Neural Bases of Cognitive ERPs: More than Phase Reset

Abstract

Up to now, two conflicting theories have tried to explain the genesis of averaged event-related potentials (ERPs): Whereas one hypothesis claims that ERPs originate from an event-related activation of neural assemblies distinct from background dynamics, the other hypothesis states that ERPs are produced by phase resetting of ongoing oscillatory activity. So far, this question has only been addressed for early ERP components. Late ERP components, however, are generally thought to represent superimposed activities of several anatomically distinct brain areas. Thus, the question of which mechanism underlies the genesis of late ERP components cannot be easily answered based on scalp recordings. In contrast, two well-investigated late ERP components recorded invasively from within the human medial temporal lobe (MTL) in epilepsy patients, the so-called MTL-P300 and the anterior MTL-N400 (AMTL-N400), are based on single source activity. Hence, we investigated whether the MTL-P300 and the AMTL-N400 are based on an event-related activity increase, a phase reset of ongoing oscillatory activity or both. ERPs were recorded from the hippocampus and rhinal cortex in subjects performing a visual oddball paradigm and a visual word recognition paradigm. With wavelet techniques, stimulus-related phase-locking and power changes were analyzed in a frequency range covering 2 to 48 Hz. We found that the MTL-P300 is accompanied by both phase reset and power increase and that both effects overlap partly in time. In contrast, the AMTL-N400 is initially associated with phase locking without power increase and only later during the course of the AMTL-N400 we observed an additional power increase. In conclusion, both aspects, event-related activation of neural assemblies and phase resetting of ongoing activity seem to be involved in the generation of late ERP components as recorded in cognitive tasks. Therefore, separate analysis of event-related power and phase-locking changes might reveal specific insights into the mechanisms underlying different cognitive functions.

Single-trial EEG–fMRI reveals the dynamics of cognitive function

“…Moreover, it has been reported that eventrelated EEG responses are due to a partial phase resetting of, and thus not independent from, ongoing activity. Therefore, ERPs originating from partial phase resetting [35,48,49] might not induce major changes in local brain metabolism, and thus might not correlate with the BOLD signal. By contrast, additive event-related responses due to a larger neural assembly should be correlated with the local event-related fMRI BOLD signal [48].…”

Delta and theta oscillations as risk markers in adolescent offspring of alcoholics

“…The debate is ongoing; however, it is likely that both aspects -event-related activation of neural assemblies and phase resetting of ongoing activity, are involved in the generation of late ERP components. Studies suggest that separate analysis of event-related power and phase-locking changes in cognitive tasks might reveal specific insights into the mechanisms underlying different cognitive functions (Fell et al, 2004;Shah et al, 2004). Although an enhancement of EEG power is thought to correspond to an activation of a larger number of neural assemblies (Lopes da Silva, 1993), the interpretation of increased intertrial phase locking is more difficult define.…”

Event-Related Oscillations in Offspring of Alcoholics: Neurocognitive Disinhibition as a Risk for Alcoholism

“…Basar 1980;Karakas et al 2000aKarakas et al , 2000bMakeig et al 2002;Jansen et al 2003;Klimesch et al 2004). This view has recently been challenged with evidence that ERPs are produced by processes separate from and additive to ongoing brain activity (Yeung et al 2004;Fell et al 2004;Makinen et al 2005).…”