The preservation of the primary visual areas on both sides of the calcarine sulcus is in some cases of utmost importance. Indeed, it should be reminded to the patient that the driving license is no more valid in case of hemianopsia. As initially reported in 1968 [38], electrical stimulation generates mainly positive visual phenomena, like phosphene, flash of light, … Of note, as for any primary motor or sensory areas, there is almost no plasticity, meaning that anatomical landmarks allow to locate primary visual areas reliably.

Language is the most widely mapped function during awake surgery. Since the very first reports by Penfield, the technology did not change that much: applying electrical stimulation for a period of 3–4 s disturbs the language task. However, the classification of the different types of observed errors has been only recently clarified: it is of utmost importance to distinguish motor arrest, speech arrest, and anomia. A simple algorithm has been proposed, in order to optimize the initial testing for differentiating the areas to these different types of error [39]. Most commonly, the language assessment is performed through a picture naming task. This raises a still debated question: is it enough to assess a function as complex as language just by naming pictures? In a first response, it is important to notice that picture naming allows to investigate the two main components of language: phonology and semantics [40]. Indeed, the speech therapist can make the on-line distinction between phonological paraphasia (e.g. log instead of dog) versus semantic paraphasia (for e.g. cat instead of dog) [41]. The current anatomo-functional model maps these two components on the dorsal pathway for phonology and ventral stream for semantics [42]. However, on the cortical surface, there is no clear-cut spatial segregation between phonological and semantic sites [43]. There is for each domain, three clusters in the left hemisphere: one in the pars triangularis, one at the posterior end of the middle frontal gyrus, and one in the posterior part of the superior temporal gyrus. The great well known inter-patients variability [44] proves once again the necessity of intraoperative mapping in an awake patient. Of note, there is a striking resemblance between this map and the clusters evidenced by a meta-analysis of T-fmRI studies [45]. This overlapping suggests that the phonological and semantic systems are not anatomically separable at the 5 mm scale at the cortical level. Of note, when the patient’s response is an anomia, one cannot conclude whether this is a disturbance of the phonological or semantic system, or both.

Hence, the ability to name a picture insures that both phonological and lexico-semantic abilities are preserved. However, it should be kept in mind that naming does not warrant full semantic judgment. Indeed, a dissociation has been shown between picture naming and pyramid-palm-tree test (PPTT), a supramodal semantic association task: in some fronto-opercular and superior temporal sites (see also Fig. 5.1), patients were able to name pictures, without being able to make semantic association [46, 47].

Moreover, language requires more than phonology and basic semantics: syntax, that is the ability to find the meaning of a sentence from rules and links between words, is also an essential part of language. Interestingly, although some patients exhibit difficulties to understand syntactically complex sentences in the immediate postoperative course, permanent syntactic deficit after picture-naming based awake surgery are rarely observed [48]. Similarly, there are no reported case of patients with a long lasting deficit of repetition (i.e. a language task with an auditory rather than visual input). Still, many authors have proposed numerous tasks for better evaluating language intraoperatively (see following review papers [49–51]), and some new protocols are still under investigation [52, 53].

Nevertheless, it should be mentioned that proper name deficits have been reported after left anterior temporal lobectomy [54, 55]. Whereas this deficit can be prevented by adding a task of naming famous faces, this should be balanced on a case-by-case basis with the oncological benefit: indeed, the impact in daily life of an inability to name proper names is highly variable for each patient.

Similarly, one should not forget to test patients in mother-tongue and secondary learned languages for bilingual patients [56–60].

