Dana Boatman

Induced electrocorticographic gamma activity during auditory perception

Abstract Objective : To define the spatial, temporal, and functional characteristics of induced gamma (>30 Hz) activity during functional activation of the left superior temporal gyrus. Methods : Electrocorticographic (ECoG) recordings were made in 4 clinical subjects during auditory tone and phoneme discrimination tasks, and event-related changes in the ECoG band power were calculated. The topography and temporal sequence of event-related power changes in different gamma bands were contrasted with those of auditory evoked potentials (AEPs), and with those of event-related power changes in the alpha band (8–12 Hz). Results : Auditory stimuli induced a broadband power augmentation that included 40 Hz, as well as higher (80–100 Hz) gamma frequencies. The topography of gamma augmentation was similar, but not identical, to that of the AEP, and was more focused than that of alpha power suppression. Its temporal onset coincided with the N100, but outlasted it. Phonemes produced greater gamma augmentation than tones, while a similar difference was not observed in the N100. Conclusions : Auditory perception induces ECoG gamma activity not only at 40 Hz, but also in higher gamma frequencies. This activity appears to be an index of cortical activation that reflects task-specific processing in the human auditory cortex more closely than the AEP or alpha power suppression.

Electrocorticographic gamma activity during word production in spoken and sign language

Objective: To investigate the functional-neuroanatomic substrates of word production using signed versus spoken language. Methods: The authors studied single-word processing with varying input and output modalities in a 38-year-old woman with normal hearing and speech who had become proficient in sign language 8 years before developing intractable epilepsy. Subdural electrocorticography (ECoG) was performed during picture naming and word reading (visual inputs) and word repetition (auditory inputs); these tasks were repeated with speech and with sign language responses. Cortical activation was indexed by event-related power augmentation in the 80- to 100-Hz gamma band, and was compared with general principles of functional anatomy and with subject-specific maps of the same or similar tasks using electrical cortical stimulation (ECS). Results: Speech outputs activated tongue regions of the sensorimotor cortex, and sign outputs activated hand regions. In addition, signed word production activated parietal regions that were not activated by spoken word production. Posterior superior temporal gyrus was activated earliest and to the greatest extent during auditory word repetition, and the basal temporal-occipital cortex was activated similarly during naming and reading, reflecting the different modalities of input processing. With few exceptions, topographic patterns of ECoG gamma were consistent with ECS maps of the same or similar language tasks. Conclusions: Spoken and signed word production activated many of the same cortical regions, particularly those processing auditory and visual inputs; however, they activated different regions of sensorimotor cortex, and signing activated parietal cortex more than did speech. This study illustrates the utility of electrocorticographic gamma for studying the neuroanatomy and processing dynamics of human language.

Language recovery after left hemispherectomy in children with late-onset seizures

We investigated the language capabilities of the isolated right hemisphere in 6 children (age, 7–14 years) after left hemidecorticectomy for treatment of Rasmussens syndrome. Patients were right‐handed before surgery and had at least 5 years of normal language development before the onset of seizures. Language testing included speech sound (phoneme) discrimination, single word and phrasal comprehension, repetition, and naming. Within 4 to 16 days after surgery, patients showed improved phoneme discrimination compared with their performance shortly before surgery. Other language functions remained severely impaired until at least 6 months after surgery. By 1 year after surgery, receptive functions were comparable with, or surpassed, patient presurgery performance. Although word repetition was intact by 1 year after surgery, naming remained impaired, and patient speech was limited largely to production of single words. These results suggest that the right hemisphere is innately capable of supporting multiple aspects of phoneme processing. Recovery of higher level receptive and, to a lesser extent, expressive language functions is attributed to plasticity of the right hemisphere, which appears to persist beyond the proposed critical period for language acquisition and lateralization.

Cortical bases of speech perception:evidence from functional lesion studies

Functional lesion studies have yielded new information about the cortical organization of speech perception in the human brain. We will review a number of recent findings, focusing on studies of speech perception that use the techniques of electrocortical mapping by cortical stimulation and hemispheric anesthetization by intracarotid amobarbital. Implications for recent developments in neuroimaging studies of speech perception will be discussed. This discussion will provide the framework for a developing model of the cortical circuitry critical for speech perception.

Auditory speech processing in the left temporal lobe : an electrical interference study

Auditory syllable discrimination, identification, and comprehension were investigated by direct cortical electrical interference in three patients with indwelling subdural electrode arrays. Without electrical interference, patients performed similarly to matched normal subjects. With electrical interference, selective deficits were observed in the posterior superior temporal (PST) lobes of all three patients. At specific PST sites, only comprehension was impaired, while at proximal sites comprehension and identification were impaired, but discrimination remained intact. At a single PST site, all three auditory speech functions were impaired. These findings suggest that lower-level auditory speech functions can operate independent of higher-level processes, as claimed by traditional hierarchical models. However, analysis of discrimination errors revealed lexical-semantic and phonological effects, suggesting that higher-level functions also influence lower-level processing. These data can be explained by a bidirectional processing model, with differentially weighted connections.

