When brief periods of EEG patterns are recorded in response to a specific event what is the response called?

While the ERSP reveals new and potentially important information about event-related brain dynamics, it cannot reveal interactions between the ERP amplitude, latency variability, and EEG spectral modulation. [4] Removing a mean ERP from each ERSP epoch prior to analysis would not eliminate the effects on the ERSP of trial-to-trial variability in the ERP itself. For example, a skewed distribution of between-trial ERP variabilities could also create the appearance of augmented EEG amplitude in an ERP-corrected ERSP. These complexities cannot be resolved by ERSP analysis alone without detailed models for ERP variance and EEG modulation. Mixed models in which the phase coherence of evoked-response components is neither one (phase locked ERP) nor zero (independent EEG) are also possible. In such models the relationship between pre- and post-event EEG phase could also be perturbed by an experimental event (Jansen and Brandt, 1991; Achimowicz, 1992), and this could produce additional ERSP effects.

Further Questions

The sensitivity of the tone-induced ERSP to prior stimulation, in the absence of peripheral effects, suggests that ERSP measures might prove sensitive to the effects of previous experience on brain dynamics. A number of further questions are raised by the ERSP results of this study:

1. Prior to this experiment, data from a group of ten subjects were collected using an exposure and intensity of 83 dB (A-weighting). The ERSPs for this group were generally similar to those reported here, as were the differences between the ERSPs before and during exposure. However, for the 83 dB group these changes were smaller and less significant, possibly because of the smaller number of subjects. Little else is known about the effects of stimulus intensity on the ERSP.

2. The tone frequencies used for evoked-response testing were the same as those used in free-field exposure. It is not known whether ERSPs to tones of different frequencies would also be suppressed during exposure.

3. Throughout these experiments subjects were attending the tones in anticipation of responding to the occasional targets. The ERSPs induced by target pips were larger than those induced by tones, possibly on account of their comparative probability or task relevance. Effects of alertness, task relevance, and attention on ERSP form and amplitude remain to be explored (Makeig and Inlow, 1992).

4. Averaging over all subjects and trials yields information about only the most global event-related response patterns-- between-subject and between-trials differences are ignored. Gray et al. (1992) have recently shown that spatial coherences among event-related gamma band oscillations in the visual cortex of cats are complex transitory dynamic phenomena. It is probable that single-trial ERSPs reported here also represent transient and somewhat variable dynamic events. Cluster analysis of subjects and trials, followed by discrete or moving subaverages, might prove useful for better understanding the brain processes measured by ERSPs.

CONCLUSIONS

1. A wideband, normalized measure of event-related spectral reactivity, the event-related spectral perturbation or ERSP, can be used to measure aspects of event-related brain dynamics not found in the ERP.

2. Auditory ERSPs had similar forms and latencies for two very different inducing stimuli -- 1 s, low-frequency tones, and 25 ms, 3 kHz tone pips. A prominent feature of both ERSPs was a prominent, relatively late (1.0-1.5s) ridge of augmented 11-19 Hz EEG amplitudes which was larger in response to the target tonepips.

3. Some features of the auditory ERSP appear to be sensitive to previous sound exposure. In this experiment, some features of the tone-induced ERSPs were significantly suppressed following prolonged exposure to moderately loud pure tones, and appeared to not recover fully on the day following exposure.

The physiological significance of the ERSP features revealed in these experiments is not yet known, and many experiments may be required to determine their properties. Auditory, visual and somatosensory stimulus-induced ERSPs may be expected to differ in form, time course, and scalp distribution. Analysis of single response epochs, multi-channel source localization studies, and multi-modality experiments would also be of interest. Results of cognitive experiments, including experiments involving omitted stimuli, could add to our understanding of the relation of EEG activity to cognition. It is likely that further research using wide band ERSP measures can yield new information on brain dynamics of perception and cognition.

ACKNOWLEDGEMENTS

The author wishes to acknowledge the assistance of Steven Linnville, Anne Makeig and Candee Corwin in collecting the data, of Mark Inlow in discussing the results, and of Robert Galambos and Jerszy Achimowicz in commenting on earlier versions of this report.

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FOOTNOTES

1. Subsequent to data collection, it was found that because of a minor computer malfunction, stimuli and EEG recording had not been precisely synchronized. Using steady-state response (SSR) phase results, it was determined that stimulus presentation lagged behind EEG data collection at a rate of one frame in 7728. Using this information, moments of stimulus onset were corrected post hoc. Because ERSP analysis trades time resolution for frequency resolution, remaining uncertainty in stimulus timing (estimated from control runs as less than 25 ms (root mean squared)) could affect the ERSP results very little. However, ERP measures depend on more precise response phase synchronization, and therefore are not reported here. (Return)

2. Amplitude/baseline amplitude normalization and mean log averaging provide one of several possible stochastic estimates of mean tendency in the distribution of response epochs. Median, arithmetic mean, and root mean square averaging might also be used to give somewhat different estimates. Averaging of event-related spectra can also be carried out without normalizing, producing an event-related spectrum transform. However, this can be dominated by noisy large-amplitude epochs in which the tendency for proportionate amplitude increase or decrease is obscured or absent. (Return)

3. This confound between EEG and ERP spectral characteristics is also present in narrow-band ERDs. (Return)

4. In general, vestiges of phenomena revealed by ERSP analysis can remain in ERPs as time-dependent changes in the spectral content of the background EEG "noise" remaining in the ERP average. However, as ERP analysis is designed to remove phase-incoherent activity occurring in the response period, the vestiges of the ERSP remaining in the ERP are normally small, and in general ERSP features cannot be expected to be recaptured efficiently from the ERP by post hoc spectral analysis. (Return)

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