Atypical cortical language organization in epilepsy patients: evidence for divergent hemispheric dominance for receptive and expressive language function.

The central goal of presurgical language mapping is to identify brain regions that subserve cortical language function to minimize postsurgical language deficits. Presurgical language mapping in patients with epilepsy presents a key challenge because of the atypical pattern of hemispheric language dominance found in this population, with higher incidences of bilateral and right-biased language dominance than typical. In this prospective study, we combine magnetoencephalography with a panel of tasks designed to separately assess receptive and expressive function to provide a sensitive measure of language function in 15 candidates for resective surgery. We report the following: 4 of 15 patients (27%) showed left hemisphere dominance across all tasks, 4 of 15 patients (27%) showed right hemisphere dominance across all tasks, and 7 of 15 (46%) showed discordant language dominance, with right-dominant receptive and left-dominant expressive language. All patients with discordant language dominance showed this right-receptive and left-expressive pattern. Results provide further evidence supporting the importance of using a panel of tasks to assess separable aspects of language function. The clinical relevance of the findings is discussed, especially about current clinical operative measures for assessing language dominance, which use single hemisphere procedure (intracarotid amobarbital procedure and awake intraoperative stimulation) for determining language laterality.

Rethinking clinical language mapping approaches: discordant receptive and expressive hemispheric language dominance in epilepsy surgery candidates.

Neuroimaging studies have shed light on cortical language organization, with findings implicating the left and right temporal lobes in speech processing converging to a left-dominant pattern. Findings highlight the fact that the state of theoretical language knowledge is ahead of current clinical language mapping methods, motivating a rethinking of these approaches. The authors used magnetoencephalography and multiple tasks in seven candidates for resective epilepsy surgery to investigate language organization. The authors scanned 12 control subjects to investigate the time course of bilateral receptive speech processes. Laterality indices were calculated for left and right hemisphere late fields ∼150 to 400 milliseconds. The authors report that (1) in healthy adults, speech processes activated superior temporal regions bilaterally converging to a left-dominant pattern, (2) in four of six patients, this was reversed, with bilateral processing converging to a right-dominant pattern, and (3) in three of four of these patients, receptive and expressive language processes were laterally discordant. Results provide evidence that receptive and expressive language may have divergent hemispheric dominance. Right-sided receptive language dominance in epilepsy patients emphasizes the need to assess both receptive and expressive language. Findings indicate that it is critical to use multiple tasks tapping separable aspects of language function to provide sensitive and specific estimates of language localization in surgical patients.

The effect of rate stress on the auditory brainstem response in autism: a preliminary report.

Efforts to correlate peripheral neurophysiologic function with perceptional deficits seen in autistic disorder (AD) have resulted in mixed findings, reflecting the high degree of heterogeneity observed in these individuals. We used the auditory brainstem response to study the effect of stress (high click presentation rate) on the auditory system in 20 children with AD (7-13 years) and 20 age-matched typically developing (TD) children. We report latency prolongations in children with AD vs. TD at waves I, III, and V that differed by ear of presentation: overall, left ear showed significant prolongations by group while right ear did not. The 'stressed' condition produced prolongations for both groups at each wave. At wave V, children with AD showed significant prolongations vs. TD, particularly for the right ear. For children with AD, wave V latency prolongations corresponded to language outcome as measured by VIQ, with longer prolongations associating with lower VIQ. Preliminary results provide evidence for reduced synaptic efficiency in auditory pathways in children with AD, which may form the neural bases for sensory reactivity and language impairment.

Rightward hemispheric asymmetries in auditory language cortex in children with autistic disorder: an MRI investigation.