A pilot study transitioning children onto levetiracetam monotherapy to improve language dysfunction associated with benign rolandic epilepsy

Benign rolandic epilepsy (BRE) and Landau-Kleffner syndrome (LKS) are similar epilepsy syndromes with sleep-accentuated epileptiform activity, sporadic seizures, and language dysfunction. Levetiracetam has been associated with improved language function in LKS and seizure reduction in BRE. We hypothesized levetiracetam would improve language function in children with BRE. A pilot study was performed with six children (aged 6-12) with BRE and evidence of impaired auditory comprehension and verbal memory. Children were transitioned from their current anticonvulsant to 40 mg/kg/day levetiracetam over a 2-week period and retested at 6 months. At 6 months, three of four children with baseline auditory comprehension impairments performed normally (P=0.06), and five had improved auditory verbal memory (P=0.08). Seizures improved in five, decreasing from 2.7 to 1.0 seizure per 6 months (P=0.11). Results from this pilot study suggest that levetiracetam may have a beneficial effect on language in children with BRE.

Right hemisphere speech perception revealed by amobarbital injection and electrical interference

Objective: To investigate the right hemispheric speech perception capabilities of an adult right-handed patient with seizures. Methods: Consecutive, unilateral, intracarotid sodium amobarbital injections and left hemispheric electrical interference mapping were used to determine lateralization and localization of speech perception, measured as syllable discrimination. Results: Syllable discrimination remained intact after left and right intracarotid sodium amobarbital injections. Language otherwise strongly lateralized to the left hemisphere. Despite evidence of bilateral speech perception capabilities, electrical interference testing in the left posterior temporal lobe impaired syllable discrimination. Conclusions: The results suggest a functionally symmetric, parallel system in the adult brain with preferential use of left hemispheric pathways for speech perception.

Making sense out of jargon A neurolinguistic and computational account of jargon aphasia

Objective: To identify the cognitive and neuroanatomic bases of neologistic jargon aphasia with spared comprehension and production of written words. Methods: Detailed analysis of performance across experiments of naming, reading, writing, repetition, and word/picture matching by a 68-year-old woman (J.B.N.) served to identify which cognitive mechanisms underlying naming and word comprehension were impaired. J.B.N.’s impairments were then simulated by selectively “lesioning” a computer model of word production that has semantic, word form, and subword phonologic levels of representation (described by Dell in 1986). Results: In comprehension experiments, J.B.N. made far more errors with spoken word input than with written word or picture input (chi-square = 40–59; df = 1; p < 0.0001) despite intact auditory discrimination. In naming experiments (with picture, definition, or tactile input), J.B.N. made far more errors in spoken output relative to written output (chi-square = 14–56; df = 1; p < 0.0001). These selective impairments of spoken word processing were simulated by reducing connection strength between word-level and subword-level phonologic units but maintaining full connection strength between word-level and semantic units in Dell’s model. The simulation yielded a distribution of error types that was nearly identical to that of J.B.N., and her CT and MRI scans showed a small subarachnoid hemorrhage in the left sylvian fissure without infarct. Cerebral angiogram showed focal vasospasm in sylvian branches of the left middle cerebral artery. Conclusion: Focal left perisylvian dysfunction can result in a highly selective “disconnection” between word-level and subword-level phonologic representations manifest as neologistic jargon aphasia with intact understanding and production of written words.

Cortical Sites Critical for Speech Discrimination in Normal and Impaired Listeners

We used statistical modeling to investigate variability in the cortical auditory representations of 24 normal-hearing epilepsy patients undergoing electrocortical stimulation mapping (ESM). Patients were identified as normal or impaired listeners based on recognition accuracy for acoustically filtered words used to simulate everyday listening conditions. The experimental ESM task was a binary (same-different) auditory syllable discrimination paradigm that both listener groups performed accurately at baseline. Template mixture modeling of speech discrimination deficits during ESM showed larger and more variable cortical distributions for impaired listeners than normal listeners, despite comparable behavioral performances. These results demonstrate that individual differences in speech recognition abilities are reflected in the underlying cortical representations.

Landau–Kleffner syndrome responsive to levetiracetam

A 5-year-old girl with Landau-Kleffner syndrome is discussed. The child began having seizures at age 4 associated with language deterioration despite anticonvulsant therapy. With levetiracetam monotherapy to a dose of 60 mg/kg/day and discontinuation of carbamazepine and valproic acid, her language has improved and seizures are controlled. Levetiracetam should be considered as therapy for Landau-Kleffner syndrome.