Biomedical subjects

George A Ojemann

Publications and source records attributed to George A Ojemann.

13 recordsLinked to original sources

PubMed2006

Structured cueing on a semantic fluency task differentiates patients with temporal versus frontal lobe seizure onset.

Patients with frontal lobe dysfunction (e.g., Huntington's disease) reportedly benefit more from cueing on measures of semantic fluency than do patients with damage to temporal lobe structures (e.g., Alzheimer's disease). This differential benefit from cueing suggests that different neurocognitive functions are impaired in these two groups. Patients with frontal lobe dysfunction are presumed to have difficulty with the executive aspects of this generative fluency task, whereas patients with temporal lobe impairment are limited by deficits in semantic memory. We studied the performance of patients with complex partial seizures of frontal or temporal lobe onset, as determined by video/EEG monitoring, on standard and cued measures of semantic fluency administered in a counterbalanced sequence across groups. These groups did not differ significantly in terms of age, education, gender, age at seizure onset, total number of antiepileptic drugs, or IQ, and all patients subsequently underwent surgery for intractable epilepsy. Patients with frontal lobe dysfunction (FL group) performed significantly worse than patients with temporal lobe impairment (TL group) on the standard semantic fluency paradigm (TL group: M=18.4, SD=4.7; FL group: M=11.1, SD=5.3), t(27)=-3.75, P<0.001. Nevertheless, results of an ANCOVA demonstrated that the FL group showed significantly greater performance improvement than the TL group when provided with a cued semantic fluency format, even after controlling for baseline differences in ability on the standard semantic fluency task (TL group: M=0.45, SD=3.8; FL M=9.4, SD=5.1), F(1,29)=12.37, P=0.002. These findings support previous research suggesting that frontal and temporal structures contribute uniquely to semantic generative fluency and suggest that using a combination of standard and cued semantic fluency tasks may help confirm localization of seizure onset in partial epilepsy by localizing the associated cognitive dysfunction.

Adult↗

PubMed2005

Gender differences in handedness and speech lateralization related to early neurologic insults.

Left-handedness is a normal variant but also may result from early-life brain insults. Speech typically resides in the left cerebral hemisphere in normal subjects. In 170 subjects with past brain injuries, bilateral or right hemisphere speech lateralization, determined by the intracarotid amobarbital procedure, was more frequent in females (19 vs 5, p = 0.003) as was left-handedness (26 vs 10, p = 0.004). This could be attributed to greater plasticity or functional symmetry in females.

Age Factors↗

PubMed2005

Dissociation of action and object naming: evidence from cortical stimulation mapping.

This cortical stimulation mapping study investigates the neural representation of action and object naming. Data from 13 neurosurgical subjects undergoing awake cortical mapping is presented. Our findings indicate clear evidence of differential disruption of noun and verb naming in the context of this naming task. At the individual level, evidence was found for punctuate regions of perisylvian cortex subserving noun and verb function. Across subjects, however, the location of these sites varied. This finding may help explain discrepancies between lesion and functional imaging studies of noun and verb naming. In addition, an alternative coding of these data served to highlight the grammatical class vulnerability of the target response. The use of this coding scheme implicates a role for the supramarginal gyrus in verb-naming behavior. These data are discussed with respect to a functional-anatomical pathway underlying verb naming.

Adult↗

PubMed2005

Invasive recordings from the human brain: clinical insights and beyond.

Although non-invasive methods such as functional magnetic resonance imaging, electroencephalograms and magnetoencephalograms provide most of the current data about the human brain, their resolution is insufficient to show physiological processes at the cellular level. Clinical approaches sometimes allow invasive recordings to be taken from the human brain, mainly in patients with epilepsy or with movement disorders, and such recordings can sample neural activity at spatial scales ranging from single cells to distributed cell assemblies. In addition to their clinical relevance, these recordings can provide unique insights into brain functions such as movement control, perception, memory, language and even consciousness.

Brain↗

PubMed2004

Different neurons in different regions of human temporal lobe distinguish correct from incorrect identification or memory.

