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Ole Jensen

Publications and source records attributed to Ole Jensen.

12 recordsLinked to original sources
PubMed2006

Posterior alpha activity is not phase-reset by visual stimuli.

There is currently a debate as to whether event-related potentials and fields measured by using electroencephalography or magnetoencephalography are generated by ongoing oscillatory activity becoming phase-reset in response to a given stimulus. We performed a magnetoencephalography study measuring brain activity in response to visual stimuli. Using a measure termed the phase-preservation index we investigated the phase of oscillatory alpha activity (8-13 Hz) before and after the stimulus. We found that in single trials the alpha oscillations after visual stimuli preserve their phase relationship with respect to the phase before the stimuli. This finding argues against phase-resetting of ongoing oscillations as being responsible for visually evoked responses. The event-related field can be explained primarily by stimulus-locked activity in the band that is absent before the stimulus. These findings suggest that different neuronal events are responsible for generating the ongoing oscillations and the visually evoked responses.

Adult↗
PubMed2005

Reading the hippocampal code by theta phase-locking.

Both the prefrontal cortex and the hippocampus are crucial for memory encoding and recall. However, it remains unclear how these brain regions communicate to exchange information. Recent findings using simultaneous recordings from the hippocampus and prefrontal cortex of the behaving rat have demonstrated that prefrontal cells' firing is phase-locked to the hippocampal theta rhythm. This suggests that phase synchronization clocked by the theta rhythm could be crucial for the communication between hippocampal and prefrontal regions.

Animals↗
PubMed2005

Altered generation of spontaneous oscillations in Alzheimer's disease.

Slowing of spontaneous alpha oscillations and an anterior shift of a source of alpha activity (8-13 Hz) have been consistently reported in the EEG studies of Alzheimer's disease (AD). It is unknown whether these changes are associated with a gradual shift in location and frequency of existing sources or rather with the involvement of a new set of oscillators. We addressed this question by applying source modeling (minimum current estimates, MCE) to spontaneous alpha activity recorded with a 306-channel MEG system from eleven non-medicated AD patients with mild to moderate cognitive impairment and twelve age-matched controls during the eyes-closed session. AD patients had predominant lower alpha band sources in the temporal regions, whereas in the controls, robust alpha sources were found near the parieto-occipital sulcus. Activation within the parieto-occipital region was significantly weaker, and activation in the right temporal area was significantly enhanced in the AD patients. These results suggest an increased temporal-lobe contribution coinciding with parieto-occipital deficits. We propose that MCE, which provides simultaneous mapping of several oscillatory sources, might be useful for detecting neurophysiological abnormalities associated with AD in combination with other neuropsychological and neurological measures.

Aged↗
PubMed2005

Hippocampal sequence-encoding driven by a cortical multi-item working memory buffer.

Encoding and recall of memory sequences is an important process. Memory encoding is thought to occur by long-term potentiation (LTP) in the hippocampus; however, it remains unclear how LTP, which has a time window for induction of approximately 100 ms, could encode the linkage between sequential items that arrive with a temporal separation >100 ms. Here, we argue that LTP can underlie the learning of such memory sequences, provided the input to the hippocampus is from a cortical multi-item working memory buffer in which theta and gamma oscillations have an important role. In such a buffer, memory items that occurred seconds apart are represented with a temporal separation of 20-30 ms, thereby bringing them within the LTP window. The physiological and behavioral evidence for such a buffer will be reviewed.

Animals↗
PubMed2003

Phase locking between human primary and secondary somatosensory cortices.

