Anthony T. Herdman

Intracerebral Sources of Human Auditory Steady-State Responses

The objective of this study was to localize the intracerebral generators for auditory steady-state responses. The stimulus was a continuous 1000-Hz tone presented to the right or left ear at 70 dB SPL. The tone was sinusoidally amplitude-modulated to a depth of 100% at 12, 39, or 88 Hz. Responses recorded from 47 electrodes on the head were transformed into the frequency domain. Brain electrical source analysis treated the real and imaginary components of the response in the frequency domain as independent samples. The latency of the source activity was estimated from the phase of the source waveform. The main source model contained a midline brainstem generator with two components (one vertical and lateral) and cortical sources in the left and right supratemporal plane, each containing tangential and radial components. At 88 Hz, the largest activity occurred in the brainstem and subsequent cortical activity was minor. At 39 Hz, the initial brainstem component remained and significant activity also occurred in the cortical sources, with the tangential activity being larger than the radial. The 12-Hz responses were small, but suggested combined activation of both brainstem and cortical sources. Estimated latencies decreased for all source waveforms as modulation frequency increased and were shorter for the brainstem compared to cortical sources. These results suggest that the whole auditory nervous system is activated by modulated tones, with the cortex being more sensitive to slower modulation frequencies.

Thresholds determined using the monotic and dichotic multiple auditory steady-state response technique in normal- hearing subjects

Auditory steady-state responses (ASSRs) were elicited by presenting single or multiple, 77-105 Hz amplitude-modulated 0.5, 1, 2, and 4 kHz tones to one or both ears. Objectives of this study were to (i) replicate and extend previous multiple ASSR studies in a quiet double-walled sound booth, and (ii) discover differences (if any) between thresholds assessed in monotic and dichotic conditions, which ranged between 15 and 22 dB SPL. The present studys behavioural and ASSR thresholds are 0-10 dB lower (better) than results of previous monotic studies. Further, there are no significant differences in ASSR thresholds between dichotic and monotic stimulus conditions. Therefore, dichotic multiple AM tone stimulation does not produce a change in the ASSR that affects threshold estimation in a clinically significant manner. Thus, at least for detecting normal hearing, the dichotic multiple ASSR technique is a feasible method for estimating hearing thresholds that would substantially reduce recording time compared to conventional single-stimulus techniques.

Children's Event-Related Potentials of Auditory Selective Attention Vary With Their Socioeconomic Status

Past research suggests a link between socioeconomic status (SES) and brain processes in children, but direct evidence from neuroimaging is scarce. The authors investigated the relationships among SES, performance, and the neural correlates of auditory selective attention, by comparing event-related potentials (ERPs) in lower- and higher-SES preadolescent children during a task in which they attended to two types of pure tones but ignored two other types. Our hypothesis was that, at comparable performance levels, higher-SES children ignore distracters (the unattended, irrelevant tones) while lower-SES children attend equally to distracters and to targets (the attended, relevant tones). The authors found that ERP waveform differences between attended and unattended tones (Nd, difference negativity) were significant in the higher-SES but not in the lower-SES group. However, the groups did not differ in reaction times or accuracy. Electroencephalographic power analysis revealed a differential pattern of theta activity concomitant with irrelevant tones for the two groups, indicating that although they performed similarly the children from these groups recruited different neural processes. Lower-SES children, the authors suggest, deployed supplementary resources to also attend to irrelevant information.

Auditory steady-state response thresholds of adults with sensorineural hearing impairments.

