The superior colliculus subserves interhemispheric neural summation in both normals and patients with a total section or agenesis of the corpus callosum

To verify the possibility that the superior colliculus (SC) subserves interhemispheric neural summation, we presented single or double white visual targets to one or both hemifields in normal participants and in patients lacking the corpus callosum (one with total callosotomy and one with callosal agenesis). Simple reaction time was typically faster with double than single stimuli, a phenomenon known as the redundant target effect (RTE); moreover, confirming previous results, we found a larger RTE in patients without callosum than in normals. In both groups, the redundancy gain was related to neural coactivation rather than to probability summation. The novel finding was that, when using monochromatic purple stimuli that are invisible to the SC, we found a similar redundancy gain in both groups; moreover, this redundancy gain was probabilistic rather than neural. Control experiments with monochromatic red stimuli yielded a RTE of the neural type similar to that with white stimuli and this confirmed that the probabilistic RTE found was specific for purple stimuli. In conclusion, visual input to the SC is necessary for interhemispheric neural summation in both normals and in individuals without the corpus callosum while probabilistic summation can occur without a collicular contribution.

[Corpus callosum, interhemispheric interaction and brain right hemisphere function]

A complex clinico-neuropsychological study, using A.R. Luria's method, has been conducted before and after surgery in 36 patients with arteriovenous malformations of the corpus callosum. Symptoms of local lesions in different compartments of the corpus callosum are described. The symptoms of the partial corpus callosum lesion are shown to be modally specific, but only to a certain extent. Isolated appearance of dyscopia or dysgraphia or symptoms is possible. A combination of lesions in the medial brain areas (cingulated gyrus, frontal lobes) with corpus callosum malformation significantly augmented their dysfunction. In case of impairment in frontal parts of the corpus callosum, symptoms of frontal lobes dysfunction were observed. The corpus callosum malformations resulted in right hemisphere dysfunction in the sphere of emotions, perception and spatial activity. As it was shown earlier, right hemisphere integrates impulses from both sides of the space, being first involved in activity, realizing its initial stages. The author suggests that such a synthetic right hemisphere activity, in the presence of closed relations to "conscious" left hemisphere, is necessary for formation of the whole picture of the separate objects, as well as of certain types of activity. In this view, right hemisphere may be relatively considered as a dominant one but not vice versa.

The endocrinology of hypothalamic hamartoma surgery for intractable epilepsy

Intractable epilepsy has replaced central precocious puberty (CPP) as the main indication for surgery in patients with hypothalamic hamartoma (HH). However, concern about endocrine complications and the paucity of published endocrine data may dissuade clinicians from recommending HH surgery. We report the preoperative endocrine status and postoperative endocrine findings of patients undergoing HH surgery at our centre. Twenty-nine patients aged 4-23 years (mean 10 years) underwent detailed clinical assessment and biochemical testing of the hypothalamic-pituitary axis before and after transcallosal resection of their HH. The perioperative evaluation included comprehensive evaluation of pubertal status, growth, weight, thyroid and adrenal function, and osmoregulation. Forty-five percent of patients had CPP at presentation and this was not altered by HH surgery. Asymptomatic deficiencies in thyroid hormone, growth hormone and cortisol response were identified in several patients prior to surgery, and biochemical CPP was present in four, clinically prepubertal children. Free thyroxine fell after surgery in the majority, and to clinically significant levels prompting treatment in 5 patients. Low growth hormone was present in 5/8 patients who had had previous HH surgery and in 6/29 following transcallosal surgery at our centre; short stature did not result during the period of follow-up. Hypernatraemia developed in most patients postoperatively with sodium >150 mmol/L seen in 16 (55%) patients; however, this was asymptomatic, not often associated with polyuria, and transient; no patient required ongoing antidiuretic hormone replacement. Appetite stimulation and early postoperative weight gain occurred in 45% patients, but resolved in half. Disturbance of endocrine function may be clinically silent and should be routinely evaluated prior to HH surgery for intractable epilepsy. Following surgery, hypernatraemia, low thyroxine, low growth hormone, and weight gain are the main endocrine problems encountered. Prior, unsuccessful surgery may be a risk factor for endocrinopathy. Except for weight gain in some patients, these postoperative endocrine disturbances appear to be transient, mild or asymptomatic, and easily treated where necessary. Long term follow-up of growth and sexual development in a large series of patients is required.

Transcallosal resection of hypothalamic hamartomas in patients with intractable epilepsy

A variety of surgical treatments for intractable epilepsy with hypothalamic hamartoma (HH) are described, although most are derived from limited patient experience gathered from several centres. We describe the results of transcallosal resection of HH in 29 consecutive patients undergoing surgery at one centre. Twenty-nine patients aged 4-23 years (mean 10 years) underwent HH surgery with a minimum of 12 months follow-up. A comprehensive, presurgical epilepsy evaluation, supplemented with endocrine and ophthalmological assessments was performed in all cases. HH were resected via a transcallosal, interforniceal approach to the third ventricle, with the assistance of frameless stereotaxy, limiting the resection to the margins of the third ventricular walls and floor and minimising traction and diathermy. Complete or near-complete (>95%) resection of the HH was achieved in 18/29 patients, 75-95% resection was achieved in seven patients (four of whom had complete or near-complete disconnection of residual HH) and less than 50% resection was achieved in four. Postoperatively (follow-up 12-70 months, mean 30 months), 15 became seizure-free (nine off antiepileptic medication), seven had >90% reduction in seizure frequency, three had 55-80% reduction in seizure frequency, and four had less than 40% reduction in seizure frequency. Of 16 patients who had seizures in the early postoperative period, six became seizure. No patient or lesion characteristics were associated with postoperative seizure freedom, including features of symptomatic generalised epilepsy. Neurobehavioural improvement and resolution of EEG abnormalities were seen in the majority. Complications were transient hemiparesis in two, transient hypernatraemia in 17, short-term memory impairment in 14 (persistent in four), weight gain in ten (persistent in five), need for supplemental thyroxine in five, and lowered growth hormone (uncertain clinical significance) in six. Transcallosal resection of HH is an effective treatment for intractable epilepsy, with 76% patients in our seizures being seizure-free or having >90% seizure reduction. The operative risks include stroke, short-term memory disturbance, weight gain and minor endocrine disturbances. Based on published data, the transcallosal approach appears to be safer and more effective than other operative strategies.