Unilateral stimulation of human peripheral nerves activates the primary somatosensory cortex (SI) contralaterally and the secondary somatosensory cortex (SII) bilaterally. We aimed at characterizing phase locking between SI and SII in response to electric stimuli applied once every 3 s to the right median nerve at the wrist; phase locking between brain regions has been proposed to either reflect joined processing or information exchange. Ongoing neuromagnetic activity of healthy volunteers was recorded with 204 planar gradiometers covering the whole scalp. After selecting a sensor maximally sensitive to activity in the left (contralateral) SI, phase locking between this sensor and the other 203 sensors was examined from single trial data. Statistically significant phase locking was found at approximately 20 Hz, 80-90 ms after the stimuli between the left SI and the right SII in 9 of 10 subjects. Sensors with high phase-locking values over the left SI and right SII were separated by sensors with no phase-locked activity over the scalp midline, indicating that the phase locking was not caused by the sensors seeing activity from the same sources. The observed SI-SII phase locking would not be reflected in the evoked responses because a considerable part of it was not time-locked to the stimuli. Thus, our finding reveals a unique interaction in the sensorimotor system.

Adult↗
PubMed2003

When neurons form memories.

Although long-term memory is central among our cognitive functions, the search for a direct neurophysiological correlate to it has proven difficult. The formation of new memories depends on the hippocampus and adjacent cortex, but the final storage is thought to be in a widely distributed neocortical network. Recent experiments, using simultaneous recordings from hundreds of sites in monkey neocortex, have revealed the activation of such a distributed network -- probably reflecting the consolidation of long-term memory storage.

Animals↗
PubMed2002

Exposure to genotoxins present in ambient air in Bangkok, Thailand--particle associated polycyclic aromatic hydrocarbons and biomarkers.

Exposure to genotoxic compounds in ambient air has been studied in Bangkok, Thailand, by analysis of polycyclic aromatic hydrocarbons (PAHs) associated with particles and using different biomarkers of exposure. Eighty-nine male, non-smoking Royal Thai police officers were investigated. The police officers were divided into a high exposure group (traffic police) and low exposure (office duty). Particulate matter was collected using personal pumps (2 l/min) and the eight carcinogenic PAHs were analysed by standard procedures. The traffic police was exposed to a 20-fold higher level of total PAHs than office police (74.25 ng/m3 vs. 3.11; P= 0.001). A two-fold variation was observed between the different police stations. The major PAHs in all groups was benzo[g,h,l]pyrelene. Large inter-individual differences in biomarker levels were observed, but the level of all markers was statistically significantly higher in the traffic police group than in the office group. The level of 1-hydroxypyrene (1-HOP) was 0.181+/-0.078 (range 0.071-0.393) micromol/mol creatinine in the traffic group and 0.173+/-0.151 (P = 0.044) in the office group. The bulky carcinogen DNA-adduct level, determined by P32-post-labelling, was 1.6+/-0.9 (range 0.4-4.3) adducts/10(8) nucleotides in the traffic group and 1.2+/-1.0 (0.2-4.9) in the office group (P = 0.029; Mann-Whitney U-test). The serum PAH-albumin adduct level was 1.76 (0.51-3.07) fmol adducts/microg albumin in the traffic group and 1.35+/-0.77 (0.11-3.45; P = 0.001) in the office group. Lower biomarker levels were observed during the period when the traffic police officers were wearing a simple facemask, indicating that these masks protect against particle-associated PAHs. No statistically significant correlations were observed between biomarker levels and the level of individual PAHs or total PAH. Our data show, that people in Bangkok, who spend most of the day outside air-conditioned offices, are exposed to high levels of genotoxic PAHs. However, for people who spend their working day in offices, the exposure is similar to people living in other metropolitan areas.

Adult↗
PubMed2002

Visually evoked gamma responses in the human brain are enhanced during voluntary hyperventilation.

