Cross-task and cross-manipulation stability in shifting the decision criterion.

In recognition memory experiments participants must discriminate between old and new items, a judgment influenced by response bias. Research has shown substantial individual differences in the extent to which people will strategically adjust their response bias to diagnostic cues such as the prior probability of an old item. Despite this significant between subject variability, shifts in bias have been found to be relatively predictive within individuals across memory tests. Experiment 1 sought to determine whether this predictability extends beyond memory. Results revealed that the amount a subject shifted response bias in a recognition memory task was significantly predictive of shifting in a visual perception task, suggesting that shifting can generalise outside of a specific testing domain. Experiment 2 sought to determine how predictive shifting would be across two manipulations well known to induce shifts in bias: a probability manipulation and a response payoff manipulation. A modest positive relationship between these two methods was observed, suggesting that shifting behaviour is relatively predictive across different manipulations of shifting. Overall, results from both experiments suggest that response bias shifting, like response bias setting, is a relatively stable behaviour within individuals despite changes in test domain and test manipulation.

Age-dependent changes in task-based modular organization of the human brain.

As humans age, cognition and behavior change significantly, along with associated brain function and organization. Aging has been shown to decrease variability in functional magnetic resonance imaging (fMRI) signals, and to affect the modular organization of human brain function. In this work, we use complex network analysis to investigate the dynamic community structure of large-scale brain function, asking how evolving communities interact with known brain systems, and how the dynamics of communities and brain systems are affected by age. We analyze dynamic networks derived from fMRI scans of 104 human subjects performing a word memory task, and determine the time-evolving modular structure of these networks by maximizing the multislice modularity, thereby identifying distinct communities, or sets of brain regions with strong intra-set functional coherence. To understand how community structure changes over time, we examine the number of communities as well as the flexibility, or the likelihood that brain regions will switch between communities. We find a significant positive correlation between age and both these measures: younger subjects tend to have less fragmented and more coherent communities, and their brain regions tend to change communities less often during the memory task. We characterize the relationship of community structure to known brain systems by the recruitment coefficient, or the probability of a brain region being grouped in the same community as other regions in the same system. We find that regions associated with cingulo-opercular, somatosensory, ventral attention, and subcortical circuits have a significantly higher recruitment coefficient in younger subjects. This indicates that the within-system functional coherence of these specific systems during the memory task declines with age. Such a correspondence does not exist for other systems (e.g. visual and default mode), whose recruitment coefficients remain relatively uniform across ages. These results confirm that the dynamics of functional community structure vary with age, and demonstrate methods for investigating how aging differentially impacts the functional organization of different brain systems.

P3a from visual stimuli: task difficulty effects.

The P3a event-related brain potential (ERP) was elicited using a visual three-stimulus oddball paradigm (target, standard, distracter) in which participants responded only to the target. Discrimination task difficulty between the target and the standard was manipulated by varying the size of the standard stimulus circle relative to a constant target stimulus circle across three conditions (easy, medium, hard). A large checkerboard pattern was employed for the distracter stimulus across all tasks. Error rate and response time increased with increases in task difficulty, so that the task difficulty manipulation was successful. Distracter P3a amplitude increased and target P3b decreased somewhat with increases in task difficulty. The findings suggest that increased perceptual discrimination difficulty between the target and standard stimuli increases P3a amplitude. Theoretical implications are discussed.