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Biomedical subjects

Sylvie Chokron

Publications and source records attributed to Sylvie Chokron.

At least 19 recordsLinked to original sources
PubMed2007

Impaired orienting of attention in left unilateral neglect: a componential analysis.

Twenty-six patients suffering from damage to the right side of the brain, 19 of whom exhibited signs of left neglect, as well as 32 matched controls, ran 3 spatial cuing tasks. Patients were also tested with 2 cancellation tests, a line-bisection test, the copy of a complex drawing, and a visual extinction procedure. Results first showed correlations between extinction and cancellation tests performance on one hand, and between line bisection and copy on the other hand. Second, results demonstrated that an engagement deficit toward contralesional targets appeared to be the most striking feature of neglect, and the engagement score was correlated with the cancellation score and extinction. Most patients with neglect also presented a deficit in disengagement, a deficit of inhibition of return, and probably a deficit of alertness. Deficits in engagement and in disengagement, as well as poor scores in cancellation tests, seemed to be related with posterior cortical and subcortical lesions. Most important, even if an endogenous deficit (frequently related with a thalamic lesion) could aggravate the neglect behavior, neglect syndrome was mainly explained by a deficit of exogenous attention.

Aged↗
PubMed2006

Thalamocortical circuits: fMRI assessment of the pulvinar and medial dorsal nucleus in normal volunteers.

This fMRI study investigates the activation of the thalamic nuclei in a spatial focusing-of-attention task previously shown to activate the pulvinar with FDG-PET and assesses the connectivity of the thalamic nuclei with cortical areas. Normal right-handed subjects (eight men, eight women, average age=32 years) viewed four types of stimuli positioned to the right or left of the central fixation point (left hemifield-large letter, left hemifield-small letter display with flanking letters; right hemifield-large letter, right hemifield-small letter display with flankers). BOLD responses to small letters surrounded by flankers were compared with responses to large isolated letters. To examine maximum functional regional connectivity, we modeled "subject" as a random effect and attained fixed effect parameter estimates and t-statistics for functional connectivity between each of the thalamic nuclei (pulvinar, medial dorsal, and anterior) as the seed region and each non-seed voxel. Greater BOLD activation for letters surrounded by flankers than for large letters was observed in the pulvinar as anticipated and was also marked in the medial dorsal nucleus (MDN), anterior and superior cingulate (BA24 and BA24'), dorsolateral prefrontal cortex, and frontal operculum and insula. For the MDN, maximal functional connectivity was with the dorsolateral prefrontal cortex; correlations with left superior temporal, parietal, posterior frontal, and occipital regions were also observed. For the pulvinar, maximal functional connectivity was with parietal BA39; for anterior thalamus, with anterior cingulate.

Adult↗
PubMed2006

Effect of temporal constraints on hemispheric asymmetries during spatial frequency processing.

Studies on functional hemispheric asymmetries have suggested that the right vs. left hemisphere should be predominantly involved in low vs. high spatial frequency (SF) analysis, respectively. By manipulating exposure duration of filtered natural scene images, we examined whether the temporal characteristics of SF analysis (i.e., the temporal precedence of low on high spatial frequencies) may interfere with hemispheric specialization. Results showed the classical hemispheric specialization pattern for brief exposure duration and a trend to a right hemisphere advantage irrespective of the SF content for longer exposure duration. The present study suggests that the hemispheric specialization pattern for visual information processing should be considered as a dynamic system, wherein the superiority of one hemisphere over the other could change according to the level of temporal constraints: the higher the temporal constraints of the task, the more the hemispheres are specialized in SF processing.

Adult↗
PubMed2006

Neural correlates of spatial frequency processing: A neuropsychological approach.