PURPOSE: determine if language disorder in children with autistic disorder (AD) corresponds to abnormalities in hemispheric asymmetries in auditory language cortex. METHODS: MRI morphometric study in children with AD (n = 50) to assess hemispheric asymmetries in auditory language cortex. A key region of interest was the planum temporale (PT), which is larger in the left hemisphere in most healthy individuals. RESULTS: (i) Heschl's gyrus and planum polare showed typical hemisphere asymmetry patterns; (ii) posterior Superior Temporal Gyrus (pSTG) showed significant rightward asymmetry; and (iii) PT showed a trend for rightward asymmetry that was significant when constrained to right-handed boys (n = 30). For right-handed boys, symmetry indices for pSTG were significantly positively correlated with those for PT. PT asymmetry was age dependent, with greater rightward asymmetry with age. CONCLUSIONS: results provide evidence for rightward asymmetry in auditory association areas (pSTG and PT) known to subserve language processing. Cumulatively, our data provide evidence for a differing maturational path for PT for lower functioning children with AD, with both pre- and post-natal experience likely playing a role in PT asymmetry. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11689-009-9010-2) contains supplementary material, which is available to authorized users.

Vowel categorization induces departure of M100 latency from acoustic prediction.

MEG studies have shown that the timing (latency) of the evoked response that peaks approximately 100 ms post-stimulus onset (M100) decreases as frequency increases for sinusoidal tones. We investigated M100 latency using a continuum of synthesized vowel stimuli in which the dominant formant frequency increases from 250 Hz (perceived /u/) to 750 Hz (perceived /a/) in 50 Hz steps. While M100 latency did vary inversely with formant frequency overall, frequency modulation was flattened within each vowel category. However, for mid-continuum ambiguous tokens (i.e. those with increased reaction time/decreased accuracy in the concurrent behavioral identification task), M100 reverted to formant frequency differences, agreeing with previous findings of frequency-dependence. A theory is proposed in which phonological categorization emerges from specific spatial distribution of frequency-tuned neurons.

Cortical sound processing in children with autism disorder: an MEG investigation.

Previous work investigating frequency encoding mechanisms in human auditory cortex has provided evidence that latency of the auditory evoked M100 is strongly proportional to frequency, with low frequency (100-200 Hz) tones associated with approximately 30 ms longer latencies than mid-range frequency (1-2 kHz) tones. Motivated by pervasive speech and auditory perception deficits observed in autism spectrum disorder, we evaluated M100 frequency dependence in children with autism disorder and typically developing controls. Results indicate that for control children, the dynamic range of frequency modulation was similar to previous reports for healthy adults. Children with autism had a much reduced range of modulation in right hemisphere sites. Findings indicate that frequency encoding mechanisms may follow a differential maturational path in autism spectrum disorder.

Cortical auditory system maturational abnormalities in children with autism disorder: an MEG investigation.

UNLABELLED: Latency of electric (e.g., P1 and N1) and magnetic (e.g., M100) auditory evoked components depends on age in typically developing children, with longer latencies for younger (4-6 years) and shorter, adult-like latencies for older (14-16 years) children. Age-related changes in evoked components provide indirect measures of auditory system maturation and reflect changes that occur during development. We use magnetoencephalography (MEG) to investigate maturational changes in cortical auditory systems in left (LH) and right (RH) hemispheres in children with autism disorder (AD) and CONTROLS. We recorded auditory evoked responses over left and right temporal lobes in 17 Control and 15 AD children in the age range 8-16 years and measured M100 latency as a function of age, subject group and hemisphere. Linear regression analyses of age and M100 latency provided an estimate of the rate of latency change (ms/year) by hemisphere and subject group. CONTROLS: M100 latency for the group ranged from 100.8 to 166.1 ms and varied linearly in both hemispheres, decreasing at a rate of -4 ms/year (LH) and -4.5 ms/year (RH). AD: M100 latency ranged from 116.2 to 186.2 ms. Slopes of regression lines did not differ from zero in either LH or RH. M100 latency showed a tendency to vary with age in LH, decreasing at a rate of -4.6 ms/year. M100 latency in RH increased slightly (at a rate of 0.8 ms/year) with age. Results provide evidence for a differential auditory system development in AD children which may reflect abnormalities in cortical maturational processes in AD.

Hemispheric asymmetries in auditory evoked neuromagnetic fields in response to place of articulation contrasts.

A growing body of evidence indicates bilateral but asymmetric hemispheric involvement in speech perception. We used magnetoencephalography to record neuromagnetic evoked responses in 10 adults to consonant-vowel syllables that differ in a single phonetic feature, place of articulation. We report differential activation patterns in M100 latency, with larger differences in the right hemisphere than the left. These findings suggest that left and right auditory fields make differential contributions to speech processing.