Resective surgery for hypothalamic hamartoma

Hypothalamic hamartoma presents with precocious puberty, epilepsy or both. There are two epileptic syndromes, one presenting initially in infancy with gelastic seizures evolving rapidly into a syndrome with multiple seizures, developmental delay and a moderate to severe behaviour disorder. The other presents later with a milder epileptic syndrome, again usually including gelastic seizures, but with normal intellect and behaviour. Magnetic resonance imaging identifies and gives a detailed anatomical picture of these lesions. Direct surgery, using microsurgical techniques and neuronavigation guidance has been used for these lesions. Three surgical approaches have been used, one lateral pterional, another midline frontal through the lamina terminalis and a third is a transcallosal interforniceal approach. In addition a disconnection procedure, usually pterional, aims to disconnect the lesion without the risks of major resection. The transcallosal interforniceal approach is the most successful with 69% of patients seizure-free. There are complications in about 24% of patients, the same as other approaches, but the complications are milder and include fewer neurological deficits than the other routes. Alternate strategies include stereotactic radiosurgery and radiofrequency ablation under stereotactic control.

Callosal lesions and behavior: history and modern concepts

Callosotomy has played a unique role in the treatment of epilepsy and in the understanding of human brain function. The pioneering work of Dejerine and Liepmann presenting the first findings of callosal lesion pathology at the turn of the 20th century was accepted but then quickly forgotten. Two schools resurrected the phoenix of callosal syndromes: Roger Sperry and Michael Gazzaniga leading in experimental neuroscience, and Norman Geschwind leading in clinical neurology. Callosotomy remains an effective technique to treat atonic, tonic, and tonic-clonic seizures, especially in patients with symptomatic generalized epilepsies such as Lennox-Gastaut syndrome. Neurologic, cognitive, and behavioral complications limit its use given that precise characterization of these complications as well as their frequency is difficult. The high frequencies of developmental delays, severe seizures, head injuries, antiepileptic drug burden, and other factors limit the ability to attribute a specific change to surgical intervention, since surgery can change multiple factors. For example, subtle behavioral changes in executive function and personality are difficult to delineate in a population with preexisting neurologic and psychiatric disorders. Despite this, a clearer picture of the effects of callosotomy, as defined by clinical neurology and neuropsychology as well as cognitive neuroscience, is emerging.

Hypothalamic hamartoma with refractory epilepsy: surgical procedures and results in 18 patients

AIM

To study the surgical procedures and results on seizures, in 18 patients with refractory epilepsy due to hypothalamic hamartoma.

PATIENTS AND METHODS

Eighteen patients aged from 9 months to 32 years underwent surgery between 1997 and 2002. The mean age at seizure onset was 15.5 months. Seventeen patients had gelastic seizures, 14 had partial seizures, two had infantile spasms, 10 had tonic or atonic seizures and three had generalized seizures. The mean seizure frequency was 21 per day. Four patients had borderline intellectual disability and the others were mentally retarded. Five patients had precocious puberty, one had acromegaly and four suffered from obesity. Brain MRI, performed at least twice in each patient showed the hamartoma as a stable, homogeneous interpeduncular mass implanted either on the mammillary tubercle or on the wall of the third ventricle, with variable extension to the bottom. Ictal SPECT, performed in four patients, showed hyperperfusion within the hamartoma in two.

SURGICAL PROCEDURE

Twenty-six operations were performed in 18 patients. The first patient underwent a total removal of the hamartoma, whereas the following patients underwent a disconnection, either through open surgery (14 patients) or endoscopy (10 patients).

RESULTS

Regarding the seizure outcome with a mean follow up of three years four months (one year to 4.5 years), nine patients are seizure-free, one patient has only brief gelastic seizures and eight are dramatically improved. Surgery was uncomplicated in all but two patients: one had transitory hemiplegia and paresis of the third cranial nerve, the other presented with hemiplegia due to ischemia of the middle cerebral artery territory. The quality of life, behavior and school performance were greatly improved in most of the patients.

CONCLUSION

Our series illustrates the feasibility and relative safety of disconnecting surgery of hypothalamic hamartomas, with seizure relief in 50% and a dramatic improvement in the others. Endoscopic disconnection seems to be a very safe way to treat the intra-ventricular part of hamartomas.

The neural substrates of visual implicit memory: do the two hemispheres play different roles?

Identification of visually presented words is facilitated by implicit memory, or visual priming, for past visual experiences with those words. There is disagreement over the neuro-anatomical substrates of this form of implicit memory. Several studies have suggested that this form of priming relies on a visual word-form system localized in the right occipital lobe, whereas other studies have indicated that both hemispheres are equally involved. The discrepancies may be related to the types of priming tasks that have been used because the former studies have relied primarily on word-stem completion tasks and the latter on tasks like word-fragment completion. The present experiments compared word-fragment and word-stem measurements of visual implicit memory in patients with right occipital lobe lesions and patients with complete callosotomies. The patients showed normal visual implicit memory on fragment completion tests, but essentially no visual priming on standard stem completion tests. However, when we used a set of word stems that had only one correct solution for each test item, as was true of the items in the fragment completion tests, the patients showed normal priming effects. The results indicate that visual implicit memory for words is not solely dependent upon the right hemisphere, rather it reflects changes in processing efficiency in bilateral visual regions involved in the initial processing of the items. However, under conditions of high lexical competition (i.e., multiple completion word stems), the lexical processes, which are dominant in the left hemisphere, overshadow the visual priming supported by the left hemisphere.

Self-face processing in a callosotomy patient

Self-face recognition is reserved for humans, apes and possibly dolphins and is thought to be a marker of self-awareness. Previous data have indicated that self-face recognition may be mediated via frontal right hemisphere circuits within the brain. Testing patient M.L., who underwent a total callosotomy, we found that when searching for the self-face in a series of morphs (composite facial images made up of his own and a famous face in inversely varying percentages) the patient made (i) more true-positive and (ii) fewer false-positive responses when responding with the right hemisphere (i.e. indicating with the left hand). There was no hand difference when searching for familiar faces. These data imply a right hemisphere processing advantage for self-faces.

Cortical and subcortical contributions to the representation of temporal information

Converging evidence suggests that temporal representations of brief durations are derived subcortically. We tested split-brain patient JW in order to investigate whether these representations project bilaterally or unilaterally to cortex. Using visual stimuli to signal time intervals, JW was asked to compare the duration of a pair of standard stimuli that were presented bilaterally with a comparison stimulus that was presented to either the left or right visual field. Assuming the hand of response is controlled by the contralateral cerebral hemisphere, a hand by visual field interaction was predicted if the representation of stimulus duration was restricted to the cerebral hemisphere receiving the lateralized stimulus. However, we failed to observe this interaction for two different ranges of stimulus durations, both in the hundred (Experiment 2) to hundreds (Experiment 1) of milliseconds range. Instead, there was a consistent right hemisphere advantage in task performance. When the task then required a discrimination based on the physical size of the stimuli rather than their duration, an interaction between response hand and visual field was obtained (Experiment 3). Taken together, these results suggest that (1) even though the comparison stimulus was presented unilaterally, the representation of its duration was available to both cerebral hemispheres, and (2) a right hemisphere advantage in psychophysical tasks requiring the comparison of successive stimuli is observed for temporal and non-temporal judgments.

