Discriminative Analysis of Brain Functional Connectivity Patterns for Mental Fatigue Classification

Mental fatigue is a commonly experienced state that can be induced by placing heavy demands on cognitive systems. This often leads to lowered productivity and increased safety risks. In this study, we developed a functional-connectivity based mental fatigue monitoring method. Twenty-six subjects underwent a 20-min mentally demanding test of sustained attention with high-resolution EEG monitoring. Functional connectivity patterns were obtained on the cortical surface via source localization of cortical activities in the first and last 5-min quartiles of the experiment. Multivariate pattern analysis was then adopted to extract the highly discriminative functional connectivity information. The algorithm used in the present study demonstrated an overall accuracy of 81.5% (p < 0.0001) for fatigue classification through leave-one-out cross validation. Moreover, we found that the most discriminative connectivity features were located in or across middle frontal gyrus and several motor areas, in agreement with the important role that these cortical regions play in the maintenance of sustained attention. This work therefore demonstrates the feasibility of a functional-connectivity-based mental fatigue assessment method, opening up a new avenue for modeling natural brain dynamics under different mental states. Our method has potential applications in several domains, including traffic and industrial safety.     

Wavelet transform of the EEG reveals differences in low and high gamma responses to elementary visual stimuli.

Multiunit electrophysiological studies indicate that oscillatory activity is common in the awake mammalian central nervous system. Synchronous 20-80 Hz oscillations, so called gamma rhythms, have been proposed as a possible fundamental physiological mechanism of binding neuronal activity underlying object recognition. The purpose of this study was to determine whether or not gamma band oscillatory activity in the human brain is modulated by attributes of elementary visual stimulation. The experiment was performed on 7 normal subjects. Sinusoidal gratings were presented over a range of spatial frequencies. Evoked potentials were recorded over 5 surface electrodes placed in a horizontal occipital chain across the back of the head. Discrete wavelet transform was performed on the first 200 msec following stimulus onset on the average data of 256 sweeps. Power was analyzed with ANOVA across conditions. In our previous studies we have separated a "low" (14-28 Hz) and "high" (28-55 Hz) gamma band. The current results indicate that both gamma bands to full-field stimulation have the highest power at the midline (inion) electrode to a spatial frequency of 5.5 cpd, which is the peak spatial frequency from foveal psychophysical data. However, the spatial frequency bandwidth is considerably narrower in the HG than in the LG band. Occipital spatial frequency tuning of the massed high gamma response is narrower than the tuning of individual cortical neurons. The bandwidth difference between low and high gamma band suggests that different frequency gamma range oscillations may represent not only different functional properties of visual processing, but may also reflect underlying differences in excitatory and postsynaptic inhibitory circuits shaping the contrast sensitivity of the human observer. Our study emphasizes the importance of elementary visual filter properties for gamma responses and the need to subdivide gamma frequency ranges according to functional properties.     

Magnetoencephalographic evidence of the interhemispheric asymmetry in echoic memory lifetime and its dependence on handedness and gender

The echoic memory trace (EMT) refers to neuronal activity associated with the short-term retention of stimulus-related information, especially within the primary and association auditory cortex. Using magnetoencephalography it is possible to determine quantitatively the lifetime of the EMT. Previous studies assumed that each new stimulus drives the EMT to its full strength, which then passively decays. In this study we show the limitations of this assumption using trains of auditory stimuli designed specifically for computing the EMT lifetime and its contextual sensitivity. We estimated a time-dependent EMT using a data-driven approach, which allows contributions from a relatively wide area around the auditory cortex in our quantitative measures. We identified: (1) internally generated cortical activations during the silent period between stimuli well separated in time from each other, which had influence on the morphology of the neuromagnetic response to the next external stimulus; and (2) EMTs with different lifetimes that modulate the amplitude of the evoked responses at different latencies, suggesting the existence of multiple neural delay lines. Long EMT lifetimes were observed on the descending part of the M100 complex, which showed handedness and gender-dependent interhemispheric asymmetry. Specifically, all subjects showed longer EMT lifetimes on the left hemisphere, except left-handed males. Distributed source analysis of the data for one left- and one right-handed male subject identified a secondary generator in the right-handed subject, which was located posterior to the early primary generator and dominated the auditory response at late latencies, where EMT lifetime asymmetry was high. The identified multiple neural delay lines and their laterality may provide a link between macroneuronal activity and left hemisphere specialization for processing linguistic material.     

A programmable digital video pattern generator controlled by microprocessor for basic research and clinical applications

A programmable digital video pattern generator controlled by a microprocessor is described as a part of any commercial or specific visual evoked potential analysis system. The generator handles all kinds of patterns (uniform, orthogonal and radial) with all known presentation modes (onset, offset and reversal) and resolution 640 x 200 square pixels. The frame rate of the generator is adjusted to be compatible to either low cost 50 Hz video monitor or expensive ones of higher frame rate. The operation parameters defined by software are: pattern selection, trigger mode, field selection and stimulation rate. The development of software applications for special research demands is desirable because of the control of the generator by a microprocessor.     