Changes in the frequency of neural activity differentiating correct from incorrect responses were identified in extracellular recordings from 113 neurons at 62 sites in temporal lobe of 26 patients undergoing awake neurosurgery, during identification and recent explicit memory for object names, text or auditory words. Activity significantly differentiating correct from incorrect performance was identified in 22 neurons at 16 sites in 12 patients. Different neurons in different regions differentiated identification or memory performance. The 13 neurons differentiating identification performance were overrepresented in medial-basal recordings, the 9 neurons differentiating memory performance in superior temporal gyrus. All memory changes occurred during encoding. For both identification and memory there was separation of neurons showing differentiation early during perception and processing from those showing differentiation late, when output should occur, perhaps reflecting response monitoring. Early differentiating neurons were located more superior-laterally within the different regions related to accuracy of identification or memory.

Adolescent↗

PubMed2004

Functional separation of languages in the bilingual brain: a comparison of electrical stimulation language mapping in 25 bilingual patients and 117 monolingual control patients.

OBJECT: The aim of this investigation was to address three questions in bilingualism research: 1) are multiple languages functionally separated within the bilingual brain; 2) are these languages similarly organized; and 3) does language organization in bilinguals mirror that in monolinguals? 9: During awake dominant-hemisphere craniotomy in each of 25 bilingual patients, the authors mapped both languages by using identical object-naming stimuli. Essential sites for primary (L1) and secondary (L2) languages were compared. Sites were photographically recorded and plotted onto an anatomically referenced grid system. Language organization in bilinguals was then compared with that in 117 monolinguals and 11 monolingual children. CONCLUSIONS: The authors found distinct language-specific sites as well as shared sites that support both languages. The L1 and L2 representations were similar in total cortical extent but significantly different in anatomical distribution. The L2-specific sites were located exclusively in the posterior temporal and parietal regions, whereas the L1 and shared sites could be found throughout the mapped regions. Bilinguals possessed seven perisylvian language zones, in which L2 sites were significantly underrepresented when compared with the distribution of language sites in monolinguals. These L2-restricted zones overlapped the primary language areas found in monolingual children, indicating that these zones become dedicated to L1 processing. These findings support three conclusions. First, it is necessary to map both languages in bilinguals because L1 and L2 sites are functionally distinct. Second, differences exist in the organization of L1 and L2 sites, with L2-specific sites located exclusively in the posterior temporal and parietal lobes. Third, language organization comparisons in bilingual and monolingual brains demonstrate the presence of L2-restricted zones, which are dedicated to L1.

Adolescent↗

PubMed2003

A direct measure of human lateral temporal lobe neurons responsive to face matching.

Do the human cerebral hemispheres process faces differently? Clinical lesion observations and primate studies suggest that the right temporal lobe is critical in face processing. Yet, the nature of this relationship remains unclear. Recording from single neurons during a visuospatial (VS)-matching paradigm, we found that 100% of significantly active neurons discriminated matching from perception, bilaterally. Lateralized differences in the nature and timing of responses revealed that the right hemisphere neurons responded earlier, and with uniform frequency reductions. Additional lateralized differences favoring the right hemisphere neurons were found when subjects matched intact faces compared to scrambled faces or complex objects. We conclude that widely distributed neural ensembles are involved in 'lateralized' behaviors, but cerebral specialization of face processing is as much a function of the nature and timing of neuronal activity as anatomic location.

Action Potentials↗

PubMed2003

The neurobiology of language and verbal memory: observations from awake neurosurgery.

