Do surface DC-shifts affect epileptic hippocampal EEG activity?

Despite considerable research on EEG-feedback of slow cortical potentials (SCPs) for seizure control in epilepsy, the underlying mechanisms and the direct effects on intracerebral pathological activity within the focal area remain unclear. Intrahippocampal EEG recordings from four patients with temporal lobe epilepsy and implanted electrodes were analyzed with regard to spike activity and power in 10 frequency bands (0.5-148Hz) during SCP feedback based on surface recordings (position Cz). Trials with positive, negative and indifferent SCPs were contrasted. Three of the four patients showed changes in spike activity during SCPs, but these were inconsistent between patients, and resulted in increased and decreased activity in both positive and negative SCPs. Spectral analysis revealed that in all patients, positive surface shifts showed a bi-hemispheric higher power in the high-frequency activity above 40Hz. Two patients showed a higher power also during negative shifts, both in high-frequency activity and one in most other frequency bands. Feedback-related power effects did not differ between focal and non-focal side. The inconsistent change in spiking activity and the lack of decrease of power in pathology associated frequency bands during SCPs show that these SCPs do not decrease pathological activity within the epileptic focus. A possible relation of higher power in high-frequency activity during positive SCPs to cognitive processes, such as memory functions, is discussed.

Disease- and treatment-related effects on the pituitary-gonadal functional axis: a study in men with epilepsy.

The consideration of the patient's gender has become an established practice in the choice of antiepileptic drugs (AEDs) for the treatment of epilepsy. This study was undertaken to confirm that temporal lobe epilepsy and the use of hepatic enzyme-inducing AEDs, have a negative effect on sex hormones in men. We calculated the testosterone (T)/luteinizing hormone (LH) ratio to reflect the effects of epilepsy or its treatment on the most important androgen and its pituitary control hormone. Of 324 men with focal epilepsy identified (untreated or on AED monotherapy), 201 were eligible for inclusion (mean age 37.5 years, mean duration of epilepsy 14.7 years). A total of 105 men served as healthy controls (mean age 33.9 years). Patients with temporal lobe epilepsy were more likely to show abnormal T/LH ratios than patients with extratemporal epilepsy (p < 0.01). Patients receiving AEDs with marked hepatic enzyme-inducing effects were more likely to have low T/LH ratios than patients taking nonenzyme-inducing AEDs or healthy controls (p < 0.01). Focus localization and AED choice affect male sex hormones. Findings raise some concerns about the effect of enzyme-inducing AEDs on sexual endocrine functions.

Correlation between EEG rhythms during sleep: surface versus mediotemporal EEG.

We compared surface and intracranial electroencephalogram recordings of mediotemporal structures. These structures are critically involved in declarative memory formation and memory consolidation during sleep. As memory processing is suggested to involve the interplay between fast and slow oscillations, we hypothesized different correlations between frequency bands in surface versus mediotemporal electroencephalogram recordings. Polysomnographic recordings obtained in 10 patients with unilateral temporal lobe epilepsy were analyzed. In accordance with earlier studies, we observed that power density in surface electroencephalogram is organized reciprocally between delta/theta and fast frequencies above 16 Hz during non-rapid-eye-movement sleep (negative correlations). In contrast, we found that within the hippocampus delta/theta power alternated in parallel with fast oscillations above 16 Hz during non-rapid-eye-movement sleep (positive correlations).

Human neocortical and hippocampal near-DC shifts are interconnected.

Hippocampal DC shifts have been observed under various physiological and pathological conditions. Here, we studied the interconnection of slow shifts (0.01 Hz high-pass) in surface EEG and hippocampal shifts as emerging in an event-related EEG biofeedback paradigm. Hippocampal EEG activity was monitored by depth electrodes implanted in four epilepsy patients for presurgical evaluation. Trials were sorted according to the near-DC shifts occurring at the surface position Cz, which was the feedback electrode, into positive, indistinct (i.e., small or biphasic) and negative shifts. We found significant hippocampal near-DC shifts being positively or negatively correlated to the shifts in surface EEG in all four patients. The amplitudes of the hippocampal near-DC shifts were several times larger than the surface shifts. The polarity of the shifts appears to depend on the location of the electrode contacts with respect to the hippocampal subfields. The finding that neocortical and hippocampal near-DC shifts are interconnected may open new perspectives for the prediction and control of mediotemporal lobe seizures.

Rhinal-hippocampal connectivity determines memory formation during sleep.

Compared with waking state attention, volition and semantic processing play a minor role during sleep. Thus, investigating declarative memory formation during sleep may allow us to isolate mnemonic core processes. The most feasible approach to memory formation during sleep is the analysis of dream memories. Lesion and imaging studies have demonstrated that encoding of declarative memories, i.e. consciously accessible events and facts, depends on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Successful memory formation is accompanied by a transient rhinal-hippocampal interaction. Consequently, the ability to memorize dreams may be related to mediotemporal connectivity. Therefore, we recorded EEG during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients (eight women, mean age 41.1 +/- 6.4 years). They were awakened during rapid eye movement sleep (REM) and asked to recall their dream. Via coherence analyses we show that rhinal-hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall. This suggests that rhinal-hippocampal connectivity is a key factor in determining declarative memory formation.

Rhinal-hippocampal EEG coherence is reduced during human sleep.

The deficiency of declarative memory compared with waking state is an often overlooked characteristic of sleep. Here, we investigated whether rhinal-hippocampal coherence, an electrophysiological correlate of declarative memory formation, is significantly altered during sleep as compared with waking state. For this purpose, we analysed recordings of intracranial EEG activity during sleep obtained directly from within the medial temporal lobe in patients with unilateral temporal lobe epilepsy. We found a general reduction of rhinal-hippocampal EEG coherence during sleep compared with waking state, which was most pronounced within the upper gamma bands (average decrease up to 56%). The observed coherence changes clearly differ from findings reported for surface EEG data and thus appear to be specific for the medial temporal lobe. The decrease of rhinal-hippocampal EEG coherence from waking state towards sleep may yield an electrophysiological explanation for the sleep-related deficiency of declarative memory.

Human scalp recorded sigma activity is modulated by slow EEG oscillations during deep sleep.

The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and below 1 Hz which can be distinguished from other delta (0.5-3.5 Hz) activity. Intracranial studies showed that this slow oscillation triggers and groups cortical network firing. In the present study, we examined whether the phases of the slow oscillation during sleep stage 4 are correlated with the magnitude of sigma (12-16 Hz) and gamma (> 20 Hz) scalp activity. For this purpose, 10-min segments of uninterrupted stage 4 sleep EEG from 9 subjects were analyzed by applying wavelet techniques. We found that scalp recorded sigma, but not gamma, activity is modulated by the phases of the slow oscillation during deep sleep. Enhancement of sigma activity was observed to be triggered by the peak of the surface positive slow wave component, whereas reduction of sigma activity started around the peak of the negative component.