Biomedical subjects

S Makeig

Publications and source records attributed to S Makeig.

29 records · Page 2Linked to original sources

PubMed1994

Different event-related patterns of gamma-band power in brain waves of fast- and slow-reacting subjects.

Fast- and slow-reacting subjects exhibit different patterns of gamma-band electroencephalogram (EEG) activity when responding as quickly as possible to auditory stimuli. This result appears to confirm long-standing speculations of Wundt that fast- and slow-reacting subjects produce speeded reactions in different ways and demonstrates that analysis of event-related changes in the amplitude of EEG activity recorded from the human scalp can reveal information about event-related brain processes unavailable using event-related potential measures. Time-varying spectral power in a selected (35- to 43-Hz) gamma frequency band was averaged across trials in two experimental conditions: passive listening and speeded reacting to binaural clicks, forming 40-Hz event-related spectral responses. Factor analysis of between-subject event-related spectral response differences split subjects into two near-equal groups composed of faster- and slower-reacting subjects. In faster-reacting subjects, 40-Hz power peaked near 200 ms and 400 ms poststimulus in the react condition, whereas in slower-reacting subjects, 40-Hz power just before stimulus delivery was larger in the react condition. These group differences were preserved in separate averages of relatively long and short reaction-time epochs for each group. gamma-band (20-60 Hz)-filtered event-related potential response averages did not differ between the two groups or conditions. Because of this and because gamma-band power in the auditory event-related potential is small compared with the EEG, the observed event-related spectral response features must represent gamma-band EEG activity reliably induced by, but not phase-locked to, experimental stimuli or events.

Acoustic Stimulation↗

PubMed1993

Oscillatory brain activity during a motor task.

Changes in spectral power in neuromagnetic fields associated with a manual task requiring a high level of sensorimotor integration (SMI) were investigated by analysing spontaneous, non-invasively recorded activity during motor preparation (WAIT), task performance (SMI), and control (REST) conditions in four healthy, right-handed human subjects. Neuromagnetic fields were recorded over the left sensorimotor cortex using a 37-channel instrument. In all subjects, a prominent narrow-band motor preparation rhythm centered near 19 Hz was consistently observed during the WAIT state. During SMI, mean relative increases in 26-30 Hz activity appeared in two of the subjects, paralleling gamma band enhancement recently observed during SMI in monkeys.

Adult↗

PubMed1993

Lapses in alertness: coherence of fluctuations in performance and EEG spectrum.

Thirteen subjects detected noise burst targets presented in a white noise background at a mean rate of 10/min. Within each session, local error rate, defined as the fraction of targets detected in a 33 sec moving window, fluctuated widely. Mean coherence between slow mean variations in EEG power and in local error rate was computed for each EEG frequency and performance cycle length, and was shown by a Monte Carlo procedure to be significant for many EEG frequencies and performance cycle lengths, particularly in 4 well-defined EEG frequency bands, near 3, 10, 13, and 19 Hz, and at higher frequencies in two cycle length ranges, one longer than 4 min and the other near 90 sec/cycle. The coherence phase plane contained a prominent phase reversal near 6 Hz. Sorting individual spectra by local error rate confirmed the close relation between performance and EEG power and its relative within-subject stability. These results show that attempts to maintain alertness in an auditory detection task result in concurrent minute and multi-minute scale fluctuations in performance and the EEG power spectrum.

Acoustic Stimulation↗

PubMed1993

Auditory event-related dynamics of the EEG spectrum and effects of exposure to tones.

