Temporal Lobe Sources Research Articles

Using generative models to make probabilistic statements about hippocampal engagement in MEG

Magnetoencephalography (MEG) enables non-invasive real time characterization of brain activity. However, convincing demonstrations of signal contributions from deeper sources such as the hippocampus remain controversial and are made difficult by its depth, structural complexity and proximity to neocortex. Here, we demonstrate a method for quantifying hippocampal engagement probabilistically using simulated hippocampal activity and realistic anatomical and electromagnetic source modelling. We construct two generative models, one which supports neuronal current flow on the cortical surface, and one which supports neuronal current flow on both the cortical and hippocampal surface. Using Bayesian model comparison, we then infer which of the two models provides a more likely explanation of the dataset at hand. We also carry out a set of control experiments to rule out bias, including simulating medial temporal lobe sources to assess the risk of falsely positive results, and adding different types of displacements to the hippocampal portion of the mesh to test for anatomical specificity of the results. In addition, we test the robustness of this inference by adding co-registration error and sensor level noise. We find that the model comparison framework is sensitive to hippocampal activity when co-registration error is <3 mm and the sensor-level signal-to-noise ratio (SNR) is >−20 dB. These levels of co-registration error and SNR can now be achieved empirically using recently developed subject-specific head-casts.

Source analysis of short and long latency vestibular-evoked potentials (VsEPs) produced by left vs. right ear air-conducted 500 Hz tone pips

Todd et al. (2014) have recently demonstrated the presence of vestibular dependent changes both in the morphology and in the intensity dependence of auditory evoked potentials (AEPs) when passing through the vestibular threshold as determined by vestibular evoked myogenic potentials (VEMPs). In this paper we extend this work by comparing left vs. right ear stimulation and by conducting a source analysis of the resulting evoked potentials of short and long latency. Ten healthy, right-handed subjects were recruited and evoked potentials were recorded to both left- and right-ear sound stimulation, above and below vestibular threshold. Below VEMP threshold, typical AEPs were recorded, consisting of mid-latency (MLR) waves Na and Pa followed by long latency AEPs (LAEPs) N1 and P2. In the supra-threshold condition, the expected changes in morphology were observed, consisting of: (1) short-latency vestibular evoked potentials (VsEPs) which have no auditory correlate, i.e. the ocular VEMP (OVEMP) and inion response related potentials; (2) a later deflection, labelled N42/P52, followed by the LAEPs N1 and P2. Statistical analysis of the vestibular dependent responses indicated a contralateral effect for inion related short-latency responses and a left-ear/right-hemisphere advantage for the long-latency responses. Source analysis indicated that the short-latency effects may be mediated by a contralateral projection to left cerebellum, while the long-latency effects were mediated by a contralateral projection to right cingulate cortex. In addition we found evidence of a possible vestibular contribution to the auditory T-complex in radial temporal lobe sources. These last results raise the possibility that acoustic activation of the otolith organs could potentially contribute to auditory processing.

Group-level spatial independent component analysis of Fourier envelopes of resting-state MEG data

We developed a data-driven method to spatiotemporally and spectrally characterize the dynamics of brain oscillations in resting-state magnetoencephalography (MEG) data. The method, called envelope spatial Fourier independent component analysis (eSFICA), maximizes the spatial and spectral sparseness of Fourier energies of a cortically constrained source current estimate. We compared this method using a simulated data set against 5 other variants of independent component analysis and found that eSFICA performed on par with its temporal variant, eTFICA, and better than other ICA variants, in characterizing dynamics at time scales of the order of minutes.We then applied eSFICA to real MEG data obtained from 9 subjects during rest. The method identified several networks showing within- and cross-frequency inter-areal functional connectivity profiles which resemble previously reported resting-state networks, such as the bilateral sensorimotor network at ~20Hz, the lateral and medial parieto-occipital sources at ~10Hz, a subset of the default-mode network at ~8 and ~15Hz, and lateralized temporal lobe sources at ~8Hz.Finally, we interpreted the estimated networks as spatiospectral filters and applied the filters to obtain the dynamics during a natural stimulus sequence presented to the same 9 subjects. We observed occipital alpha modulation to visual stimuli, bilateral rolandic mu modulation to tactile stimuli and video clips of hands, and the temporal lobe network modulation to speech stimuli, but no modulation of the sources in the default-mode network.We conclude that (1) the proposed method robustly detects inter-areal cross-frequency networks at long time scales, (2) the functional relevance of the resting-state networks can be probed by applying the obtained spatiospectral filters to data from measurements with controlled external stimulation.

