Biomedical subjects

M Lassonde

Publications and source records attributed to M Lassonde.

At least 37 records · Page 2Linked to original sources

PubMed2000

Divided visuo-spatial attention systems with total and anterior callosotomy.

The role of the corpus callosum in the inter-hemispheric integration of the visuo-spatial attention system, was investigated in patients with a total callosotomy or with an anterior callosal section. Subjects produced simple reaction times (RTs) to visual targets shown to the left or right visual hemifield. Preceding the target by an interval of 500 ms, arrow cues predicting the target location were shown left and right of the point of ocular fixation. For a majority of total and anterior callosotomy patients, results with valid focused cues (both arrows pointing to the target location) and with divided-attention cues (arrows pointing away from fixation) did not differ and both conditions produced shorter RTs than with neutral cues (equal signs). In contrast, neurologically intact subjects showed equal RTs with divided-attention and neutral cues, whereas valid focused cues produced reduced RTs relative to neutral cues. These results indicate that most split-brains, in contrast to normal observers, are capable of directing their attention to left and right visual field locations simultaneously, and therefore that each cerebral hemisphere controls its own visuo-spatial attention mechanism.

Adult↗

PubMed2000

Absence of interhemispheric transfer of unilateral visuomotor learning in young children and individuals with agenesis of the corpus callosum.

This study was undertaken to investigate the role of the corpus callosum in interhemispheric transfer of unilateral visuomotor learning. In the first experiment, the cross-manual performance of 4 callosal agenesis participants was compared to that of 4 age- and IQ-matched controls. In the second experiment, normal children of different ages (6-7 and 11-12 years) and adults were submitted to the same task to assess the impact of callosal maturation on interhemispheric transfer. Participants had to make aiming movements from a starting position toward either a central or a lateral target on the same side as the hand used, while maintaining central fixation. Prior to training, a pretest was performed with the hand contralateral to the hand used during learning. Participants were then submitted to a posttest with the untrained hand. All participants learned the unilateral aiming task in the learning phase, as evidenced by a reduction in spatial errors with an increasing number of practice trials. However, acallosal participants and children aged 6 to 7 years failed to transfer the acquired skill from the trained to the untrained hemisphere. These findings suggest that interhemispheric transfer of visuomotor skills cannot be assumed by other structures in the case of agenesis or morphological immaturity of the corpus callosum. The results further indicate that unilateral visuomotor learning leads to the formation of a single, unihemispheric engram in the absence, whether functional or anatomical, of the corpus callosum.

Adolescent↗

PubMed2000

Neuropsychology of childhood epilepsy: pre- and postsurgical assessment.

Childhood epilepsy is one of the most prevalent forms of chronic and disabling childhood disorders. Because it disrupts brain maturation, it has long been thought to produce non-specific consequences such as mental deficiency and behavioral problems. However, advances in medical knowledge have shown that childhood epilepsy should not be considered as a single disorder, and it is now becoming apparent that various clinical entities have different cognitive expressions that yet need to be specified. The purpose of this paper is to provide an up-to-date analysis of this multi-faceted pathology. The first section is devoted to the characterization of the neuropsychological profile that accompanies focal epilepsies, as defined by the site of the epileptic process. We report the first group study of children with frontal lobe epilepsy. The results indicate that frontal lobe epilepsy produces symptoms (deficits of planning, attention and motor dexterity) that are akin to those found in frontal-lesioned adults. Similarly, like in adults, temporal lobe epilepsy produces memory impairment in children as well as behavioral and academic disturbances. Occipito-parietal lobe epilepsy is rare in children and its effects still need to be specified. The second section deals with the neuropsychological techniques used in presurgical evaluation. Finally, the various neurosurgical procedures that are increasingly being used as part of the arsenal of epilepsy treatment are described along with the neuropsychological findings that are associated with these interventions. It can be concluded that the beneficial effects of epilepsy surgery (callosotomy, hemispherectomy, temporal and extra-temporal resections) by far outweigh the few cognitive deficits that are occasionally reported following these interventions.