Last but not least, reading is an essential part of language abilities. Removal of visual word form area (VWFA) has been shown to result in long lasting reading deficit [61], while its identification by DES allows to preserve reading abilities [62, 63]. Apart in the basal temporo-occipital areas, different studies reported specific reading disturbances when stimulating posterior superior left temporal gyrus and left supramarginal gyrus [64], but also posterior part of inferior and middle left frontal gyrus [65]. These latter sites were generally superimposed or close to naming sites, meaning that they would have been preserved even if lecture would not have been tested. Finally, stimulation of FEF areas also induced reading troubles, by generating involuntary ocular movements (but this area can be detected just by looking at eyes movements [66]). On a practical point of view, these results show that reading per se needs to be tested only in (left) dominant posterior temporo-occipital regions. Of note, disturbances were very infrequently observed in the anterior temporal lobe. This is somehow in disagreement with the triangle model, in which it has been shown recently that the semantico-phonological conversion in reading is thought to be supported by the anterior temporal lobe [67]. Moreover, in this model, the role of posterior inferior and middle frontal regions was not assigned. Hence, electrostimulation datas should be better integrated in the currents neurocomputational models of reading.

It has been well known from stroke studies that right parietal lobe lesions can result in spatial neglect of the opposite hemi-space. Because such deficit can be as debilitating in daily life, the importance of preventing this syndrome cannot be overemphasized. In a seminal report in 2005, Thiebaut de Schotten et al. described two cases of right parietal tumor resection with spatial consciousness monitoring [68]. The bisection test is very simple: the patient is asked to draw the middle of a 20 cm line. In case of unilateral spatial neglect (transiently generated by the electrical stimulation), patient will deviate on the right of the line. Two cortical deviations sites were identified, in the caudal superior temporal gyrus and in the supramarginal gyrus. Since then, two studies reported larger series of spatial consciousness mapping [69, 70]. Roux et al. found several cortical regions with rightward but also leftward deviations. Those regions were centered around the temporo-parietal junction and deviations in the superior parietal lobule were uncommon. On the contrary, Vallar et al. observed rightward deviations mainly in Broadman’s area 7b. Like for language, it is likely that spatial consciousness is supported by a distributed set of interconnected cortical areas.

The neural correlates of mental calculation have been intensively studied by T-fMRI studies. It involves a large set of areas, grouped in three different networks [71]: the bilateral horizontal segment of intraparietal sulcus for quantity processing, the left angular gyrus for numbers manipulation in verbal modality, and the bilateral superior parietal regions for focusing attention (and this network is not specific to number manipulations). Accordingly, several authors reported disruption of calculation in various cortical areas, such as left angular gyrus [72–74], right angular gyrus [75, 76], left superior parietal lobule [77], horizontal portion of left intraparietal sulcus and left supramarginal gyrus [74]. In all these studies, some sites were specific to calculation and even to a subtype of computations (susbtraction vs. multiplication), while some others were also related to language disturbances. It should be noted that, in comparison with language mapping, there are very few reports about mental calculation mapping. This might be in relation to the relative rarity of parietal diffuse low-grade glioma. Moreover, the functional benefit of preserving mental calculation should be balanced with the oncological benefit to remove these areas: indeed, except for some very specific professions requiring mathematical abilities, patients usually do not care to keep their mental calculation abilities (probably because we are more and more relying on computers for daily arithmetic).

The importance in efficient cognition of being able to perform simultaneously two tasks cannot be overemphasized. Fortunately, this can be easily tested intraoperatively: a motor task can be added to any other cognitive task. Hence, patients can be asked to do picture naming or PPTT or mental calculation while simultaneously performing a repetitive movement of the upper limb. This double task greatly enhances the sensitivity to electrical stimulation, that is, some sites will respond only to the double task, while each task separately would be efficiently performed [42]. Again, depending on the preoperative discussion with the patient, one can decide to preserve or not such areas requiring a high-level of cognitive functioning.

Emotions have a major influence in daily life, particularly for decision making. Assessing the subjective experience of others in terms of mental states is a brain function referred to as mentalizing. Recent theories hypothesizes a two levels hierarchical network: a low-level network for emotion recognition or motor intention (theory of mind), and a higher-level network of mentalizing per se for complex inferences about other’s state of mind and intentions. The low-level network is supposed to be linked to the mirror neuron system, whereas the higher-level is related to the default-mode network, in particular the ability to attribute the intentions of others [78]. Interestingly, the low-level network can be monitored intraoperatively by the Read the Mind in the Eyes test. A first study reported responses in the right superior temporal gyrus, middle temporal gyrus and supramarginal gyrus [79]. In another study, responses were found in the pars opercularis and triangularis of the right frontal operculum [80]. If it is safer to preserve these sites, it should be noted that this function is likely to be distributed over redundant areas, ensuring a high plastic potential: indeed, in a series without intraoperative testing, only two patients out of ten kept an impairment on the assessment 3 months after surgery [81]. Of note, whereas mentalizing per se can be objectively evaluated with the comic strips task, there is currently no way to test the higher-level network in an intraoperative setting. It could be anticipated that this higher-level function is even more prone to plasticity than its lower-level counterpart. This would explain the very low risk of permanent deficit (10%).

