Topographical gradients of semantics and phonology revealed by temporal lobe stimulation.

Word retrieval is a fundamental component of oral communication, and it is well established that this function is supported by left temporal cortex. Nevertheless, the specific temporal areas mediating word retrieval and the particular linguistic processes these regions support have not been well delineated. Toward this end, we analyzed over 1000 naming errors induced by left temporal cortical stimulation in epilepsy surgery patients. Errors were primarily semantic (lemon → "pear"), phonological (horn → "corn"), non-responses, and delayed responses (correct responses after a delay), and each error type appeared predominantly in a specific region: semantic errors in mid-middle temporal gyrus (TG), phonological errors and delayed responses in middle and posterior superior TG, and non-responses in anterior inferior TG. To the extent that semantic errors, phonological errors and delayed responses reflect disruptions in different processes, our results imply topographical specialization of semantic and phonological processing. Specifically, results revealed an inferior-to-superior gradient, with more superior regions associated with phonological processing. Further, errors were increasingly semantically related to targets toward posterior temporal cortex. We speculate that detailed semantic input is needed to support phonological retrieval, and thus, the specificity of semantic input increases progressively toward posterior temporal regions implicated in phonological processing. Hum Brain Mapp 38:688-703, 2017. © 2016 Wiley Periodicals, Inc.

Functional differences among stimulation-identified cortical naming sites in the temporal region.

To preserve postoperative language, electrical stimulation mapping is often conducted prior to surgery involving the language-dominant hemisphere. Object naming is the task most widely used to identify language cortex, and sites where stimulation elicits naming difficulty are typically spared from resection. In clinical practice, sites classified as positive undergo no further testing regarding the underlying cause of naming failure. Word production is a complex function involving multiple mechanisms that culminate in the identification of the target word. Two main mechanisms, i.e., semantic and phonological, underlie the retrieval of stored information regarding word meaning and word sounds, and naming can be hampered by disrupting either of these. These two mechanisms are likely mediated by different brain areas, and therefore, stimulation-identified naming sites might not be functionally equivalent. We investigated whether further testing at stimulation-identified naming sites would reveal an anatomical dissociation between these two mechanisms. In 16 patients with refractory temporal lobe epilepsy (TLE) with implanted subdural electrodes, we tested whether, despite inability to produce an item name, patients could reliably access semantic or phonological information regarding objects during cortical stimulation. We found that stimulation at naming sites in superior temporal cortex tended to impair phonological processing yet spared access to semantic information. By contrast, stimulation of inferior temporal naming sites revealed a greater proportion of sites where semantic access was impaired and a dissociation between sites where stimulation spared or disrupted semantic or phonological processing. These functional-anatomical dissociations reveal the more specific contribution to naming provided by these cortical areas and shed light on the often profound, interictal word-finding deficit observed in temporal lobe epilepsy. Additionally, these techniques potentially lay the groundwork for future studies to determine whether particular naming sites that fall within the margins of the desired clinical resection might be resected without significant risk of decline.

Extraoperative neurostimulation mapping: results from an international survey of epilepsy surgery programs.

OBJECTIVE: Extraoperative electrical stimulation mapping (ESM) to identify functional cortex is performed prior to neurosurgical resection at epilepsy surgery programs worldwide. However, the procedure remains unstandardized, with no established clinical guidelines. We sought to determine the current range in ESM practice parameters across established epilepsy surgery centers. METHODS: We developed and distributed a 31-question survey to 220 epilepsy centers worldwide regarding current practice parameters of ESM. Questions addressed preoperative assessment, technical stimulation parameters, language testing protocols, criteria for identification of positive or negative functional sites, management of mapping complications, and postoperative functional outcome. RESULTS: Survey responses were obtained from 56 centers. These revealed marked practice variability in virtually all aspects of the ESM procedure. These aspects included critical procedure components such as electrical stimulation settings, the types of language functions tested, the operational definition of a language error, size of surgical resection margin, cortical locations mapped for language, testing in the presence of afterdischarges, and medical management of mapping complications. Forty-one percent of centers reported at least one persistent adverse language outcome despite preserving all eloquent sites defined by their stimulation mapping procedure. SIGNIFICANCE: The striking variations in practice across centers are likely to influence mapping results, which directly affect the boundaries of cortical resection and, consequently, might worsen either seizure or functional outcomes. Clearly, adverse functional outcomes occur despite mapping procedures that were perceived to be adequate. Investigation of critical technical and procedural aspects of stimulation mapping is warranted, with the ultimate goal of establishing empirically based practice guidelines to improve the safety and efficacy of ESM and resective epilepsy surgery. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here.

Shared space, separate processes: Neural activation patterns for auditory description and visual object naming in healthy adults.

Historically, both clinicians and cognitive scientists have used visual object naming measures to study naming, and lesion-type studies have implicated the left posterior, temporo-parietal region as a critical component of naming circuitry. However, recent results from behavioral and cortical stimulation studies using auditory description naming as well as visual object naming in left temporal lobe epilepsy patients suggest that discrete sites in anterior temporal cortex are critical for description naming, whereas posterior temporal regions mediate both visual object naming and description naming. To determine whether this task specificity reflects normal cerebral organization and processing, 13 healthy adults performed description naming and visual naming during functional neuroimaging. In addition to standard univariate analysis, multivariate, ordinal trend analysis examined the network character of the regions involved in task-specific naming. Univariate analysis indicated posterior temporal activation for both visual naming and description naming, whereas multivariate analysis revealed broader networks for both tasks, with both overlapping and task-specific regions, as well as task-related differences in the way the tasks utilized common regions. Additionally, multivariate analysis revealed unique, task-specific, regionally covarying activation patterns that were strikingly consistent in all 13 subjects for visual naming and 12/13 subjects for description naming. Results suggest a common neural substrate, yet differentiable neural processes underlying visual naming and description naming in neurologically intact individuals. These findings support the use of both types of tasks for clinical assessment and may have application in the treatment of neurologically based naming deficits. Inc.

Distribution of auditory and visual naming sites in nonlesional temporal lobe epilepsy patients and patients with space-occupying temporal lobe lesions.

PURPOSE: Current knowledge regarding the topography of essential language cortex is based primarily on stimulation mapping studies of nonlesional epilepsy patients. We sought to determine whether space-occupying temporal lobe lesions are associated with a similar topography of language sites, as this information would be useful in surgical planning. METHODS: We retrospectively compared the topography of auditory and visual naming sites in 25 nonlesional temporal lobe epilepsy patients ("nonlesional") and 18 patients with space-occupying lesions ("lesional") who underwent cortical language mapping before left temporal resection. RESULTS: Both groups exhibited a similar pattern of auditory naming sites anterior to visual and dual (auditory-visual) naming sites; no group differences were specific to auditory or visual naming sites. However, significantly fewer lesional (10 of 20) compared with nonlesional patients (21 of 25) exhibited any naming sites in the temporal region (p=0.04). Although the proportion of naming sites on the superior temporal gyrus was similar between groups, naming sites were found on the middle temporal gyrus in 13 of 25 nonlesional patients, yet in only one of 18 lesional patients (p=0.002). Across groups, patients with visual naming sites were older than patients without visual naming sites identified (p=0.02). CONCLUSIONS: The precise location of essential language cortex cannot be reliably inferred from anatomic landmarks or patient-related variables. As time constraints are a common quandary in stimulation mapping, the different patterns reported here for patients with and without space-occupying lesions can be used to guide mapping strategies, thereby increasing the efficiency by which positive naming sites are identified.