Adult↗

PubMed1999

Localization of moving sounds by hemispherectomized subjects.

The objective of the present study was to evaluate the localization abilities of the residual auditory structures in hemispherectomized subjects (Hs) during monaural and binaural listening. The monaural stimulus presentations were aimed at evaluating the relative contribution of crossed and uncrossed afferents to the perception of a moving auditory target. Three Hs and ten control subjects (Cs) were asked to localize a simulated moving sound source of fixed intensity presented on the horizontal plane. The moving stimulus was delivered randomly through 16 loudspeakers which were mounted at 10 degree intervals on a calibrated perimeter frame located inside an anechoic chamber. The apparent movement traveled in either direction over three different distances in the peri-central and lateral fields. Listeners had to report the movement trajectory by pointing with the index finger where they thought the beginning and the ending of the stimulus occurred on the perimeter. Hs were less accurate than Cs in the binaural condition, suggesting that the residual hemisphere and/or subcortical structures are not by themselves as efficient to accurately analyze motion as when the two hemispheres are present. In the monaural testing conditions, most of the subjects showed a prominent lateral displacement of the perceived localization towards the functional ear. However, all subjects, particularly the Cs, were able to perceive different lengths of trajectories although they did not discriminate the direction of the motion. For the Hs, the pattern of performance differed for each subject. One right Hs was more accurate for localizing the sources delivered on the side of the functional ear when relying on crossed-input. A left Hs was able to detect the appropriate side of the moving sound source even when it was presented on the side to the obstructed ear. The other left Hs who had developed normally until the age of 5, was more affected by the hemidecortication in both the binaural and the monaural listening conditions. These findings suggest that it is possible to detect moving sounds in the absence of binaural cues and without the integration carried out by both cerebral hemispheres. However it appears that interaural disparities and bihemispheric integration are required for finer analysis.

Acoustic Stimulation↗

PubMed1999

Callosal and cortical contribution to procedural learning.

Acallosal and callosotomized subjects usually show impairments on tasks requiring bilateral interdependent motor control. However, few studies have assessed the ability of these subjects to learn a skill that requires the simultaneous contribution of each hemisphere in its acquisition. The present study examined whether acallosal and callosotomized subjects could learn a visuomotor skill that involved a motor control from either both or a single hemisphere. Eleven adult patients, six acallosal and five callosotomized, participated in this study. Seven of these patients had epileptic foci located in the frontal and/or temporal areas and one of the acallosal patients showed bilateral prefrontal atrophy following surgical removal of an orbitofrontal cyst. The performance of the experimental subjects was compared with that of 11 matched control subjects, on a modified version of a serial reaction time task developed by Nissen and Bullemer (Cogn Psychol 1987; 19: 1-32). This skill acquisition task involved bimanual or unimanual key-pressing responses to a sequence of 10 visual stimuli that was repeated 160 times. A declarative memory task was then performed to assess explicit knowledge of the sequence. None of the experimental subjects learned the task in the bimanual condition. Patients with frontal epileptic foci or orbitofrontal damage also failed to learn the task in the unimanual condition when they were using the hand contralateral to the damaged hemisphere. All other subjects, including the acallosal and callosotomized patients with temporal foci, learned the visuomotor skill as well as their controls in the unimanual condition. In spite of the absence of transfer and interhemispheric integration of procedural learning, some of the acallosal and callosotomized patients were able to learn the sequence explicitly. These findings indicate that the corpus callosum and the frontal cortical areas are important for procedural learning of a visuomotor skill. They also confirm the dissociation described by Squire (Science 1986; 232: 1612-9 and J Cogn Neurosci 1992; 4: 232-43) between the declarative and procedural memory systems and extend this dissociation to processes involving simultaneous bihemispheric co-operation.