Evidence that juvenile myoclonic epilepsy is a disorder of frontotemporal corticothalamic networks

The purpose of this study is to determine regions of cerebral cortex activated during the onset and propagation of dense array electroencephalographic (dEEG) epileptiform discharges in patients with juvenile myoclonic epilepsy (JME), through the use of 256 channel, dense array scalp EEG recordings. Ten patients (16–58 years old) with the clinical diagnosis of JME comprised the study group. In all cases the MRI and neurological exams were normal, while standard EEG recordings documented typical “generalized” 4–6 Hz epileptiform patterns. Outpatient dEEG recordings captured epileptiform discharges in each patient. Localization of onset and spread of discharges in relation to a standard MRI model was accomplished by applying dipole fits and a distributed linear inverse method of cortically constrained source analysis. All patients showed epileptiform discharges that localized to sources that included orbitofrontal/medial frontopolar cortex, while basal–medial temporal lobe sources were observed in 5/10 subjects. In many ways similar to discharges of typical absence, epileptiform patterns in JME are usually irregular and frequently include temporal lobe structures as the dominant contributors to the discharges. We find that epileptiform discharges in patients with JME are not “generalized” in the sense of bilaterally synchronous diffuse onset. Rather, discharges have both localized onsets and a restricted cortical network during propagation that includes regions of frontal and temporal cortex.

A complementary analytic approach to examining medial temporal lobe sources using magnetoencephalography

Neuropsychological and neuroimaging findings reveal that the hippocampus is important for recognition memory. However, it is unclear when and whether the hippocampus contributes differentially to recognition of previously studied items (old) versus novel items (new), or contributes to a general processing requirement that is necessary for recognition of both types of information. To address this issue, we examined the temporal dynamics and spectral frequency underlying hippocampal activity during recognition of old/new complex scenes using magnetoencephalography (MEG). In order to provide converging evidence to existing literature in support of the potential of MEG to localize the hippocampus, we reconstructed brain source activity using the beamformer method and analyzed three types of processing-related signal changes by applying three different analysis methods: (1) Synthetic aperture magnetometry (SAM) revealed event related and non-event-related spectral power changes; (2) Inter-trial coherence (ITC) revealed time-locked changes in neural synchrony; and (3) Event-related SAM (ER-SAM) revealed averaged event-related responses over time. Hippocampal activity was evident for both old and new information within the theta frequency band and during the first 250 ms following stimulus onset. The early onset of hippocampal responses suggests that general comparison processes related to recognition of new/old information may occur obligatorily.

Scalp-Recorded Auditory P300 Event-Related Potentials in New-Onset Untreated Temporal Lobe Epilepsy

We studied scalp-recorded auditory event-related potentials (ERPs) of 30 untreated patients with new-onset temporal lobe epilepsy and 30 age- and sex-matched normal controls. This study was designed to eliminate the effects of intractability of seizures and chronic use of antiepileptic drugs on P300 auditory ERPs. There were no statistically significant differences in both latency and amplitude of P300 between the two groups. Similar methods were also used to analyze component latencies and amplitudes of ERPs of 9 patients who had hippocampal sclerosis with comparison to control subjects. There were no statistically significant differences between these two groups as well. Our study evidently does not support temporal lobe sources of P300 scalp-recorded auditory ERPs. We also conclude that the scalp-recorded auditory ERPs procedure is not a useful tool to evaluate temporal lobe epilepsy.

Auditory change detection in schizophrenia: sources of activity, related neuropsychological function and symptoms in patients with a first episode in adolescence, and patients 14 years after an adolescent illness-onset

BackgroundThe event-related brain response mismatch negativity (MMN) registers changes in auditory stimulation with temporal lobe sources reflecting short-term echoic memory and frontal sources a deviance-induced switch in processing. Impairment, controversially present at the onset of schizophrenia, develops rapidly and can remain independent of clinical improvement. We examined the characteristics of the scalp-recorded MMN and related these to tests of short-term memory and set-shifting. We assessed whether the equivalent dipole sources are affected already at illness-onset in adolescence and how these features differ after a 14-year course following an adolescent onset. The strength, latency, orientation and location of frontal and temporal lobe sources of MMN activity early and late in the course of adolescent-onset schizophrenia are analysed and illustrated.MethodsMMN, a measure of auditory change-detection, was elicited by short deviant tones in a 3-tone oddball-presentation and recorded from 32 scalp electrodes. Four dipole sources were placed following hypothesis-led calculations using brain electrical source analysis on brain atlas and MR-images. A short neuropsychological test battery was administered. We compared 28 adolescent patients with a first episode of schizophrenia and 18 patients 14 years after diagnosis in adolescence with two age-matched control groups from the community (n = 22 and 18, respectively).ResultsMMN peaked earlier in the younger than the older subjects. The amplitude was reduced in patients, especially the younger group, and was here associated with negative symptoms and slow set-shifting. In first-episode patients the temporal lobe sources were more ventral than in controls, while the left cingular and right inferior-mid frontal sources were more caudal. In the older patients the left temporal locus remained ventral (developmental stasis), the right temporal locus extended more antero-laterally (illness progression), and the right frontal source moved antero-laterally (normalised).ConclusionFrom the start of the illness there were differences in the dipole-model between healthy and patient groups. Separate characteristics of the sources of the activity differences showed an improvement, stasis or deterioration with illness-duration. The precise nature of the changes in the sources of MMN activity and their relationship to selective information processing and storage depend on the specific psychopathology and heterogeneous course of the illness.