Visual consciousness in health and disease

Conscious experience is an essential part of normal human life and interaction with the environment. Yet the nature of consciousness and conscious perception remains a mystery. Because of its subjective nature, consciousness has been difficult to investigate scientifically, but clues have been gained through studies involving patients with cortical lesions. During the past decade, the development of event-related fMRI has provided insights into aspects of conscious perception in control subjects and patients with cortical lesions by correlating awareness and performance with neural activity during visual tasks. This article reviews how recent research has advanced understanding of conscious perception, its relationship to neural activity and visual performance, and how this relationship can be altered by visual dysfunction. It also presents recent research about how conscious awareness of vision might be represented at a neural level in the central nervous system.

Split-brain patients neglect left personal space during right-handed gestures

Since some patients with right hemisphere damage or with spontaneous callosal disconnection neglect the left half of space, it has been suggested that the left cerebral hemisphere predominantly attends to the right half of space. However, clinical investigations of patients having undergone surgical callosal section have not shown neglect when the hemispheres are tested separately. These observations question the validity of theoretical models that propose a left hemispheric specialisation for attending to the right half of space. The present study aims to investigate neglect and the use of space by either hand in gestural demonstrations in three split-brain patients as compared to five patients with partial callosotomy and 11 healthy subjects. Subjects were asked to demonstrate with precise gestures and without speaking the content of animated scenes with two moving objects. The results show that in the absence of primary perceptual or representational neglect, split-brain patients neglect left personal space in right-handed gestural demonstrations. Since this neglect of left personal space cannot be explained by directional or spatial akinesia, it is suggested that it originates at the conceptual level, where the spatial coordinates for right-hand gestures are planned. The present findings are at odds with the position that the separate left hemisphere possesses adequate mechanisms for acting in both halves of space and neglect results from right hemisphere suppression of this potential. Rather, the results provide support for theoretical models that consider the left hemisphere as specialised for processing the right half of space during the execution of descriptive gestures.

Transcortical or transcallosal approach to ventricle-associated lesions: a clinical study on the prognostic role of surgical approach

Most entities in and around the anterior two-thirds of the supratentorial ventricles can be reached via transcortical or transcallosal approach. This study examined the effect of surgical approach on the postoperative neurological outcome. Thirty-eight patients with intra- and periventricular supratentorial lesions were operated on by either frontal transcortical or anterior transcallosal approach. Postoperative diencephalic damage occurred in 22% of patients in the transcortical group and in 36% in the transcallosal group; transient mutism was virtually equivalent in the two groups. Postoperative epilepsy (26%) and subdural fluid collections (30%) occurred only in the transcortical group. The incidence of postoperative hemiparesis was higher in the transcallosal group. There was a high correlation between postoperative Glasgow Outcome Score of 5 and preoperative severity of neurological disease but no correlation between postoperative Glasgow Outcome Score of 5 and location of the lesion or between postoperative clinical course and surgical approach. Surgical outcome of ventricle-associated lesions depends mainly on the severity of preoperative symptoms and not on surgical approach. Additionally, the incidence of postoperative seizures and subdural fluid collections after transcortical surgery is high.

Quantitative analysis of cortical pyramidal neurons after corpus callosotomy

This study quantitatively explored the dendritic/spine extent of supragranular pyramidal neurons across several cortical areas in two adult male subjects who had undergone a callosotomy several decades before death. In all cortical areas, there were numerous atypical, supragranular pyramidal neurons with elongated "tap root" basilar dendrites. These atypical cells could be associated with an underlying epileptic condition and/or could represent a compensatory mechanism in response to deafferentation after callosotomy.

[Complications during epilepsy surgery. Experience after 102 interventions between 1997 and 2001]

OBJECTIVES

To describe perioperative complications in different approaches to surgery for epilepsy.

PATIENTS AND METHODS

All patients undergoing surgery related to epilepsy between January 1997 and December 2001 were studied. We gathered information on patient characteristics, diagnosis, anticonvulsant treatment, surgical procedure, type of anesthesia and perioperative complications.

RESULTS

Ninety-eighty patients underwent 90 procedures under general anesthesia and 12 procedures with local anesthesia and sedation. Surgery was for anteromedial temporal resection in 74 patients, electrode implantation through the foramen ovale in 10 patients, extratemporal excisions in 7 patients, callosotomy in 3 patients, functional hemispherectomy in 3 patients, implantation of electrode grids in 2 patients, and craniotomy with an awake patient in 3 cases. Complications related to the surgical procedure were intraoperative bradycardia (5 cases), dural tension at the start of surgery (3), bleeding (2) and seizure (1). Complications related to anesthesia were bronchospasm (2 cases), histamine-releasing reaction upon administration of the muscle relaxant (1), and difficult intubation (1). During recovery we saw 1 case of postoperative aphasia, 1 of polyuria, 1 of pulmonary condensation, and 1 of factor VII deficit requiring plasma transfusion.

CONCLUSIONS

The rate of perioperative complications in surgery for drug-resistant epilepsy is low, the most common complication being self-limiting bradycardia related to surgical maneuvers.

Alternative surgical procedures to help drug-resistant epilepsy - a review

The concepts of pathophysiology of epilepsy which underly the non-resective surgical treatment of epilepsy are reviewed. The available techniques, lesioning, disconnection and stimulation are described and reviewed critically. Stereotactic lesioning, popular in the 1950's has been largely abandoned but stereotactic radiosurgery emerges as a useful technique, especially in the treatment of mesial temporal sclerosis. Disconnection by callosotomy has fewer applications than previously and multiple subpial transection (MST) has limited applications. Stimulation is a technique with increasing usefulness. Vagus nerve stimulation(VNS) is an accepted method of treatment with low morbidity and mortality, which improves seizure control in at least 30% of patients, together with concomitant improvements in QOL and economic advantages. Stimulation of deep brain targets in the thalamus, subthalamus and mesial temporal structures is practical. There are indications that this improves seizure control in groups of patients previously un helped by surgery, and this methodology has enormous potential.

Validation of a near-infrared probe for detection of thin intracranial white matter structures

OBJECT

The authors have developed an intracranial near-infrared (NIR) probe that analyzes the scattering of light emitted from its tip to measure the optical properties of cerebral tissue. Despite its success in distinguishing graymatter from white matter in humans during stereotactic surgery, the limits of this instrument's resolution remain unclear. In this study, the authors determined the spatial resolution of this new probe by using a rodent model supplemented with phantom measurements and computer simulation.