A major advantage of on-line monitoring during awake surgery is the possibility for the patient to continuously report any inner conscious and subjective feeling induced by electrical stimulation. For example, intention to move (by stimulation in the posterior parietal cortex [82]) or out-of-body experiments (interpreted as a disruption of multisensory information by stimulating the (right) temporo-parietal junction [83]) have been described by patients.

In the same vein, disruption of consciousness of the external environment (induced by stimulating the ventral part of the posterior cingulate cortex) has been reported [84]. After recovering from the stimulation, the patients could describe their state as if «in a dream».

Whenever the resection has been pushed until encountering functional responses in an awake patient, an immediate postoperative decline is usually observed in one or several cognitive domains. The onset of this deterioration is not always immediate, and usually takes place between postoperative day 1 and 3 [125]. The classical explanation of this delay is that deterioration is concomitant to the peak of edema. In addition, it can be hypothesized that in the first postoperative days, the brain is massively reorganizing its connection weights, resulting in a transient abnormal functioning. Recovery usually occurs in two steps: a rapid spontaneous postoperative recovery in the first week after deterioration, and a slow rehabilitation-guided long-term recovery (lasting about 3 months). The importance of intensive rehabilitation cannot be overemphasized. It should be started as soon as possible, right after the surgery [126, 127], and it should be under the supervision of a trained speech therapist or neuropsychologist. On a patho-physiological point of view, this long-term recovery is directly related to the patient’s potential of plasticity, and it has been observed that it took longer time in older patients.

All in all, for low-grade glioma, postoperative plasticity is driven first by rehabilitation and then by the slow regrowth of the tumor. As a consequence, it should be kept in mind that, thanks to this plasticity, responsive sites identified during a first surgery might be unresponsive some years later. This opens the possibility to reoperate on low-grade glioma and to remove the second time some areas found eloquent at first surgery—thus to increase the extent of resection without eliciting permanent functional deficits [128, 129].

Advances in brain mapping techniques have allowed a better understanding of the dynamic organization of human brain, i.e. in large-scale, parallel, delocalized and interactive sub-networks. Therefore, anatomical landmarks are not enough to preserve an optimal quality of life in brain tumor patients undergoing maximal resective surgery: individual functional mapping is mandatory to tailor the resection according to cortical and subcortical eloquent structures for each patient. Because non-invasive preoperative functional neuroimaging is currently not reliable enough to identify the cortices and white matter tracts crucial for brain processing, in particular with regard to high order cognitive functions, intraoperative mapping using direct electrical stimulation is the goal standard to remove diffuse gliomas. In awake patients, it is now possible to achieve an extensive mapping, not only of sensorimotor and language functions, but also of cognitive and emotional functions. Cortical and axonal stimulation enables a precise investigation of the neural circuits of glioma patients, to detect a possible remapping elicited by the tumor itself, in addition to the interindividual variability, and to define in real-time the boundaries of surgical resection in order to improve both the oncological results (e.g. by performing a supratotal resection, extended beyond the enhancement in high-grade gliomas and beyond the FLAIR abnormalities in low-grade glioma) as well as the functional outcomes. The ultimate goal is to increase the quantity of quality of life, based upon a personalized surgical strategy taking into account the wishes of the patient. Therefore, a comprehensive explanation of the natural history of the disease, but also the determination of the individual quality of life (according notably to the job and hobby of the patient) is essential before the surgical act, with the aim to adapt the selection of cognitive tasks during resection, and then to optimize the onco-functional balance of surgery.