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Neurophysiology of language and cognitive mapping
7.1
Introduction
Language and cognitive mapping is recommended in patients with intrinsic tumors involving language or cognitive pathways. Traditionally, this type of mapping was limited to patients harboring lesions in the so-called dominant hemisphere, where language was thought to be uniquely located. Nowadays, however, due to the evolving knowledge of language and cognitive network organization, mapping has been extended also to patients with tumors located in the so-called nondominant hemisphere, in order to maximally preserve patients’ functional integrity, maintaining functions allocated in this hemisphere, with the aim of preserving full quality of life (QoL).
The need for functions preservation while obtaining a maximal tumor resection is nowadays considered imperative in intrinsic brain tumor surgery. This aim, achieved through brain mapping techniques, has been shown to have the most relevant oncological impact, both in terms of amount of tissue removal and functional preservation. This has been obtained by the introduction of the brain mapping techniques . This term refers to a group of techniques, which allow to safely and effectively remove tumors preserving at the same time the functional integrity of the patients. This can be achieved by the identification and preservation at time of surgery of cortical and subcortical sites, which are involved in different separate functions. The identification of functions during surgery is performed by applying electrical stimulation at cortical and subcortical level during resection, and by looking for evoked responses, either when the patient is at rest or when is performing a task. Task selection depends on the function to be explored. The concept of detecting and preserving the essential functional cortical and subcortical sites has been recently defined as functional neuro-oncological surgery, and it is characterized by extending resection till functional boundaries are encountered, independently from where they are located. This functional approach exploits the functional reorganization of patient brain, allowing to remove as much as tumor is feasible, possibly extending resection far beyond tumor margins visible and detectable on conventional MR images, preserving at the same time the full patient functional integrity . Language and cognitive functions need to be preserved during this approach, and in this chapter, we will summarize the main activities related to this process.
7.2
Language mapping
Language mapping is a part of the cognitive mapping, and it aims at detecting language functional sites at cortical and subcortical level during intrinsic brain tumor surgery. Language is a complex function that includes different components such as speech (language articulation) along with semantic and phonemic components, reading and writing.
In the following section, we will describe the most accepted model of language organization and the workup usually requested to perform a language mapping during brain surgery, along with the main activities requested to identify the various language components during the surgical procedure.
7.2.1
Current model of language organization
Various models of language organization have been proposed in the recent past (for deep insight read Hickok, Nat Rev Neurosci 2012 ). According to the current view, human language does not depend on specific cortical areas and it is not side dependent, but it is based on a complex, mostly bilateral, subcortical system. The level of complexity depends also on gender, education, and presumably race factors. According to this view a ventral and a dorsal stream can be identified ( Fig. 7.1 ). The ventral stream maps acoustic speech input onto conceptual and semantic representations; it supports the perception and recognition of auditory objects such as speech; it has a bilateral activity, although the dominant hemisphere seems to be predominant. It comprises different fascicles, the main represented by the inferior fronto-occipital fasciculus, running from the occipital to the frontal lobe and mainly involved in semantic inputs, the inferior longitudinalis fasciculus (involved in the processing of auditive and acoustic input into semantic one), and the uncinatus (involved in proper name retrieval). Stimulation of the ventral stream results in anomia or hesitation. The dorsal stream supports an interface with the motor system (sensory motor integration), and it is mostly unilateral being mainly located within the dominant hemisphere. It is composed mainly by the arcuatus, a fascicle connecting the temporal and parietal lobe with the frontal lobe, running into and over the deep insula. Stimulation of the dorsal stream results in speech arrest or phonological paraphasia. Additional fascicles involved in language production are the superior longitudinalis and subcallosum. Both systems are characterized by a certain degree of lateralization and are predominant into the dominant hemisphere. However, female brains are usually characterized by a higher degree of bilateral organization. Two main hubs characterized the current model of language organization: a posterior one, located into the parietal lobe (temporo-parietal connection) and involved in decoding auditory inputs onto conceptual and semantic representation; an anterior one, the ventral premotor area (vPM) mainly involved in sensory motor integration and in transforming semantic and phonemic inputs into words (speech articulation).
7.2.2
Language mapping workup
Language mapping workup includes all the activities to be put in place to perform a language mapping during glioma surgery. The workup is comprehensive set of pre- and postoperative activities, along with a specific intraoperative part. The intraoperative part is comprehensive of the neurophysiological approach.
7.2.2.1
Preoperative workup
The aim of preoperative workup is to evaluate the degree of functional reorganization reached by the brain of the individual patient, to examine patient language, and to prepare the patient for the intraoperative testing. In addition, it also evaluates the possible ability of the patient to perform the testing, specifically preparing the patient for the awake phase. Neuropsychologists or speech therapists are in charge of performing these activities and their presence is a mandatory both pre- and intraoperatively.
Preoperative workup mainly consists of neuropsychological testing and language examination.
7.2.2.2
Neuropsychological evaluation
The neuropsychological evaluation assesses the degree of functional reorganization achieved by the individual patient and prepares the patient for the intraoperative testing. Extensive testing is used and usually explores all functional domains, comprehensive of memory, language, praxis, executive functions, and fluid intelligence ( Table 7.1 ) . The set of tests carried out varies according to language and population (to which must be normalized), taking also into account the age and level of education. It is mandatory to use an extensive and standardized neuropsychological assessment in order to evaluate how specific brain functions might have been affected and to highlight residual cognitive strengths that may help patients to adapt and cope with their eventual deficits. Moreover, a detailed assessment should cover also behavioral, cognitive, and psychological aspects to identify the most effective treatment extending testing also to those nonverbal or motor functions that sometime are ignored but potentially have a strong impact on patient’s daily activities and QoL. The selection of the intraoperative task must be based both on the eventual network(s) involved and on patient’s characteristics (job and hobbies) in order to preserve the individual needs of each patient.
Table 7.1
Subcortical language mapping tasks.
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| Temporal lobe | Insula |
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7.2.2.3
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