A functional MRI study of face recognition in patients with prosopagnosia.

An fMRI investigation was conducted to determine whether patients with impaired face recognition, a deficit known as prosopagnosia, would show functional activation in the fusiform gyrus, the neural substrate for face processing, when viewing faces. While the patients did show activation in the fusiform gyrus, with significantly more voxels in posterior areas than their control subjects, this activation was not sufficient for face processing. In one of the patients, the posterior activation was particularly evident in the left hemisphere, which is thought to be involved in feature-based strategies of face perception. We conclude that an increased reliance on feature-based processing in prosopagnosia leads to a recruitment of neurons in posterior regions of the fusiform gyrus, regions that are not ideally suited for processing faces.

Maturation of widely distributed brain function subserves cognitive development.

Cognitive and brain maturational changes continue throughout late childhood and adolescence. During this time, increasing cognitive control over behavior enhances the voluntary suppression of reflexive/impulsive response tendencies. Recently, with the advent of functional MRI, it has become possible to characterize changes in brain activity during cognitive development. In order to investigate the cognitive and brain maturation subserving the ability to voluntarily suppress context-inappropriate behavior, we tested 8-30 year olds in an oculomotor response-suppression task. Behavioral results indicated that adult-like ability to inhibit prepotent responses matured gradually through childhood and adolescence. Functional MRI results indicated that brain activation in frontal, parietal, striatal, and thalamic regions increased progressively from childhood to adulthood. Prefrontal cortex was more active in adolescents than in children or adults; adults demonstrated greater activation in the lateral cerebellum than younger subjects. These results suggest that efficient top-down modulation of reflexive acts may not be fully developed until adulthood and provide evidence that maturation of function across widely distributed brain regions lays the groundwork for enhanced voluntary control of behavior during cognitive development.

A comparison of linear and nonlinear statistical techniques in performance attribution.

Performance attribution is usually conducted under the linear framework of multifactor models. Although commonly used by practitioners in finance, linear multifactor models are known to be less than satisfactory in many situations. After a brief survey of nonlinear methods, nonlinear statistical techniques are applied to performance attribution of a portfolio constructed from a fixed universe of stocks using factors derived from some commonly used cross sectional linear multifactor models. By rebalancing this portfolio monthly, the cumulative returns for procedures based on standard linear multifactor model and three nonlinear techniques-model selection, additive models, and neural networks-are calculated and compared. It is found that the first two nonlinear techniques, especially in combination, outperform the standard linear model. The results in the neural-network case are inconclusive because of the great variety of possible models. Although these methods are more complicated and may require some tuning, toolboxes are developed and suggestions on calibration are proposed. This paper demonstrates the usefulness of modern nonlinear statistical techniques in performance attribution.

Cortical networks subserving pursuit and saccadic eye movements in humans: an FMRI study.

High-field (3 Tesla) functional magnetic resonance imaging (MRI) was used to investigate the cortical circuitry subserving pursuit tracking in humans and compare it to that for saccadic eye movements. Pursuit performance, relative to visual fixation, elicited activation in three areas known to contribute to eye movements in humans and in nonhuman primates: the frontal eye field, supplementary eye field, and intraparietal sulcus. It also activated three medial regions not previously identified in human neuroimaging studies of pursuit: the precuneus and the anterior and posterior cingulate cortices. All six areas were also activated during saccades. The spatial extent of activation was similar for saccades and pursuit in all but two regions: spatial extent was greater for saccades in the superior branch of the frontal eye field and greater for pursuit in posterior cingulate cortex. This set of activations for smooth pursuit parallels the network of oculomotor areas characterized in nonhuman primates and complements recent studies showing that common cortical networks subserve oculomotor functions and spatial attention in humans.

Evaluation of respiratory artifact correction techniques in multishot spiral functional MRI using receiver operator characteristic analyses.

Navigator corrections and low-spatial frequency (LSF) oversampling are investigated as methods for reducing respiration-related effects in multishot functional MRI. Both techniques take advantage of the smoothly varying or nearly constant phase variations linked to the respiration cycle. These techniques were tested in functional MRI studies with spiral k-space acquisitions. Receiver operator characteristic (ROC) analyses and the temporal variance averaged across the brain were used to evaluate their effectiveness. Both methods were found to increase the area under the ROC curve and to reduce the standard deviation, with the LSF oversampling method being more effective.

Dorsal cortical regions subserving visually guided saccades in humans: an fMRI study.

Neurophysiological studies in non-human primates have identified saccade-related neuronal activity in cortical regions including frontal (FEF), supplementary (SEF) and parietal eye fields. Lesion and neuroimaging studies suggest a generally homologous mapping of the oculomotor system in humans; however, a detailed mapping of the precise anatomical location of these functional regions has not yet been achieved. We investigated dorsal frontal and parietal cortex during a saccade task vs. central fixation in 10 adult subjects using functional magnetic resonance imaging (fMRI). The FEF were restricted to the precentral sulcus, and did not extend anteriorly into Brodmann area 8, which has traditionally been viewed as their location in humans. The SEF were located in cortex along the interhemispheric fissure and extended minimally onto the dorsal cortical surface. Parietal activation was seen in precuneus and along the intraparietal sulcus, extending into both superior and inferior parietal lobules. These findings localize areas in frontal and parietal cortex involved in saccade generation in humans, and indicate significant differences from the macaque monkey in both frontal and parietal cortex. These differences may have functional implications for the roles these areas play in visuomotor processes.

Estimating test-retest reliability in functional MR imaging. I: Statistical methodology.

A common problem in the analysis of functional magnetic resonance imaging (fMRI) data is quantifying the statistical reliability of an estimated activation map. While visual comparison of the classified active regions across replications of an experiment can sometimes by informative, it is typically difficult to draw firm conclusions by inspection; noise and complex patterns in the estimated map make it easy to be misled. Here, several statistical models, of increasing complexity, are developed, under which "test-retest" reliability can be meaningfully defined and quantified. The method yields global measures of reliability that apply uniformly to a specified set of brain voxels. The estimates of these reliability measures and their associated uncertainties under these models can be used to compare statistical methods, to set thresholds for detecting activation, and to optimize the number of images that need to be acquired during an experiment.

Estimating test-retest reliability in functional MR imaging. II: Application to motor and cognitive activation studies.

Functional magnetic resonance imaging (fMRI) using blood oxygenation contrast has rapidly spread into many application areas. In this paper, a new statistical model is used to evaluate the reliability of fMRI activation in a finger opposition motor paradigm for both within-session and between-session data and in a working memory paradigm for between-session data. A slice prescription procedure for between-session reproducibility is introduced. Estimates are made for the probabilities of correctly and falsely classifying voxels as active or inactive and receiver operator characteristic curves are generated. In the motor paradigm, estimated between-session reliability was found to be somewhat reduced relative to within-session reliability; however, this includes additional sources of variation and may not reflect intrinsically lower reliability. After matching false-positive classification probabilities, between-session reliability was found to be nearly identical for both motor and cognitive activation paradigms.