We examined the neural correlates of spatial frequency (SF) processing through a gender and neuropsychological approach, using a recognition task of filtered (either in low spatial frequencies/LSF or high spatial frequencies/HSF) natural scene images. Experiment 1 provides evidence for hemispheric specialization in SF processing in men (the right hemisphere is predominantly involved in LSF analysis and the left in HSF analysis) but not in women. Experiment 2 aims to investigate the role of the right occipito-temporal cortex in LSF processing with a neurological female patient who had a focal lesion of this region due to an embolization of an arterioveinous malformation. This study was conducted 1 week before and 6 months after the surgical intervention. As expected, after the embolization, LSF scene recognition was more impaired than HSF scene recognition. These data support the hypothesis that the right occipito-temporal cortex might be preferentially specialized for LSF information processing and more generally suggest a hemispheric specialization in SF processing in females, although it is difficult to demonstrate in healthy women.

Adult↗
PubMed2005

Time to imagine space: a chronometric exploration of representational neglect.

When describing known places from memory, patients with left spatial neglect may mention more right- than left-sided items, thus showing representational, or imaginal, neglect. This suggests that these patients cannot either build or explore left locations in visual mental imagery. However, in place description there is no guarantee that patients are really employing visual mental imagery abilities, rather than verbal-propositional knowledge. Thus, patients providing symmetrical descriptions might be using other strategies than visual mental imagery. To address this issue, we devised a new test which strongly encourages the use of visual mental imagery. Twelve participants without brain damage and 12 right brain-damaged patients, of whom 7 had visual neglect, were invited to conjure up a visual mental image of the map of France. They subsequently had to state by pressing a left- or a right-sided key whether auditorily presented towns or regions were situated to the left or right of Paris on the imagined map. This provided measures of response time and accuracy for imagined locations. A further task, devised to assess response bias, used the words "left" or "right" as stimuli and the same keypress responses. Controls and non-neglect patients performed symmetrically. Neglect patients were slower for left than for right imagined locations. On single-case analysis, two patients with visual neglect had a greater response time asymmetry on the geographical task than predicted by the response bias task, but with symmetrical accuracy. The dissociation between response times and accuracy suggests that, in these patients, the left side of the mental map of space was not lost, but only "explored" less efficiently.

Adult↗
PubMed2004

Independent effects of endogenous and exogenous spatial cueing: inhibition of return at endogenously attended target locations.

Inhibition of return (IOR) is thought to reflect a bias against returning attention to previously attended locations. According to this view, IOR should occur only if attention is withdrawn from the target location prior to target appearance. In the present study, endogenous attention and exogenous cueing were manipulated orthogonally. IOR was observed both when a target appeared at an unexpected location, and when a target appeared at the expected location. A similar pattern of results was obtained in a reanalysis of data from a study with Neglect patients. These results suggest that IOR is independent of endogenous orienting.

Adult↗
PubMed2004

Active versus passive proprioceptive straight-ahead pointing in normal subjects.

Eighty blindfolded healthy female subjects participated in an active and a passive straight-ahead pointing task to study the estimation of the subjective sagittal middle in the presence or absence of an active haptic exploration. Subjects were to point straight-ahead with their left or right index finger starting from different right- or left-sided locations, while performing either an active movement or with the hand being passively moved by the examiner. Results showed: (i) a significant effect of the hand used only on the active straight-ahead pointing task performance; (ii) a significant effect of the starting position both on the active and passive pointing performance; (iii) a significant correlation between the two protocols only in terms of spatial bias (algebraic error) but not in terms of precision (absolute errors). These results are discussed with regards to normal and neuropsychological studies of the egocentric frame of reference.

Adult↗
PubMed2004

The role of vision in spatial representation.