METHODS

A phantom consisting of Intralipid and gelatin was constructed to resemble a layer of white matter overlying a layer of gray matter. Near-infrared measurements were obtained as the probe was inserted through the gray-white matter transition. A computer simulation of NIR measurements through a gray-white matter transition was also performed using Monte Carlo techniques. The NIR probe was then used to study 19 tracks from the cortical surface through the corpus callosum in an in vivo rodent preparation. The animals were killed and histological sections through the tracks were obtained. Data from the phantom models and computer simulations showed that the NIR probe samples a volume of tissue extending 1 to 1.5 mm in front of the probe tip (this distance is termed the "lookthrough" distance). Measurements obtained from an NIR probe passing through a thin layer of white matter consisted of an initial segment of increasing values, a maximum (peak) value, and a trailing segment of decreasing values. The length of the initial segment is the lookthrough distance, the position of the peak indicates the location of the superficial white matter boundary, and the length of the trailing segment is the thickness of the layer. These considerations were confirmed in experiments with rodents. All tracks passed through the corpus callosum, which was demonstrated as a broad peak on each NIR graph. The position of the dorsal boundary of the corpus callosum and its width (based on histological measurements) correlated well with the peak of the NIR curve and its trailing segment, respectively. The initial segments correlated well with estimates of the lookthrough distance. Five of the tracks transected the smaller anterior commissure (diameter 0.2 mm), producing a narrow NIR peak at the correct depth.

CONCLUSIONS

Data in this study confirm that the NIR probe can reliably detect and measure the thickness of layers of white matter as thin as 0.2 mm. Such resolution should be adequate to detect larger structures of interest encountered during stereotactic surgery in humans.

Interhemispheric neural summation in the split brain: effects of stimulus colour and task

Four split-brained subjects, two subjects with agenesis of the corpus callosum, and 14 normal subjects performed two tasks requiring responses to red or green disks, briefly presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. In Experiment 1, simple reaction times to these stimuli, regardless of colour, were recorded (the Go-Both Task), and found to be faster to bilateral-redundant stimulus pairs, than to single stimuli. This so-called "redundancy gain" was much larger for acallosal or split-brained subjects than for normal subjects and exceeded the predictions of a race model, implying neural summation. Experiment 2 used the same stimuli, but subjects were required to respond only to stimuli of a designated colour (the Go/No-Go Task). Redundant target stimuli produced neural summation, while stimuli pairs that included a non-target stimulus did not. These results suggest that neural summation in the acallosal or split brain involves the convergence of response-associated activation, and that redundant sensory processes are not sufficient.

Surgical treatment of multifocal epilepsy involving eloquent cortex

PURPOSE

This report describes our long-term follow-up for combined resective surgery and multiple subpial transections (MSTs) in patients with refractory epilepsy involving eloquent and noneloquent cortex in multiple lobes. Multiple independent seizure foci made these patients poor candidates for conventional surgery.

METHODS

MST and resective surgery were used in 13 patients to treat localization-related refractory epilepsy involving eloquent and noneloquent cortex of two or more lobes. Preoperative investigation was followed by invasive monitoring.

RESULTS

Eleven patients had MST plus resection involving two different lobes, and two patients had MST plus resection involving three different lobes. MSTs were performed on the primary sensorimotor cortex (eight patients), temporal language area (two patients), Broca's area (one patient), and on both frontal motor and temporal language areas (two patients). Nine patients had a two-stage procedure, and four patients had a three-stage procedure (two consecutive subdural grid studies followed by resections). Average follow-up was 59.2 months (range, 42-98 months). With a modified Engel Outcome Scale, four patients (31%) had a class I outcome; three (23%), class II; three (23%), class III; and three (23%), class IV. Ten (77%) patients had a >50% reduction of seizure burden.

CONCLUSIONS

Combined MST and resection can meaningfully improve seizure control in patients with multifocal epilepsy involving eloquent cortex. Prospective randomized studies are needed.

Interhemispheric visual interaction in a patient with posterior callosectomy

The role of anterior commissure (AC) and anterior parts of corpus callosum in visual interactions was investigated in a partial split-brain patient whose posterior and middle parts of the corpus callosum were resected surgically leaving intact only a thin portion of anterior corpus callosum. Although the primary visual areas of the two hemispheres are disconnected in the patient, we found that visual distracters presented to one hemisphere (in a crowding paradigm) impaired recognition of the target stimulus presented to the other hemisphere. The normal control group showed the same result. To rule out the possible contribution of subcortical areas to this interaction, we repeated the same crowding task with texture-defined stimuli. The patient again showed an interhemispheric interaction, even though subcortical structures respond poorly or do not respond at all to texture defined shapes. Despite the evidence for interhemispheric interaction, a classic match-to-sample task confirmed that the patient was unable to explicitly report when stimuli in left and right hemifields were the same or different. Similarly, in a search task, the patient's reaction time was unaffected by distracters in the hemifield opposite the target whereas normals' response time was affected. Considering the dissociation between these two tasks, we conclude that the anterior commissure and/or the anterior corpus callosum contribute to interhemispheric interactions in the attentional selection of location.

Mortality in a population-based cohort of epilepsy surgery patients

PURPOSE

To investigate mortality and especially the incidence of sudden unexpected death in epilepsy (SUDEP) in a population-based cohort of epilepsy surgery patients.

METHODS

All patients who underwent epilepsy surgery treatment between January 1990 and December 1998 (surgery patients) or whose presurgical evaluation started, although not leading to an operation, during the same period (nonsurgery patients) were identified through the Swedish National Epilepsy register. All subjects were followed up through the Cause of Death Register until December 1998. Standardized mortality ratios (SMRs) for all causes of death and incidence of SUDEP were calculated.

RESULTS

During the study period, 651 surgical operations were carried out on 596 patients (316 male). Of those, 14 patients died (six in SUDEP), rendering a total SMR of 4.9 [95% confidence interval (CI), 2.7-8.3]. SUDEP incidence was 2.4 per 1,000 person years. No major differences were found in SMRs or SUDEP rates between subgroups when stratifying for type of operation and for seizure outcome 2 years after surgery. SMR and SUDEP rates were higher in right-sided temporal lobe resections for gliosis than in left-sided, but the number of deaths was small. Among 212 nonsurgery patients, five died (four in SUDEP). The SMR for all causes was 7.9 (2.6-18.4), and SUDEP incidence, 6.3 per 1,000 person years.

CONCLUSIONS

In this large and strictly population-based cohort, SMR for all causes and SUDEP incidence among surgery patients were similar to those of other studies. No differences in overall mortality emerged by seizure outcome, but none of the SUDEP cases was seizure free at the time of death. Four of five deaths in the nonsurgery group occurred during the surgery evaluation period. Mortality appeared to be lower for surgery than for nonsurgery patients, and the interpretation of this finding is discussed.