Neurosurgical operations under local anesthesia provide a unique opportunity to investigate the neurobiology of human cognition. We have studied the cortical organization of language and verbal memory in this setting, using two different techniques: electrical stimulation mapping and extracellular microelectrode recording of activity of individual neurons. The two techniques provide very different perspectives. Stimulation mapping identifies brain areas that are essential for a behavior, while changes in neuronal activity can occur in non-essential regions. Stimulation mapping identifies multiple discrete areas in perisylvian cortex of the dominant hemisphere as essential for a function, with separation of areas for different aspects of language including naming in two languages, different semantic classes, naming compared to reading, and language from verbal memory. There is substantial individual variation in the location of these essential areas, variability that in part relates to subjects age, gender and verbal abilities. Neurons changing activity with language or verbal memory are widely distributed, in both hemispheres. However, individual neurons usually change activity with only one function, including naming in only one of two languages, only naming or reading, or with recent verbal memory encoding but not identification of similar items. A few lateralized changes in neuronal activity have been identified, including a predominance of inhibition in dominant hemisphere with naming, and polymodal memory responses in dominant hemisphere, unimodal in nondominant. Specific neuronal populations have been identified that are related to different aspects of memory, that differentiate correct from incorrect identification or memory performance and differentiate learned from unlearned associations, with some evidence of differences in neuronal activity related to subjects' ability.

Animals↗

PubMed2003

Factors predicting outcome of surgery for intractable epilepsy with pathologically verified mesial temporal sclerosis.

PURPOSE: To examine the subgroup of patients with medically intractable epilepsy receiving temporal lobectomies who have pathologically verified mesial temporal sclerosis (MTS) and to determine the relation of demographic and clinical factors, results of diagnostic testing, and details of the surgical procedure with prognosis for achieving control of seizures. METHODS: All patients receiving surgical treatment for intractable epilepsy between 1991 and 1998 at the University of Washington were reviewed. There were 118 patients who met inclusion criteria of adequate pathological analysis showing MTS without a progressive process and a minimum of 1-year follow-up. RESULTS: Only personal history of status epilepticus demonstrated significant (p = 0.0276) prediction of outcome, increasing the risk of surgical failure. No other factors were significant predictors of outcome, including history of febrile seizures, possible etiologic factors, EEG, magnetic resonance imaging (MRI) or neuropsychological testing results, or extent of resection. CONCLUSIONS: Many factors that have been previously described to predict favorable outcome in the overall group of patients receiving temporal lobe resections for intractable epilepsy are, in fact, predictors of MTS and lose their predictive value when the subgroup of patients with confirmed MTS is examined. Neurosurgical treatment of MTS can be very effective even in the presence of significant etiologic factors, or of bilateral or extratemporal abnormalities on EEG or MRI.

Adult↗

PubMed2003

Identifying potential surgical candidates in patients with evidence of bitemporal epilepsy.

PURPOSE: To determine which patients with evidence of medically refractory bitemporal epilepsy are potentially good candidates for surgical therapy. METHODS: We reviewed 42 adults with intractable seizures who were found to have bitemporal ictal onsets, based on scalp video-EEG long-term monitoring (LTM). All underwent invasive LTM before surgery. Surgical outcomes were classified as seizure free, >75% reduction in seizures, or <75% reduction in seizures, >or=1 year after resection. We related the following factors to outcome: (a). >75% preponderance of interictal scalp EEG discharges to one temporal region; (b). magnetic resonance imaging (MRI) findings; (c). lateralizing deficits on verbal or visual reproduction memory testing; and (d). memory failure with injection contralateral to side of surgery on Wada testing. RESULTS: Twenty-six (62%) of 42 patients had unilateral ictal onsets based on intracranial studies. Seizure freedom (occurring in 64% of this group), or >75% seizure reduction (found in 12% of subjects) occurred only when at least one of the following three factors was concordant with the side of surgery: preponderance of interictal scalp EEG discharges, unilateral temporal lesion on MRI, or lateralizing verbal or visual reproduction memory deficits on neuropsychological tests (p = 0.004). Seven subjects with bilateral ictal onsets based on intracranial studies had resections based on preponderance of seizures to one side, or other lateralizing noninvasive abnormality. Five of these (all of whom had >or=80% of seizures originating from one side) had >75% reduction in seizures. CONCLUSIONS: Invasive monitoring to pursue possible surgical therapy for patients with surface EEG evidence of bitemporal epilepsy may be justified only when some lateralizing feature is found in other noninvasive assessments.

Adolescent↗

PubMed2003

Localization of language function in children: results of electrical stimulation mapping.

