Motivation-dependent responses in the human caudate nucleus.

Motivation is a complex process that leads to completion or avoidance of a behavior. Past research strongly implicates the basal ganglia in a circuit integral for the control of motivation. Specifically, the human striatum has been shown to process reward information, differentiating between monetary rewards and punishments in recent neuroimaging experiments. It is unclear, however, how the dorsal striatum, particularly the caudate nucleus, responds to changes in the motivational context of a task. Using an event-related design, where participants were given positive and negative feedback upon guessing the value of an unknown card, we manipulated the motivational context of the task by dividing trials into periods of high incentive (where visual feedback indicated monetary rewards and punishments) and low incentive (where visual feedback indicated only accuracy). We found that activity in the caudate nucleus was strongly influenced by the different incentive periods. The hemodynamic response was characterized by a larger rise at the onset of trials and larger differences between positive and negative feedback during periods of high incentive. These results suggest that changes in motivation are capable of modulating basal ganglia activity, and further support an important role for the caudate nucleus in affective processing.

Dorsal striatum responses to reward and punishment: effects of valence and magnitude manipulations.

The goal of this research was to further our understanding of how the striatum responds to the delivery of affective feedback. Previously, we had found that the striatum showed a pattern of sustained activation after presentation of a monetary reward, in contrast to a decrease in the hemodynamic response after a punishment. In this study, we tested whether the activity of the striatum could be modulated by parametric variations in the amount of financial reward or punishment. We used an event-related fMRI design in which participants received large or small monetary rewards or punishments after performance in a gambling task. A parametric ordering of conditions was observed in the dorsal striatum according to both magnitude and valence. In addition, an early response to the presentation of feedback was observed and replicated in a second experiment with increased temporal resolution. This study further implicates the dorsal striatum as an integral component of a reward circuitry responsible for the control of motivated behavior, serving to code for such feedback properties as valence and magnitude.

Anterior cingulate cortex, conflict monitoring, and levels of processing.

It has been hypothesized that the anterior cingulate cortex (ACC) contributes to cognition by detecting conflicts that might occur during information processing, to signal the need to engage top-down attentional processes. The present study was designed to investigate which levels of processing are being monitored by the ACC for the presence of conflict. Event-related fMRI was used to measure the response of the ACC during an interference task in which distracting information could be congruent, conflicting at the level of stimulus identification, or conflicting at the response level. Although both types of conflict caused reaction time interference, the fMRI data showed that the ACC is responsive only to response conflict, even when controlling for reaction times. These results suggest a highly specific contribution of the ACC to executive functions, through the detection of conflicts occurring at later or response-related levels of processing.

Anterior cingulate cortex activity and impaired self-monitoring of performance in patients with schizophrenia: an event-related fMRI study.

OBJECTIVE: The authors examined brain activity associated with the internal monitoring of performance to test the hypothesis that error-related activity in the anterior cingulate cortex is impaired in patients with schizophrenia. METHOD: Seventeen patients with schizophrenia and 16 healthy comparison subjects underwent event-related functional magnetic resonance imaging during a continuous performance task; stimulus degradation was used to increase error rates. RESULTS: Comparison subjects, but not schizophrenic patients, showed error-related activity in the anterior cingulate cortex, and this difference in brain activity was significantly different across the two groups. Patients also showed less slowing of reaction time after error commission. CONCLUSIONS: Lower error-related activity in the anterior cingulate cortex and less performance adjustment after error commission are consistent with the hypothesis that disturbances in anterior cingulate cortex function are related to a specific alteration in an evaluative component of executive functioning-the internal monitoring of performance.

The role of prefrontal cortex and posterior parietal cortex in task switching.

Human ability to switch from one cognitive task to another involves both endogenous preparation without an external stimulus and exogenous adjustment in response to the external stimulus. In an event-related functional MRI study, participants performed pairs of two tasks that are either the same (task repetition) or different (task switch) from each other. On half of the trials, foreknowledge about task repetition or task switch was available. On the other half, it was not. Endogenous preparation seems to involve lateral prefrontal cortex (BA 46/45) and posterior parietal cortex (BA 40). During preparation, higher activation increases in inferior lateral prefrontal cortex and superior posterior parietal cortex were associated with foreknowledge than with no foreknowledge. Exogenous adjustment seems to involve superior prefrontal cortex (BA 8) and posterior parietal cortex (BA 39/40) in general. During a task switch with no foreknowledge, activations in these areas were relatively higher than during a task repetition with no foreknowledge. These results suggest that endogenous preparation and exogenous adjustment for a task switch may be independent processes involving different brain areas.

