John Gore

P300 is an event-related potential elicited by infrequent target events whose amplitude is dependent on the context provided by the immediately preceding sequence of stimuli, suggesting its dependence on working memory. We employed magnetic resonance imaging sequences sensitive to blood oxygenation level to identify regional changes evoked by infrequent visual target stimuli presented in a task typically used to elicit P300. Targets evoked transient event-related activation bilaterally in the middle frontal gyrus, in the inferior parietal lobe, and near the inferior aspect of the posterior cingulate gyrus beginning within 1.5 s of target onset and peaking between 4.5 and 6 s. These regions have been identified in previous neuroimaging studies in humans, and in single-unit recordings in monkeys, as components of a neural system mediating working memory, which suggests that this system may be activated by the same events that evoke P300.

P300 is an event-related potential elicited by infrequent target events whose amplitude is dependent on the context provided by the immediately preceding sequence of stimuli, suggesting its dependence on working memory. We employed magnetic resonance imaging sequences sensitive to blood oxygenation level to identify regional changes evoked by infrequent visual target stimuli presented in a task typically used to elicit P300. Targets evoked transient event-related activation bilaterally in the middle frontal gyrus, in the inferior parietal lobe, and near the inferior aspect of the posterior cingulate gyrus beginning within 1.5 s of target onset and peaking between 4.5 and 6 s. These regions have been identified in previous neuroimaging studies in humans, and in single-unit recordings in monkeys, as components of a neural system mediating working memory, which suggests that this system may be activated by the same events that evoke P300.

The importance to MR image quality of the order of acquisition of different phaseencoded views with sequences that have variable TR and TE has been recently reported. It has been shown that the effective point spread function (PSF) may be manipulated by varying TE or TR, or both, with each phase-encoding step. This paper explores the behavior of the PSF in a variable TE sequence and its dependence on both imaging and tissue parameters. It is shown that the PSF is different for each tissue type and that its effect on tissue contrast is a function of both the shape and size of the structure. The important problem of signal loss from small objects that arises when the effective PSF is broad and the difficulty in detecting this phenomenon in practical MR images is illustrated. It is shown that the PSF can produce significant blurring and loss of object contrast in fast spin-echo images but that this blurring may be not be obvious in practice because the noise is unaffected by the PSF. It is also shown that the signal from small lesions with short T2 can easily be lost through this blurring mechanism. The importance of signal loss from small objects and its implication for the clinical use of such sequences as fast spinecho or rapid acquisition relaxation-enhanced and echo planar imaging is stressed.© 1992 Academic Press,Inc.

For diffusion-weighted magnetic resonance imaging and under circumstances where patient movement can be modeled as rigid body motion, it is shown both theoretically and experimentally that translations and rotations produce phase errors which are zero- and first-order, respectively, in position. Whlile a navigator echo can be used to correct the imaging data for arbitrary translations, only when the diffusion gradient is applied in the phase encode direction is there sufficient information to correct for rotations around all axes, and therefore for general rigid body motion. Experiments in test objects and human brain imaging confirm theoretical predictions and demonstrate that appropriate corrections dramatically improve image quality in vivo.

We sought to determine whether regions of extrastriate visual cortex could be activated in subjects viewing eye and mouth movements that occurred within a stationary face. Eleven subjects participated in three to five functional magnetic resonance imaging sessions in which they viewed moving eyes, moving mouths, or movements of check patterns that occurred in the same spatial location as the eyes or mouth. In each task, the stimuli were superimposed on a radial background pattern that continually moved inward to control for the effect of movement per se. Activation evoked by the radial background was assessed in a separate control task. Moving eyes and mouths activated a bilateral region centered in the posterior superior temporal sulcus (STS). The moving check patterns did not appreciably activate the STS or surrounding regions. The activation by moving eyes and mouths was distinct from that elicited by the moving radial background, which primarily activated the posterior-temporal-occipital fossa and the lateral occipital sulcus-a region corresponding to area MT/V5. Area MT/V5 was also strongly activated by moving eyes and to a lesser extent by other moving stimuli. These results suggest that a superior temporal region centered in the STS is preferentially involved in the perception of gaze direction and mouth movements. This region of the STS may be functionally related to nearby superior temporal regions thought to be involved in lip-reading and in the perception of hand and body movement.

Functional connectivity among brain regions has been investigated via an analysis of correlations between regional signal fluctuations recorded in magnetic resonance (MR) images obtained in a steady state. In comparison with studies of functional connectivity that utilize task manipulations, the analysis of correlations in steady state data is less susceptible to confounds arising when functionally unrelated brain regions respond in similar ways to changes in task. A new approach to identifying interregional correlations in steady state data makes use of two independent data sets. Regions of interest (ROIs) are defined and hypotheses regarding their connectivity are generated in one data set. The connectivity hypotheses are then evaluated in the remaining (independent) data set by analyzing low frequency temporal correlations between regions. The roles of the two data sets are then reversed and the process repeated, perhaps multiple times. This method was illustrated by application to the language system. The existence of a functional connection between Broca's area and Wernicke's area was confirmed in healthy subjects at rest. An increase in this functional connection when the language system was actively engaged (when subjects were continuously listening to narrative text) was also confirmed. In a second iteration of analyses, a correlation between Broca's area and a region in left premotor cortex was found to be significant at rest and to increase during continuous listening. These findings suggest that the proposed methodology can reveal the presence and strength of functional connections in high-level cognitive systems. Hum. Brain Mapping 15:247–262, 2002. © 2002 Wiley-Liss, Inc.

Functional magnetic resonance (MR) imaging was performed using a 1.5-tesla MR system to localize sensorimotor cortex. Six neurologically normal subjects were studied by means of axial gradient-echo images with a motor task and one or more sensory tasks: 1) electrical stimulation of the median nerve; 2) continuous brushing over the thenar region; and 3) pulsed flow of compressed air over the palm and digits. An increased MR signal was observed in or near the central sulcus, consistent with the location of primary sensory and motor cortex. Four patients were studied using echo planar imaging sequences and motor and sensory tasks. Three patients had focal refractory seizures secondary to a lesion impinging on sensorimotor cortex. Activation seen on functional MR imaging was coextensive with the location of the sensorimotor area determined by evoked potentials and electrical stimulation. Functional MR imaging provides a useful noninvasive method of localization and functional assessment of sensorimotor cortex.

A much debated question is whether sex differences exist in the functional organization of the brain for language. A long-held hypothesis posits that language functions are more likely to be highly lateralized in males and to be represented in both cerebral hemispheres in females, but attempts to demonstrate this have been inconclusive. Here we use echo-planar functional magnetic resonance imaging to study 38 right-handed subjects (19 males and 19 females) during orthographic (letter recognition), phonological (rhyme) and semantic (semantic category) tasks. During phonological tasks, brain activation in males is lateralized to the left inferior frontal gyrus regions; in females the pattern of activation is very different, engaging more diffuse neural systems that involve both the left and right inferior frontal gyrus. Our data provide clear evidence for a sex difference in the functional organization of the brain for language and indicate that these variations exist at the level of phonological processing.