Adult↗

PubMed1998

Early-blind human subjects localize sound sources better than sighted subjects.

Do blind persons develop capacities of their remaining senses that exceed those of sighted individuals? Besides anecdotal suggestions, two views based on experimental studies have been advanced. The first proposes that blind individuals should be severely impaired, given that vision is essential to develop spatial concepts. The second suggests that compensation occurs through the remaining senses, allowing them to develop an accurate concept of space. Here we investigate how an ecologically critical function, namely three-dimensional spatial mapping, is carried out by early-blind individuals with or without residual vision. Subjects were tested under monaural and binaural listening conditions. We find that early-blind subjects can map the auditory environment with equal or better accuracy than sighted subjects. Furthermore, unlike sighted subjects, they can correctly localize sounds monaurally. Surprisingly, blind individuals with residual peripheral vision localized sounds less precisely than sighted or totally blind subjects, confirming that compensation varies according to the aetiology and extent of blindness. Our results resolve a long-standing controversy in that they provide behavioural evidence that totally blind individuals have better auditory ability than sighted subjects, enabling them to compensate for their loss of vision.

Adaptation, Physiological↗

PubMed1998

Interhemispheric disconnection syndrome in Alzheimer's disease.

It is commonly acknowledged that patients with Alzheimer's disease show memory and cognitive deficits that result from their cerebral histopathological abnormalities. We report new evidence showing that they also manifest deficits in interhemispheric integration of information, probably reflecting a corpus callosum dysfunction. Patients were given a battery of motor, somatosensory, and visual tests that had to be carried out by using either one or both hemispheres. Tasks were chosen such that subjects with Alzheimer's disease performed normally when using intrahemispheric processing. They, however, performed poorly when interhemispheric communication was required. This observation attests to the presence of a disconnection syndrome and suggests that these interhemispheric tasks can serve as diagnostic tools for the early assessment of their dementia.

Alzheimer Disease↗

PubMed1997

Unilateral and bilateral temperature comparisons in acallosal and split-brain subjects.

Therapeutic section of the corpus callosum in adult epileptic patients typically results in their incapacity to carry out interhemispheric comparisons of lateralized information. The fact that acallosal and early split-brain subjects display few of these symptoms when tested in the tactile modality has led to the suggestion that these patients may use ipsilateral projections of the somatosensory system more effectively. Compensation, however, is limited by the fact that the lemniscal pathway is strongly lateralized, especially for the distal parts of the body, where few ipsilaterally projecting fibres have been demonstrated. The pathway carrying temperature information has a larger ipsilateral component. Bilateral comparisons within the same hemisphere in subjects who are lacking the corpus callosum should be more common and the development of compensatory mechanisms in early-sectioned or acallosal subjects should be more likely. The objective of the present experiment was to evaluate differential thresholds for thermal stimuli applied on a number of regions either on the same side or on corresponding sites on opposite sides of the body. One subject callosotomized as an adult and one split-brain subject who underwent callosotomy in childhood, as well as three acallosal subjects, were compared to IQ-matched and normal-IQ control subjects. The fingers, forearm and trunk were tested. The comparison temperature was 30 degrees C and the other was varied in an ascending or descending fashion using a modified method of limits. Differential thresholds were similar for within- and between-side comparisons, and comparable to those of the IQ-matched subjects. The results indicate that comparisons involving temperature discrimination for stimuli applied to the two sides of the body do not require the integrity of the corpus callosum.

Adolescent↗

PubMed1997

Neuropsychological alterations after split-brain surgery.

