Basal temporal language area

Support for anterior temporal involvement in semantic error production in aphasia: new evidence from VLSM

Semantic errors in aphasia (e.g., naming a horse as "dog") frequently arise from faulty mapping of concepts onto lexical items. A recent study by our group used voxel-based lesion-symptom mapping (VLSM) methods with 64 patients with chronic aphasia to identify voxels that carry an association with semantic errors. The strongest associations were found in the left anterior temporal lobe (L-ATL), in the mid- to anterior MTG region. The absence of findings in Wernicke's area was surprising, as were indications that ATL voxels made an essential contribution to the post-semantic stage of lexical access. In this follow-up study, we sought to validate these results by re-defining semantic errors in a manner that was less theory dependent and more consistent with prior lesion studies. As this change also increased the robustness of the dependent variable, it made it possible to perform additional statistical analyses that further refined the interpretation. The results strengthen the evidence for a causal relationship between ATL damage and lexically-based semantic errors in naming and lend confidence to the conclusion that chronic lesions in Wernicke's area are not causally implicated in semantic error production.

The interactive account of ventral occipitotemporal contributions to reading

The ventral occipitotemporal cortex (vOT) is involved in the perception of visually presented objects and written words. The Interactive Account of vOT function is based on the premise that perception involves the synthesis of bottom-up sensory input with top-down predictions that are generated automatically from prior experience. We propose that vOT integrates visuospatial features abstracted from sensory inputs with higher level associations such as speech sounds, actions and meanings. In this context, specialization for orthography emerges from regional interactions without assuming that vOT is selectively tuned to orthographic features. We discuss how the Interactive Account explains left vOT responses during normal reading and developmental dyslexia; and how it accounts for the behavioural consequences of left vOT damage.

The interactive account of ventral occipitotemporal contributions to reading

The ventral occipitotemporal cortex (vOT) is involved in the perception of visually presented objects and written words. The Interactive Account of vOT function is based on the premise that perception involves the synthesis of bottom-up sensory input with top-down predictions that are generated automatically from prior experience. We propose that vOT integrates visuospatial features abstracted from sensory inputs with higher level associations such as speech sounds, actions and meanings. In this context, specialization for orthography emerges from regional interactions without assuming that vOT is selectively tuned to orthographic features. We discuss how the Interactive Account explains left vOT responses during normal reading and developmental dyslexia; and how it accounts for the behavioural consequences of left vOT damage.

Rapid online language mapping with electrocorticography

OBJECT

Emerging research in evoked broadband electrocorticographic (ECoG) measurement from the cortical surface suggests that it might cleanly delineate the functional organization of cortex. The authors sought to demonstrate whether this could be done in a same-session, online manner to identify receptive and expressive language areas.

METHODS

The authors assessed the efficacy of simple integration of "χ-band" (76-200 Hz) change in the ECoG signal by implementing a simple band-pass filter to estimate broadband spectral change. Following a brief (less than 10-second) period to characterize baseline activity, χ-band activity was integrated while 7 epileptic patients with implanted ECoG electrodes performed a verb-generation task.

RESULTS

While the patients were performing verb-generation or noun-reading tasks, cortical activation was consistently identified in primary mouth motor area, superior temporal gyrus, and Broca and Wernicke association areas. Maps were robust after a mean time of 47 seconds (using an "activation overlap" measure). Correlation with electrocortical stimulation was not complete and was stronger for noun reading than verb generation.

CONCLUSIONS

Broadband ECoG changes can be captured online to identify eloquent cortex. This demonstrates the existence of a powerful new tool for functional mapping in the operative and chronic implant setting.

Patterns of brain reorganization subsequent to left fusiform damage: fMRI evidence from visual processing of words and pseudowords, faces and objects

Little is known about the neural reorganization that takes place subsequent to lesions that affect orthographic processing (reading and/or spelling). We report on an fMRI investigation of an individual with a left mid-fusiform resection that affected both reading and spelling (Tsapkini & Rapp, 2010). To investigate possible patterns of functional reorganization, we compared the behavioral and neural activation patterns of this individual with those of a group of control participants for the tasks of silent reading of words and pseudowords and the passive viewing of faces and objects, all tasks that typically recruit the inferior temporal lobes. This comparison was carried out with methods that included a novel application of Mahalanobis distance statistics, and revealed: (1) normal behavioral and neural responses for face and object processing, (2) evidence of neural reorganization bilaterally in the posterior fusiform that supported normal performance in pseudoword reading and which contributed to word reading (3) evidence of abnormal recruitment of the bilateral anterior temporal lobes indicating compensatory (albeit insufficient) recruitment of mechanisms for circumventing the word reading deficit.

The organization and dissolution of semantic-conceptual knowledge: is the 'amodal hub' the only plausible model?

In recent years, the anatomical and functional bases of conceptual activity have attracted a growing interest. In particular, Patterson and Lambon-Ralph have proposed the existence, in the anterior parts of the temporal lobes, of a mechanism (the 'amodal semantic hub') supporting the interactive activation of semantic representations in all modalities and for all semantic categories. The aim of then present paper is to discuss this model, arguing against the notion of an 'amodal' semantic hub, because we maintain, in agreement with the Damasio's construct of 'higher-order convergence zone', that a continuum exists between perceptual information and conceptual representations, whereas the 'amodal' account views perceptual informations only as a channel through which abstract semantic knowledge can be activated. According to our model, semantic organization can be better explained by two orthogonal higher-order convergence systems, concerning, on one hand, the right vs. left hemisphere and, on the other hand, the ventral vs. dorsal processing pathways. This model posits that conceptual representations may be mainly based upon perceptual activities in the right hemisphere and upon verbal mediation in the left side of the brain. It also assumes that conceptual knowledge based on the convergence of highly processed visual information with other perceptual data (and mainly concerning living categories) may be bilaterally represented in the anterior parts of the temporal lobes, whereas knowledge based on the integration of visual data with action schemata (namely knowledge of actions, body parts and artefacts) may be more represented in the left fronto-temporo-parietal areas.