This study evaluated the use of multiple auditory steady-state responses (ASSRs) to estimate the degree and configuration of behavioral audiograms of subjects with sensorineural hearing impairments. Place specificity of the multiple-ASSR method was also assessed. Multiple amplitudemodulated (77–105 Hz) tones (500, 1000, 2000 and 4000 Hz) were simultaneously presented to one ear. The results showed that, on average, multiple-ASSR thresholds were 14±13, 8±9, 10±10 and 3±10 dB above behavioral thresholds for 500, 1000, 2000 and 4000 Hz, respectively. Behavioral and multiple-ASSR thresholds were significantly correlated (r =0.75–0.89). There were no significant differences between behavioral and multiple-ASSR measures of the audiogram configuration. In subjects with steep-sloping ≥30 dB/octave) hearing losses, multiple-ASSR thresholds did not underestimate behavioral thresholds, revealing good place specificity. These results indicate that the multiple-ASSR method provides good estimates of the degree and configuration of hearing in individuals with sensorineural hearing impairments. Este estudio evaluó el uso de las repuestas auditivas de estado estable (ASSR) para estimar el grado y la configuración de la audiometria conductual en sujetos con hipoacusia sensorineural. También se evaluó la especificidad tonal del método de ASSR múltiple. Se presentaron tonos múltiples de amplitud modulada (77–105 Hz) en 500, 1000, 2000 y 4000 Hz en forma simultánea para cada oído. Los resultados muestran que en promedio, los umbrales de ASSR-múltiple son 14±13, 8±9, 10±10 y 3±10 dB por encima del umbral conductual para las frecuencias 500, 1000, 2000 y 4000Hz respectivamente. No hubo una diferencia significativa (r = 0.75–0.89) entre los umbrales conductuales y los umbrales por ASSR-múltiple en la configuración del audiograma. En sujetos con curvas de perfil descendente abrupto (≥30 dB/octave), los umbrales obtenidos por ASSR-múltiple no subestimaron los umbrales conductuales y demostraron buena relación de especificidad frecuencial. Estos resultados indican que el método ASSR-múltiple proporciona un buen estimado del grado y la configuración de la audición de los individuos con hipoacusia sensorineural.

Inversion and contrast-reversal effects on face processing assessed by MEG.

The processing of upright, inverted and contrast-reversed faces was investigated using MEG. Peak and global field power analyses revealed that the M100, M170 and M220 components were delayed for inverted and contrast-reversed compared to normal upright faces but no amplitude modulations were found. Source analyses using an event-related SAM beamformer technique revealed bilateral occipital sources for the M100 and M220 components. For the M170, two distinct sources simultaneously active were found, a bilateral and posterior source (M170A) and a right lateralized ventral and more anterior source (M170B) around the fusiform gyrus. None of the sources varied in location or intensity between face types. However, although different from the M100, the location of the M170A was not significantly different from that of the M220, suggesting the latter could be a reactivation of the former. Confirming previous ERP results on the processing of inverted faces, the present study extends the findings to contrast-reversed face stimuli and suggests that deviations from the standard upright face format do not activate extra areas but simply result in the delayed activation of the sources generating the M100, M170 and M220 components. The data confirm the sensitivity of the M100 to face manipulations and further suggest that the M170 is generated by two distinct sources, one of which situated in occipital extrastriate areas (M170A) could be reactivated around 220 ms to generate the M220 component.

Neonatal pain-related stress, functional cortical activity and visual-perceptual abilities in school-age children born at extremely low gestational age

Summary Neonatal pain‐related stress is associated with altered brain activity and visual‐perceptual abilities in school‐age children born at extremely low gestational age. Abstract Children born very prematurely (≤32 weeks) often exhibit visual‐perceptual difficulties at school‐age, even in the absence of major neurological impairment. The alterations in functional brain activity that give rise to such problems, as well as the relationship between adverse neonatal experience and neurodevelopment, remain poorly understood. Repeated procedural pain‐related stress during neonatal intensive care has been proposed to contribute to altered neurocognitive development in these children. Due to critical periods in the development of thalamocortical systems, the immature brain of infants born at extremely low gestational age (ELGA; ≤28 weeks) may have heightened vulnerability to neonatal pain. In a cohort of school‐age children followed since birth we assessed relations between functional brain activity measured using magnetoencephalogragy (MEG), visual‐perceptual abilities and cumulative neonatal pain. We demonstrated alterations in the spectral structure of spontaneous cortical oscillatory activity in ELGA children at school‐age. Cumulative neonatal pain‐related stress was associated with changes in background cortical rhythmicity in these children, and these alterations in spontaneous brain oscillations were negatively correlated with visual‐perceptual abilities at school‐age, and were not driven by potentially confounding neonatal variables. These findings provide the first evidence linking neonatal pain‐related stress, the development of functional brain activity, and school‐age cognitive outcome in these vulnerable children.