OBJECT: The authors examined the localization of language sites and the frequency of naming errors at these sites in a population of children undergoing electrical stimulation mapping during surgeries in which epileptic foci and dominant hemisphere neoplasms were resected. The frequency with which essential language sites were found (that is, "the frequency of language sites") in children was compared with that of a population of adults who had undergone this procedure, to assess the relationship of age to the distribution of essential areas for language. METHODS: The results of electrical stimulation mapping to determine sites of naming and speech arrest in 26 children ranging in ages from 4 to 16 years are presented in this report. Mapping was performed in the intraoperative setting in eight patients and in the extraoperative setting, by stimulation across a subdural grid, in 18 patients. The frequency and distribution of essential language areas were analyzed in populations of different ages and according to the method used to obtain the map. Considerable variability was found in the localization of language sites. When the language site distribution in pediatric patients was compared with the language site distribution found previously in a population of patients older than 16 years of age, a relative paucity of language sites was found in all perisylvian cortices in the younger age group. This relationship was also found within the group of patients 16 years of age and younger, when segregated into two groups: those patients 8 years of age or younger, and those patients between 9 and 16 years of age. These findings are relevant to theories of the intrahemispheric organization of the cortex devoted to language function. CONCLUSIONS: The differences found between groups of younger and older patients in the frequencies of sites where stimulation produces naming errors was identified suggests the possibility that, with advancing age, maturational processes contribute new foci of cortex essential for language.

Adolescent↗

PubMed2002

Frameless stereotaxy without rigid pin fixation during awake craniotomies.

OBJECTIVE: The accuracy and precision of resection with awake brain mapping is augmented when combined with frameless stereotaxy. Frameless stereotactic assisted surgery, however, typically involves immobilization in Mayfield pins. Rigid fixation, however, may be problematic for individuals undergoing awake craniotomy. We describe an alternate method of frameless stereotaxy without the use of pin fixation. METHODS: The MRI fiducials are placed in proximity to the proposed incision and prepared and draped in the operative field. After craniotomy, the epidural skull clamp (standardly used to support the electrode holder during corticography) is attached. Using the Stealth Navigational System, a spinal reference arc is then clamped onto the skull clamp base. After the fiducials and arc are registered, the probe is used for frameless guidance. RESULTS: In 14 of 15 cases (93%), this technique was successful. One case failed because of close proximity of the fiducials to the incision. The accuracy of the 14 successful cases was less than 4 mm. The error within the lesion itself was always less than 2 mm. Overall, these results are comparable to those achieved using a Mayfield head holder. In all tumor cases, postoperative imaging concurred with intraoperative assessment of a total versus subtotal resection. CONCLUSIONS: By affixing the spinal arc to the skull, the complications and discomfort associated with pin fixation are avoided completely. The patient is free to move without affecting the accuracy. The spine arc, in combination with the skull clamp, provides an efficient, well-tolerated, and accurate method of frameless navigation for the awake patient undergoing craniotomy.

Awareness↗

PubMed2002

Cortical stimulation mapping of language cortex by using a verb generation task: effects of learning and comparison to mapping based on object naming.

OBJECT: Cortical stimulation mapping has traditionally relied on disruption of object naming to define essential language areas. In this study, the authors reviewed the use of a different language task, verb generation, in mapping language. This task has greater use in brain imaging studies and may be used to test aspects of language different from those of object naming. METHODS: In 14 patients, cortical stimulation mapping performed using a verb generation task provided a map of language areas in the frontal and temporoparietal cortices. These verb generation maps often overlapped object naming ones and, in many patients, different areas of cortex were found to be involved in the two functions. In three patients, stimulation mapping was performed during the initial performance of the verb generation task and also during learned performance of the task. Parallel to findings of published neuroimaging studies, a larger area of stimulated cortex led to disruption of verb generation in response to stimulation during novel task performance than during learned performance. CONCLUSIONS: Results of cortical stimulation mapping closely resemble those of functional neuroimaging when both implement the verb generation task. The precise map of the temporoparietal language cortex depends on the task used for mapping.

Adolescent↗