Postscript

As is evident from the scientific chapters of this book, the technology of magnetoencephalography offers a combination of spatial, temporal, and spectral resolution, unique among neuroimaging technologies. While functional magnetic resonance imaging (fMRI) accommodates spatial resolution, it lacks the millisecond resolution (because of the reliance on a slow hemodynamic response) to identify subtle latency shifts, or the specificity to distinguish theta- versus alpha- versus gamma-band oscillatory activity. While electroencephalography (EEG) offers the needed temporal resolution, it fails to adequately localize brain sources, owing to the physics of inverse modeling and the dependence of scalp electric potentials on tissue electrical conductivity. Thus, although fMRI may see “activity,” it cannot characterize important attributes of its nature. Conversely, EEG may detect “anomalies” but not be able to attribute them to a particular spatial source....

APOE4 status is related to differences in memory-related brain function in asymptomatic older adults: Baseline analysis of the PREVENT-AD task fMRI dataset

Episodic memory decline is one of the earliest symptoms of late-onset Alzheimer’s Disease (AD) and older adults with the apolipoprotein E e4 (+APOE4) genetic risk factor for AD may exhibit altered patterns of memory-related brain activity years prior to initial symptom onset. In the current study we report the baseline episodic memory task fMRI results from the PRe-symptomatic EValuation of Experimental or Novel Treatments for Alzheimer’s Disease (PREVENT-AD) study in Montreal, Canada, in which 327 healthy older adults, within 15 years of the parent’s conversion to AD, were scanned. During the task fMRI protocol volunteers were scanned as they encoded and retrieved object-location spatial source associations. The task was designed to discriminate between brain activity related to successful spatial source recollection and failures in spatial source recollection, with memory for only item (object) memory. Multivariate task-related partial least squares (task PLS) was used to test the hypothesis that +APOE4 adults with a family history of AD would exhibit altered patterns of brain activity in the recollection-related memory network, comprised of medial frontal, parietal and medial temporal cortices, compared to APOE4 non-carriers (-APOE4). We also tested for group differences in the correlation between event-related brain activity and memory performance in +APOE4 compared to -APOE4 adults using behavioral-PLS (B-PLS). We found group similarities in memory performance and in task-related brain activity in the recollection network. However, the B-PLS results indicated there were group differences in brain activity-behavior correlations in ventral occipito-temporal, medial temporal, and medial prefrontal cortices during episodic encoding. These findings are consistent with previous literature on the influence of APOE4 on brain activity and provide new perspective on potential gene-based differences in brain-behavior relationships in people with parental history of AD. Future research should further investigate the potential to distinguish risk of AD development based on memory performance and associated patterns of brain activity.

Second-order time integration of the wave equation with dispersion correction procedures

Second-order time integration of the wave equation is numerically efficient with time steps close to the limit set by the stability criterion. However, dispersion errors over realistic propagation distances are unacceptable with time steps in this range. Dispersion-free results, using second-order time integration of the wave equation, can be achieved by applying a time-domain prepropagation filter to all source time functions followed by a time-domain postpropagation filter applied to the simulated wavefield at all recording positions. The time-domain implementation of the filtering process is an order of magnitude more effective, in terms of CPU time, compared to an implementation via discrete and fast Fourier transforms. Pre- and postpropagation filters are valid for any simulation time step. The two filters can be calculated once because they are independent of the simulation time step, and they can be applied with any modeling scheme that uses second-order time integration. Second-order time integration results in traveltime and amplitude errors. The amplitude errors depend on the spatial source distribution. The combined application of the two filters removes traveltime-related errors and amplitude-related errors independently of the spatial source distribution.