Human entorhinal gamma and theta oscillations selective for remote autobiographical memory

Current source densities in different layers of the human entorhinal cortex (ER) were recorded using a linear array of 24 microelectrodes during three memory conditions: a remote autobiographical condition eliciting recollections of events that occurred 10 or more years ago in the participant's past, a semantic icon condition invoking the mental image of a well‐known object, and a semantic knowledge condition asking about general information. Our data demonstrate theta, gamma, and delta oscillations in left ER particularly for remote autobiographical memory. Gamma is predominant in hippocampally projecting layers during presentation of the memory cue, whereas theta is prolonged and dominant in cortically projecting layers during memory retrieval. Gamma interactions between ER and hippocampal formation (HF) may elicit an HF contribution to recall under influences relayed from the association cortex (AC). This contribution may then be relayed back to AC during retrieval of the memory orchestrated by theta interactions with ER. Accordingly, theta oscillations were recorded simultaneously in frontal and temporal cortices. © 2009 Wiley‐Liss, Inc.     

Modulation of alpha and gamma oscillations related to retrospectively orienting attention within working memory

Selective attention mechanisms allow us to focus on information that is relevant to the current behavior and, equally important, ignore irrelevant information. An influential model proposes that oscillatory neural activity in the alpha band serves as an active functional inhibitory mechanism. Recent studies have shown that, in the same way that attention can be selectively oriented to bias sensory processing in favor of relevant stimuli in perceptual tasks, it is also possible to retrospectively orient attention to internal representations held in working memory. However, these studies have not explored the associated oscillatory phenomena. In the current study, we analysed the patterns of neural oscillatory activity recorded with magnetoencephalography while participants performed a change detection task, in which a spatial retro‐cue was presented during the maintenance period, indicating which item or items were relevant for subsequent retrieval. Participants benefited from retro‐cues in terms of accuracy and reaction time. Retro‐cues also modulated oscillatory activity in the alpha and gamma frequency bands. We observed greater alpha activity in a ventral visual region ipsilateral to the attended hemifield, thus supporting its suppressive role, i.e. a functional disengagement of task‐irrelevant regions. Accompanying this modulation, we found an increase in gamma activity contralateral to the attended hemifield, which could reflect attentional orienting and selective processing. These findings suggest that the oscillatory mechanisms underlying attentional orienting to representations held in working memory are similar to those engaged when attention is oriented in the perceptual space.     

Human verbal working memory impairments associated with thalamic damage

Although animal studies, human neuroimaging studies, and numerous theoretical models suggest possible contributions of the thalamus to working memory, there are very few reported deficits in human working memory following thalamic lesions. The present study examined working memory performance in six individuals with isolated thalamic stroke and found evidence of impairment on a number of working memory span tasks, but not on a forward digit-span measure. Examination of additional aspects of working memory performance (e.g., spatial and object working memory), analysis of subjects with other sites of thalamic stroke, and functional neuroimaging suggest a role of the thalamus in working memory.     

The temporal, spatial, and frequency dimensions of neural oscillations associated with verbal working memory

Cognitive operations engage neural generators oscillating at different frequencies distributed in time and space. Accordingly, oscillatory activity detected by magnetoencephalography (MEG)/electroencephalography (EEG) should be analyzed along frequency, time, and spatial dimensions. MEG data were obtained from 19 healthy individuals while performing a modified Sternberg paradigm. The stimuli were letters, which constituted words or pronounceable nonwords. We applied tridimensional analysis of oscillations and also computed event-related fields (ERFs) in areas where significant changes in oscillatory activity were observed. Verbal working memory for visual verbal stimuli was associated with oscillatory interplay between the bilateral occipital lobes and the left frontoparietotemporal areas. Spatially stable occipital desynchrony was noted during information encoding, while a left hemisphere desynchronization, increasing in amplitude and spatial extent over time, was observed during information encoding and maintenance. No ERF changes were detected during information maintenance. Oscillatory activity associated with verbal working memory is consistent with the above hypothesis. These findings underscore the importance of multidimensional evaluation of oscillations. The findings also indicate that combining electrophysiological methods increase the chance of signal detection.     