Neuropsychological changes following corpus callosotomy depend on the degree of the callosal section, the portion sectioned and the patient's age at the time of the surgery. Anterior section frequently results in transient hemiparesis of the non-dominant leg and temporary difficulties in initiating speech. Posterior section is followed by disconnection symptoms in the sensory modalities which can be demonstrated when input is lateralized and one hemisphere is denied access to the information received by the other. Visual and tactile stimuli presented to the non-dominant hemisphere are no longer verbally identified due to disconnection from the language-dominant hemisphere. Total callosotomy additionally interrupts interhemispheric communication between the motor regions. This results in deficits in bimanual coordination and apraxia of the non-dominant hand to verbal commands. Some of the symptoms subside, probably due to increased use of ipsilateral sensory and motor pathways. Others are permanent. However, they are not disabling since unrestricted scanning of the environment ensures bilateral representation of sensory experience. Cognitive functions are frequently improved, although preexisting lateralized deficits may be exacerbated. Learning of new material is difficult for some patients with lateralized temporal lobe dysfunction in whom interhemispheric compensation is abolished by the surgery. Language deficits are observed mainly in patients with crossed dominance. Studies in children reveal that callosotomy performed before puberty is not followed by permanent disconnection deficits. This may be attributable to the greater neural plasticity of the immature brain.

Child↗

PubMed1997

Neuropsychological outcome of corpus callosotomy in children and adolescents.

The effect of corpus callosotomy on cognitive functioning, social adjustment and motor behavior was studied in 25 pediatric patients. In order to assess age-related differences in cerebral adjustment, the sample was divided into two groups, one group operated on before and the other after the age of 13 years. Pre- and postoperative data were collected from all patients by means of standardized neuropsychological tests. When possible, the children were also submitted to simple interhemispheric transfer tasks in the tactile modality. Results revealed that all patients benefited from the callosotomy, regardless of their age and their mental capacities at the time of the surgery. The greatest improvements were observed in social adjustment. Generally, neuropsychological outcome paralleled neurological outcome. In keeping with the plasticity hypothesis, the young group showed greater gains than the older group. The latter showed similar sequelae as the adult split-brain patients described in the literature. Number of years of uncontrolled seizures was not related to neuropsychological outcome. Higher IQ was associated with better outcome in the young group, mainly because this group was more severely retarded and had more to gain from the surgery than the older group. However, mental retardation should not be an exclusion criterion for corpus callosotomy in view of the potential benefit of the surgery for the patient and his/her family.

Adolescent↗

PubMed1996

Atypical hemispheric specialization in intellectual deficiency.

In order to determine the relation between hemispheric specialization for language and intellectual deficiency, two groups of lower-IQ subjects were compared to normal-IQ controls on word-dichotic listening tasks. Two conditions in which stimuli differed by their level of phonological complexity were used. Normal controls showed the expected right-ear advantage in both conditions. Moreover, they showed a greater magnitude of ear difference on the condition requiring higher-order phonological processing. In the mentally deficient group, almost half the subjects exhibited a left-ear advantage and they showed no difference between the two conditions in terms of the magnitude of ear difference. These results point to the presence of atypical hemispheric specialization in mentally deficient subjects.

Adolescent↗

PubMed1995

Extent and limits of callosal plasticity: presence of disconnection symptoms in callosal agenesis.

Although earlier studies have emphasized the absence of 'split-brain' symptoms in callosal agenesis patients, the notion of an 'asymptomatic' acallosal brain has lately been challenged. We report a number of findings that are indicative of an interruption of interhemispheric communication and integration in individuals lacking the corpus callosum. Several groups of patients with callosal pathology (acallosals, patients with commissurotomy or callosotomy, either complete or partial) were compared to matched controls. Interhemispheric transfer was tested in two different experiments involving pointing to a light source while maintaining central fixation. In the first experiment, a learning paradigm was used to measure transfer of a motor skill from the trained to the untrained hand. In the second experiment, subjects pointed to visual targets at different locations on a perimeter. Midline fusion, a recurrent theme when describing callosal function, was assessed using tasks which included depth perception with binocular and/or monocular cues, two-point discrimination thresholds and sound localization in the peri-central and lateral fields. Subjects with callosal pathology were impaired on all tasks involving transfer of motor and visuo-spatial skills and on some of the tasks requiring sensory integration of visual and tactile information across the body midline. We conclude that these functions require an intact corpus callosum since none of these deficits were seen in controls equated for IQ.

