Lawrence Ward

One of the most striking features of human consciousness is its ability to foster an ongoing and seemingly continuous stream of thought. How do mental states unfold over time as the mind moves from one thought to another? The Dynamic Framework of Thought (DFT) has suggested a taxonomy of thought based on the way it moves over time (i.e., degree and type of constraints applied on thought) rather than on features of its content (e.g., task-relatedness). This dissertation explored two kinds of dynamics proposed in the DFT framework – deliberately constrained (i.e., goal-directed) and relatively unconstrained (i.e., spontaneous) thought – using first-person reports through experience sampling and third-person data from fMRI. In the first part, I investigated whether people have introspective access to these two types of thought dynamics. Using experience sampling, across various contexts, I found that the two dynamics were experienced as being phenomenally distinct from each other and shared a moderately negative correlation. Subsequently, I characterized the brain regions and networks involved in these phenomenally distinct transitions between mental states. In this real-time experience sampling paradigm, participants were asked to let their thoughts unfold naturally as they were intermittently probed to rate their thoughts on the degree of free-movement and active-direction while in an fMRI scanner. Results showed that regions in the medio-temporal and core subcomponents of the default network were more engaged when thoughts were freely-moving. Regions in the frontoparietal control network, on the other hand, were more engaged when thoughts were not freely-moving and when deliberately-constrained. These findings suggest that executive regions are more associated with strong constraints on the stream of thought, whereas default network regions are more associated with weak constraints on the stream of thought. This work sheds new light on our current understanding of the relationship between the default network and frontoparietal control network during self-generated thought. Overall, this dissertation represents progress in broadening the use of first-person, introspective accounts in neuroscience to advance our understanding of the stream of thought in the broader context of neurophenomenology.

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Here I report evidence of awareness among unresponsive hospice patients at the end of life. Chapter 2 describes the neurotypical neural networks underlying some EEG measures used to assess awareness, particularly activations of different attention networks associated with the P3a and P3b event-related potential (ERP) sub-components of the P300 ERP. I found that the primary neural generators of the P3a were frontal regions associated with exogenous attention processes, such as the Ventral Attention (Corbetta & Shulman, 2002) and Saliency (Menon & Uddin, 2010) networks. The neural generators of the P3b, by contrast, were parietal regions associated with endogenous attention processes, such as the Dorsal Attention Network (Corbetta & Shulman, 2002), and regions often involved in detecting oddball targets (H. Kim, 2014). Chapter 3 reports evidence of auditory cortical processing (Näätänen et al., 2007) among all unresponsive patients, and attention orienting or context updating (Polich, 2007) among some unresponsive patients. Chapter 4 describes the spatio-temporal dynamics of attention networks reported in Chapter 2 among individual neurotypical control participants, and both responsive and unresponsive hospice patients. While these results are highly nuanced, some unresponsive patients showed some fronto-parietal connectivity, and may have engaged in motor imagery. Chapter 5 reports evidence of stimulus-independent cognition occurring in the default mode network (Buckner et al., 2008a; Christoff et al., 2016; Raichle et al., 2001; Smallwood et al., 2012) among unresponsive patients during a period of rest. Chapter 6 reports evidence of a decrease in alpha-band oscillation power in the posterio-parietal cortex in response to music among unresponsive patients. As such a decrease in alpha-band power is associated with orienting of attention, this implies that these patients may be paying attention to the music. The results of this research suggest that some unresponsive hospice patients at the end of life may be aware. These results lend credence to the belief that “hearing is the last to go”, and that loved ones at the bedside should be encouraged to interact with their dying relatives for as long as possible. Furthermore, attention to music could be a promising new biomarker of awareness that has potential for clinical adaptation.