Impairment of hippocampal gamma-frequency oscillations in vitro in mice overexpressing human amyloid precursor protein (APP)

Alzheimer's disease is associated with a dramatic decline in cognitive performance including hippocampal-dependent memory. We have investigated one feature of hippocampal activity related to memory, the gamma (30-80 Hz)-frequency rhythm. Hippocampal slices from mice overexpressing the human amyloid precursor protein (APP)(SWE) mutation (TAS10) were compared at 8 and 16 months of age with wild-type littermates. In slices obtained from TAS10 mice aged 8 months the gamma-frequency activity evoked with bath application of 200 nm kainate was significantly (P < 0.05; n = 8 slices, five animals) impaired (area power, 5956 +/- 2487 microV(2)) compared to slices from wild-type animals (area power, 18 256 +/- 7880 microV(2)). At 16 months of age there was no longer a significant difference (P > 0.05; n = 11 slices from five animals) between slices from TAS10 and wild-type control mice as the wild-type mice now exhibited a marked age-dependent reduction in gamma-frequency activity (TAS10 area power, 5751 +/- 1573 microV(2); wild-type area power = 5379 +/- 1454 microV(2)). Although no dense-core plaques were evident at 8 months there was detectable amyloid labelling in the TAS10 mice which might account for the deficits in gamma activity observed at this age. Dense plaques were clearly evident in the TAS10, but not wild-type, mice at 16 months of age but no further reductions in gamma-frequency activity were seen in the TAS10 mice. These data suggest that deficits in network function in Alzheimer's disease occur early and are not directly correlated to amyloid load.     

Post-traumatic stress disorder is associated with altered modulation of prefrontal alpha band oscillations during working memory

ObjectiveTo investigate the relationship between the severity of PTSD symptoms, modulation of alpha band oscillations, and behavioral performance in a working memory task.MethodsMagnetoencephalography data were recorded in 35 participants with combat exposure and various degrees of PTSD symptom severity while they performed a modified Sternberg working memory task: briefly presented sets of two or six letters had to be held in memory and participants indicated whether subsequent probe letters were present or absent from these sets.ResultsPTSD scores were positively correlated with the false positive rate in the high memory load condition. Higher rates of false recognition were associated with negative probes that were seen in recent previous trials (negative probe recency effect) or were physically similar with the list letters. The relative alpha band power in the left middle frontal gyrus was negatively correlated with both PTSD scores and false positive rates.ConclusionsReduced task specific modulation of alpha band oscillations in left middle frontal cortex may reflect alterations in the functions of pattern separation and suppression of memory traces for irrelevant or no longer relevant information in PTSD.SignificanceThe lower amplitude of prefrontal alpha band oscillations may represent an important physiological basis for core PTSD symptoms and can provide a target for interventions to augment response to treatment.     

Brain regions associated with successful and unsuccessful retrieval of verbal episodic memory as revealed by divided attention

Which brain regions are implicated when words are retrieved under divided attention, and what does this tell us about attentional and memory processes needed for retrieval? To address these questions we used fMRI to examine brain regions associated with auditory recognition performed under full and divided attention (DA). We asked young adults to encode words presented auditorily under full attention (FA), and following this, asked them to recognize studied words while in the scanner. Attention was divided at retrieval by asking participants to perform either an animacy task to words, or odd-digit identification task to numbers presented visually, concurrently with the recognition task. Retrieval was disrupted significantly by the word-, but not number-based concurrent task. A corresponding decrease in brain activity was observed in right hippocampus, bilateral parietal cortex, and left precuneus, thus demonstrating, for the first time, involvement of these regions in recognition under DA at retrieval. Increases in activation of left prefrontal cortex (PFC), associated with phonological processing, were observed in the word- compared to number-based DA condition. Results suggest that the medial temporal lobe (MTL) and neo-cortical components of retrieval, believed to form the basis of episodic memory traces, are disrupted when phonological processing regions in left PFC are engaged simultaneously by another task. Results also support a component-process model of retrieval which posits that MTL-mediated retrieval does not compete for general cognitive resources but does compete for specific structural representations.     