Through the choroidal fissure: a quantitative anatomic comparison of 2 incisions and trajectories (transsylvian transchoroidal and lateral transtemporal)

BACKGROUND

We compared the transsylvian transchoroidal (TSTC) approach with the lateral transtemporal (LTT) approach. Both approaches proceed through the choroidal fissure but through different incisions and along different trajectories.

METHODS

Four fixed, silicon-injected heads (8 sides) were used. Nine strategic anatomic points within the dissections were compared between the TSTC and LTT approaches in 7 other silicon-injected heads (14 sides). Neuronavigation was used to gather coordinates from selected points of both approaches to calculate surgical angles and distances to common targets.

RESULTS

The surgical angle of the TSTC approach for the inferior choroidal point was wider compared with the LTT approach (P < .05). The surgical angles for the P2a-P2p point were similar for both approaches. In the TSTC approach, the P2-P3 point angle was smaller than in the LTT approach (P < .05). The TSTC approach provided (except for the P2-P3 point) significantly shorter distances to all defined anatomic targets compared with the LTT approach. When the posterior cerebral artery was the target in the TSTC approach, the hippocampus was retracted 3 to 8 mm compared with 8 to 13 mm in the LTT approach.

CONCLUSION

We quantitatively described anatomic features of the TSTC approach and compared them with the LTT approach. For approaching the mesial temporal region, the TSTC approach offers an adequate surgical angle and shorter or similar distances proximal to P2-P3 and requires less temporal lobe and hippocampal retraction than the LTT approach. Such information can help surgeons select the optimal approach to the mesial temporal lobe and its surrounding structures. The TSTC approach should be considered for lesions located in the medial temporal region.

Watching TV news as a memory task--brain activation and age effects

BACKGROUND

Neuroimaging studies which investigate brain activity underlying declarative memory processes typically use artificial, unimodal laboratory stimuli. In contrast, we developed a paradigm which much more closely approximates real-life situations of information encoding.

METHODS

In this study, we tested whether ecologically valid stimuli--clips of a TV news show--are apt to assess memory-related fMRI activation in healthy participants across a wide age range (22-70 years). We contrasted brain responses during natural stimulation (TV news video clips) with a control condition (scrambled versions of the same clips with reversed audio tracks). After scanning, free recall performance was assessed.

RESULTS

The memory task evoked robust activation of a left-lateralized network, including primarily lateral temporal cortex, frontal cortex, as well as the left hippocampus. Further analyses revealed that--when controlling for performance effects--older age was associated with greater activation of left temporal and right frontal cortex.

CONCLUSION

We demonstrate the feasibility of assessing brain activity underlying declarative memory using a natural stimulation paradigm with high ecological validity. The preliminary result of greater brain activation with increasing age might reflect an attempt to compensate for decreasing episodic memory capacity associated with aging.

The role of functional magnetic resonance imaging in brain surgery

New functional neuroimaging techniques are changing our understanding of the human brain, and there is now convincing evidence to move away from the classic and clinical static concepts of functional topography. In a modern neurocognitive view, functions are thought to be represented in dynamic large-scale networks. The authors review the current (limited) role of functional MR imaging in brain surgery and the possibilities of new functional MR imaging techniques for research and neurosurgical practice. A critique of current clinical gold standard techniques (electrocortical stimulation and the Wada test) is given.

Anterior temporal involvement in semantic word retrieval: voxel-based lesion-symptom mapping evidence from aphasia

Analysis of error types provides useful information about the stages and processes involved in normal and aphasic word production. In picture naming, semantic errors (horse for goat) generally result from something having gone awry in lexical access such that the right concept was mapped to the wrong word. This study used the new lesion analysis technique known as voxel-based lesion-symptom mapping to investigate the locus of lesions that give rise to semantic naming errors. Semantic errors were obtained from 64 individuals with post-stroke aphasia, who also underwent high-resolution structural brain scans. Whole brain voxel-based lesion-symptom mapping was carried out to determine where lesion status predicted semantic error rate. The strongest associations were found in the left anterior to mid middle temporal gyrus. This area also showed strong and significant effects in further analyses that statistically controlled for deficits in pre-lexical, conceptualization processes that might have contributed to semantic error production. This study is the first to demonstrate a specific and necessary role for the left anterior temporal lobe in mapping concepts to words in production. We hypothesize that this role consists in the conveyance of fine-grained semantic distinctions to the lexical system. Our results line up with evidence from semantic dementia, the convergence zone framework and meta-analyses of neuroimaging studies on word production. At the same time, they cast doubt on the classical linkage of semantic error production to lesions in and around Wernicke's area.

The orthography-specific functions of the left fusiform gyrus: evidence of modality and category specificity

We report on an investigation of the cognitive functions of an individual with a resection of the left fusiform gyrus. This individual and a group of control participants underwent testing to examine the question of whether or not there are neural substrates within the left fusiform gyrus that are dedicated to orthographic processing. We evaluated the modality specificity (written vs spoken language) and the category specificity (written language vs other visual categories) of this individual's impairments. The results clearly reveal deficits affecting lexical processes in both reading and spelling. Specifically, we find disruption of normal, rapid access to meaning from print in reading and of accurate retrieval of the spellings of words from their meaning in writing. These deficits stand in striking contrast with intact processing of spoken language and categories of visual stimuli such as line drawings of objects and faces. The modality and category specificity of the deficits provide clear evidence of neural substrates within the left-mid-fusiform gyrus that are specialized and necessary for normal orthographic processing.

Direct electrical stimulation as an input gate into brain functional networks: principles, advantages and limitations

BACKGROUND

While the fundamental and clinical contribution of direct electrical stimulation (DES) of the brain is now well acknowledged, its advantages and limitations have not been re-evaluated for a long time.

METHOD

Here, we critically review exactly what DES can tell us about cerebral function.