Adolescent↗

PubMed1994

Cognitive and sensori-motor functioning in the absence of the corpus callosum: neuropsychological studies in callosal agenesis and callosotomized patients.

The aim of the present study was to investigate the role of the corpus callosum in cognitive and sensori-motor functioning as measured by a neuropsychological test battery. After a brief review and analysis of the literature, we report our own studies in acallosal subjects (n = 9) and callosotomized patients (n = 25). The main instrument of evaluation was the Michigan Neuropsychological Test Battery. This battery was supplemented by age-appropriate intelligence tests. The performance of the acallosal group was compared to that of two matched control groups: one group consisting of children and adolescents that attended the same school as the acallosals and a second group of subjects recruited from regular schools. The callosotomized patients, tested pre- and post-operatively, served as their own controls. Taken together, the results of the reviewed and personal studies suggest that absence of the corpus callosum does not necessarily impede cognitive functioning. However, samples drawn from clinical populations tend to show a larger variability as to their mental abilities. In keeping with previous findings, our results indicate that the corpus callosum does play a role in bimanual motor coordination although other pathways (probably ipsilateral and/or subcortical) may provide adequate compensation in many cases. The data further suggest that the corpus callosum may be important for interhemispheric transfer of tactuo-motor learning when a spatial component is involved. Finally, our results are consistent with a facilitatory role of the corpus callosum in cognitive and sensori-motor functioning which allows for interhemispheric compensation as part of cerebral reorganization in the case of unilateral brain damage.

Adolescent↗

PubMed1994

Interhemispheric depth judgement.

Interhemispheric depth comparisons were studied by requiring subjects to align in depth two textured plates, one presented to the left hemifield and the other to the right. Callosal agenesis subjects and neurologically-normal control subjects adjusted the plates so that they appeared to be at the same distance. Subjects viewed the plates monocularly or binocularly while keeping their head still, moving it side-to-side or moving it up and down. Subjects fixated a target located between the two plates while performing the task. For all subjects, the results showed that the deviations from veridical settings were significantly smaller for the binocular than for the monocular viewing conditions. Moreover, there were no significant differences among the three binocular viewing conditions (horizontal, vertical or no head movement), indicating that neither vertical nor horizontal motion parallax improves the precision of depth judgement when binocular disparity is available. These results further suggest that the precision of interhemispheric comparison for binocular depth is not affected by the absence of the corpus callosum. Looking at the plates monocularly, the control subjects judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that motion parallax is a useful depth cue when relative motion is extracted from different hemifields. Unlike the control subjects, the callosal agenesis subjects did not judge the relative depth between the plates more precisely when they moved their head than when they kept it still. These results show that interhemispheric comparison of depth using relative motion is not possible without the corpus callosum.

Adult↗

PubMed1994

Sound localization in hemispherectomized patients.

In order to precisely evaluate the consequences of cortical damage on free-field sound localization in humans, the present study examined response accuracy to auditory targets in three hemispherectomized patients and IQ-matched controls. Listeners reported sound location by pointing with their dominant hand to the apparent sound location in an anechoic chamber. Two conditions were tested: (i) localization of a fixed-sound source and (ii) localization of the beginning and the end of a simulated moving stimulus. In both conditions, the responses of the patients were less accurate than those of the controls in the hemifield contralateral to their removed hemisphere. Moreover, the single-case analyses revealed that the performances obtained with fixed sources were generally more precise than those obtained with moving sources. This result is discussed in terms of a differential involvement of cortical and subcortical pathways in the processing of stationary and moving sounds. Finally, the age at surgery and the post-surgical interval were related with the magnitude of the deficits, suggesting the possible influences of functional reorganization and cerebral plasticity.

Adolescent↗

PubMed1994

Control of proximal and distal components of prehension in callosal agenesis.

