Feature clustering of intracranial pressure time series for alarm function estimation in traumatic brain injury.

OBJECTIVE: The conventional application of intracranial pressure (ICP) monitoring of traumatic brain injury (TBI) patients consists merely in the acquisition of ICP values in discrete time and their comparison to the established ICP threshold. An exceeding of this threshold triggers a special emergency treatment protocol. This paper addresses the possibility of making use of the rich information latent in the ICP records of known vital and fatal outcomes gathered during real clinical practice of treating TBI patients. Our assumption was that the proposed algorithmic procedure derived from this information could, in addition to ICP monitoring itself, provide a complementary added value. This might help clinicians to make better decisions during a patient's treatment. APPROACH: We concentrated on studying specific clustering schemes for subsequences of ICP time series. The clusterization problem was formulated for feature vectors which are introduced to represent ICP time subsequences. The ICP transformation to a feature space uses global and local definitions of time subsequences. For clusterization itself, we adopted hierarchical Gaussian mixture models (hGMMs). By using posterior probabilities of the clusters, we introduced three novel alarm functions. We explored two alternative methods of searching for optimum alarm function thresholds (ROC analysis and a novel efficiency measure). MAIN RESULTS: We performed extensive cross-validation experiments on a clinical retrospective data set. The results of the optimization over several hGMMs, various feature space dimensionality and all the types of the novel alarm functions show the potential of the novel alarm functions for supplementing conventional ICP monitoring. SIGNIFICANCE: In conclusion, the paper provides a prospective extended ICP monitoring technique for real TBI patients, based on the proposed methodology of ICP subsequence clustering and thresholding of the optimum novel alarm function.

The long-term effects of phase advance shifts of photoperiod on cardiovascular parameters as measured by radiotelemetry in rats.

Cardiovascular parameters, such as blood pressure and heart rate, exhibit both circadian and ultradian rhythms which are important for the adequate functioning of the system. For a better understanding of possible negative effects of chronodisruption on the cardiovascular system we studied circadian and ultradian rhythms of blood pressure and heart rate in rats exposed to repeated 8 h phase advance shifts of photoperiod. The experiment lasted 12 weeks, with three shifts per week. Spectral power as a function of frequency for both circadian and harmonic ultradian rhythms was expressed as the circadian-ultradian power ratio. The circadian rhythms of blood pressure, heart rate and locomotor activity were recorded during the control light:dark (LD) regimen with higher values during the D in comparison with the L. Phase advance shifts resulted in a diminished circadian-ultradian power ratio for blood pressure, heart rate and locomotor activity indicating suppressed circadian control of these traits greater in heart rate than blood pressure. In conclusion, rats kept under irregular LD conditions have suppressed circadian control of heart rate, blood pressure and locomotor activity and rely more on an acute response to the LD regime. Their ability to anticipate regular loads can be weakened and in this way chronodisruption can contribute to the pathogenesis of cardiovascular diseases.

Direct effects of audio-visual stimulation on EEG.

In the course of 2 months, 25 repetitions of a 20 min audio-visual stimulation (AVS) program with stimulations at 17, 9, 4, and 2 Hz were applied to 6 volunteers. EEG data were recorded from 6 scalp locations prior, during and after AVS. In order to identify direct and transient changes in EEG under influence of AVS, total power, relative frequency band powers and magnitude-squared coherences were estimated. Intense brain wave entrainment as a direct reaction to AVS was significant through increase of spectral powers and coherences around the stimulating frequency bands in the occipital areas, spreading also to the central and frontal regions. However, these excitations were 'short-lived'. On the other hand some signs of interhemispheric cooperation (coherences in the narrow bands around 2, 4, and 17 Hz at parieto-occipital areas) remained increased during the investigated 3 min after AVS. As going through further AVS sessions the driving response progressively enhanced for 2 and 4 Hz stimulation in centro-parietal locations. Progress was also found in the left and right hemisphere synchronization examined by coherences. In perspective, the results contribute to deeper comprehension of photic stimulation approaches as a technique of guided entrainment of the brain waves or intermediate increase of hemispheres' synchronization.

Phase synchronization in human EEG during audio-visual stimulation.

Synchrony of EEG data recorded under influence of audio-visual stimulation was investigated. Instantaneous phases were derived from Wavelet transform method. Phase synchronization is assessed quantitatively via measure derived from uniformity of phase difference distribution. EEG data came from 6 healthy volunteers repeatedly exposed to 20 min AVS program with intervals with stable stimulation at 4 and 17 Hz. Phase synchronization during AVS significantly increased in comparison to non stimulation conditions in all examined cortex locations. The lowest increases in synchronization occurred in the frontal areas. In central region no dumping in synchronization was recognizable in comparison to backward locations where visual processing centers are situated.

EEG responses to long-term audio-visual stimulation.

In this study, linear and nonlinear electroencephalogram (EEG) changes due to long-term audio-visual stimulation (AVS) were investigated. In the course of 2 months, 25 repetitions of a 20-min AVS program with stimulation frequencies in the range 2-18 Hz were applied to six healthy volunteers. EEG data were recorded from six head locations during relaxed wakefulness prior to AVS. Then linear spectral measures (total power, frequency band powers, spectral edge frequency, and spectral entropy), nonlinear measures of complexity (histogram-based entropy and correlation dimension), interdependency measures (linear correlation coefficient, mutual information, and coherence), and measures of subjective assessment were estimated. Evolution of these measures during the whole experiment period was analyzed with respect to the significance of their linear regression. Our results confirm that repetitive training with audio-visual stimulation does induce changes in the electro-cortical activity of the brain. Long-term AVS significantly increased power in theta-1, theta-2, and alpha-1 bands in the frontal and central cortex locations. Total power increased in the right central region. Interhemispheric coherence in alpha-1 band displayed a significant increase between frontal parts in contrast to the decrease of both linear correlation and mutual information. Correlation dimension significantly decreased in some locations while entropy displayed an ascending trend.