Investigations into the Neural Basis of Consciousness
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The overarching goal of this dissertation was to improve our understanding of the neural basis of consciousness by approaching the problem along two separate, complementary facets: examining the levels of consciousness and the contents of consciousness.
Chapter 2 examines how the level of consciousness changes under general anesthesia for surgery, and how neural (EEG) markers of this change relate to postoperative cognitive impairments afflicting many older adults. Older adult patients underwent neurocognitive testing before and after surgery, and their 32-channel EEG was recorded both before and during general anesthesia for surgery. Results showed that one of the most profound changes from the awake to the anesthetized brain—the anteriorization of alpha-band (8-12 Hz) activity—correlated with preoperative cognitive scores, which are themselves predictors for postoperative cognitive impairments. These results have added to our understanding of how manipulations of the level of consciousness under general anesthesia ramify into potentially long-lasting impairments to cognition, and how these impairments might be monitored and avoided.
Chapters 3 and 4 examined how the contents of consciousness relate to the selection mechanism of attention. Chapter 3 investigated the dissociability of these two phenomena by examining the neural mechanisms underlying the orienting of spatial attention without awareness. High-density (64-channel) EEG was recorded while subjects performed a novel task that combined classic spatial cueing with object-substitution masking to manipulate subjects’ awareness of the cues on ~half of the trials, allowing a direct comparison of orienting with and without awareness, controlled for having identical sensory stimulation. Results confirmed that attention could be oriented without awareness, leading to improved behavior (faster reaction times and better accuracy) and enhanced sensory processing (indexed by the P1 event-related potential, ERP) for validly (compared to invalidly) cued targets. Interestingly, the hallmark ERP for the orienting of attention in response to a cue, the N2pc, was only observed for conscious orienting, pointing to an alternate mechanism for unconscious orienting, such as via the subcortical retinotectal pathway.
Chapter 4 investigated the mechanisms and temporal dynamics of the attentional selection of conscious internal representations in working memory. EEG was recorded while subjects performed a modified delayed match-to-sample task where one of two sample objects, a face or a house, was retroactively cued on each trial. A multivariate classifier was trained on the pattern of alpha-band activity to determine if and when information about the selected object could be decoded from the alpha signal following the retrocue. Results showed that alpha could be used to decode the selected object, pointing to its general role as a top-down attentional control signal. This decoding was relatively transient, rather than sustained, which accords with recent proposals of “activity-silent” working memory and argues against accounts of working memory that posit sustained internal attention as the underlying mechanism. Together the results of Chapters 3 and 4 help inform our understanding of how attention operates both externally and internally to select the contents of consciousness.
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