From alpha to gamma: electrophysiological correlates of meditation-related states of consciousness.

Meditation practice is difficult to access because of its countless forms of appearances originating from the complexity of cultures it has to serve. This makes a suitable categorization for scientific use almost impossible. However, empirical data suggest that different forms of meditation show similar steps of development in terms of their neurophysiological correlates. Some electrophysiological alterations can be observed on the beginner/student level, which are closely related to non-meditative processes. Others seem to correspond to an advanced/expert level, and seem to be unique for meditation-related states of consciousness. Meditation is one possibility to specialize brain/mind functions using the brain's immanent neural plasticity. This plasticity is probably recruited by certain EEG patterns observed during or as a result of meditation, for instance, synchronized gamma oscillations. While meditation formerly has been understood to comprise mainly passive relaxation states, recent EEG findings suggest that meditation is associated with active states which involve cognitive restructuring and learning.

Intracranial EEG correlates of expectancy and memory formation in the human hippocampus and nucleus accumbens.

The human brain is adept at anticipating upcoming events, but in a rapidly changing world, it is essential to detect and encode events that violate these expectancies. Unexpected events are more likely to be remembered than predictable events, but the underlying neural mechanisms for these effects remain unclear. We report intracranial EEG recordings from the hippocampus of epilepsy patients, and from the nucleus accumbens of depression patients. We found that unexpected stimuli enhance an early (187 ms) and a late (482 ms) hippocampal potential, and that the late potential is associated with successful memory encoding for these stimuli. Recordings from the nucleus accumbens revealed a late potential (peak at 475 ms), which increases in magnitude during unexpected items, but no subsequent memory effect and no early component. These results are consistent with the hypothesis that activity in a loop involving the hippocampus and the nucleus accumbens promotes encoding of unexpected events.

Electrophysiological signature of working and long-term memory interaction in the human hippocampus.

Recent findings indicate that the hippocampus is not only crucial for long-term memory (LTM) encoding, but plays a role for working memory (WM) as well. In particular, it has been shown that the hippocampus is important for WM maintenance of multiple items or associations between item features. Previous studies using intracranial electroencephalography recordings from the hippocampus of patients with epilepsy revealed slow positive potentials during maintenance of a single item and during LTM encoding, but slow negative potentials during maintenance of multiple items. These findings predict that WM maintenance of multiple items interferes with LTM encoding, because these two processes are associated with slow potentials of opposing polarities in the hippocampus. Here, we tested this idea in a dual-task paradigm involving a LTM encoding task nested into a WM Sternberg task with either a low (one item) or a high (three items) memory load. In the high WM load condition, LTM encoding was significantly impoverished, and slow hippocampal potentials were more negative than in the low WM load condition. Time-frequency analysis revealed that a reduction of slow hippocampal activity in the delta frequency range supported LTM formation in the low load condition, but not during high WM load. Together, these findings indicate that multi-item WM and LTM encoding interfere within the hippocampus.

Interference of working memory load with long-term memory formation.

Traditionally, it has been assumed that the medial temporal lobe (MTL) is indispensable for long-term memory (LTM) encoding, but only plays a minor role for working memory (WM) maintenance. Recently, however, an increasing number of studies questioned this seemingly clear distinction by showing that the MTL does participate in some WM processes, especially if multiple items are being maintained. This would predict that WM maintenance of multiple items interferes with simultaneous LTM encoding. Here, we tested this idea in a functional magnetic resonance imaging paradigm that required subjects to encode stimuli into LTM during simultaneous WM maintenance of either single or multiple items. Indeed, we found that maintenance of multiple items deteriorates simultaneous LTM encoding as compared with maintenance of single items. WM-related activation of the hippocampus was more pronounced in the condition with high WM load; in contrast, hippocampal activation related to LTM encoding was stronger in the low WM load condition. Successful LTM encoding was associated with a high level of activity in the adjacent parahippocampal cortex (PHC), leading to pronounced parahippocampal subsequent memory effects in the high load condition. This suggests that the PHC is a locus of WM-LTM interaction. Functional connectivity analysis with a seed in the PHC confirmed this result by revealing strong connectivity with the medial frontal cortex, which was only active in the high WM load condition. Taken together, these findings suggest that high WM demands interfere with LTM encoding and thus support the idea that WM and LTM processes interact in the MTL.

Activation of the caudal anterior cingulate cortex due to task-related interference in an auditory Stroop paradigm.