Source localization of mesial temporal interictal epileptiform discharges: Correlation with intracranial foramen ovale electrode recordings

Objective We have investigated the localization accuracy of low-resolution electromagnetic tomography (LORETA) for mesial temporal interictal epileptiform discharges (IED) on a statistical basis by using clinical electroencephalographic (EEG) data of simultaneous scalp and intracranial foramen ovale (FO) electrode recordings. Methods We retrospectively analyzed the IED of 15 patients who underwent presurgical assessment for intractable temporal lobe epilepsy. All patients have subsequently undergone amygdalohippocampectomy. The scalp signals were averaged time-locked to the peak activity in bilateral 10-contact FO electrode recordings. Source modeling was carried out by using statistical non-parametric mapping (SNPM) of LORETA values and by calculating raw LORETA values of averaged IED. The results were compared to intracranial data obtained from FO electrode recordings. Results Two thousand six hundred and fifteen discharges could be attributed to 19 different patterns of intracranial mesial temporal IED. SNPM of LORETA revealed confined ipsilateral mesial temporal solutions for 14 (73.7%) and no significant solutions for five (26.3%) of these patterns. Raw LORETA current density distributions of the 19 averaged IED patterns revealed ipsilateral basal to lateral temporal solutions for the 14 IED patterns with a sufficient signal to noise ratio (SNR), but spurious results for those five IED with a low SNR. Conclusions SNPM of LORETA but not LORETA analysis of averaged IED patterns accurately localizes the source generators of mesial temporal IEDs. Significance SNPM of raw LORETA values might be appropriate for localizing restricted mesial temporal lobe sources.

Clinical Neurophysiology: EEG–Video Monitoring

Clinical Neurophysiology: EEG–Video Monitoring

Maturation processes in automatic change detection as revealed by event-related brain potentials and dipole source localization: Significance for adult AD/HD

Mismatch negativity (MMN) is an event-related potential reflecting automatic attention-related information processing marking the detection of auditory change. The bilateral scalp distribution develops by 14 years of age, and is elicited with adult latencies by 17 years. But consistent with reports of continued brain maturation after adolescence, we show here that features of the temporal and frontal lobe dipole sources also continue to develop in the third decade of life. This has consequences for studies of the developmental course of MMN anomalies, from childhood into adulthood, in attention-deficit/hyperactivity disorder. Two groups of healthy subjects with mean ages of 17 and 30 years were presented with a 3-tone auditory oddball. The duration-deviant MMN was recorded during attention to a visual discrimination (auditory-passive condition) and an active auditory discrimination. MMN amplitudes were smaller in the older subjects and the MMN lasted longer over the right hemisphere. Latencies and moments of the four dipoles in the temporal and frontal lobes did not distinguish the two subject-groups. But both temporal lobe sources were located significantly more ventrally and further left in the young adult than in the adolescent subjects. The left cingular source moved posteriorly and the right inferior frontal source moved antero-medially in the older subjects. Brain development in the third decade may cause the two frontal sources to move apart on the rostro-caudal axis but the temporal lobe sources to move left on the lateral and down on the dorsoventral axes. Thus special care is necessary in interpreting putative dysfunctional neurobiological changes in developmental attention-deficit disorders where as-yet-unspecified sub-groups may show a late developmental lag, partial lag, or no lag at all, associated with other impairments.

Frontal and temporal lobe sources for a marker of controlled auditory attention: the negative difference (Nd) event-related potential.

Frontal and temporal lobe sources for electrical activity associated with auditory controlled attention (negative difference, Nd) were sought for comparison with those reported to arise from the earlier detection of stimulus-change (mismatch negativity, MMN: Jemel et al. 2002). In two sessions a month apart (T1 and T2), 14 subjects were presented with a 3-tone oddball passively, then as a discrimination task. In EEG recordings (32 sites), Nd was calculated by subtraction of the event-related potential elicited by a non-attended stimulus from that after the same frequency-deviant as target Putative generators in the 180-228 ms latency-range were modelled with brain electrical source analysis and mapped to the modified Montreal brain-atlas. Initial T1-analyses located bilateral Nd dipoles in the superior temporal gyrus (BA22) and the dorsolateral prefrontal cortex (BA8). Re-test allowed estimates of the temporal and spatial extension of activity. Peak activity occurred 14 ms later. Step-by-stepanalysis showed that the best spatial fit for the inverse-solutions extended 3-6 mm from the point sources, but for temporal lobe sources this increased 15 mm caudally. The right mid-frontal source (BA10) was rostral and ventral from that in the left superior frontal gyrus (BAB). T1 and T2 dipole strengths were well correlated. Nd measures of controlled attention localised to areas associated with sustained attention, problem-solving and working-memory. Temporal lobe sources were later and more posterior and medial than for automatic change-detection. Frontal Nd sources were more dorsal on the right and more rostral on the left than MMN dipoles reported for the right inferior frontal and left anterior cingulate. The sequence of information processing is reviewed.