A complex link exists between vision and unilateral spatial neglect (USN). Firstly, USN is not a perceptual deficit, secondly, USN is not necessarily accompanied by a visual deficit and finally, USN can be observed in non-visual modalities as well as in mental spatial imagery. This apparent supramodality of USN stands in sharp contrast to the fact that neglect signs are often more severe and more durable in the visual than in other sensory modalities (Chokron et al., 2002). The influence of vision on spatial representation has rarely been studied. In the present study we assessed six right brain-damaged patients suffering from left USN on two tasks involving spatial representations: a clock-drawing task and a drawing from memory task in two experimental conditions, with and without visual control. We confirm that even in mental imagery, the absence of visual feedback may decrease and even suppress left neglect signs (Bartolomeo and Chokron, 2001b; 2002). Since vision is largely involved in the orientation of attention in space, suppressing visual control could reduce the magnetic attraction towards the right ipsilesional hemispace and in this way could allow a re-orientation of attention towards the left neglected hemispace. We discuss the theoretical and therapeutic implications of these findings.

Adult↗
PubMed2004

Neglected attention in apparent spatial compression.

Halligan and Marshall [Cortex 27 (1991) 623] devised a new test to evaluate the hypothesis that in visual neglect, left space is systematically compressed rightwards. In the critical condition of the original study, rows of horizontally arranged numbers with a target arrow pointing to one of them from the opposite margin of the display were presented. When asked to verbally identify the number indicated by the arrow, a right brain-damaged patient with left neglect and hemianopia often indicated a number to the right of the target. The more the target was located on the left, the greater the response shift rightward, as if rightward compression were linearly proportional to the co-ordinates of Euclidian space. However, a possible alternative account could be that the patient's attention was attracted by the numbers located to the right of the target digit, thus biasing her responses toward numbers on the right. To explore this hypothesis, we asked normal participants and patients with right hemisphere lesions, with and without neglect or hemianopia, to mark on the margin of a sheet the approximate location indicated by an arrow situated on the opposite margin. In three different conditions, the arrow indicated either one of several numbers or lines in a row, or a blank location on the sheet margin. Only patients with left neglect, and especially those with associated hemianopia, deviated rightward, and then crucially only on those conditions where visible targets were present, consistent with the attentional bias account.

Adult↗
PubMed2003

Right parietal lesions, unilateral spatial neglect, and the egocentric frame of reference.

Recently, the hypothesis has been proposed that the crucial mechanism leading to neglect is the disturbed transformation of sensory input into a supramodal egocentric frame of reference (ER), which causes in turn a deviation of this reference frame toward the side ipsilateral to the brain lesion. This egocentric coordinate system is normally centered on the midsagittal plane, but a unilateral brain lesion may cause a deviation of the egocentric reference due to an imbalance between the differentially lateralized neural processes which build this representation. Although neglect signs are often defined in an egocentric frame, and an ER shift may be observed in some right-brain-damaged patients (RBD), I present here several data that rule out any causal link between the deviation of the egocentric reference and the presence and/or severity of left-neglect signs in RBD patients.

Brain↗
PubMed2003

Selective attention, inhibition for repeated events and hemispheric specialization.

When two visual events appear consecutively in the same spatial location, our response to the second event is slower than to the first. This inhibition for repeated events may reflect a bias toward sampling novel locations, a bias useful for exploring visual space. It has been shown that the left hemisphere is more specialized in selective attentional processes than the right one. The aim of the present experiment was to test if this hemispheric specialization for selective attention may also affect the inhibition for repeated events. For this purpose, we asked 11 normal subjects to perform an identity-based discrimination task in which the target to be detected could appear alone or surrounded by flanking letters, in the left or in the right visual field. Results show that inhibition for repeated events is present only when selective attention is required and when the task is performed in the right specialized visual field.

Adult↗
PubMed2003

Hemispheric specialization for spatial frequency processing in the analysis of natural scenes.

Experimental data coming from visual cognitive sciences suggest that visual analysis starts with a parallel extraction of different visual attributes at different scales/frequencies. Neuropsychological and functional imagery data have suggested that each hemisphere (at the level of temporo-parietal junctions-TPJ) could play a key role in spatial frequency processing: The right TPJ should predominantly be involved in low spatial frequency (LFs) analysis and the left TPJ in high spatial frequency (HFs) analysis. Nevertheless, this functional hypothesis had been inferred from data obtained when using the hierarchical form paradigm, without any explicit spatial frequency manipulation per se. The aims of this research are (i) to investigate, in healthy subjects, the hemispheric asymmetry hypothesis with an explicit manipulation of spatial frequencies of natural scenes and (ii) to examine whether the 'precedence effect' (the relative rapidity of LFs and HFs processing) depends on the visual field of scene presentation or not. For this purpose, participants were to identify either non-filtered or LFs and HFs filtered target scene displayed either in the left, central, or right visual field. Results showed a hemispheric specialization for spatial frequency processing and different 'precedence effects' depending on the visual field of presentation.

