Functional heterogeneity within Broca's area during verbal working memory.

In previous fMRI research, we found that two subregions of the left inferior frontal cortex showed distinct patterns of activity during a verbal working memory task. Specifically, a more dorsal region tracked with performance, while a more ventral region was sensitive to lexical status. To test the veracity of this finding, we developed a new method for meta-analysis of neuroimaging results. In this method, Gaussian probability distributions are formed around stereotaxic coordinates obtained from published neuroimaging studies. These probability distributions are then combined to identify regions of convergence across studies. When this method was applied to prior studies of working memory, the results largely paralleled those from earlier reviews of the literature on working memory, but also confirmed our empirical findings showing distinct foci within Broca's area. Further application of this meta-analytic technique substantiated the dissociation in these regions for performance and sublexical processing. These results help to validate a novel approach for meta-analysis of neuroimaging findings that avoids many of the subjective assumptions involved in alternative approaches.

Conflict monitoring versus selection-for-action in anterior cingulate cortex.

The anterior cingulate cortex (ACC), on the medial surface of the frontal lobes of the brain, is widely believed to be involved in the regulation of attention. Beyond this, however, its specific contribution to cognition remains uncertain. One influential theory has interpreted activation within the ACC as reflecting 'selection-for-action', a set of processes that guide the selection of environmental objects as triggers of or targets for action. We have proposed an alternative hypothesis, in which the ACC serves not to exert top-down attentional control but instead to detect and signal the occurrence of conflicts in information processing. Here, to test this theory against the selection-for-action theory, we used functional magnetic resonance imaging to measure brain activation during performance of a task where, for a particular subset of trials, the strength of selection-for-action is inversely related to the degree of response conflict. Activity within the ACC was greater during trials featuring high levels of conflict (and weak selection-for-action) than during trials with low levels of conflict (and strong selection-for-action), providing evidence in favour of the conflict-monitoring account of ACC function.

A comparison of the von Mises and Gaussian basis functions for approximating spherical acoustic scatter.

This paper compares the approximation accuracy of two basis functions that share a common radial basis function (RBF) neural network architecture used for approximating a known function on the unit sphere. The basis function types considered are that of a new spherical basis function, the von Mises function, and the now well-known Gaussian basis function. Gradient descent learning rules were applied to optimize (learn) the solution for both approximating basis functions. A benchmark approximation problem was used to compare the performance of the two types of basis functions, in this case the mathematical expression for the scattering of an acoustic wave striking a rigid sphere.