Classic work with split-brain monkeys suggests that the reaching limb can be controlled by either cerebral hemisphere, but that finger control is largely crossed (Haaxma and Kuypers, 1974). Accordingly, one might predict that acallosal subjects should have little difficulty grasping objects presented in the visual field ipsilateral to the hand used, but should have great difficulty forming their grasp when reaching into crossed space. In the present study, we carried out a kinematic analysis of reaching and grasping movements executed by four acallosal subjects and four matched control subjects. Subjects maintained central fixation while reaching with either hand for objects placed in left, central and right space. Relative to controls, acallosal subjects took longer to complete reaches directed across the body midline, and spent more time decelerating. Moreover, unlike controls, their grip formation appeared to be impaired in all regions of space, although this deficit was most pronounced during reaches into crossed space. These results suggest that congenital absence of the corpus callosum is associated with deficits in the control of both the proximal and distal musculature.

Adolescent↗

PubMed1993

Somesthetic discrimination thresholds in the absence of the corpus callosum.

The aim of this study was to investigate how the absence of the corpus callosum affects somesthetic sensation on the axial midline and in proximal and distal body regions. For this purpose, two-point discrimination ability was evaluated in four acallosal subjects, four callosotomized subjects, six IQ-matched subjects and 10 control subjects with average and above average IQ. Sensory thresholds were established in the distal (index, palm), proximal (forearm), cranio-axial (forehead) and axial (dorsal trunk) body regions. The threshold was defined as the smallest separation at which the two points were perceived at a 70% accuracy level. Results showed that the thresholds of the acallosal and the callosotomized subjects were not significantly different from those of the IQ-matched control groups in the distal, proximal and cranio-axial body regions. However, thresholds in the dorsal trunk were significantly higher in the two experimental groups. It thus appears that the axial regions of the body that are normally densely represented in the corpus callosum function abnormally when this structure is absent or transected. Moreover, compensatory mechanisms normally seen in cases of early brain injury do not seem to apply in the present case since the acallosals showed the same impairments as the callosotomized subjects.

Adult↗

PubMed1993

Sound localization in acallosal human listeners.

In order to evaluate the callosal and hemispheric involvement in sound localization, the present study examined response accuracy to auditory targets in acallosal subjects. The primary interest was to determine whether the congenital absence of the corpus callosum affects auditory localization, especially for sounds situated near the midline of auditory space or moving across it. A corollary objective was to examine the possible existence of an hemispheric asymmetry on audio-spatial localization tasks. Four subjects with callosal agenesis paired to four age and IQ-matched controls and 16 normal control subjects were asked to locate broad band noise bursts at fixed intensity (52 dB sound pressure level) in the horizontal plane in an anechoic chamber. Broad band noise bursts were delivered randomly through 16 loudspeakers, which were mounted at approximately 10 degrees intervals on a perimeter frame. Two conditions were tested: (i) localization of a fixed-sound source; (ii) localization of the beginning and the end of a simulated moving stimulus. Two response modes were used. Listeners reported the apparent stimulus location either (i) by pointing with the ipsilateral index finger or (ii) by calling out the estimated angles indicated on the calibrated sound perimeter. Aiming accuracy was assessed by calculating the mean deviation of the response from the objective target position. The results indicated that the responses of the acallosal subjects were less accurate than those of the controls. The deficit was observed not only at the midline but throughout the auditory field. This points to possible compensatory mechanisms following the early absence of the corpus callosum which are, however, limited. The results obtained with manual pointing were generally more precise than those obtained through oral responses. This difference suggests that the remapping of spatial positions onto a verbally based coordinate system involves a supplementary cognitive step which affects the precision of the response. Comparing the performance to stimulus presentation in the left and right fields indicated that no hemispheric asymmetry was apparent under any of the conditions for either the acallosal subjects or the IQ-matched and normal control subjects.

Adolescent↗