Successful information processing requires the focusing of attention on a certain stimulus property and the simultaneous suppression of irrelevant information. The Stroop task is a useful paradigm to study such attentional top-down control in the presence of interference. Here, we investigated the neural correlates of an auditory Stroop task using fMRI. Subjects focused either on tone pitch (relatively high or low; phonetic task) or on the meaning of a spoken word (high/low/good; semantic task), while ignoring the other stimulus feature. We differentiated between task-related (phonetic incongruent vs. semantic incongruent) and sensory-level interference (phonetic incongruent vs. phonetic congruent). Task-related interference activated similar regions as in visual Stroop tasks, including the anterior cingulate cortex (ACC) and the presupplementary motor-area (pre-SMA). More specifically, we observed that the very caudal/posterior part of the ACC was activated and not the dorsal/anterior region. Because identical stimuli but different task demands are compared in this contrast, it reflects conflict at a relatively high processing level. A more conventional contrast between incongruent and congruent phonetic trials was associated with a different cluster in the pre-SMA/ACC which was observed in a large number of previous studies. Finally, functional connectivity analysis revealed that activity within the regions activated in the phonetic incongruent vs. semantic incongruent contrast was more strongly interrelated during semantically vs. phonetically incongruent trials. Taken together, we found (besides activation of regions well-known from visual Stroop tasks) activation of the very caudal and posterior part of the ACC due to task-related interference in an auditory Stroop task.

Medial frontal cortex and response conflict: evidence from human intracranial EEG and medial frontal cortex lesion.

The medial frontal cortex (MFC) has been implicated in the monitoring and selection of actions in the face of competing alternatives, but much remains unknown about its functional properties, including electrophysiological oscillations, during response conflict tasks. Here, we recorded intracranial EEG during a modified Flanker task from the MFC of two patients undergoing pre-surgical evaluation for the treatment of epilepsy. Performance on the task was associated with a suppression of beta (15-30 Hz) frequency oscillation power prior to and just following the response and an enhancement of theta (4-8 Hz) frequency power following the response. Beta (theta) power was anatomically distributed towards more dorsal/caudal (rostral/ventral) electrode sites along the cortex, suggesting an anatomical/functional specialization along the medial frontal wall for pre-response versus post-response action monitoring. Inter-site phase coherence analyses demonstrated that the ventral/rostral MFC theta oscillations were coupled with theta oscillations observed at scalp electrodes Fz and Cz. One patient was tested before and after having epileptogenic tissue in the MFC surgically removed; task performance increased from chance levels to near-perfect, and an ERP conflict effect was observed only following surgery. These findings provide novel evidence for the role of MFC oscillations and their relation to surface EEG-recorded potentials during action monitoring.

The role of sleep in declarative memory consolidation--direct evidence by intracranial EEG.

Two step theories of memory formation assume that an initial learning phase is followed by a consolidation stage. Memory consolidation has been suggested to occur predominantly during sleep. Very recent findings, however, suggest that important steps in memory consolidation occur also during waking state but may become saturated after some time awake. Sleep, in this model, specifically favors restoration of synaptic plasticity and accelerated memory consolidation while asleep and briefly afterwards. To distinguish between these different views, we recorded intracranial electroencephalograms from the hippocampus and rhinal cortex of human subjects while they retrieved information acquired either before or after a "nap" in the afternoon or on a control day without nap. Reaction times, hippocampal event-related potentials, and oscillatory gamma activity indicated a temporal gradient of hippocampal involvement in information retrieval on the control day, suggesting hippocampal-neocortical information transfer during waking state. On the day with nap, retrieval of recent items that were encoded briefly after the nap did not involve the hippocampus to a higher degree than retrieval of items encoded before the nap. These results suggest that sleep facilitates rapid processing through the hippocampus but is not necessary for information transfer into the neocortex per se.

Rhinal-hippocampal coupling during declarative memory formation: dependence on item characteristics.

Lesion and imaging studies have demonstrated that encoding and retrieval of declarative memories, i.e. consciously accessible events and facts, depend on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Analysis of intracranially recorded EEG in presurgical epilepsy patients revealed that successful memory formation is accompanied within one second by a transient enhancement and later decrease of Rhinal-hippocampal phase synchronization in the gamma range, as well as enhanced connectivity in the low-frequency range. In these studies, words with a high frequency of occurrence were used as stimulus material. Here, we re-examined these effects in another group of 10 presurgical epilepsy patients, this time not only for high-frequency, but also for low-frequency words. For successfully memorized compared to later forgotten high-frequency words we again observed an early phase coupling and later decoupling within the gamma range, as well as enhanced coupling within the sub-gamma range. However, for remembered as compared to forgotten low-frequency words clear synchronization increases were only observed for the delta band, but not for the gamma band. Our data suggest, that broadband Rhinal-hippocampal coupling including the gamma range only occurs, when significant semantic associations are processed within rhinal cortex, as is the case for high-frequency words.