RESUMEN
Since years there is a hotly discussed dispute whether event-related potentials are either generated by an evoked component or by resetting of ongoing phase. We argue that phase-reset must not be proven in order to accept the general involvement of phase in ERP-generation as it is only one of several possible mechanisms influencing or generating certain ERP-components. Supporting data are presented showing that positive peaks of ongoing pre-stimulus alpha activity are not randomly distributed in time across trials. Most importantly, we found that a certain kind of pre-stimulus phase concentration that represents a continuous development of an alpha wave up to the time window where the P1 is generated is associated with an enlarged event-related component. We conclude that ongoing oscillations cannot be considered random background noise (even before stimulus onset) and that there are probably more phase-mechanisms that can contribute to the ERP-generation.
Asunto(s)
Ritmo alfa/fisiología , Potenciales Evocados/fisiología , Adolescente , Adulto , Electroencefalografía , Femenino , Humanos , Masculino , Adulto JovenRESUMEN
OBJECTIVE: Although behavioral evaluation of awareness in disorders of consciousness is difficult it remains the clinical standard. We believe that the refinement of EEG and analyses techniques would improve our characterization of those patients. METHODS: We focused on cognitive processing in a sample of 12 control subjects, eight vegetative-state patients, and 13 patients in the minimally consciousness state using EEG. We used an 'active paradigm' which asks subjects to follow instructions, specifically to actively count own or other names as compared to passively listening to them. EEG data was then analyzed using an advanced EEG analysis technique. RESULTS: Results revealed that all groups exhibit a stronger theta-synchronization to their own names when forced to count them. We also observed a delay in theta power in response to targets relative to non-targets when participants were instructed to count their own name. CONCLUSION: Active paradigms are able to induce a different oscillatory activity compared to passive paradigms. Differences between controls and the pathologic groups are prominent in the theta- and alpha-band. SIGNIFICANCE: Time-frequency analyses allow to focus on distinct cognitive processes in patients with disorders of consciousness and thereby contribute to a refined understanding of severely brain-injured patients.
Asunto(s)
Cognición/fisiología , Electroencefalografía/métodos , Estado Vegetativo Persistente/diagnóstico , Estado Vegetativo Persistente/fisiopatología , Adulto , Anciano , Concienciación/fisiología , Ondas Encefálicas/fisiología , Estado de Conciencia/fisiología , Potenciales Evocados/fisiología , Femenino , Humanos , Masculino , Persona de Mediana Edad , Tiempo de Reacción/fisiología , Procesamiento de Señales Asistido por ComputadorRESUMEN
In this study we investigated the hypothesis whether P2-related differences tested in a visual priming paradigm are associated with theta phase-locking. We recorded the EEG from 31 electrodes and calculated phase-locking index and total power differences for frequencies between 2 and 20 Hz. ERPs (event-related potentials) were analyzed for P1, N1 and P2 components. P2 showed strongest task-related amplitude differences between congruent and incongruent targets. A source analyses was performed for the P2 component using sLoreta that revealed local generators of the P2 in parieto-occipital regions. Phase-locking analyses showed specific effects in the theta range (4-6 Hz) appearing in time windows at around the P2 component. We draw the conclusion that phase-locked theta reflect top-down regulation processes mediating information between memory systems and is in part involved in the modulation of the P2 component.
Asunto(s)
Encéfalo/fisiología , Potenciales Evocados/fisiología , Ritmo Teta , Adulto , Femenino , Humanos , MasculinoRESUMEN
The event-related potential (ERP) is one of the most popular measures in human cognitive neuroscience. During the last few years there has been a debate about the neural fundamentals of ERPs. Two models have been proposed: The evoked model states that additive evoked responses which are completely independent of ongoing background electroencephalogram generate the ERP. On the other hand the phase reset model suggests a resetting of ongoing brain oscillations to be the neural generator of ERPs. Here, evidence for either of the two models is presented and validated, and their possible impact on cognitive neuroscience is discussed. In addition, future prospects on this field of research are presented.
Asunto(s)
Encéfalo/fisiología , Potenciales Evocados/fisiología , Periodicidad , Electroencefalografía/métodos , Humanos , Modelos BiológicosRESUMEN
In a task switching design, we investigated the question whether long-range theta coupling primarily reflects top-down control processes. Switch and stay trials did not differ with respect to memory load or global working memory (WM) demands. The results revealed significantly stronger theta coupling (in a range of 4-7 Hz) between prefrontal and posterior regions during switch as compared to stay trials. Power differences, reflecting more local effects, were largest in the upper alpha band (10-13 Hz) and over posterior brain areas, possibly reflecting long-term memory activation. The conclusion of the present study is that long-range coherent oscillatory activity in the theta band reflects top-down activation rather than global WM functions.