Magnetic resonance imaging enables the noninvasive mapping of both anatomical white matter connectivity and dynamic patterns of neural activity in the human brain. We examine the relationship between the structural properties of white matter streamlines (structural connectivity) and the functional properties of correlations in neural activity (functional connectivity) within 84 healthy human subjects both at rest and during the performance of attention-and memory-demanding tasks. We show that structural properties, including the length, number, and spatial location of white matter streamlines, are indicative of and can be inferred from the strength of resting-state and task-based functional correlations between brain regions. These results, which are both representative of the entire set of subjects and consistently observed within individual subjects, uncover robust links between structural and functional connectivity in the human brain.uman cognitive function is supported by large-scale interactions between different regions of the brain. The anatomical scaffolding that mediates these interactions can be described by a structural connectome that maps the spatial layout of white matter (1). Structural connectivity (SC), defined by the physical properties of these direct anatomical connections, supports the relay of electrical signals between brain regions. Neurophysiological events can similarly be described by a functional connectome that maps coordinated changes in neuronal activity, field potentials, blood flow, or energy consumption (2). Functional connectivity (FC), defined by temporal correlations in such neurophysiological events, reflects the resting-state and task-dependent strengths of correlated activity in different brain regions (3-5). The estimation of structural and functional connectivity from different experimental techniques raises two complementary questions about the quantitative relationships between structural and functional connectomes: (i) to what extent can the resting-state and task-dependent strengths of functional correlations between brain regions be inferred from structural connectomes, and (ii) to what extent can the physical properties of anatomical connections be inferred from functional connectomes?Connectomes, whether examined at the neural or systems level, are networks whose structural properties, such as the length and number of connections, can differentially impact functional properties, such as local or global correlations in temporal dynamics. Whereas the length and density of anatomical connections are thought to impact functional processes such as information segregation and integration (6, 7), the extent to which such relationships are robustly observed in the human brain is not well understood. Previous studies have been limited in scope to specific anatomical connections and brain regions, small sample sizes, and resting-state neural activity (8-13) and have consequently left several fundamental questions unanswered. How do variations in structural features, such a...
Accumulating evidence suggests that the brain can efficiently process both external and internal information. The processing of internal information is a distinct “offline” cognitive mode that requires not only spontaneously generated mental activity; it has also been hypothesized to require a decoupling of attention from perception in order to separate competing streams of internal and external information. This process of decoupling is potentially adaptive because it could prevent unimportant external events from disrupting an internal train of thought. Here, we use measurements of pupil diameter (PD) to provide concrete evidence for the role of decoupling during spontaneous cognitive activity. First, during periods conducive to offline thought but not during periods of task focus, PD exhibited spontaneous activity decoupled from task events. Second, periods requiring external task focus were characterized by large task evoked changes in PD; in contrast, encoding failures were preceded by episodes of high spontaneous baseline PD activity. Finally, high spontaneous PD activity also occurred prior to only the slowest 20% of correct responses, suggesting high baseline PD indexes a distinct mode of cognitive functioning. Together, these data are consistent with the decoupling hypothesis, which suggests that the capacity for spontaneous cognitive activity depends upon minimizing disruptions from the external world.
An ability to flexibly shift a decision criterion can be advantageous. For example, a known change in the base rate of targets and distractors on a recognition memory test will lead optimal decision makers to shift their criterion accordingly. In the present study, 95 individuals participated in two recognition memory tests that included periodic changes in the base rate probability that the test stimulus had been presented during the study session. The results reveal a wide variability in the tendency to shift decision criterion in response to this probability information, with some appropriately shifting and others not shifting at all. However, participants were highly reliable in their tendency to shift criterion across tests. The goal of the present study was to explain what factors account for these individual differences. To accomplish this, over 50 variables were assessed for each individual (e.g., personality, cognitive style, state of mind). Using a regression model that incorporated different sets of factors, over 50% of the variance was accounted for. The results of the analysis describe the total, direct, and mediating effects on criterion shifting from factors that include memory strength, strategy, and inherent characteristics such as a fun-seeking personality, a negative affect, and military rank. The results are discussed with respect to understanding why participants rarely chose an optimal decision-making strategy and provide greater insight into the underlying mechanisms of recognition memory.
The anatomical connectivity of the human brain supports diverse patterns of correlated neural activity that are thought to underlie cognitive function. In a manner sensitive to underlying structural brain architecture, we examine the extent to which such patterns of correlated activity systematically vary across cognitive states. Anatomical white matter connectivity is compared with functional correlations in neural activity measured via blood oxygen level dependent (BOLD) signals. Functional connectivity is separately measured at rest, during an attention task, and during a memory task. We assess these structural and functional measures within previously-identified resting-state functional networks, denoted task-positive and task-negative networks, that have been independently shown to be strongly anticorrelated at rest but also involve regions of the brain that routinely increase and decrease in activity during task-driven processes. We find that the density of anatomical connections within and between task-positive and task-negative networks is differentially related to strong, task-dependent correlations in neural activity. The space mapped out by the observed structure-function relationships is used to define a quantitative measure of separation between resting, attention, and memory states. We find that the degree of separation between states is related to both general measures of behavioral performance and relative differences in task-specific measures of attention versus memory performance. These findings suggest that the observed separation between cognitive states reflects underlying organizational principles of human brain structure and function.
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