Adaptation of the viscoelastic behavior of bone cement to bone as a stabilizing factor in endoprostheses

Incompatibility of viscoelastic properties is a major factor contributing to the development and enlargement of microscopic cracks between bone and bone cement. In the study reported here the viscoelastic characteristics of fresh long bones were studied in vitro and compared with those of bone cement. Using strain gauges, creep elongation was measured over time spans ranging from 10(-2) to 10(3) seconds with an applied stress of 10 MPa. On the basis of these measurements creep compliance and permanent elongation at two points on a sheep femur were calculated by means of a rheologic Kelvin model with three parameters. The possibility of obtaining an optimal bone/cement bond was studied in the light of these results and of data published in the literature on bone cement creep related to cement composition factors.     

Experimentelle Methode zur Diagnose versp?teter Knochenneubildung mittels der Analyse von Biegungs- und Axialschwingungen von Knochen

Ohne Zusammenfassung     

Investigation of the effects of ischemic preconditioning on the HRV response to transient global ischemia using linear and nonlinear methods

Ischemic preconditioning (IP) has been used as a strategy to prevent cell death in various organs, including the brain and the heart. Investigation of the effects of ischemic preconditioning mostly employed models with reduced complexity, such as cell cultures, tissue slices or perfused organ preparations. Although such models can provide valuable insight into the protective mechanism of preconditioning, the functional (re)organization of the control mechanisms at the level of the living organism cannot be assessed. The purpose of the present animal model study was to evaluate the effect of global ischemic preconditioning on the heart rate variability (HRV) response to the asphyxia insult. The data consisted of 4 h RR interval measurements recorded in five preconditioned and five non-preconditioned Wistar rats. Using linear (time and frequency domain) and nonlinear (approximate entropy and parameters of Poincare plots) measures, we evaluated the dynamic time course of the HRV response to the asphyxia insult and the effect of preconditioning on the autonomic neurocardiac control. Both the linear and nonlinear parameters indicate a faster recovery of the baseline HRV corresponding to the preconditioned groups, though only the spectral analysis identifies a statistically significant difference between the two groups. For the preconditioned group, at about 90 min after the asphyxic insult, the autonomic neural balance (measured by LF/HF ratio) appears fully recovered. The small variation of the rest of the parameters indicates the necessity of further investigation including the design of a larger study with a higher statistical power. Our results show for the first time that global ischemic preconditioning influences the HRV response to the asphyxia injury. The neuroprotective effect of preconditioning translates into a faster recovery of the basal HRV and the autonomic modulation of the heart.     

Removal of ECG interference from the EEG recordings in small animals using independent component analysis.

In experiments involving small animals, the electroencephalogram (EEG) recorded during severe injury and accompanying resuscitation exhibit the strong presence of electrocardiogram (ECG). For improved quantitative EEG (qEEG) analysis, it is therefore imperative to remove ECG interference from EEG. In this paper, we validate the use of independent component analysis (ICA) to effectively suppress the interference of ECG from EEG recordings during normal activity, asphyxia and recovery following asphyxia. Two channels of EEG from five rats were recorded continuously for 2 h. Simultaneous recording of one channel ECG was also made. Epochs of 4 s and 1 min were selected from baseline, asphyxia and recovery (every 10 min) and their independent components and power spectra were calculated. The improvement in normalized power spectrum of EEG obtained for all animals was 7.71+/-3.63 db at the 3rd minute of recovery and dropped to 1.15+/-0.60 db at 63rd minute. The application of ICA has been particularly useful when the power of EEG is low, such as that observed during early brain hypoxic-asphyxic injury. The method is also useful in situations where accurate indications of EEG signal power and frequency content are needed.     

Functional properties of sub-bands of oscillatory brain waves to pattern visual stimulation in man.

The scalp recorded transient visual evoked potential (VEP) represents the massed activity of a large number of neurons of the human visual cortex. Animal studies show that intracerebrally-recorded high frequency electrical activity represents binding between neurons participating in a cooperative response. We evaluated the relationship between scalp recorded high frequency activity and transient VEPs elicited by a repetitive (grating) pattern. Stimuli were 1 and 4 cycles/degree sinusoidal gratings, presented in an on/off mode. Following conventional averaging, the discrete wavelet transform (DWT) was applied. Multi-resolution decomposition was used to divide the responses into 6 orthogonal frequency bands. The results show that high frequency oscillatory activity in the beta and gamma frequency range is closely related in time to the N70 peak of the simultaneous VEP. Power in both bands is modulated by spatial frequency. Beta range response to hemifield stimulation recorded over a chain of electrodes over the occipital area lateralizes in the same manner as N70, while gamma range activity is insensitive to lateralization and is more closely linked to foveal stimulation. This dissociation between beta and gamma range activity suggests that different bands of high frequency oscillatory activity in humans, linked to visual stimulation, may represent different aspects of visual processing.