RESULTS

First, we show that DES is highly sensitive for detecting the cortical and axonal eloquent structures. Moreover, DES also provides a unique opportunity to study brain connectivity, since each area responsive to stimulation is in fact an input gate into a large-scale network rather than an isolated discrete functional site. DES, however, also has a limitation: its specificity is suboptimal. Indeed, DES may lead to interpretations that a structure is crucial because of the induction of a transient functional response when stimulated, whereas (1) this effect is caused by the backward spreading of the electro-stimulation along the network to an essential area and/or (2) the stimulated region can be functionally compensated owing to long-term brain plasticity mechanisms.

CONCLUSION

In brief, although DES is still the gold standard for brain mapping, its combination with new methods such as perioperative neurofunctional imaging and biomathematical modeling is now mandatory, in order to clearly differentiate those networks that are actually indispensable to function from those that can be compensated.

Neurophysiological mechanisms involved in language learning in adults

Little is known about the brain mechanisms involved in word learning during infancy and in second language acquisition and about the way these new words become stable representations that sustain language processing. In several studies we have adopted the human simulation perspective, studying the effects of brain-lesions and combining different neuroimaging techniques such as event-related potentials and functional magnetic resonance imaging in order to examine the language learning (LL) process. In the present article, we review this evidence focusing on how different brain signatures relate to (i) the extraction of words from speech, (ii) the discovery of their embedded grammatical structure, and (iii) how meaning derived from verbal contexts can inform us about the cognitive mechanisms underlying the learning process. We compile these findings and frame them into an integrative neurophysiological model that tries to delineate the major neural networks that might be involved in the initial stages of LL. Finally, we propose that LL simulations can help us to understand natural language processing and how the recovery from language disorders in infants and adults can be accomplished.

Where is the semantic system? A critical review and meta-analysis of 120 functional neuroimaging studies

Semantic memory refers to knowledge about people, objects, actions, relations, self, and culture acquired through experience. The neural systems that store and retrieve this information have been studied for many years, but a consensus regarding their identity has not been reached. Using strict inclusion criteria, we analyzed 120 functional neuroimaging studies focusing on semantic processing. Reliable areas of activation in these studies were identified using the activation likelihood estimate (ALE) technique. These activations formed a distinct, left-lateralized network comprised of 7 regions: posterior inferior parietal lobe, middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Secondary analyses showed specific subregions of this network associated with knowledge of actions, manipulable artifacts, abstract concepts, and concrete concepts. The cortical regions involved in semantic processing can be grouped into 3 broad categories: posterior multimodal and heteromodal association cortex, heteromodal prefrontal cortex, and medial limbic regions. The expansion of these regions in the human relative to the nonhuman primate brain may explain uniquely human capacities to use language productively, plan, solve problems, and create cultural and technological artifacts, all of which depend on the fluid and efficient retrieval and manipulation of semantic knowledge.

Distributed source modeling of language with magnetoencephalography: application to patients with intractable epilepsy

PURPOSE

To examine distributed patterns of language processing in healthy controls and patients with epilepsy using magnetoencephalography (MEG), and to evaluate the concordance between laterality of distributed MEG sources and language laterality as determined by the intracarotid amobarbital procedure (IAP).

METHODS

MEG was performed in 10 healthy controls using an anatomically constrained, noise-normalized distributed source solution (dynamic statistical parametric map, dSPM). Distributed source modeling of language was then applied to eight patients with intractable epilepsy. Average source strengths within temporoparietal and frontal lobe regions of interest (ROIs) were calculated, and the laterality of activity within ROIs during discrete time windows was compared to results from the IAP.

RESULTS

In healthy controls, dSPM revealed activity in visual cortex bilaterally from approximately 80 to 120 ms in response to novel words and sensory control stimuli (i.e., false fonts). Activity then spread to fusiform cortex approximately 160-200 ms, and was dominated by left hemisphere activity in response to novel words. From approximately 240 to 450 ms, novel words produced activity that was left-lateralized in frontal and temporal lobe regions, including anterior and inferior temporal, temporal pole, and pars opercularis, as well as bilaterally in posterior superior temporal cortex. Analysis of patient data with dSPM demonstrated that from 350 to 450 ms, laterality of temporoparietal sources agreed with the IAP 75% of the time, whereas laterality of frontal MEG sources agreed with the IAP in all eight patients.

DISCUSSION

Our results reveal that dSPM can unveil the timing and spatial extent of language processes in patients with epilepsy and may enhance knowledge of language lateralization and localization for use in preoperative planning.

Dynamics of event-related causality in brain electrical activity

A new method (Event-Related Causality, ERC) is proposed for the investigation of functional interactions between brain regions during cognitive processing. ERC estimates the direction, intensity, spectral content, and temporal course of brain activity propagation within a cortical network. ERC is based upon the short-time directed transfer function (SDTF), which is measured in short EEG epochs during multiple trials of a cognitive task, as well as the direct directed transfer function (dDTF), which distinguishes direct interactions between brain regions from indirect interactions via brain regions. ERC uses new statistical methods for comparing estimates of causal interactions during prestimulus "baseline" epochs and during poststimulus "activated" epochs in order to estimate event-related increases and decreases in the functional interactions between cortical network components during cognitive tasks. The utility of the ERC approach is demonstrated through its application to human electrocorticographic recordings (ECoG) of a simple language task. ERC analyses of these ECoG recordings reveal frequency-dependent interactions, particularly in high gamma (>60 Hz) frequencies, between brain regions known to participate in the recorded language task, and the temporal evolution of these interactions is consistent with the putative processing stages of this task. The method may be a useful tool for investigating the dynamics of causal interactions between various brain regions during cognitive task performance.

Hemispheric asymmetry of the arcuate fasciculus: a preliminary diffusion tensor tractography study in patients with unilateral language dominance defined by Wada test

OBJECTIVE

Lateralization of language function is a prominent feature of human brain function, and its underlying structural asymmetry has been recently reported in normal right-handed subjects. By means of diffusion tensor tractography (DTT), we investigated the asymmetry of the language network, namely, the arcuate fasciculus in patients in whom the unilateral language dominance was defined by Wada test.

