Biomedical subjects

Anthony T Herdman

Publications and source records attributed to Anthony T Herdman.

7 recordsLinked to original sources

PubMed2007

Task-related modulation of early cortical responses during language production: an event-related synthetic aperture magnetometry study.

We used whole-head magnetoencephalography measurements to investigate the spatiotemporal pattern of neural activity related to language production. Eight participants overtly responded by repeating aloud or vocalizing an internally generated verb to auditorily or visually presented nouns. Activity peaked within primary sensory (auditory or visual) cortices between 75 and 130 ms after stimulus onset, association cortices (inferior and superior temporal gyri) between 130 and 170 ms, and inferior frontal and premotor areas between 150 and 240 ms. Common to auditory and visual modalities, peak activity at about 220 ms was significantly larger in bilateral inferior frontal and left precentral regions when participants generated a verb than when they repeated a noun. These early differences in frontal regions may reflect the allocation of resources to the processing of low-level perceptions that are projected to the premotor areas early in the preparation of language production.

Adult↗

PubMed2006

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.

Adult↗

PubMed2006

Cortical oscillations related to processing congruent and incongruent grapheme-phoneme pairs.

In this study, we investigated changes in cortical oscillations following congruent and incongruent grapheme-phoneme stimuli. Hiragana graphemes and phonemes were simultaneously presented as congruent or incongruent audiovisual stimuli to native Japanese-speaking participants. The discriminative reaction time was 57 ms shorter for congruent than incongruent stimuli. Analysis of MEG responses using synthetic aperture magnetometry (SAM) revealed that congruent stimuli evoked larger 2-10 Hz activity in the left auditory cortex within the first 250 ms after stimulus onset, and smaller 2-16 Hz activity in bilateral visual cortices between 250 and 500 ms. These results indicate that congruent visual input can modify cortical activity in the left auditory cortex.

Acoustic Stimulation↗

PubMed2003

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.

Acoustic Stimulation↗

PubMed2003

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 amplitude-modulated (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 > or = 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.

Adult↗

PubMed2002

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 dBSPL. 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.

Acoustic Stimulation↗

PubMed2002

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.

Acoustics↗