Neurosurgery for intractable obsessive-compulsive disorder and depression: critical issues

Intractable OCD and depression cause tremendous suffering in those affected and in their families. The impaired ability to function of those affected imposes a heavy burden on society as a whole. Existing data suggest that lesion procedures offer benefit to a large proportion (ranging from about 35%-70%) of patients with intractable OCD and depression. The literature also suggests that although serious long-term adverse events have occurred, these are relatively infrequent overall. Methodologic limitations of the earlier reports on any of these procedures were described previously in this article. The major academic centers conducting this work have since been obtaining systematic prospective data using modern assessment tools. Nevertheless, even with improved methodologies, more recent studies confront some remaining issues that have been difficult to overcome fully. First, the number of patients who have received any one procedure has been relatively small, constraining statistical power. This limits the ability of researchers to enhance patient selection based on clinical characteristics. This is important, because patients with intractable OCD and depression referred for neurosurgery have high rates of comorbid Axis I diagnoses, personality disorders, and functional impairments, which may have value in predicting response. Other features, such as age of onset, chronicity, and symptom subtypes, may be likewise useful. Another key factor in response may be postoperative management, which has varied most over time but also across patients enrolled in trials. As noted previously, randomized controlled trials of neurosurgical treatment for intractable psychiatric illness have not been reported, although one has been proposed for gamma knife capsulotomy in intractable OCD [23]. The development of deep brain stimulation has also made sham-controlled studies possible and also allows within-patient designs to be considered. Bearing these problems in mind, the literature does provide important guidance on a number of key points, including approaches to referral, patient selection, and the need for long-term prospective follow-up and postoperative management. Nevertheless, important gaps in knowledge remain in all these areas. Research is expected to narrow these gaps in a number of ways, including patient selection, optimizing the procedures themselves, and understanding the mechanisms of therapeutic action. Neuroimaging studies will play a key role in achieving these aims (see the article by Rauch in this issue). So will cross-species translational research on the anatomy and physiology of the pathways implicated in the pathophysiology and response to treatment in these disorders. Future research in psychiatric neurosurgery must proceed cautiously. A recent editorial statement of the OCD-DBS Collaborative Group [26] recommends a minimum set of standards for any multidisciplinary teams contemplating work in this domain. The rationale for those standards is found throughout this issue and is especially developed in the article by Fins. The need for safe and effective therapeutic options for people suffering with these severe illnesses is just as clear. The experience over the last several decades provides grounds for careful optimism that refined lesion procedures or reversible deep brain stimulation may relieve suffering and improve the lives of people with these devastating disorders.

Obliteration of giant corpus callosum AVM with linac based stereotactic radiosurgery

Successful management of large AVMs is difficult. Traditionally they are considered less responsive and even unsuitable for radiosurgery. This case report demonstrates total angiographic obliteration in a complex, large corpus callosum AVM (volume 36.52 cc) in a 39-year-old male. Stereotactic radiation was delivered with a Linear Accelerator using ultra conformal treatment planning. Large volume AVMs can be subjected to stereotactic radiosurgery if the shape and location makes it possible to deliver an adequate radiation dose.

Therapeutics in pediatric epilepsy, Part 2: Epilepsy surgery and vagus nerve stimulation

When antiepileptic drugs fail to relieve seizures adequately in children and adolescents, more invasive therapies such as epilepsy surgery and an implanted device to stimulate the vagus nerve should be considered. Temporal lobectomy is an effective treatment of complex partial and secondarily generalized tonic-clonic seizures arising in the mesial structures or lateral temporal neocortex. Excellent outcomes (seizure free or rare, nondisabling seizures) are achieved in at least 70% of children. The most common adverse effect is a superior quadrant field cut that is usually asymptomatic. Transient and more long-lasting language difficulties have been reported when the surgery involves the dominant temporal lobe. The excellent outcome rate for extratemporal surgery ranges from approximately 20% to 80%, with better results seen in patients with an identifiable lesion. Potential morbidity is related to the region of resected neocortex. Corpus callosotomy is an excellent procedure for palliation but is not a cure for seizures that cause falls, with substantial improvement seen in more than 80% of patients. Potential adverse effects include more intense focal seizures and dysphasia, depending on the developmental level of the individual. Hemispherectomy provides seizure relief in 60% to 80% of patients with hemispherical pathologies such as Sturge-Weber or Rasmussen syndromes. Operative mortality has been reported in the range of 0% to 6%; other morbidities include infection and hydrocephalus. Stimulation of the vagus nerve has reduced partial seizures by 50% or more in approximately one third of patients. No adverse cognitive or systemic effects are associated with use of the implanted vagus nerve stimulator.

Endoscope-assisted microsurgery of the corpus callosum

The authors describe the use of a rigid endoscope lens to enhance microsurgical visualization of the corpus callosum. Using cadaver preparations, endoscopic dissection was carried out through a narrow corridor without significant retraction of the brain. The endoscope improves the operative exposure in corpus callosotomy and other transcallosal procedure.

Pantomime to visual presentation of objects: left hand dyspraxia in patients with complete callosotomy

Investigations of left hand praxis in imitation and object use in patients with callosal disconnection have yielded divergent results, inducing a debate between two theoretical positions. Whereas Liepmann suggested that the left hemisphere is motor dominant, others maintain that both hemispheres have equal motor competences and propose that left hand apraxia in patients with callosal disconnection is secondary to left hemispheric specialization for language or other task modalities. The present study aims to gain further insight into the motor competence of the right hemisphere by investigating pantomime of object use in split-brain patients. Three patients with complete callosotomy and, as control groups, five patients with partial callosotomy and nine healthy subjects were examined for their ability to pantomime object use to visual object presentation and demonstrate object manipulation. In each condition, 11 objects were presented to the subjects who pantomimed or demonstrated the object use with either hand. In addition, six object pairs were presented to test bimanual coordination. Two independent raters evaluated the videotaped movement demonstrations. While object use demonstrations were perfect in all three groups, the split-brain patients displayed apraxic errors only with their left hands in the pantomime condition. The movement analysis of concept and execution errors included the examination of ipsilateral versus contralateral motor control. As the right hand/left hemisphere performances demonstrated retrieval of the correct movement concepts, concept errors by the left hand were taken as evidence for right hemisphere control. Several types of execution errors reflected a lack of distal motor control indicating the use of ipsilateral pathways. While one split-brain patient controlled his left hand predominantly by ipsilateral pathways in the pantomime condition, the error profile in the other two split-brain patients suggested that the right hemisphere controlled their left hands. In the object use condition, in all three split-brain patients fine-graded distal movements in the left hand indicated right hemispheric control. Our data show left hand apraxia in split-brain patients is not limited to verbal commands, but also occurs in pantomime to visual presentation of objects. As the demonstration with object in hand was unimpaired in either hand, both hemispheres must contain movement concepts for object use. However, the disconnected right hemisphere is impaired in retrieving the movement concept in response to visual object presentation, presumably because of a deficit in associating perceptual object representation with the movement concepts.