METHODS

DTT was performed in 24 patients with a focal lesion or an epileptic focus outside the C-shaped segment of the arcuate fasciculus. The arcuate fasciculus was reconstructed by placing two regions-of-interest in the deep white matter lateral to the corona radiata. The pathway was then divided into one terminating in the temporal lobe (FT tract) and the other in the parietal lobe (FP tract). The relative number and volume of the FT and FP tracts in each hemisphere were submitted to repeated measure ANOVA separately, with the hemisphere as a within-subject factor and with the side of pathology as a between subject factor.

RESULTS

The FT tract showed a significantly larger number and volume in the language dominant hemisphere than in the non-dominant hemisphere, while, for the FP tract, no significant hemispheric difference was observed in the relative number or volume. There was a tendency that the FT tract was less lateralized when the pathology was located in the dominant hemisphere than in the non-dominant hemisphere.

CONCLUSION

Dominance of the FT tract in the language dominant hemisphere was demonstrated for the first time in a patient population and implicated a clinical utility of DTT for non-invasive evaluation of language lateralization. Our preliminary study might indicate reorganization of the language network in conjunction with pathology.

[Corticectomy: technical considerations]

The surgical treatment of epilepsy requires careful preparation and presents a certain number of technical specificities. The neurosurgeon must master not only the technical aspects but also the therapeutic and functional trade-off in order to modulate the procedure according to morphological and electrophysiological intraoperative data. A large number of technical variants have been developed to correspond to epileptological or functional anatomical considerations. Until this point, the choice of a particular technique does not seem to have a significant impact on the therapeutic effectiveness of surgery, and differences in results can be related to the presurgical evaluation and surgical indications. On the other hand, technical development promises to play an important role in limiting the long-term neurocognitive consequences of surgery.

Microsurgical Anatomy of the Supracerebellar Transtentorial Approach to the Posterior Mediobasal Temporal Region: Technical Considerations With a Case Illustration

Objective:

Surgical access to the posterior portion of the mediobasal temporal lobe presents a formidable challenge to neurosurgeons, and much controversy still exists regarding the selection of the surgical approach to this region. The supracerebellar transtentorial (SCTT) approach to the posterior mediobasal temporal region can be used as an alternative to the subtemporal or transtemporal approaches. The aim of this study was to demonstrate the surgical anatomy of the SCTT approach and review the gyral, sulcal, and vascular anatomy of the posterior mediobasal temporal lobe. The use of this approach in the resection of a ganglioglioma located in the left posterior parahippocam-pal gyrus is illustrated.

Methods:

The SCTT approach to the posterior parahippocampal gyrus was performed on three silicone-injected cadaveric heads. The gyral, sulcal, and arterial anatomy of the posterior mediobasal temporal lobe was studied in six formalin-fixed injected hemispheres.

Results:

The SCTT approach provided a direct path to the posterior mediobasal temporal lobe and exposed the posterior parahippocampal gyrus as well as the adjacent gyri in all of the cadaveric specimens. Through this approach, gross total resection of the ganglioglioma was possible in our patient.

Conclusion:

The SCTT approach provided a viable surgical route to the posterior mediobasal temporal lobe in the cadaveric studies. This approach provides an advantage over the subtemporal and transtemporal routes in that there is less temporal lobe retraction.

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

Update on epilepsy and cerebral localization

The field of epilepsy has contributed significantly to localization of neurologic function, particularly in the neocortex. Methodologies such as cortical stimulation, positron emission tomography, functional MRI, trans-cranial magnetic stimulation, surgical resection, and magnetoencephalography have been used successfully in patients with epilepsy to locate specific functions, primarily for the purpose of defining eloquent cortex before surgical resections. The left hemisphere serves language-related functions and verbal memory in most people, whereas the right hemisphere serves some language function in addition to perceiving most components of music and other forms of nonverbal material. Both hemispheres cooperate in understanding spatial relationships. Studies in patients with developmental abnormalities have enriched our understanding of localization of function within the cortex. Future studies may help us understand the sequence in which specific regions are activated during specific tasks and determine which regions are necessary for tasks and which are supplementary. The ability to predict preoperatively the effect of removal of specific tissues would benefit surgical planning for all patients who undergo cortical resections, including those with epilepsy.

Cortical language mapping in epilepsy: a critical review

One challenge in dominant hemisphere epilepsy surgery is to remove sufficient epileptogenic tissue to achieve seizure freedom without compromising postoperative language function. Electrical stimulation mapping (ESM) of language was developed specifically to identify essential language cortex in pharmacologically intractable epilepsy patients undergoing left hemisphere resection of epileptogenic cortex. Surprisingly, the procedure remains unstandardized, and limited data support its clinical validity. Nevertheless, ESM for language mapping has likely minimized postoperative language decline in numerous patients, and has generated a wealth of data elucidating brain-language relations. This article reviews the literature on topographical patterns of language organization inferred from ESM, and the influence of patient characteristics on these patterns, including baseline ability level, age, gender, pathology, degree of language lateralization and bilingualism. Questions regarding clinical validity and limitations of ESM are discussed. Finally, recommendations for clinical practice are presented, and theoretical questions regarding ESM and the findings it has generated are considered.

Review of language organisation in bilingual patients: what can we learn from direct brain mapping?

Although the majority of people worldwide are bilingual, the brain representation of language in bilingual persons is still a matter of debate. Since the beginning of the studies conducted on bilinguals, most authors denied that learning a new language requires a new semantic processing or the involvement of new cortical areas. In this paper, we review neurosurgical studies using direct electrocortical or subcortical stimulation techniques for brain mapping in bilingual subjects and compare this data with that obtained from other brain mapping methods. The authors focused on the most controversial issue whether multiple languages are represented in common or distinct cerebral areas. Seven direct brain mapping studies from different teams focused on bilingualism and multilingualism. All these studies showed that even if cerebral representation of language in multilingual patients could be grossly located in the same cortical region, it was possible to individualise distinct language-specific areas by direct cortical stimulation in the dominant frontal and temporo-parietal regions. Task- and language-specific sites were also described, demonstrating an important specialisation of some cortical areas. Using subcortical stimulation, some authors were able to find specific white matter tracts for different languages. Finally, all authors recommend in bilingual patients who need brain mapping for neurosurgical purpose to test all languages in which the subjects are fluent.