Cognition↗
PubMed2002

Visually- and motor-based knowledge of letters: evidence from a pure alexic patient.

We describe a patient, VSB, whose reading was impaired as a consequence of a left temporal-parietal lesion, whereas writing was relatively preserved. At variance with other pure alexic patients described in the literature, VSB claimed to have become unable to mentally visualise letters and words. Indeed, his performance on a series of tests tapping visual mental imagery for orthographic material was severely impaired. However, performance on the same tests was dramatically ameliorated by allowing VSB to trace each item with his finger. Visual mental imagery for non-orthographic items was comparatively spared. The pattern of dissociation shown by VSB between impaired visual mental imagery and relatively preserved motor-based knowledge for orthographic material lends support to the view that separate codes, respectively based on visual appearance and on motor engrams, may be used to access knowledge of the visual form of letters and words.

Alexia, Pure↗
PubMed2002

Visual, proprioceptive and tactile performance in left neglect patients.

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect. For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks. In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect. These results are discussed with regard to the egocentric and attentional hypothesis of neglect.

Aged↗
PubMed2002

Orienting of attention in left unilateral neglect.

After right posterior brain damage, patients may ignore events occurring on their left, a condition known as unilateral neglect. Although deficits at different levels of impairment may be at work in different patients, the frequency and severity of attentional problems in neglect patients have been repeatedly underlined. Recent advances in the knowledge of the mechanisms of spatial attention in normals may help characterizing these deficits. The present review focuses on studies exploring several aspect of attentional processing in unilateral neglect, with particular reference to the dichotomy between 'exogenous', or stimulus-related, and 'endogenous', or strategy-driven, orienting of attention. A large amount of neuropsychological evidence suggests that a basic mechanism leading to left neglect behavior is an impaired exogenous orienting toward left-sided targets. In contrast, endogenous processes seem to be relatively preserved, if slowed, in left unilateral neglect. Other component deficits, such as a general slowing of the operations of spatial attention, might contribute to neglect behavior. These results are presented and discussed, and their implications for hemispheric specialization in attentional orienting and for the mechanisms of visual consciousness are explored.

Attention↗
PubMedDate not supplied

The influence of handedness on profile and line drawing directionality in children, young, and older normal adults.

One hundred twenty-two normal subjects--children, young and older adults (77 right-handers and 45 left-handers)--performed two drawing tasks in order to study their directionality trends. The first task consisted of drawing the profile of a face and in the second task subjects had to link two points in order to draw a line. The analysis of the drawing directionality revealed a significant effect of the age when right-handers drew profiles whereas this effect reached significance in left-handed subjects only when drawing lines. Left- and right-handers' performances across the two tasks were found to significantly differ. Results are discussed in terms of hemispheric and environmental factors.

Adult↗
PubMedDate not supplied

Effect of gaze orientation on tactilo-kinesthetic performance.

Twenty-two normal right-handed subjects indicated with their index finger the midpoint of a horizontal rod that they could not see. Subjects performed this task while directing their gaze either centrally or toward four different locations (5 degrees or 30 degrees to the left or to the right of the midline). Results showed an overall leftward bias in rod bisection, which increased when subjects used their right hand and fixated a right-sided visual target. Thus, orienting of gaze can affect a nonvisual, tactilo-kinesthetic spatial task. The possible mechanisms of this interaction are discussed with respect to activation-orienting theories egocentric hypotheses and directional trends.

Adult↗