Search

Background Deep brain stimulation (DBS) of the human entorhinal area using 50 Hz pulses has revealed conflicting results regarding memory performance. Moreover, its impact on memory-related hippocampal potentials has not yet been investigated. Methods We recorded data from seven epilepsy patients implanted with depth electrodes in the entorhinal cortex, hippocampus, amygdala, and parahippocampal cortex. Entorhinal DBS (bipolar, biphasic 50 Hz pulses, on- and off-cycles of 15 s) was applied with low amplitude (0.1 mA) to resemble physiologic conditions. During DBS on- and off-periods, patients learned noun-color associations that were later tested. Results During entorhinal DBS we observed more positive deflections of event-related potentials (ranging from 700 to 950 ms) in the anterior hippocampus for the on- vs. off-condition. We detected no effects in the amygdala, mid hippocampus and parahippocampal cortex. On the behavioral level, no differences in memory performance (item and source memory) were apparent in the on- vs. off-condition, neither across all trials nor across patients. Discussion Our findings indicate that entorhinal DBS with low amplitude has an impact on memory encoding-related potentials within the anterior hippocampus, but not on memory performance per se. [ABSTRACT FROM AUTHOR]

ABSTRACT Studies on hippocampal cell loss in epilepsy have produced diverging evidence as to which subfields are specifically related to memory. This may be due to rather small and often heterogeneous samples, or to different memory measures. Therefore, the current study examined hippocampal cell densities and memory in a large sample of patients with solely mesial temporal lobe epilepsy (mTLE), employing measures with proven sensitivity to mesiotemporal pathology. In 104 patients who had undergone epilepsy surgery for mTLE, we evaluated the role of segmental hippocampal cell loss and its underlying factor structure with regard to presurgical verbal and figural memory while controlling for side-of-surgery and hemispheric dominance. First of all, patients showed material-specific memory impairment concordant with the lateralization of epilepsy. Factor analysis of segmental cell loss revealed a single factor reflecting the overall integrity of the hippocampus. The overall pathological status of the left hippocampus correlated with verbal memory parameters ( r = 0.33-0.34, P < 0.05), especially when controlling for atypical hemispheric dominance ( r = 0.50-0.57, P < 0.01), and explained up to 33% of the observed variance. Further analyses revealed no superior role of a single subfield or cell loss pattern for memory performance. No systematic relations between neuronal cell densities of the right hippocampus and memory function were found, nor did left or right hippocampal pathology explain figural memory parameters. The results suggest that the overall pathological status of the left hippocampus - rather than a specific subfield pathology - is predictive for verbal memory in mTLE. The finding that figural memory parameters, although sensitive to right mTLE, were not related to neuronal cell densities of the right hippocampus, puts the left/right hippocampus verbal/nonverbal memory dichotomy into perspective. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The presentation of rare target stimuli results in P300 scalp event-related potentials (ERPs). Generators of this ERP component were found in various brain areas, indicating that multiple cortical and subcortical areas subserve target detection. One of these structures is the mediotemporal lobe (MTL). In the hippocampus, large negative MTL-P300 potentials are usually observed, whereas reports concerning the rhinal cortex and subiculum are inconsistent. The aim of the present study was to investigate the topography of the mediotemporal P300. ERPs were recorded in epilepsy patients from multicontact depth electrodes, implanted along the longitudinal axis of MTL. Patients had to respond to rare visual target stimuli by a button press. ERP data from the nonfocal hemisphere of 53 patients were included in the analysis. Target detection resulted in large MTL-P300 potentials in the hippocampus and subiculum. Their latencies did not differ. The hippocampal P300 amplitude increased linearly from anterior to posterior hippocampal body (HB). In contrast, an inverse gradient with larger mean amplitudes in anterior parts was observed for the subiculum. Our results indicate two separate generators of the MTL-P300, one in the anterior subiculum and one in the posterior HB. Since latencies did not differ, a parallel activation via the entorhinal cortex might have initiated the simultaneous MTL-P300. Hippocampus and subiculum are essential parts of the MTL-memory system. Their function within target detection might be to maintain a template of previous stimuli for a comparison with incoming sensory stimuli. © 2009 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]