Determination of activation areas in the human auditory cortex by means of synthetic aperture magnetometry

In this study we applied synthetic aperture magnetometry (SAM) to investigate active cortical areas associated with magnetically recorded transient and steady-state auditory evoked responses. For transient evoked responses, SAM images reveal an activated volume of cortical tissue within the lateral aspect of the superior temporal plane. The volume of cortical activation for steady-state responses was located more medially than that for transient evoked responses. Additionally, SAM also reveals a small overlap of activated areas between transient and steady-state evoked responses, which has not be demonstrated when using equivalent current dipole (ECD) source modeling. Source waveforms from SAM and ECD analyses show comparable temporal information. Results from this study suggest that SAM is a useful technique for imaging cortical structures involved in processing perceptual information.

A complementary analytic approach to examining medial temporal lobe sources using magnetoencephalography

Neuropsychological and neuroimaging findings reveal that the hippocampus is important for recognition memory. However, it is unclear when and whether the hippocampus contributes differentially to recognition of previously studied items (old) versus novel items (new), or contributes to a general processing requirement that is necessary for recognition of both types of information. To address this issue, we examined the temporal dynamics and spectral frequency underlying hippocampal activity during recognition of old/new complex scenes using magnetoencephalography (MEG). In order to provide converging evidence to existing literature in support of the potential of MEG to localize the hippocampus, we reconstructed brain source activity using the beamformer method and analyzed three types of processing-related signal changes by applying three different analysis methods: (1) Synthetic aperture magnetometry (SAM) revealed event related and non-event-related spectral power changes; (2) Inter-trial coherence (ITC) revealed time-locked changes in neural synchrony; and (3) Event-related SAM (ER-SAM) revealed averaged event-related responses over time. Hippocampal activity was evident for both old and new information within the theta frequency band and during the first 250 ms following stimulus onset. The early onset of hippocampal responses suggests that general comparison processes related to recognition of new/old information may occur obligatorily.

Place specificity of multiple auditory steady-state responses

Auditory steady-state responses (ASSRs) were elicited by simultaneously presenting multiple AM (amplitude-modulated) tones with carrier frequencies of 500, 1000, 2000, and 4000 Hz and modulation frequencies of 77, 85, 93, and 102 Hz, respectively. Responses were also evoked by separately presenting single 500- or 2000-Hz AM tones. The objectives of this study were (i) to determine the cochlear place specificity of single and multiple ASSRs using high-pass noise masking and derived-band responses, and (ii) to determine if there were any differences between single- and multiple-stimulus conditions. For all carrier frequencies, derived-band ASSRs for 1-octave-wide derived bands ranging in center frequency from 0.25 to 8 kHz had maximum amplitudes within a 1/2 octave of the carrier frequency. For simultaneously presented AM tones of 500, 1000, 2000, and 4000 Hz, bandwidths for the function of derived-band ASSR amplitude by derived-band center frequency were 476, 737, 1177, and 3039 Hz, respectively. There were no significant differences when compared to bandwidths of 486 and 1371 for ASSRs to AM tones of 500 or 2000 Hz presented separately. Results indicate that ASSRs to moderately intense stimuli (60 dB SPL) reflect activation of reasonably narrow cochlear regions, regardless of presenting AM tones simultaneously or separately.

Cortical oscillatory power changes during auditory oddball task revealed by spatially filtered magnetoencephalography.

OBJECTIVE To investigate the neural sources and associated changes in oscillatory activity involved in auditory attention and memory updating processing using spatially filtered magnetoencephalography. METHODS We recorded magnetic responses during an auditory oddball task in 12 normal subjects. Synthetic aperture magnetometry (SAM)-permutation analysis was used to visualize the multiple brain regions associated with event-related magnetic fields (ERFs), and event-related oscillations during target detection processing. RESULTS SAM-permutation results showed the topographical distribution of N1m over the bilateral primary auditory cortex. Post-stimulus delta (1.5-4 Hz) activity sources, likely related to the P300 slow-waveform, were distributed over the right frontocentral and parietal regions. Source locations of theta (4-8 Hz) and alpha (8-13 Hz) event-related synchronization (ERS) were identified over the dorsolateral and medial prefrontal cortex. We visualized bilateral central-Rolandic suppresions for mu (8-15 Hz), beta (15-30 Hz), and low-gamma (30-60 Hz) activities, more dominant in the hemisphere contralateral to the moving hand (button-pressing in response to target stimuli). CONCLUSIONS Prefrontal theta and alpha ERS, and frontocentral-parietal delta ERS are functionally engaged in auditory attention and memory updating process. SIGNIFICANCE Spatially filtered MEG is valuable for detection and source localization of task-related changes in the ongoing oscillatory activity during oddball tasks.