Callosal neglect

BACKGROUND

According to the interhemispheric inhibition model of neglect, the uninjured hemisphere inhibits (via the corpus callosum) the injured hemisphere but the injured hemisphere can no longer inhibit the opposite hemisphere, which becomes hyperactive and produces an ipsilesional attentional bias. Alternatively, according to the compensation hypothesis, the uninjured hemisphere helps compensate for the damaged hemisphere, which is impaired in directing attention to contralateral stimuli. If the inhibition model of neglect is correct, callosal disconnection should reduce neglect. If the compensation model is correct, however, it may increase or induce neglect.

PATIENT

A 32-year-old woman, at age 14 years, developed a right frontal astrocytoma and was treated with surgery and radiation but had a cardiopulmonary arrest secondary to aspiration. Subsequent imaging studies revealed damage to the frontal, parietal, and occipital regions of the right hemisphere and damage to the temporal region of the left hemisphere. After discharge, she was able to return to school and drive a car, without any evidence of neglect. About 10 years later, she developed complex partial and atonic seizures that were multifocal and medically intractable. She underwent a complete section of her corpus callosum at age 31 years.

RESULTS

One year after the callosal section, she demonstrated (1) diminished spontaneous saccades to the left, hypometric leftward saccades, and left gaze impersistence; (2) left arm hemispatial limb akinesia; (3) unilateral spatial neglect; and (4) motor and cognitive impersistence.

CONCLUSION

In patients with right hemisphere injury, callosal section may induce or enhance motor-intentional deficits and hemispatial neglect.

Persistence of ictal activity after functional hemispherectomy in Rasmussen syndrome

A 15-year-old girl with a 3-year history of Rasmussen syndrome (RS) underwent left functional hemispherectomy by central disconnection. Clinical seizures then ceased. Five months postoperatively, ictal EEG discharges were associated with focal hyperperfusion on SPECT within the disconnected hypoperfused left hemisphere, suggesting that the basic mechanisms of RS may continue, only to remit later. EEG and SPECT may complement studies of these in seizure-free surgically treated patients in whom clinical follow-up may be unrevealing.

Learned dynamics of reaching movements generalize from dominant to nondominant arm

Accurate performance of reaching movements depends on adaptable neural circuitry that learns to predict forces and compensate for limb dynamics. In earlier experiments, we quantified generalization from training at one arm position to another position. The generalization patterns suggested that neural elements learning to predict forces coded a limb's state in an intrinsic, muscle-like coordinate system. Here, we test the sensitivity of these elements to the other arm by quantifying inter-arm generalization. We considered two possible coordinate systems: an intrinsic (joint) representation should generalize with mirror symmetry reflecting the joint's symmetry and an extrinsic representation should preserve the task's structure in extrinsic coordinates. Both coordinate systems of generalization were compared with a naïve control group. We tested transfer in right-handed subjects both from dominant to nondominant arm (D-->ND) and vice versa (ND-->D). This led to a 2 x 3 experimental design matrix: transfer direction (D-->ND/ND-->D) by coordinate system (extrinsic, intrinsic, control). Generalization occurred only from dominant to nondominant arm and only in extrinsic coordinates. To assess the dependence of generalization on callosal inter-hemispheric communication, we tested commissurotomy patient JW. JW showed generalization from dominant to nondominant arm in extrinsic coordinates. The results suggest that when the dominant right arm is used in learning dynamics, the information could be represented in the left hemisphere with neural elements tuned to both the right arm and the left arm. In contrast, learning with the nondominant arm seems to rely on the elements in the nondominant hemisphere tuned only to movements of that arm.

An overview of surgery for chronic seizures

Intracranial surgical intervention offers the potential to reduce or eliminate seizures in a substantial number of patients with medically intractable epilepsy. Although some patients with epilepsy have a pathologic condition that can be imaged radiographically, this is not a requirement for the patient to be considered a surgical candidate. Evaluation in a comprehensive epilepsy center includes defining the characteristics of the seizures; integrating medical criteria, psychosocial factors, imaging findings, and surgical risks; and weighing these factors against the risks of continued, uncontrolled epilepsy. Several neurosurgical procedures are available to help these patients, including frontal and temporal lobectomy, lesionectomy, subpial transection, and callosotomy.

Basic science and epilepsy: experimental epilepsy surgery

Epilepsy surgery, as is employed for the management of intractable seizures, was performed in animals harboring a seizure focus induced by a local application of kainic acid (KA). Amygdalo-hippocampectomy failed to stop spontaneous seizures in the contralateral hippocampus. Callosotomy inhibited seizure propagation to the contralateral sensori-motor cortex. However, epileptic activity ipsilateral to the focus, including subcortical structures, persisted even after the callosotomy. Multiple subpial transection (MST) around the epileptic cortical focus suppressed the seizure activity of the cortex. However, seizure propagations in subcortical structures remained, even after MST. Nefiracetam (a new nootropic agent) was tested in these models, and its promising effect on the intractable extratemporal epilepsy is reported.

Is a cortical spike discharge "transferred" to the contralateral cortex via the corpus callosum?: An intraoperative observation of electrocorticogram and callosal compound action potentials

PURPOSE

By means of the intraoperative electrophysiologic observation, we reevaluated the "transfer" theory that a transcallosal volley invoked by a cortical spike discharge in one hemisphere directly causes its contralateral counterpart via the corpus callosum (CC).

METHODS

Twenty-six patients who underwent corpus callosotomy were the subjects of this study. Intraoperatively, electrocorticograms from both hemispheres were simultaneously monitored with callosal compound action potentials (CCAPs) from the CC. Analysis was conducted on (a) the interhemispheric delay of bilaterally synchronous spike-and-wave discharges (BSSWs), and (b) the chronological relation between BSSWs and CCAPs.

RESULTS

The side of prior spike discharges was never fixed but was occasionally reversed. Interhemispheric delays between the BSSWs were not constant, regardless of direction, and fluctuated in all patients. Most of the interhemispheric delays were distributed within 20 ms with a mode of 0 ms. The waveform of the CCAP was characterized by slow-rising negative potential change that attained its peak after a cortical spike discharge. These findings were identical in all the patients regardless of whether the BSSWs were changed or unchanged after callosotomy.