Microsurgical approaches to the medial temporal region: an anatomical study

OBJECTIVE

To describe the surgical anatomy of the anterior, middle, and posterior portions of the medial temporal region and to present an anatomic-based classification of the approaches to this area.

METHODS

Twenty formalin-fixed, adult cadaveric specimens were studied. Ten brains provided measurements to compare different surgical strategies. Approaches were demonstrated using 10 silicon-injected cadaveric heads. Surgical cases were used to illustrate the results by the different approaches. Transverse lines at the level of the inferior choroidal point and quadrigeminal plate were used to divide the medial temporal region into anterior, middle, and posterior portions. Surgical approaches to the medial temporal region were classified into four groups: superior, lateral, basal, and medial, based on the surface of the lobe through which the approach was directed. The approaches through the medial group were subdivided further into an anterior approach, the transsylvian transcisternal approach, and two posterior approaches, the occipital interhemispheric and supracerebellar transtentorial approaches.

RESULTS

The anterior portion of the medial temporal region can be reached through the superior, lateral, and basal surfaces of the lobe and the anterior variant of the approach through the medial surface. The posterior group of approaches directed through the medial surface are useful for lesions located in the posterior portion. The middle part of the medial temporal region is the most challenging area to expose, where the approach must be tailored according to the nature of the lesion and its extension to other medial temporal areas.

CONCLUSION

Each approach to medial temporal lesions has technical or functional drawbacks that should be considered when selecting a surgical treatment for a given patient. Dividing the medial temporal region into smaller areas allows for a more precise analysis, not only of the expected anatomic relationships, but also of the possible choices for the safe resection of the lesion. The systematization used here also provides the basis for selection of a combination of approaches.

What's in a word? (... and why it matters)

Brain Stimulation Reveals Critical Auditory Naming Cortex

Hamberger MJ, Seidel WT, McKhann GM 2nd, Perrine K, Goodman RR

Brain 2005;128(Pt 11):2742–2749.

One challenge in dominant temporal lobe epilepsy surgery is to remove sufficient epileptogenic tissue without compromising postoperative language functioning. Preresection electrical stimulation mapping enables identification of language areas that can be spared from resection, and also provides a unique opportunity to investigate brain–language relationships. Visual object naming is the gold standard for identifying “essential” language cortex; however, sparing visual naming (VN) sites has not reliably prevented postoperative language decline. In addition to visual object naming, we included a more “ecologically valid” auditory description naming task in our preresection cortical mapping protocol. Of the seven patients who had auditory naming (AN) sites removed, six declined postoperatively, whereas of the 12 patients who did not have AN sites removed, only 3 declined postoperatively ( p = 0.02), suggesting an association between AN site removal and postoperative naming decline. Interestingly, although VN sites were preserved in all patients, AN site removal resulted in decline in both auditory and VN tasks. These findings not only have potentially critical clinical significance, but also argue for modality specificity, with considerable integration within the semantic system.

Does the left inferior longitudinal fasciculus play a role in language? A brain stimulation study

Although advances in diffusion tensor imaging have enabled us to better study the anatomy of the inferior longitudinal fasciculus (ILF), its function remains poorly understood. Recently, it was suggested that the subcortical network subserving the language semantics could be constituted, in parallel with the inferior occipitofrontal fasciculus, by the left ILF, joining the posterior occipitotemporal regions to the temporal pole, then relayed by the uncinate fasciculus connecting the anterior temporal pole to the frontobasal areas. Nevertheless, this hypothesis was solely based on neurofunctional imaging, allowing a cortical mapping but with no anatomofunctional information regarding the white matter. Here, we report a series of 12 patients operated on under local anaesthesia for a cerebral low-grade glioma located within the left temporal lobe. Before and during resection, we used the method of intraoperative direct electrostimulation, enabling us to perform accurate and reliable anatomofunctional correlations both at cortical and subcortical levels. In order to map the ILF. Using postoperative MRI, we correlated these functional findings with the anatomical locations of the sites where language disturbances were elicited by stimulations, both at cortical and subcortical levels. Our goal was to study the potential existence of parallel and distributed language networks crossing the left dominant temporal lobe, subserved by distinct subcortical pathways--namely the inferior occipitofrontal fasciculus and the ILF. Intraoperative stimulation of the anterior and middle temporal cortex elicited anomia in four patients. At the subcortical level, semantic paraphasia were induced in seven patients during stimulation of the inferior occipitofrontal fasciculus, and phonological paraphasia was generated in seven patients by stimulating the arcuate fasciculus. Interestingly, subcortical stimulation never elicited any language disturbances when performed at the level of the ILF. In addition, following a transient postoperative language deficit, all patients recovered, despite the resection of at least one part of the ILF, as confirmed by control MRI. On the basis of these results, we suggest that the "semantic ventral stream" could be constituted by at least two parallel pathways within the left dominant temporal lobe: (i) a direct pathway, the inferior occipitofrontal fasciculus, that connects the posterior temporal areas and the orbitofrontal region, crucial for language semantic processing, since it elicits semantic paraphasia when stimulated; (ii) and also possibly an indirect pathway subserved by the ILF, not indispensable for language, since it can be compensated both during stimulation and after resection.

Impact of modality and linguistic complexity during reading and listening tasks

Reading and understanding speech are usually considered as different manifestations of a single cognitive ability, that of language. In this study, we were interested in characterizing the specific contributions of input modality and linguistic complexity on the neural networks involved when subjects understand language. We conducted an fMRI study during which 10 right-handed male subjects had to read and listen to words, sentences and texts in different runs. By comparing reading to listening tasks, we were able to show that the cerebral regions specifically recruited by a given modality were circumscribed to unimodal and associative unimodal cortices associated with the task, indicating that higher cognitive processes required by the task may be common to both modalities. Such cognitive processes involved a common phonological network as well as lexico-semantic activations as revealed by the conjunction between all reading and listening tasks. The restriction of modality-specific regions to their corresponding unimodal cortices was replicated when looking at brain areas showing a greater increase during the comprehension of more complex linguistic units than words (such as sentences and texts) for each modality. Finally, we discuss the possible roles of regions showing pure effect of linguistic complexity, such as the anterior part of the superior temporal gyrus and the ventro-posterior part of the middle temporal gyrus that were activated for sentences and texts but not for isolated words, as well as a text-specific region found in the left posterior STS.