Three-dimensional tailored RF pulses for the reduction of susceptibility artifacts in T(*)(2)-weighted functional MRI.

A three-dimensional tailored RF pulse method for reducing intravoxel dephasing artifacts in T *(2)-weighted functional MRI is presented. A stack of spirals k-space trajectory is employed to excite a disk of magnetization for small tip angles. Smaller disks with a linear through-plane phase are inserted into the disk to locally refocus regions which are normally dephased due to susceptibility variations. Numerical simulations and imaging experiments which use the tailored RF pulses are presented. Limitations of the method and improvements are also discussed.

Dissociating the role of the dorsolateral prefrontal and anterior cingulate cortex in cognitive control.

Theories of the regulation of cognition suggest a system with two necessary components: one to implement control and another to monitor performance and signal when adjustments in control are needed. Event-related functional magnetic resonance imaging and a task-switching version of the Stroop task were used to examine whether these components of cognitive control have distinct neural bases in the human brain. A double dissociation was found. During task preparation, the left dorsolateral prefrontal cortex (Brodmann's area 9) was more active for color naming than for word reading, consistent with a role in the implementation of control. In contrast, the anterior cingulate cortex (Brodmann's areas 24 and 32) was more active when responding to incongruent stimuli, consistent with a role in performance monitoring.

Parsing executive processes: strategic vs. evaluative functions of the anterior cingulate cortex.

Event-related functional MRI and a version of the Stroop color naming task were used to test two conflicting theories of anterior cingulate cortex (ACC) function during executive processes of cognition. A response-related increase in ACC activity was present when strategic processes were less engaged, and conflict high, but not when strategic processes were engaged and conflict reduced. This is inconsistent with the widely held view that the ACC implements strategic processes to reduce cognitive conflicts, such as response competition. Instead, it suggests that the ACC serves an evaluative function, detecting cognitive states such as response competition, which may lead to poor performance, and representing the knowledge that strategic processes need to be engaged.

Complementary category learning systems identified using event-related functional MRI.

Event-related fMRI was used to dissociate the neural systems involved in category learning with and without awareness. Ten subjects performed a speeded response category learning task. Functional MR images were acquired during both explicit and implicit learning conditions. Behavioral data showed evidence of learning in both conditions. Functional imaging data showed different activation patterns in implicit and explicit trials. Decreased activation in extrastriate region V3 was found with implicit learning, and increased activation in V3, the medial temporal lobe, and frontal regions were found with explicit learning. These results support the theory that implicit and explicit learning utilize dissociable neural systems. Moreover, in both the implicit and explicit conditions a similar pattern of decreased activation was found in parietal regions. This commonality suggests that these dissociable systems also operate in parallel.

3D spiral cardiac/respiratory ordered fMRI data acquisition at 3 Tesla.

Three-dimensional (3D), multi-shot functional magnetic resonance imaging (fMRI) data acquisitions are desirable because of higher resolution and reduced susceptibility artifacts, due to shorter readouts and thinner slices. However, 3D multi-shot techniques are more susceptible to physiological noise, which can increase inter-image variance and lead to inaccurate assessment of activation. This work presents a 3D spiral fMRI data acquisition method at 3 T in which the acquisition of views was ordered to match the phase of either the respiratory or the cardiac cycle. For the acquisition timing parameters used in this work, cardiac ordering was found to reduce inter-image variance by 19%. Cardiac ordered data acquisitions showed the same reduction in variance as sequentially ordered data with cardiac contributions estimated and removed using an externally acquired reference prior to reconstruction. Respiratory ordering showed no reduction in fluctuation noise due to poor alignment of views to the respiratory phase.

Partial Fourier reconstruction for three-dimensional gradient echo functional MRI: comparison of phase correction methods.

Partial Fourier (PF) methods take advantage of data symmetry to allow for either faster image acquisition or increased image resolution. Faster acquisition and increased spatial resolution are advantageous for fMRI because of increased temporal resolution and/or reduced partial volume effects, respectively. Standard PF methods, which use a phase reference obtained from a low resolution image, are adequate for the reconstruction of time-stationary images acquired using either spin echoes or short TE gradient echoes. In fMRI, however, multiple images are acquired using long TE gradient echoes, which introduces possible phase drifts in the fMRI data and high spatial frequencies in the phase reference. This work investigates several techniques developed to reconstruct fMRI data obtained with PF acquisitions. PF methods that account for both high-frequency spatial variations and time-dependent drifts in the phase reference are discussed and are quantitatively evaluated using receiver operator characteristic curve analysis.