A model of gamma-frequency network oscillations induced in the rat CA3 region by carbachol in vitro

Carbachol (> 20 microM) and kainate (100 nM) induce, in the in vitro CA3 region, synchronized neuronal population oscillations at approximately 40 Hz having distinctive features: (i) the oscillations persist for hours; (ii) interneurons in kainate fire at 5-20 Hz and their firing is tightly locked to field potential maxima (recorded in s. radiatum); (iii) in contrast, pyramidal cells, in both carbachol and kainate, fire at frequencies as low as 2 Hz, and their firing is less tightly locked to field potentials; (iv) the oscillations require GABAA receptors, AMPA receptors and gap junctions. Using a network of 3072 pyramidal cells and 384 interneurons (each multicompartmental and containing a segment of unmyelinated axon), we employed computer simulations to examine conditions under which network oscillations might occur with the experimentally determined properties. We found that such network oscillations could be generated, robustly, when gap junctions were located between pyramidal cell axons, as suggested to occur based on studies of spontaneous high-frequency (> 100 Hz) network oscillations in the in vitro hippocampus. In the model, pyramidal cell somatic firing was not essential for the oscillations. Critical components of the model are (i) the plexus of pyramidal cell axons, randomly and sparsely interconnected by gap junctions; (ii) glutamate synapses onto interneurons; (iii) synaptic inhibition between interneurons and onto pyramidal cell axons and somata; (iv) a sufficiently high rate of spontaneous action potentials generated in pyramidal cell axons. This model explains the dependence of network oscillations on GABA(A) and AMPA receptors, as well as on gap junctions. Besides the existence of axon-axon gap junctions, the model predicts that many of the pyramidal cell action potentials, during sustained gamma oscillations, are initiated in axons.     

Verbal cognition and attention deficits do not explain the verbal memory decline associated with pharmacoresistant partial epilepsy

The aim of this study was to explore whether change in verbal memory with time in patients with epilepsy is influenced by performance on tasks assessing verbal cognition or attention/processing speed. Thirty-six patients and twenty-five healthy controls were tested twice with median retest intervals of 4.8 and 3.1 years, respectively. Aspects of verbal memory, verbal cognition, and attention/processing speed were assessed. Decline in one verbal memory variable (Cronholm-Molander Memory Test Paired Associates -- Delayed Recall) was the strongest correlate of epilepsy. The second strongest correlate was a decrease in one attention/processing speed variable (Digit Symbol). The relationship between decline in verbal memory and epilepsy was not influenced by the decline in attention/processing speed, and the results did not support the notion that limited mental reserves as reflected in impaired verbal cognition or attention/processing speed can explain the relationship between verbal memory and epilepsy.     

Building attention from memory

The processes that lead to the selection of information for additional processing, which characterizes attention, depend upon previous experiences and expectancies generated in the light of stored past experiences regularities and plans for action. It is proposed that conceptual association between memory and attention brings advantages for explaining a diversity of attentional phenomena and allow to generate testable predictions relating past experiences and performance in tests for attention. A model is presented relating the trained neural network (memories) and attention during performance of behavioral tasks.     

Computer-based assessment of memory and attention: evaluation of the memory and attention test (MAT)

Objective A computer-based test for the assessment of working and short-term memory and selective attention was evaluated in Alzheimer patients and controls.Methods By means of the MAT, working and short-term memory for verbal, figural and episodic material are assessed through recognition tasks. Selective attention is tested through visual samples in an oddball paradigm. In 42 Alzheimer patients and 42 controls, we applied the MAT and usual reference methods for the assessment of attention and the respective memory domains.Results The correlations of the scores of the MAT memory subtests with the scores of the reference tests were moderate to high (r: 0.63-0.84). Test-retest correlations were between 0.79 and 0.90. Differentiation of Alzheimer patients and controls was good, particularly in episodic and figural short-term memory (p?相似文献   

Prefrontal activity associated with working memory and episodic long-term memory

Many recent neuroimaging studies have highlighted the role of prefrontal regions in the sustained maintenance and manipulation of information over short delays, or working memory (WM). In addition, neuroimaging findings have highlighted the role of prefrontal regions in the formation and retrieval of memories for events, or episodic long-term memory (LTM), but it remains unclear whether these regions are distinct from those that support WM. We used event-related functional magnetic resonance imaging (fMRI) to identify patterns of prefrontal activity associated with encoding and recognition during WM and LTM tasks performed by the same subjects. Results showed that the same bilateral ventrolateral prefrontal regions (at or near Brodmann's Areas [BA] 6, 44, 45, and 47) and dorsolateral prefrontal regions (BA 9/46) were engaged during encoding and recognition within the context of WM and LTM tasks. In addition, a region situated in the left anterior middle frontal gyrus (BA 10/46) was engaged during the recognition phases of the WM and LTM tasks. These results support the view that the same prefrontal regions implement reflective processes that support both WM and LTM.     