A new measure of event-related brain dynamics, the event-related spectral perturbation (ERSP), is introduced to study event-related dynamics of the EEG spectrum induced by, but not phase-locked to, the onset of the auditory stimuli. The ERSP reveals aspects of event-related brain dynamics not contained in the ERP average of the same response epochs. Twenty-eight subjects participated in daily auditory evoked response experiments during a 4 day study of the effects of 24 h free-field exposure to intermittent trains of 89 dB low frequency tones. During evoked response testing, the same tones were presented through headphones in random order at 5 sec intervals. No significant changes in behavioral thresholds occurred during or after free-field exposure. ERSPs induced by target pips presented in some inter-tone intervals were larger than, but shared common features with, ERSPs induced by the tones, most prominently a ridge of augmented EEG amplitude from 11 to 18 Hz, peaking 1-1.5 sec after stimulus onset. Following 3-11 h of free-field exposure, this feature was significantly smaller in tone-induced ERSPs; target-induced ERSPs were not similarly affected. These results, therefore, document systematic effects of exposure to intermittent tones on EEG brain dynamics even in the absence of changes in auditory thresholds.

Acoustic Stimulation↗

PubMed1992

Physiological studies of central masking in man. I: The effects of noise on the 40-Hz steady-state response.

In a typical masking situation, two Békésy waves overlap on the basilar membrane, and each of them initiates a stream of nerve impulses that enters the brain via the auditory nerve. Much is known about the overlapping of the cochlear waves, but much less about where, how, and even if at all, the impulse streams interact once they get inside the brain. In these experiments the incoming impulses are measured electrophysiologically using the auditory brainstem response (ABR), and, simultaneously, using the 40-Hz auditory steady-state response (SSR) to monitor events at a probable site of their interaction, the auditory cortex. The principal finding is that, when progressively increasing levels of continuous noise are presented to the contralateral ear, the SSR to the signal drops to about half its control amplitude. Second, low levels of ipsilateral noise reliably enhance SSR amplitude. Third, moderate levels of ipsilateral noise reduce SSR latency. In none of these cases does the ABR show similar effects. These findings are interpreted to mean that, in each case, impulses excited by the signal interact with impulses excited by the noise, and regardless of ear of origin the interactions take place beyond the brainstem level where ABR wave V is generated, either before the impulses reach the cortex, or in the cortex itself.

Acoustic Stimulation↗

PubMed1992

Physiological studies of central masking in man. II: Tonepip SSRs and the masking level difference.

The auditory steady-state response (SSR), an evoked response generated in the auditory cortex, was initiated by monaural trains of 500-Hz tonepips repeated at rates near 40 Hz while wideband noise was being delivered to the same or opposite ear. Contralateral noise reduced SSR amplitudes in an intensity-dependent manner, whereas ipsilateral noise enhanced the SSR amplitudes at low levels and depressed them at high levels. Systematic phase changes accompanied the amplitude changes. These results, obtained with tonepips, closely resemble those previously reported for clicks. A third experiment, a masking level difference (MLD) experiment, examined changes in the SSR measures during four successive tonepip-plus-noise conditions: (1) monaural tonepips alone; (2) adding ipsilateral noise; (3) then adding contralateral noise; (4) finally, adding contralateral tonepips. The SSR amplitude changes measured in the experiment did not always correspond with the changes in perception reported by the subject.

Acoustic Stimulation↗

PubMed1991

Human auditory evoked gamma-band magnetic fields.

We have discovered a ca. 40-Hz transient magnetic oscillatory response, evoked in the human brain by the onset of auditory stimuli, consisting of four or more cycles locked in phase to stimulus onset in approximately the 20- to 130-ms poststimulus interval. The response originates in the supratemporal auditory cortex, some millimeters deeper and anterior to the source of the larger-amplitude slow-wave M100 component of the evoked magnetic field and moves in a posterior arcing trajectory 1 cm or more in length. The oscillatory cortical activation elicited by auditory stimuli may be similar to the gamma-band cortical oscillations elicited by olfactory and visual stimuli and may represent an essential component of auditory perceptual processing.

Acoustic Stimulation↗

PubMed1987

Auditory steady-state responses: threshold prediction using phase coherence.