Modulation of inferotemporal cortex activation during verbal working memory maintenance

Regions of the left inferotemporal cortex are involved in visual word recognition and semantics. We utilized functional magnetic resonance imaging to localize an inferotemporal language area and to demonstrate that this area is involved in the active maintenance of visually presented words in working memory. Maintenance activity in this inferotemporal area showed an effect of memory load for words, but not pseudowords. In the absence of visual input, the selective modulation of this language-related inferotemporal area for the maintenance of words is accompanied by an increased functional connectivity with left prefrontal cortex. These results demonstrate an involvement of inferotemporal cortex in verbal working memory and provide neurophysiological support for the notion that nonphonological language representations can be recruited in the service of verbal working memory. More generally, they suggest that verbal working memory should be conceptualized as the frontally guided, sustained activation of pre-existing cortical language representations.

Subtemporal Hippocampectomy Preserving the Basal Temporal Language Area for Intractable Mesial Temporal Lobe Epilepsy: Preliminary Results

PURPOSE

Decline in verbal memory as a surgical complication remains an unresolved problem in mesial temporal lobe epilepsy. Some areas in the temporal lobe associated with the language function, often including the basal temporal language area, have been removed or transected by conventional surgical procedures. The authors defined the basal temporal language area and removed only the epileptogenic zone with a subtemporal approach.

METHODS

The basal temporal language area was evaluated by using long-term subdural electrodes in five patients with language-dominant-side mesial temporal lobe epilepsy. While preserving this area, the hippocampus and the parahippocampal gyrus were removed by using a combined subtemporal, transventricular, transchoroidal fissure approach. Verbal memory performance was assessed with the Wechsler Memory Scale-Revised (WMS-R) before and after the operation.

RESULTS

The basal temporal language area, defined as a part of the inferior temporal gyrus, the fusiform gyrus, and the parahippocampal gyrus, was spared by entering the temporal horn via collateral sulcus. Verbal memory was significantly improved by 3 months and 1 year after the operation.

CONCLUSIONS

In language-dominant-side mesial temporal lobe epilepsy, preserving the basal temporal language area would have potential to improve verbal memory outcomes after removal of the epileptogenic zone.

Language lateralization and the role of the fusiform gyrus in semantic processing in young children

We used blood oxygen-dependent (BOLD) fMRI technique at 1.5 T to examine brain regions associated with language comprehension in normally developing children, age 5 to 10 years. Twenty-three children participated in the study using an auditory semantic decision task which varied in task difficulty. Analysis of individual participants' data showed patterns of activation largely consistent with previous neuroimaging findings in adult language processing. Group data analysis also showed a strong left-lateralized pattern of activation that closely resembles those typically observed in adults. In addition, significant activation in the left fusiform gyrus was observed and was associated with task accuracy. This finding suggests that auditory semantic processing in young children may recruit cortical regions associated with word reading in adults prior to the initiation of a semantic category decision, a process which is consistent with patterns of early word recognition process and language development.

Direct intracranial, FMRI, and lesion evidence for the causal role of left inferotemporal cortex in reading

Models of the "visual word form system" postulate that a left occipitotemporal region implements the automatic visual word recognition required for efficient reading. This theory was assessed in a patient in whom reading was explored with behavioral measures, fMRI, and intracranial local field potentials. Prior to surgery, when reading was normal, fMRI revealed a normal mosaic of ventral visual selectivity for words, faces, houses, and tools. Intracranial recordings demonstrated that the left occipitotemporal cortex responded with a short latency to conscious but also to subliminal words. Surgery removed a small portion of word-responsive occipitotemporal cortex overlapping with the word-specific fMRI activation. The patient developed a marked reading deficit, while recognition of other visual categories remained intact. Furthermore, in the post-surgery fMRI map of visual cortex, only word-specific activations disappeared. Altogether, these results provide direct evidence for the causal role of the left occipitotemporal cortex in the recognition of visual words.

The intersubject and intrasubject reproducibility of FMRI activation during three encoding tasks: implications for clinical applications

The goal of the present study was to evaluate the inter- and intrasubject reproducibility of FMRI activation for three memory encoding tasks previously used in the context of presurgical functional mapping. The primary region of interest (ROI) was the medial temporal lobe (MTL). Comparative ROIs included the inferior frontal and fusiform gyri which are less affected by susceptibility-induced signal losses than the MTL regions. Eighteen subjects were scanned using three memory encoding paradigms: word-pair, pattern, and scene encoding. Nine subjects underwent repeat scanning. Intersubject reproducibility of FMRI activation was evaluated by examining the percent of subjects who showed activation within a given ROI and the range to which individual laterality indices (LIs) varied from the mean. Intrasubject test-retest reproducibility was evaluated by examining the LI test-retest correlation, the average difference between LIs from two separate imaging sessions, and concordance ratios of activation volumes (R(volume) and R(overlap)). For scene encoding the reproducibility of activation volume and LIs within the MTL were as good as or better than the reproducibility within the fusiform and inferior frontal ROIs. For pattern encoding and word-pair encoding, the reproducibility of activation volume and LIs within the MTL tended to be worse compared to the fusiform and inferior frontal ROIs. The differences in FMRI reproducibility appeared more dependent on the task than the susceptibility effects. The results of this study suggest that FMRI-based assessment of the neural substrates of memory using a scene encoding task may be a useful clinical tool.

Category-specific cortical mapping: color-naming areas

OBJECT

It has been hypothesized that a certain degree of specialization exists within language areas, depending on some specific lexical repertories or categories. To spare hypothetical category-specific cortical areas and to gain a better understanding of their organization, the authors studied patients who had undergone electrical stimulation mapping for brain tumors and they compared an object-naming task with a category-specific task (color naming).