Hypocapnia induced by hyperventilation (HV) has powerful effects on neuronal excitability and synaptic transmission. We have studied the effect of hyperventilation on the phase-locked oscillatory components of the evoked responses in the human brain. We recorded visually evoked magnetoencephalographic responses before, during, and after voluntary hyperventilation to pattern-reversal checkerboard stimuli. Gamma-band (30-45 Hz) responses phase-locked to the stimuli were generated in the occipital visual cortex. A wavelet-based time-frequency analysis revealed that the gamma responses increased during HV whereas their frequency did not change significantly. A recent in vitro study in the rat hippocampus demonstrated that the stability of spontaneous gamma activity increases during hypocapnia as a result of enhanced GABAergic transmission. To test if a similar mechanism could account for our findings, we performed simulations on a network of 100 Hodgkin-Huxley neurons connected by inhibitory synapses. We found that enhanced GABA(A) transmission, paired with enhanced excitability, can explain the increase in evoked gamma activity without changing the frequency.

Adult↗
PubMed2002

A new method to identify multiple sources of oscillatory activity from magnetoencephalographic data.

Identifying the sources of oscillatory activity in the human brain is a challenging problem in current magnetoencephalography (MEG) and electroencephalography (EEG) research. The fluctuations in phase and amplitude of cortical oscillations preclude signal averaging over successive sections of the data without a priori assumptions. In addition, several sources at different locations often produce oscillatory activity at similar frequencies. For example, spontaneous oscillatory activity in the 8- to 13-Hz band is produced simultaneously at least in the posterior parts of the brain and bilaterally in the sensorimotor cortices. The previous approaches of identifying sources of oscillatory activity by dipole modeling of bandpass filtered data are quite laborious and require that multiple criteria are defined by an experienced user. In this work we introduce a convenient method for source localization using minimum current estimates in the frequency domain. Individual current estimates are calculated for the Fourier transforms of successive sections of continuous data. These current estimates are then averaged. The algorithm was tested on simulated and measured MEG data and compared with conventional dipole modeling. The main advantage of the proposed method is that it provides an efficient approach for simultaneous estimation of multiple sources of oscillatory activity in the same frequency band.

Algorithms↗
PubMed2002

Frontal theta activity in humans increases with memory load in a working memory task.

Recent theoretical work has suggested that brain oscillations in the theta band are involved in active maintenance and recall of working memory representations. To test this theoretical framework we recorded neuromagnetic responses from 10 subjects performing the Sternberg task. Subjects were required to retain a list of 1, 3, 5 or 7 visually presented digits during a 3-s retention period. During the retention period we observed ongoing frontal theta activity in the 7-8.5-Hz band recorded by sensors over frontal brain areas. The activity in the theta band increased parametrically with the number of items retained in working memory. A time-frequency analysis revealed that the task-dependent theta was present during the retention period and during memory scanning. Following the memory task the theta activity was reduced. These results suggest that theta oscillations generated in frontal brain regions play an active role in memory maintenance.

Adult↗
PubMed2002

Oscillations in the alpha band (9-12 Hz) increase with memory load during retention in a short-term memory task.

To study the role of brain oscillations in working memory, we recorded the scalp electroencephalogram (EEG) during the retention interval of a modified Sternberg task. A power spectral analysis of the EEG during the retention interval revealed a clear peak at 9-12 Hz, a frequency in the alpha band (8-13 Hz). In apparent conflict with previous ideas according to which alpha band oscillations represent brain "idling", we found that the alpha peak systematically increased with the number of items held in working memory. The enhancement was prominent over the posterior and bilateral central regions. The enhancement over posterior regions is most likely explained by the well known alpha rhythm produced close to the parietal-occipital fissure, whereas the lateral enhancement could be explained by sources in somato-motor cortex. A time-frequency analysis revealed that the enhancement was present throughout the last 2.5 s of the 2.8 s retention interval and that alpha power rapidly diminished following the probe. The load dependence and the tight temporal regulation of alpha provide strong evidence that the alpha generating system is directly or indirectly linked to the circuits responsible for working memory. Although a clear peak in the theta band (5-8 Hz) was only detectable in one subject, other lines of evidence indicate that theta occurs and also has a role in working memory. Hypotheses concerning the role of alpha band activity in working memory are discussed.

Adolescent↗