These experiments add a measure of response phase variance--'phase coherence'--to the analysis procedures applied to auditory steady-state responses (SSR). The effects of stimulus frequency, intensity, rate and total number (i.e., recording time) were studied using 11 normal adult subjects. In a first experiment, SSR phase coherence was found to be highest at presentation rates near 40/sec, even when response amplitudes were higher at other rates. Further, phase coherence was observed to be linearly related (r = 0.91) to signal-to-noise ratio. Two further experiments demonstrated that phase coherence can correctly detect responses to near-threshold stimuli. In 15 min runs, significant phase coherence was detected within 6 dB of behavioral threshold in 6 subjects for 0.5 and 2.0 kHz signals, while phase coherence in no-stimulus control runs did not reach significance. Minimum data collection time required to record significant (P less than 0.01) responses was studied for 10 subjects. In 2 of 40 recordings at 10 dB SL phase coherence remained insignificant after even 15 min. However, average recording time to reach significance at 10 dB SL was less than 4 min in 38 of 40 recordings, and less than 1 min at 25 dB SL (18 of 18 recordings). These results indicate that using phase coherence to detect the presence of the 40/sec auditory steady-state response, efficient threshold search procedures may be devised to provide fast, accurate, and objective estimates of auditory behavioral thresholds in nearly all normal adults.

Acoustic Stimulation↗

PubMed1981

A 40-Hz auditory potential recorded from the human scalp.

Computer techniques readily extract from the brainwaves an orderly sequence of brain potentials locked in time to sound stimuli. The potentials that appear 8 to 80 msec after the stimulus resemble 3 or 4 cycles of a 40-Hz sine wave; we show here that these waves combined to form a single, stable, composite wave when the sounds are repeated at rates around 40 per sec. This phenomenon, the 40-Hz event-related potential (ERP), displays several properties of theoretical and practical interest. First, it reportedly disappears with surgical anesthesia, and it resembles similar phenomena in the visual and olfactory system, facts which suggest that adequate processing of sensory information may require cyclical brain events in the 30- to 50-Hz range. Second, latency and amplitude measurements on the 40-Hz ERP indicate it may contain useful information on the number and basilar membrane location of the auditory nerve fibers a given tone excites. Third, the response is present at sound intensities very close to normal adult thresholds for the audiometric frequencies, a fact that could have application in clinical hearing testing.

Auditory Pathways↗

PubMedDate not supplied

Inconsistency of auditory middle latency and steady-state responses in infants.

Auditory middle latency and steady-state responses (MLR/SSRs) were recorded in normal infants (aged 3 weeks to 28 months) and adults. SSR amplitudes were maximum using stimulus presentation rates near 40 Hz in adults. By contrast, the infant data showed no consistent amplitude maximum across the rates tested (9-59 Hz). With the exception of the brain-stem response wave V to MLR Na deflection, MLR components in infants' responses to 10.85 Hz clicks did not show any consistent pattern. To investigate the hypothesis that the 40 Hz SSR is derived from overlapping of the 10 Hz MLR components, 43.4 Hz SSRs were synthesized from the responses recorded at 10.85 Hz and compared with those recorded at 43.4 Hz. The predictive accuracy of the synthesized 43.4 Hz SSRs was significantly better in adults than in infants. The results of these studies indicate the presence of large age-related differences in the auditory MLR and SSR, and in the relationship between the two responses.

Acoustic Stimulation↗

PubMedDate not supplied

Relationship of transient and steady-state auditory evoked fields.

Transient and steady-state auditory evoked fields (AEFs) to brief tone pips were recorded over the left hemisphere at 7 different stimulus rates (0.125-39 Hz) using a 37-channel biomagnetometer. Previous observations of transient auditory gamma band response (GBR) activity were replicated. Similar rate characteristics and equivalent dipole locations supported the suggestion that the steady-state response (SSR) at about 40 Hz represents the summation of successive overlapping (10 Hz) middle latency responses (MLRs). On the other hand, differences in equivalent dipole locations and habituation effects suggest that the magnetically recorded GBR is a separate phenomenon which occurs primarily at low stimulus rates and is unrelated to either the magnetically recorded MRL or SSR.

Acoustic Stimulation↗