METHODS

Thirty-six patients with no significant preoperative language deficit were prospectively studied during a 2-year period. Along with a reading task, both object- and color-naming tasks were used in brain mapping. During color naming, patients were asked to identify 11 visually presented basic colors. The modality specificity of the color-naming sites found was subsequently tested by asking patients to retrieve the color attributes of objects. High individual variability was observed in language organization among patients and in the tasks performed. Significant interferences in color naming were found in traditional language regions-that is, Broca (p < 0.003) and Wernicke centers (p = 0.05)--although some color-naming areas were occasionally situated outside of these regions. Color-naming interferences were exclusively localized in small cortical areas (< 1 cm2). Anatomical segregation of the different naming categories was apparent in 10 patients; in all, 13 color-specific naming areas (that is, sites evoking no object-naming interference) were detected in the dominant-hemisphere F3 and the supramarginal, angular, and posterior parts of the temporal gyri. Nevertheless, no specific brain region was found to be consistently involved in color naming (p > 0.05). At five sites, although visually presented color-naming tasks were impaired by stimulation, auditory color naming (for example, "What color is grass?") was performed with no difficulty, showing that modality-specific areas can be found during naming.

CONCLUSIONS

Within language areas, a relative specialization of cortical language areas for color naming can be found during electrical stimulation mapping.

Meta-analyzing left hemisphere language areas: phonology, semantics, and sentence processing

The advent of functional neuroimaging has allowed tremendous advances in our understanding of brain-language relationships, in addition to generating substantial empirical data on this subject in the form of thousands of activation peak coordinates reported in a decade of language studies. We performed a large-scale meta-analysis of this literature, aimed at defining the composition of the phonological, semantic, and sentence processing networks in the frontal, temporal, and inferior parietal regions of the left cerebral hemisphere. For each of these language components, activation peaks issued from relevant component-specific contrasts were submitted to a spatial clustering algorithm, which gathered activation peaks on the basis of their relative distance in the MNI space. From a sample of 730 activation peaks extracted from 129 scientific reports selected among 260, we isolated 30 activation clusters, defining the functional fields constituting three distributed networks of frontal and temporal areas and revealing the functional organization of the left hemisphere for language. The functional role of each activation cluster is discussed based on the nature of the tasks in which it was involved. This meta-analysis sheds light on several contemporary issues, notably on the fine-scale functional architecture of the inferior frontal gyrus for phonological and semantic processing, the evidence for an elementary audio-motor loop involved in both comprehension and production of syllables including the primary auditory areas and the motor mouth area, evidence of areas of overlap between phonological and semantic processing, in particular at the location of the selective human voice area that was the seat of partial overlap of the three language components, the evidence of a cortical area in the pars opercularis of the inferior frontal gyrus dedicated to syntactic processing and in the posterior part of the superior temporal gyrus a region selectively activated by sentence and text processing, and the hypothesis that different working memory perception-actions loops are identifiable for the different language components. These results argue for large-scale architecture networks rather than modular organization of language in the left hemisphere.

High-frequency gamma oscillations and human brain mapping with electrocorticography

Invasive EEG recordings with depth and/or subdural electrodes are occasionally necessary for the surgical management of patients with epilepsy refractory to medications. In addition to their vital clinical utility, electrocorticographic (ECoG) recordings provide an unprecedented opportunity to study the electrophysiological correlates of functional brain activation in greater detail than non-invasive recordings. The proximity of ECoG electrodes to the cortical sources of EEG activity enhances their spatial resolution, as well as their sensitivity and signal-to-noise ratio, particularly for high-frequency EEG activity. ECoG recordings have, therefore, been used to study the event-related dynamics of brain oscillations in a variety of frequency ranges, and in a variety of functional-neuroanatomic systems, including somatosensory and somatomotor systems, visual and auditory perceptual systems, and cortical networks responsible for language. These ECoG studies have confirmed and extended the original non-invasive observations of ERD/ERS phenomena in lower frequencies, and have discovered novel event-related responses in gamma frequencies higher than those previously observed in non-invasive recordings. In particular, broadband event-related gamma responses greater than 60 Hz, extending up to approximately 200 Hz, have been observed in a variety of functional brain systems. The observation of these "high gamma" responses requires a recording system with an adequate sampling rate and dynamic range (we use 1000 Hz at 16-bit A/D resolution) and is facilitated by event-related time-frequency analyses of the recorded signals. The functional response properties of high-gamma activity are distinct from those of ERD/ERS phenomena in lower frequencies. In particular, the timing and spatial localization of high-gamma ERS often appear to be more specific to the putative timing and localization of functional brain activation than alpha or beta ERD/ERS. These findings are consistent with the proposed role of synchronized gamma oscillations in models of neural computation, which have in turn been inspired by observations of gamma activity in animal preparations, albeit at somewhat lower frequencies. Although ECoG recordings cannot directly measure the synchronization of action potentials among assemblies of neurons, they may demonstrate event-related interactions between gamma oscillations in macroscopic local field potentials (LFP) generated by different large-scale populations of neurons engaged by the same functional task. Indeed, preliminary studies suggest that such interactions do occur in gamma frequencies, including high-gamma frequencies, at latencies consistent with the timing of task performance. The neuronal mechanisms underlying high-gamma activity and its unique response properties in humans are still largely unknown, but their investigation through invasive methods is expected to facilitate and expand their potential clinical and research applications, including functional brain mapping, brain-computer interfaces, and neurophysiological studies of human cognition.

