Ph.D. Thesis Defense - Neural Correlates of Cognitive Workload and Anesthetic Depth: fNIR Spectroscopy Investigation in Humans
Date: July 18, 2008
Time: 10:00 AM
Location: Bossone Research Enterprise Center, Room: 709
Advisors: Banu Onaral, Ph.D., and Scott C Bunce, Ph.D.
Functional near-infrared spectroscopy (fNIR), a non-invasive neuroimaging modality designed to monitor the hemodynamic change, can help identify neural correlates of human brain functioning mediated by different events. In this thesis, fNIR has been used to monitor prefrontal cortical activity with the primary objective to determine a set of neurophysiological markers that detect changes in neural activation elicited by levels of mental engagement. Two studies were selected to assess the high and the low ends of engagement: at the high level of neural activation, human performance studies were conducted to assess cognitive workload; the low end, representing minimal level of neural activation, was tested in patients undergoing general anesthesia. In the human performance study, participants were cognitively challenged by a complex task. By contrast, in anesthetic depth assessment study, cognitive activity was deliberately suppressed by anesthetic agents. In both assessments, neurophysiological markers of hemodynamic changes were extracted from the fNIR measurements.
The hypothesis underlying the human performance study is the positive correlation of blood oxygenation in the prefrontal cortex with increasing task difficulty and sustained cognitive effort. In addition, increased blood oxygenation demonstrates a positive relationship with behavioral performance measures in this task. A naval air warfare management and control task with varying difficulty levels has been chosen to test this engagement condition. This study showed that changes in blood oxygenation in relevant areas of the prefrontal cortex are associated with increasing cognitive workload, defined as attention in a verbal and spatial working memory and decision-making task. The results suggest a reliable, positive association between cognitive workload and increases in the oxygenation responses (F = 16.24, p<0.001). The data analysis also support the hypothesis that rate of oxygenation change in dorsolateral prefrontal cortex as measured by fNIR can provide an index of sustained attention in a complex working memory and decision-making task. Furthermore, this study reveals that a drop in the rate of oxygenation change in dorsolateral prefrontal cortex under high workload conditions can predict a user’s decline in performance.
Anesthesia awareness is an unintended mental alertness during general anesthesia. In the exploratory investigation reported in this thesis, the neuromarker(s) derived from the fNIR system measurements help quantify transition from deep anesthesia to light anesthesia. This feasibility study suggests that the rate of deoxygenated hemoglobin change can be used as a descriptive neuromarker to differentiate between deep and light anesthesia stages (F = 7.61, p<0.01). This marker is proposed as an index to monitor depth of anesthesia.
Furthermore, the thesis introduces customized algorithms and procedures implemented to record fNIR signals in real time under noisy operating room conditions. Independent and principal component analyses (ICA, PCA) combined in a novel procedure and based on the use of dark current as a reference measurement, provided improved signal-to-noise ratio for the hemodynamic measurements acquired in the operating room. The same approach can be exploited to enhance signal quality under other field situations and natural environments.
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