Mechanisms of human attention: event-related potentials and oscillations

Electrophysiological and hemodynamical responses of the brain allow investigation of the neural origins of human attention. We review attention-related brain responses from auditory and visual tasks employing oddball and novelty paradigms. Dipole localization and intracranial recordings as well as functional magnetic resonance imaging reveal multiple areas involved in generating and modulating attentional brain responses. In addition, the influence of brain lesions of circumscribed areas of the human cortex onto attentional mechanisms are reviewed. While it is obvious that damaged brain tissue no longer functions properly, it has also been shown that functions of non-lesioned brain areas are impaired due to loss of modulatory influence of the lesioned area. Both early (P1 and N1) and late (P3) event-related potentials are modulated by excitatatory and inhibitory mechanisms. Oscillatory EEG-correlates of attention in the alpha and gamma frequency range also show attentional modulation.     

Delayed episodic memory recall after one week is associated with executive functions and divided attention in pediatric epilepsy patients

AimRecent studies suggest that although children with epilepsy may show normal learning and memory performance, accelerated long-term forgetting (ALF) may become evident over time. Our study examined associations between delayed episodic memory performance (recall 1-week after learning) and executive functions.MethodA consecutive sample of children with a diagnosis of idiopathic epilepsy with focal or generalized seizures, without morphologic or metabolic abnormalities (n = 20, mean age: 11.70 years) was compared to an IQ-matched healthy control group (n = 20, mean age: 11.55 years). We also assessed parents’ and children’s rating of forgetting in everyday life and explored its association with delayed episodic memory recall.ResultsSimilar to results from recent studies of pediatric patients with temporal lobe epilepsy or genetic generalized epilepsy, our pediatric epilepsy patients showed a significantly elevated recall loss over time, although verbal learning, immediate and 30-minute recall was comparable to the matched control group. Additionally, delayed memory recall in patients was moderately associated with their subjective rating of forgetting, as well as with executive functions (verbal fluency and switching) and divided attention.InterpretationWe assume that executive functions play a crucial role in deep memory encoding, facilitating stronger and more enduring memory traces. Given that approximately 20% of epilepsy patients – compared to a healthy reference sample – had a significantly reduced delayed recall and due to the clinical relevance of long-term memory, age-appropriate standard norms for free memory recall after 1-week are desirable.     

Perilymph fistula associated with pulse-synchronous eye oscillations.

Three-dimensional eye movements (scleral search coil system) were recorded in a patient with a surgically acquired perilymph fistula of the left horizontal semicircular canal. Spontaneous horizontal pendular nystagmus was found to be related to the heart rate and may be caused by pressure transfer of blood pulses to the labyrinth. In addition, a contralesional horizontal jerk nystagmus was elicited by Valsalva maneuver, indicating that Ewald's first law may not only be valid for excitation but also for inhibition.     

Intelligence, attention, and memory in patients with myasthenia gravis

BACKGROUND: In patients with myasthenia gravis (MG), poor performance on cognitive tests has been found. Performance on memory tasks has been reported to correlate with disease activity, but memory in MG patients was also recently found to be unimpaired. PATIENTS AND METHODS: Cognitive functioning was examined in 23 patients with MG and 23 healthy controls. The patients were assessed for IQ, memory, attention, and motor performance. Immunoglobulin G antiacetylcholine receptor autoantibody titers were determined. Event-related potentials were generated for patients and controls. RESULTS: Mean IQs of patients were at average. Memory and attention were not more impaired in patients than controls. Event-related potentials were not delayed in either group. Patients with elevated autoantibody concentrations similarly showed unimpaired neurocognitive and motor functioning. CONCLUSION: In this study, no evidence of neuropsychological impairment was found in MG, arguing against the involvement of higher cortical functions. Sleep abnormalities rather than central mechanisms may explain the memory impairments reported by some studies.