, lingual, and parietal regions in successive discrimination. Our results provide further evidence for the functional specialization of the human visual system because the cortical regions involved in direction discrimination partially differ from those involved in orientation discrimination. They also support the principle of task-dependent visual processing and indicate that the right fusiform gyrus participates in temporal comparison, irrespective of the stimulus attribute.

Acoustic Stimulation↗
PubMed1998

Functional magnetic resonance imaging (fMRI) visualises the brain at work.

Functional Magnetic Resonance Imaging (fMRI) is a recent MRI technique capable of visualising neuronal activity in humans in a non-invase way. The technique visualises the physiological changes in oxy- and deoxyhemoglobin concentration changes in small cortical blood vessels upon neuronal activation without the need for radiation or the administration of contrast media or radioactive tracers. The spatial accuracy of the technique is of the order of millimeters and the temporal resolution of the order of one second. The concept has captured the interest of neuroradiologists as well as neuroscientists, who now have a means to visualise their theories in human volunteers. In the clinical environment the non-invasive studies should aid neurosurgeons in adopting a safe course into the brain and assist neurologists in unraveling neurological hypotheses. This report describes the technical principals of fMRI and presents some of our clinical results on the mapping of several cortical functions, such as motor, auditory and language functions, in a large group of patients.

Brain↗
PubMed1997

FMRI studies of the supplementary motor area and the premotor cortex.

Brain activation patterns associated with three motor tasks, differing in the mode of movement selection, were studied in seven right-handed subjects, using functional magnetic resonance imaging (fMRI). The tasks consisted of sequences of finger movements in which the next finger was selected (i) according to a fixed sequence (FIX), (ii) in response to an external sensory cue (RAND), or (iii) on the basis of free, internal selection (SELF). Periods of hand relaxation (REST) alternating with the tasks served as a control. Functional maps resulting from comparison of the motor tasks with REST reveal activation in primary sensorimotor cortex, medial and lateral premotor areas, cingulate cortex, and parietal cortex. The task activation level, defined as the percentage MR signal increase for each task relative to REST, and the differential activation, defined as the percentage MR signal increase for RAND and SELF relative to FIX, were calculated in each area. All areas showed a higher activation level for RAND and SELF than for FIX. A significant difference in activation level or differential activation between SELF and RAND was found in the posterior part of the superior frontal sulcus, in a part of the premotor cortex on the lateral brain surface, in the anterior cingulate motor cortex, and in the posterior part of the superior parietal cortex. The high-resolution and single-subject approach, provided by fMRI, allowed the distinguishing of multiple foci in medial frontal areas, premotor cortex, and parietal cortex, reflecting the functional heterogeneity of these areas suggested by previous studies.

Adult↗
PubMed1997

Evaluation of manual vs semi-automated delineation of liver lesions on CT images.

In this paper we compare a semi-automated delineation method with totally manual delineation for area quantification, with respect to efficiency, quality, and intra- and interobserver variability. Liver lesions on 28 CT images were delineated by three observers, twice using completely manual delineation and twice using a semi-automated method. Quantitative comparisons were performed with respect to delineated area and time required for the delineation tasks. Subjective comparisons were performed with respect to efficiency and perceived quality of the semi-automated method. The areas obtained using semi-automated delineation were significantly smaller (11 %) than those obtained using totally manual delineation. Intraobserver and interobserver variability with the semi-automated method were approximately three times lower than with manual delineation. Efficiency of the semi-automated method was subjectively rated favorable, although further improvements are possible. With respect to quality, the semi-automated method was ranked better than the manual method in 73 % of cases.

Humans↗
PubMedDate not supplied

The kinetic occipital (KO) region in man: an fMRI study.

We used functional magnetic resonance imaging to explore, in individual subjects, the properties of the kinetic occipital (KO) region, which previous position emission tomography studies have shown to be involved in the processing of kinetic boundaries. The KO region was significantly activated in 23/25 subjects tested in the subtraction of uniform motion from kinetic gratings. The KO region is genuinely specialized for processing kinetic boundaries since it is significantly more activated by kinetic gratings than by luminance-defined gratings, uniform motion or transparent motion. This leaves only the kinetic boundaries, created by discontinuities in motion direction, as the specific stimulus aspect, activating the KO region. The KO region is anatomically and functionally distinct from areas MT/V5, V3 and V3A. It also has minimal overlap with the lateral occipital (LO) region. The selective activation of the KO region is robust and relatively immune to changes in stimulus size, spatial frequency and type of kinetic boundary. These results strongly argue for the view that the KO region is a new, separate, functional region in human occipital cortex.

Adolescent↗