CONCLUSIONS

If the "transfer" role of the CC is true, interhemispheric delays between BSSWs must be longer than interhemispheric axonal conduction time (about 20 ms), and a preceding cortical spike discharge must produce a CCAP and then a contralateral one in order of time. However, this hypothesis was not confirmed in the present study. We propose the interhemispheric recruitment of the epileptogenic state as a different role of the CC on epileptogenesis.

Paradoxical interhemispheric summation in the split brain

We measured simple reaction time (RT) to light flashes, presented either singly or in pairs, in two people who had undergone callosotomy, one person with agenesis of the corpus callosum, and 17 normal subjects. The three split-brained subjects' RTs were decreased to bilateral pairs beyond predictions based on a simple race between independent unilateral processes, while those of the normal subjects were actually longer than predicted by the race model. This effect was present whether the bilateral pairs were in mirror-image locations or not, but was not present when the pairs were presented unilaterally. Since summation does not depend on close spatial correspondence, and also occurs when inputs are staggered in time, we suggest that it is due to cortical projection to a subcortical arousal system, and is normally inhibited by the corpus callosum.

Surgery in Lennox-Gastaut syndrome. Corpus callosum division for children

A tonic or "drop" attacks--often seen in Lennox-Gastaut syndrome--are some of the most feared childhood seizures, since they may lead to head injuries. Corpus callosum division will often control atonic seizures, and other types of generalized seizure.

Interhemispheric neural summation in the split brain with symmetrical and asymmetrical displays

The present study, investigates interhemispheric integration in the split brain. Four split-brained, two acallosal and 14 normal subjects carried out a simple reaction time task in which they responded to stimuli presented either singly in the left visual field, singly in the right visual field, or simultaneously in both visual fields. Stimuli were white against a black background and bilateral stimuli were either symmetrical or asymmetrical around the central vertical meridian. For unilateral stimuli, the difference in response time (RT) between crossed and uncrossed hand-field combinations (crossed-uncrossed difference, or CUD) measured interhemispheric transfer time. RTs to bilateral and unilateral stimulus displays were compared to provide a measure of redundancy gain (RG). Normal subjects exhibited small CUDs and RGs. Split-brained and acallosal subjects were found to have much longer CUDs, and to show enhanced RGs which could not be explained by a probability (race) model, implying subcortical neural summation. This summation did not depend on the preservation of symmetry, suggesting that it may not occur at the retinotopically organized superior colliculus, but at another site such as the pons or reticular formation.

Are eyes special? It depends on how you look at it

Recent behavioral data have shown that central nonpredictive gaze direction triggers reflexive shifts of attention toward the gazed-at location (e.g., Friesen & Kingstone, 1998). Friesen and Kingstone suggested that this reflexive orienting effect is unique to biologically relevant stimuli. Three experiments were conducted to test this proposal by comparing the attentional orienting produced by nonpredictive gaze cues (biologically relevant) with the attentional orienting produced by nonpredictive arrow cues (biologically irrelevant). Both types of cues produced reflexive orienting in adults (Experiment 1) and preschoolers (Experiment 2), suggesting that gaze cues are not special. However, Experiment 3 showed that nonpredictive arrows produced reflexive orienting in both hemispheres of a split-brain patient. This contrasts with Kingstone, Friesen, and Gazzaniga's (2000) finding that nonpredictive gaze cues produce reflexive orienting only in theface-processing hemisphere of split-brain patients. Therefore, although nonpredictive eyes and arrows may produce similar behavioral effects, they are not subserved by the same brain systems. Together, these data provide important insight into the nature of the representations of directional stimuli involved in reflexive attentional orienting.

Suppressive effect of callosotomy on epileptic seizures is due to the blockade of enhancement of cortical reactivity by transcallosal volleys

The present work demonstrates that the cortical reactivity of the rat, monitored by thalamocortical responses, can be enhanced by repetitive transcallosal volleys (5-20 Hz). This effect can be completely inhibited by callosotomy. We believe that the interhemispheric epileptogenesis and the suppressive effect of callosotomy for intractable epilepsy can be explained by this facilitatory effect of the corpus callosum on the cerebral cortex.

Hemispheric encoding asymmetry is more apparent than real

Previous neuroimaging studies have claimed a left hemisphere specialization for episodic "encoding" and a right hemisphere specialization for episodic "retrieval." Yet studies of split-brain patients indicate relatively minor memory impairment after disconnection of the two hemispheres. This suggests that both hemispheres are capable of encoding and retrieval. In the present experiment, we examined the possible limits on encoding capacity of each hemisphere by manipulating the "depth" of processing during the encoding of unfamiliar faces and familiar words in the left and right hemispheres of two split-brain patients. Results showed that only the left hemisphere benefited from deeper (more elaborate) encoding of familiar words, and only the right hemisphere benefited from deeper encoding of unfamiliar faces. Our findings are consistent with the view that hemispheric asymmetries in episodic encoding are related to hemisphere-specific processing of particular stimuli. Convergent with recent neuroimaging studies, these results with split-brain patients also suggest that these hemispheric differences are not due to unique specializations in each half brain for encoding memories, but rather, are due to preferential recruitment of the synaptically closer prefrontal cortex to posterior regions processing material-specific information.

Laminar-dependent dendritic spine alterations in the motor cortex of adult rats following callosal transection and forced forelimb use

Previously, the authors found that partial denervation of the motor cortex in adult animals can enhance this region's neuronal growth response to relevant behavioral change. Rats with partial corpus callosum transections that were forced to rely on one forelimb for 18 days had increased dendritic arborization of layer V pyramidal neurons in the opposite motor cortex compared to controls. This was not found as a result of denervation alone or of forced forelimb use alone. However, it seemed possible that each independent manipulation (i.e., forced forelimb use alone and callosal transections alone) resulted in neural structural alterations that were simply not revealed in measurements of dendritic branch number and/or not inclusive of layer V dendrites. This possibility was assessed in the current study with a reexamination of the Golgi-Cox impregnated tissue generated in the previous study. Tissue was quantified from rats that received either partial transections of the rostral two-thirds of the corpus callosum (CCX) or sham operations (Sham) followed either by 18 days of forced use of one forelimb (Use) or unrestricted use of both forelimbs (Cont). Measurements of apical and basilar dendrites from pyramidal neurons of layer II/III and layer V were performed to detect spine addition resulting from either increased spine density or the addition of dendritic material. As hypothesized, significant spine addition was found following forced forelimb use alone (Sham+Use) and callosal transections alone (CCX+Cont). However, forced use primarily increased spines on layer II/III pyramidal neurons, whereas callosal transections primarily increased dendritic spines on layer V pyramidal neurons in comparison to Sham+Cont. A much more robust increase in layer V dendritic spines was found in animals with the combination of forced forelimb use and denervation (CCX+Use). In contrast to the effects of forced use alone, however, CCX+Use rats failed to show major net increases in spines on layer II/III neurons. These results indicate that while callosal denervation may greatly enhance the neuronal growth and synaptogenic response to behavioral change in layer V, it may also limit spine addition associated with forced forelimb use in layer II/III of the motor cortex.