Lateralizing signs during seizures in focal epilepsy

This article reviews lateralizing semiological signs during epileptic seizures with respect to prediction of the side of the epileptogenic zone and, therefore, presurgical diagnostic value. The lateralizing significance of semiological signs and symptoms can frequently be concluded from knowledge of the cortical representation. Visual, auditory, painful, and autonomic auras, as well as ictal motor manifestations, e.g., version, clonic and tonic activity, unilateral epileptic spasms, dystonic posturing and unilateral automatisms, automatisms with preserved responsiveness, ictal spitting and vomiting, emotional facial asymmetry, unilateral eye blinking, ictal nystagmus, and akinesia, have been shown to have lateralizing value. Furthermore, ictal language manifestations and postictal features, such as Todd's palsy, postictal aphasia, postictal nosewiping, postictal memory dysfunction, as well as peri-ictal water drinking, peri-ictal headache, and ipsilateral tongue biting, are reviewed. Knowledge and recognition of semiological lateralizing signs during seizures is an important component of the presurgical evaluation of epilepsy surgery candidates and adds further information to video/EEG monitoring, neuroimaging, functional mapping, and neuropsychological evaluation.

Processing of Japanese morphogram and syllabogram in the left basal temporal area: electrical cortical stimulation studies

Language functions in the left basal temporal area (LBTA) were investigated using electrical cortical stimulation during functional mapping in six Japanese patients with refractory epilepsy. This study provides the first direct evidence that kana (Japanese syllabogram) is processed in the LBTA. Electrical stimulation of some areas within LBTA induced disturbance in overt reading of kana words only in the first trials, with no errors in the subsequent trials. By contrast, stimulation of the same area caused obvious disturbance in kana non-word reading in all trials. Since a kana word carries both meaning and sound while a kana non-word carries only sounds of a letter string, the contrasting results of partial and complete disturbance imply a possibility that there are two distinct pathways for kana reading: one dealing with both phonological and semantic aspects of the words and the other dealing only with phonological aspect. Kanji words (Japanese morphogram) and objects/pictures were found to be processed in an area different from the area for the kana non-word processing. Furthermore, the present study also identified the common area for processing kanji reading and object/picture naming. There were no errors in matching pictures with kanji words, indicating that concepts of pictures and meanings of kanji words were not interfered by the electrical stimulation of that area. The new insight provides a clue for partial description of processing pathways for language-related visual information in LBTA. Three types of information (morphological, phonological, and semantic) are conveyed together at some stages and are separated into different routes at some other stages.

Interictal language functions in temporal lobe epilepsy

OBJECTIVE

To evaluate interictal language functions in patients with medically intractable left and right sided mesial temporal lobe epilepsy (TLE).

METHODS

Spontaneous speech, language comprehension, confrontation naming, repetition, reading, writing, and word fluency were examined in 12 patients with left sided TLE and 11 patients with right sided TLE.

RESULTS

Four patients out of 23 displayed language deficits in more than one language domain. Three further patients exhibited isolated language deficits. Linguistic deficits were observed in both left TLE and right TLE. In quantitative analyses left and right TLE only differed in spontaneous speech (p = 0.02); no difference was found in other language functions, laterality quotient of Wada test, or overall IQ. Qualitative error analysis of object naming, however, showed typical errors associated only with left TLE. Patients with linguistic deficits were older at testing compared to patients without linguistic deficits (p = 0.003), whereas other factors including side of TLE, handedness, educational level, age at epilepsy onset, and duration of epilepsy did not differ between groups.

CONCLUSIONS

Possible explanations for these findings include neuronal cell loss and deafferentiation in cortical areas, and disruption of the basal temporal language area pathways. Our study suggests that some patients with chronic mesial TLE exhibit linguistic deficits when specifically tested, and underlines the need to routinely investigate linguistic functions in TLE.

Electrocorticographic high gamma activity versus electrical cortical stimulation mapping of naming

Subdural electrocorticographic (ECoG) recordings in patients undergoing epilepsy surgery have shown that functional activation is associated with event-related broadband gamma activity in a higher frequency range (>70 Hz) than previously studied in human scalp EEG. To investigate the utility of this high gamma activity (HGA) for mapping language cortex, we compared its neuroanatomical distribution with functional maps derived from electrical cortical stimulation (ECS), which remains the gold standard for predicting functional impairment after surgery for epilepsy, tumours or vascular malformations. Thirteen patients had undergone subdural electrode implantation for the surgical management of intractable epilepsy. Subdural ECoG signals were recorded while each patient verbally named sequentially presented line drawings of objects, and estimates of event-related HGA (80-100 Hz) were made at each recording site. Routine clinical ECS mapping used a subset of the same naming stimuli at each cortical site. If ECS disrupted mouth-related motor function, i.e. if it affected the mouth, lips or tongue, naming could not be tested with ECS at the same cortical site. Because naming during ECoG involved these muscles of articulation, the sensitivity and specificity of ECoG HGA were estimated relative to both ECS-induced impairments of naming and ECS disruption of mouth-related motor function. When these estimates were made separately for 12 electrode sites per patient (the average number with significant HGA), the specificity of ECoG HGA with respect to ECS was 78% for naming and 81% for mouth-related motor function, and equivalent sensitivities were 38% and 46%, respectively. When ECS maps of naming and mouth-related motor function were combined, the specificity and sensitivity of ECoG HGA with respect to ECS were 84% and 43%, respectively. This study indicates that event-related ECoG HGA during confrontation naming predicts ECS interference with naming and mouth-related motor function with good specificity but relatively low sensitivity. Its favourable specificity suggests that ECoG HGA can be used to construct a preliminary functional map that may help identify cortical sites of lower priority for ECS mapping. Passive recordings of ECoG gamma activity may be done simultaneously at all electrode sites without the risk of after-discharges associated with ECS mapping, which must be done sequentially at pairs of electrodes. We discuss the relative merits of these two functional mapping techniques.

Magnetoencephalographic localization of the basal temporal language area

Magnetoencephalography (MEG) recordings were made on 25 native English-speaking patients with localization-related epilepsy during a semantic language task (verb generation). Eighteen right-handed subjects with normal reading ability had MEG scans performed during the same language task. MEG data was analyzed by MR-FOCUSS, a current density imaging technique. Detectable MEG signals arising from activation in the left fusiform gyrus, also known as the basal temporal language area (BTLA), occurred at 167 +/- 18 ms (n = 43) in all subjects. The BTLA has been associated with a variety of language production and comprehension tasks involving processing of semantic, orthographic, and phonologic information. MEG may become an important tool in efforts to further define the linguistic operations of specific regions within this language area.