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There are concerns that cannabis use leads to increased risk of mental illness and neurocognitive impairment. However, empirical findings into the deleterious effects of cannabis use on cognition have been mixed and the underlying brain processes are poorly understood. The present research examined auditory cognitive processes putatively related to psychosis and cannabis use. Groups of cannabis users (CU) and non-users (NU) are compared on two novel tasks that examine electroencephalographic (EEG) measures of early somewhat pre-attentional (MMN) and later attentional (P300) auditory change detection and P300 response in auditory working memory. Additionally, sophisticated EEG source localization was used to interrogate underlying oscillatory activity and brain network connectivity. Chapter 2 introduced the novel roving dual oddball task, and revealed an asymmetry between easy and difficult task conditions, possibly due to increased attentional demands as evidenced by an interplay between dorsal and ventral attentional systems. Chapter 3 compared CU and NU on the same dual oddball task and revealed slower response times and increased P300 latency for CU on the more difficult condition. Theta-band network connectivity suggested that CU engaged in a stimulus driven strategy that became less effective in the more difficult condition due to a breakdown of fronto-temporal connectivity. Groups did not differ in early auditory processes (MMN). Chapter 4 unexpectedly revealed CU to have superior performance on working memory for basic auditory features (pitch and pattern), which was partially reflected in larger P300 amplitudes. Theta connectivity revealed different patterns of brain connectivity. Chapter 5 combined data from Chapters 3 and 4 to examine a local MMN effect across a larger sample along with measures putatively related to impairments associated with heavy cannabis use and psychosis. Again, CU did not show reduced MMN and showed only small differences on various measures. As a whole, this research suggests that proposed auditory impairments due to heavy cannabis use may not generalize to all subsets of cannabis users, and additional research is needed before any definitive conclusions can be reached regarding the impact of frequent cannabis use on cognitive processes.

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Developmental dyslexia is a language-based learning disability characterized by impaired reading speed and accuracy, poor spelling, and poor decoding abilities, despite normal intelligence. Neuroimaging investigations have identified brain regions critical for reading; few studies, however, have characterized how those regions interact to form networks and how those networks are perturbed in individuals with dyslexia. Advances in electroencephalography (EEG) analysis techniques now allow for intricate examination of these networks by focusing on individual frequency bands that comprise the brain signals. This study used EEG across several experiments to examine theta- and gamma-band connectivity patterns—first in the brains of adults, and then in dyslexic and typically-developing children during reading tasks. I investigated: 1) the ways in which the reading networks of typical and dyslexic readers differ, and 2) whether targeted reading interventions reduce these differences over time. Results show that dyslexic children generated greater occipito-temporal connectivity at critical time points in response to words and word-like stimuli, as well as increased engagement of higher-level language areas even for stimuli lacking linguistic content (e.g. consonant strings). After six months, the networks of dyslexic readers resembled those of their typically-developing counterparts for simple orthographic processing, but continued to utilize existing alternative pathways when engaging in higher-level language processes (e.g. phonology). This suggests that performance improvements in dyslexic readers are not necessarily related to changes to the typical left-lateralization of reading networks. These findings are in line with existing frameworks of dyslexia, and highlight the value of connectivity measures in understanding the neural underpinnings of word reading.

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Several cortical regions appear active when the mismatch negativity (MMN) scalp potential is evoked automatically in response to detectable auditory changes. It remains debatable whether the activation of regions beyond the auditory cortex is coincidental or functionally significant to the MMN response. We used independent component analysis (ICA) to separate high density EEG data (64-channel) prior to dipole fitting for two reasons: 1) to enhance the spatial resolution of EEG and 2) to provide temporal and frequency information about the cortical sources needed to evaluate their functional relationships during the MMN response. For a group of young adults (n = 12) passively listening to infrequent changes in complex tones while watching a silent movie, event-related activity within sources localized to the orbitofrontal cortex (OFC) and the bilateral superior temporal gyrus (STG) regions accounted for most of the scalp response variance implicating these regions as driving forces in the MMN. For a second group (n = 14) performing both passive and active listening across the same paradigm, cross-coherence (phase synchronization) during the MMN response was consistently found between the OFC and the STG bilaterally. During both paradigms the source in the right inferior frontal gryus (R IFG) was also synchronous with the STG-OFC network. When responding to deviant targets in the active paradigm, synchrony was more bilaterally distributed across the network. For a third group (n = 14) passively listening to infrequent changes in speech syllables, synchrony during the MMN response was found between the STG-OFC again as well as with regions in the R IFG and Broca’s area. This same subject group later attended to the speech syllables responding to deviants and standards with a different button press. Synchrony between the STG-OFC, and Broca’s area was found, as well synchrony with a source in the right anterior cingulate. All paradigms showed synchronous interactions both within and between the temporo-frontal regions that were modulated differentially by deviant and standard stimulus conditions as well as by task demands providing the first evidence of functional coupling within a hierarchical network coinciding with the MMN response evoked at the scalp.

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