Mirroring of a simple motor behavior in disorders of consciousness.

OBJECTIVE: In patients suffering from Disorders of Consciousness (DOC) electrophysiological recordings at bedside could serve as a complimentary and economical tool to improve diagnosis. We utilized a motor observation and imagination paradigm to gain new insights on preserved cognitive processing in DOC. METHODS: EEG brain oscillations were analyzed in 10 VS/UWS (Vegetative State/Unresponsive Wakefulness Syndrome) patients and 7 MCS (Minimally Conscious State) patients and 21 controls during observation and imagination of a grasping movement and group statistics were conducted. RESULTS: While control subjects showed a typical desynchronization at 8-15Hz during observation of a movement, MCS patients presented an analogue response at 8-10Hz, but exhibited a synchronization at 12-15Hz. The VS group did not show a systematic response. Imagery-related activation was only sustained for 1500ms even in control subjects, therefore, limiting conclusions regarding the ability to follow an instruction. Furthermore, a clinically diagnosed VS patient exhibited EEG responses indicative for MCS. CONCLUSION: Results indicate that MCS patients are still able to process an observed motor behavior on a basic sensory and perhaps even pre-motoric level, but seem not to be capable of "mirroring" the movement like healthy participants. SIGNIFICANCE: "Real-world" tasks as presented here carry the potential to identify residual cognitive functioning in DOC patients and may ultimately help to lower misdiagnosis rates.

Alpha entrainment is responsible for the attentional blink phenomenon.

The attentional blink phenomenon is the reduced ability to report a second target (T2) after identifying a first target (T1) in a rapid serial visual presentation (RSVP) of stimuli (e.g., letters), which are presented at approximately 10 items per second. Several explanations have been proposed, which focus primarily on cognitive aspects, such as attentional filter-, capacity limitation- and retrieval failure-processes. Here, we focus on the hypothesis that an entrainment of alpha oscillations (with a frequency of about 10Hz) is a critical factor for the attentional blink phenomenon. Our hypothesis is based on the fact that item presentation rate in the RSVP typically lies in the alpha frequency range and is motivated by theories assuming an inhibitory function for alpha. We predict that entrainment--during the time window of T2 presentation--is larger for attentional blink (AB) items (when T2 cannot be reported) than for NoAB trials (when T2 cannot be reported). The results support our hypothesis and show that alpha entrainment as measured by the amplitude of the alpha evoked response and the extent of alpha phase concentration is larger for AB than for NoAB trials. Together with the lack of differences in alpha power these findings demonstrate that the differences between AB and NoAB trials--during presentation onset of T2--are due to an entrainment of alpha phase and not due to an amplitude modulation. Thus, we conclude that alpha entrainment may be considered the critical factor underlying the attentional blink phenomenon.

Evoked traveling alpha waves predict visual-semantic categorization-speed.

In the present study we have tested the hypothesis that evoked traveling alpha waves are behaviorally significant. The results of a visual-semantic categorization task show that three early ERP components including the P1-N1 complex had a dominant frequency characteristic in the alpha range and behaved like traveling waves do. They exhibited a traveling direction from midline occipital to right lateral parietal sites. Phase analyses revealed that this traveling behavior of ERP components could be explained by phase-delays in the alpha but not theta and beta frequency range. Most importantly, we found that the speed of the traveling alpha wave was significantly and negatively correlated with reaction time indicating that slow traveling speed was associated with fast picture-categorization. We conclude that evoked alpha oscillations are functionally associated with early access to visual-semantic information and generate--or at least modulate--the early waveforms of the visual ERP.

Alpha oscillations and early stages of visual encoding.

For a long time alpha oscillations have been functionally linked to the processing of visual information. Here we propose an new theory about the functional meaning of alpha. The central idea is that synchronized alpha reflects a basic processing mode that controls access to information stored in a complex long-term memory system, which we term knowledge system in order to emphasize that it comprises not only declarative memories but any kind of knowledge comprising also procedural information. Based on this theoretical background, we assume that during early stages of perception, alpha "directs the flow of information" to those neural structures which represent information that is relevant for encoding. The physiological function of alpha is interpreted in terms of inhibition. We assume that alpha enables access to stored information by inhibiting task-irrelevant neuronal structures and by timing cortical activity in task relevant neuronal structures. We discuss a variety findings showing that evoked alpha and phase locking reflect successful encoding of global stimulus features in an early post-stimulus interval of about 0-150 ms.

Event-related activity and phase locking during a psychomotor vigilance task over the course of sleep deprivation.

There is profound knowledge that sleep restriction increases tonic (event-unrelated) electroencephalographic (EEG) activity. In the present study we focused on time-locked activity by means of phasic (event-related) EEG analysis during a psychomotor vigilance task (PVT) over the course of sleep deprivation. Twenty healthy subjects (10 male; mean age ± SD: 23.45 ± 1.97 years) underwent sleep deprivation for 24 h. Subjects had to rate their sleepiness hourly (Karolinska Sleepiness Scale) and to perform a PVT while EEG was recorded simultaneously. Tonic EEG changes in the δ (1-4 Hz), θ (4-8 Hz) and α (8-12 Hz) frequency range were investigated by power spectral analyses. Single-trial (phase-locking index, PLI) and event-related potential (ERP) analyses (P1, N1) were used to examine event-related changes in EEG activity. Subjective sleepiness, PVT reaction times and tonic EEG activity (delta and theta spectral power) significantly increased over the night. In contrast, event-related EEG parameters decreased throughout sleep deprivation. Specifically, the ERP component P1 diminished in amplitude, and delta and theta PLI estimates decreased progressively over the night. It is suggested that event-related EEG measures (such as the amplitude of the P1 and especially delta/theta phase-locking) serve as a complimentary method to track the deterioration of attention and performance during sleep loss. As these measures actually reflect the impaired response to specific events rather than tonic changes during sleep deprivation they are a promising tool for future sleep research.

Dissociation between phase-locked and nonphase-locked alpha oscillations in a working memory task.

The functions of human alpha oscillations ( approximately 10 Hz) were related to cognitive processes such as memory and top-down control. Recent models suggest that alpha phase serves as a mechanism especially relevant for the timing of neural activity, whereas alpha amplitude is important for the inhibition of task-irrelevant brain areas. This study investigates directly the influence of top-down modulation on phase-locked and nonphase-locked alpha rhythms. We conducted an EEG experiment where subjects performed a working memory task. In the encoding phase of the task subjects had to learn presented pictures of nonliving objects that could later be asked to be retrieved. We varied the top-down modulation by including cues indicating either to remember or to forget (not to remember) the next following item. Spectral analyses showed that nonremember cues elicited pronounced alpha amplitude increase compared to remember cues. Furthermore, phase-locking in low frequencies, especially in the alpha range (7-12 Hz), was stronger for remember as opposed to not-to-remember items. In conclusion, we propose that alpha amplitude reflects top-down modulated inhibition and that alpha phase is important for the exact timing of neural activity and can be related to binding processes.