Impact of task-related changes in heart rate on estimation of hemodynamic response and model fit.

The blood oxygen level dependent (BOLD) signal, as measured using functional magnetic resonance imaging (fMRI), is widely used as a proxy for changes in neural activity in the brain. Physiological variables such as heart rate (HR) and respiratory variation (RV) affect the BOLD signal in a way that may interfere with the estimation and detection of true task-related neural activity. This interference is of particular concern when these variables themselves show task-related modulations. We first establish that a simple movement task reliably induces a change in HR but not RV. In group data, the effect of HR on the BOLD response was larger and more widespread throughout the brain than were the effects of RV or phase regressors. The inclusion of HR regressors, but not RV or phase regressors, had a small but reliable effect on the estimated hemodynamic response function (HRF) in M1 and the cerebellum. We next asked whether the inclusion of a nested set of physiological regressors combining phase, RV, and HR significantly improved the model fit in individual participants' data sets. There was a significant improvement from HR correction in M1 for the greatest number of participants, followed by RV and phase correction. These improvements were more modest in the cerebellum. These results indicate that accounting for task-related modulation of physiological variables can improve the detection and estimation of true neural effects of interest.

Effects of Prior Knowledge on Decisions Made Under Perceptual vs. Categorical Uncertainty.

Humans use prior knowledge to bias decisions made under uncertainty. In this fMRI study we predicted that different brain dynamics play a role when prior knowledge is added to decisions made under perceptual vs. categorical uncertainty. Subjects decided whether shapes belonged to Category S - smoother - or Category B - bumpier - under both uncertainty conditions, with or without prior knowledge cues. When present, the prior knowledge cue, 80/20 or 50/50, indicated that 80 and 20% (or 50 and 50%) were the chances that responding "S" and "B" (or vice versa) would be correct. During perceptual uncertainty, shapes were degraded with noise. During categorical uncertainty, shapes were ambiguous. Adding the 80/20 cue increased activation during perceptual uncertainty in bilateral lateral occipital (LO) cortex and left middle frontal gyrus (MidFG), and decreased activity in bilateral LO cortex during categorical uncertainty. Right MidFG and other frontoparietal regions were active in all conditions. The results demonstrate that left MidFG shows activation changes, suggestive of an influence on visual cortex, that depend on the factor that makes the decisions difficult. When sensory evidence is difficult to perceive, prior knowledge increases visual cortical activity. When the sensory evidence is easy to perceive but difficult to interpret, prior knowledge decreases visual cortical activity.

Human brain activity predicts individual differences in prior knowledge use during decisions.

Studies by cognitive psychologists, psychophysicists, neuroscientists, and economists provide ample evidence that humans use prior knowledge to bias decisions adaptively. In this study, we sought to locate and investigate the brain areas mediating this behavior. Participants viewed ambiguous abstract shapes and decided whether a shape was of Category A (smoother) or B (bumpier). The decision was made in the context of one of two prior knowledge cues, 80/20 and 50/50. The 80/20 cue indicated that upcoming shapes had an 80% probability of being of one category, for example, B, and a 20% probability of being of the other. The 50/50 cue indicated that upcoming shapes had an equal probability of being of either category. The shift in bias produced by the 80/20 cue relative to the 50/50 cue was of the predicted sign for every subject but varied in magnitude. We searched for brain regions in which activity changes correlated with the extent of the bias shift; these were dorsolateral pFC (middle frontal gyrus), inferior frontal junction, anterior insula, inferior parietal lobule, intraparietal sulcus, head of the caudate, posterior cingulate cortex, and fusiform gyrus. The findings indicate that an individual's brain activity in these regions reflects the extent to which that individual makes use of prior knowledge to bias decisions. We also created within-ROI tuning curves by binning the shape curvature levels and plotting brain activity levels at each of the nine bins. In the fronto-parietal and anterior insula ROIs, the tuning curves peaked at targets contraindicated by the prior knowledge cue (e.g., Category B targets if the 80/20 cue meant 20% probability B). The increased activity in these regions likely indicates a no-go response when sufficient perceptual evidence favored the alternative contraindicated by the 80/20 cue.

Persistency of priors-induced bias in decision behavior and the FMRI signal.

It is well known that people take advantage of prior knowledge to bias decisions. To investigate this phenomenon behaviorally and in the brain, we acquired fMRI data while human subjects viewed ambiguous abstract shapes and decided whether a shape was of Category A (smoother) or B (bumpier). The decision was made in the context of one of two prior knowledge cues, 80/20 and 50/50. The 80/20 cue indicated that upcoming shapes had an 80% probability of being of one category, e.g., B, and a 20% probability of being of the other. The 50/50 cue indicated that upcoming shapes had an equal probability of being of either category. The ideal observer would bias decisions in favor of the indicated alternative at 80/20 and show zero bias at 50/50. We found that subjects did bias their decisions in the predicted direction at 80/20 but did not show zero bias at 50/50. Instead, at 50/50 the subjects retained biases of the same sign as their 80/20 biases, though of diminished magnitude. The signature of a persistent though diminished bias at 50/50 was also evident in fMRI data from frontal and parietal regions previously implicated in decision-making. As a control, we acquired fMRI data from naïve subjects who experienced only the 50/50 stimulus distributions during both the pre-scan training and the fMRI experiment. The behavioral and fMRI data from the naïve subjects reflected decision biases closer to those of the ideal observer than those of the prior knowledge subjects at 50/50. The results indicate that practice making decisions in the context of non-equal prior probabilities biases decisions made later when prior probabilities are equal. This finding may be related to the "anchoring and adjustment" strategy described in the psychology, economics, and marketing literatures, in which subjects adjust a first approximation response - the "anchor" - based on additional information, typically applying insufficient adjustment relative to the ideal observer.