The ventriloquist in motion: illusory capture of dynamic information across sensory modalities

Integrating dynamic information across the senses is crucial to survival. However, most laboratory studies have only examined sensory integration for static events. Here we demonstrate that strong crossmodal integration can also occur for an emergent attribute of dynamic arrays, specifically the direction of apparent motion. The results of the present study show that the perceived direction of auditory apparent motion is strongly modulated by apparent motion in vision, and that both spatial and temporal factors play a significant role in this crossmodal effect. We also demonstrate that a split-brain patient who does not perceive visual apparent motion across the midline is immune to this audiovisual dynamic capture effect, highlighting the importance of motion being experienced in order for this new multisensory illusion to occur.

Impairment in the control of coupled cyclic movements of ipsilateral hand and foot after total callosotomy

Integrity of both cerebral hemispheres is required to control in-phase or anti-phase coupling of ipsilateral hand and foot oscillations, as shown by the impairment of these tasks when performed on the healthy side of hemiplegic patients. On this basis, coupling of hand-foot movements was analysed in a right-handed subject (ME) who underwent a total resection of the corpus callosum. Oscillations of the prone hand and foot, paced by a metronome at different frequencies, as well as EMG activity in extensor carpi radialis (ECR) and tibialis anterior (TA) muscles were analysed by measuring the average phase difference between the hand and foot movements and EMG cycles. ME performed in-phase movements (right-hand extension coupled to right-foot dorsal flexion) at frequencies up to 3 Hz, though the hand cycle progressively lagged the foot cycle as the frequency increased. At 3 Hz the hand lag reached -142 degrees (as compared to about 25 degrees in healthy subjects). The lag increased even further after application of an inertial load to the hand, reaching 180 degrees at 1.8 Hz (about 50 degrees in healthy subjects). ME's hand lag is caused by the lack of any anticipatory reaction in hand movers. In contrast to healthy subjects, which activate the ECR earlier than the TA when the frequency increases, ME activated the ECR later than TA at all frequencies higher than 0.9 Hz. Anti-phase movements (hand extension coupled to foot plantar flexion) were performed only upto 1 Hz in unloaded conditions. At 0.6 Hz, movements were in tight phase-opposition (3 degrees), but at 1 Hz, the hand lag reached -34 degrees because of a delayed ECR activation. After hand loading ME was unable to couple movements in anti-phase. In contrast, normal subjects maintain a tight anti-phase coupling up to 2.0 Hz, both with an unloaded or loaded hand. Similar deficits were observed by ME when performing in-phase and anti-phase coupling on the left side, as well as when he was blindfolded. In normal subjects, an anticipated muscular activation of hand movers compensates for hand loading. Since this compensation must depend on monitoring the hand delay induced by loading, the absence in ME of such compensatory reaction suggests that callosal division had apparently compromised the mechanisms sustaining feedback compensation for differences in the biomechanical limb properties. They also confirm and reinforce the idea that elaboration of the afferent message, aiming at controlling the phase of the movement association, needs the co-operation of both cerebral hemispheres.

[Arteriovenous malformation of the middle and posterior third section of the corpus callosum treated with embolization and surgery]

We are presenting the case of a 58 year-old woman with an arteriovenous malformation (AVM) of the middle and posterior thirds of the corpus callosum which had produced two episodes of bleeding before admission to our Department, when the patient was neurologically intact. The nidus was fed by the anterior and posterior pericallosal arteries draining through the left medial atrial vein to the vein of Galen, by parasagittal cortical veins to the superior sagittal sinus and by right temporal veins to the sphenoparietal sinus. The patient underwent embolization of the anterior and posterior feeders in two sessions separated by a week interval, and then the AVM was removed through a left paramedial parietooccipital craniotomy in a single stage. The patient showed transient mild short term memory deficit, but the final outcome was excellent.

Corpus callosum: microsurgical anatomy and MRI

The anatomy of the corpus callosum has received renewed interest during recent years due to the increasing number of callosotomies performed to treat intraventricular lesions, as well as some forms of generalized epilepsy. We have previously reported on the microsurgical anatomy of the corpus callosum and identified specific anatomical reference landmarks that can be used during surgery. In the present study we have continued the anatomical aspect of this earlier work in a larger number of cases, with in vitro observations (brain out of skull) being compared with the corresponding in vivo features seen in sagittal MRI slices. Fifty-three in vitro microsurgical callosotomies was performed and the data collected compared with a series of 57 in vivo normal MR callosal images. Callosal dimensions were measured on both the anatomical and MRI material, thus overcoming the problems associated with in vitro callosal deformation. Of the anatomical landmarks studied the distance from the genu of the corpus callosum to the bifurcation of the columns of the fornix was found to be useful for the intraoperative evaluation of the extent of rostral callosotomy, as it is not significantly changed in in vitro. The main microsurgical features of rostral callosotomy are presented.

Sound localization in callosal agenesis and early callosotomy subjects: brain reorganization and/or compensatory strategies

In order to evaluate the callosal involvement in sound localization, the present study examined the response accuracy of acallosal and early callosotomized subjects to monaural and binaural auditory targets presented in three-dimensional space. In these subjects, bilateral localization cues, such as interaural time and level differences, are integrated at the cortical and subcortical levels without the additional support of the callosal commissure. Because acallosal and early-callosotomized subjects have developed with this reduced source of binaural activation of cortical cells, they might have perfected their ability to use monaural sound localization cues. This hypothesis was tested by assessing localization performance under both binaural and monaural listening conditions. Five subjects with callosal agenesis, one callosotomized subject operated early in life and 19 control subjects were asked to localize broad-band noise bursts (BBNBs) of fixed intensity in the horizontal plane in an anechoic chamber. BBNBs were delivered through randomly selected loudspeakers. Two conditions were tested: (i) localization of a stationary sound source; and (ii) localization of a moving sound source. Listeners had to report the apparent stimulus location by pointing to its perceived position on a graduated perimeter. The results indicated that the acallosal subjects were less accurate than controls, but only in the binaural moving sound condition. More interestingly, in the monaural testing conditions, some of the acallosal subjects and the early-callosotomized subject performed significantly better than control subjects. This suggests that, because of the absence of the corpus callosum, these subjects compensate for their reduced access to cortically determined binaural cues by making more efficient use of monaural cues.