Hih-speed magnetic resonance (MR) imaging was used to detect activation in the human prefrontal cortex induced by a spatial working memory task modeled on those used to elucidate neuronal circuits in nonhuman primates. Subjects were required to judge whether the location occupied by the current stimulus had been occupied previously over a sequence of 14 or 15 stimuli presented in various locations. Control tasks were similar in all essential respects, except that the subject's task was to detect when one of the stimuli presented was colored red (color detection) or when a dot briefly appeared within the stimulus (dot detection). In all tasks, two to three target events occurred randomly. The MR signal increased in an area of the middle frontal gyrus corresponding to Brodmann's area 46 in all eight subjects performing the spatial working memory task. Right hemisphere activation was greater and more consistent than left. The MR signal change occurred within 6-9 sec of task onset and declined within a similar period after task completion. An increase in MR signal was also noted in the control tasks, but the magnitude of change was less than that recorded in the working memory task. These differences were replicated when testing was repeated in five of the original subjects. The localization of spatial working memory function in humans to a circumscribed area of the middle frontal gyrus supports the compartmentalization of working memory functions in the human prefrontal cortex and the localization ofspatial memory processes to comparable areas in humans and nonhuman primates.Working memory (1) provides temporary storage of information for cognitive capacities including comprehension and reasoning. The neural substrates for working memory have been elucidated in studies of nonhuman primates (2). Spatial working memory tasks requiring the brief storage of spatial information throughout the visual field engage neurons in the principal sulcus in the mid-dorsolateral cortex of rhesus monkeys (3, 4). Individual neurons in this region increase their discharge for particular target locations as monkeys recall the position of a previous stimulus (memory fields) (5). The same region is innervated by cortico-cortical projections from the posterior parietal cortex (6), where spatial perceptions are consolidated (7). These studies suggest that areas of the dorsolateral prefrontal cortex, by virtue of their extrinsic connections and physiological properties, are specialized for processing of spatial information in working memory.Positron emission tomography (PET) has recently been used to study working memory in humans. Petrides et al. (8) found that object working memory using pictures activated a region of the right middle frontal gyrus, including Brod-mann's area 9 and 46 and that these regions were activated bilaterally by a verbal working memory task (9). On the other hand, Paulescu et al. (10) found more posterior frontal activation in Broca's area (area 44) and parietal cortex (area 40) in a verbal working memor...

A new scheme is proposed to edit the 3.0 ppm GABA resonance without macromolecule (MM) contamination. Like previous difference spectroscopy approaches, the new scheme manipulates J‐modulation of this signal using a selective editing pulse. The elimination of undesirable MM contribution at 3.0 ppm is obtained by applying this pulse symmetrically about the J‐coupled MM resonance, at 1.7 ppm, in the two steps of the editing scheme. The effectiveness of the method is demonstrated in vitro, using lysine to mimic MM, and in vivo. As compared to the most commonly used editing scheme, which necessitates the acquisition and processing of two distinct difference spectroscopy experiments, the new scheme offers a reduction in experimental time (–33%) and an increase in accuracy. Magn Reson Med 45:517–520, 2001. © 2001 Wiley‐Liss, Inc.

The purpose of this work was to investigate the temporal relationship between intensity changes in T2*-weighted NMR images and tissue oxygen content, measured by myoglobin proton NMR spectroscopy, in the skeletal muscle. During an ischemic stress test, the calf muscles of five healthy volunteers were studied at 3 Tesla. An interleaved NMRI-NMRS sequence was used, which made it possible to record T2*-weighted images and myoglobin spectra simultaneously. During ischemia, rapid changes in muscle signal intensity were observed on T2*-weighted images, which immediately preceded myoglobin desaturation. Bearing in mind the respective P50 of hemoglobin and myoglobin, this observation clearly favored the hypothesis that hemoglobin desaturation was responsible for the changes in T2*. This interpretation was further supported by the temporal coincidence between the experimental NMR data and a model of hemoglobin desaturation solely derived from physiological considerations.

The stability of the main magnetic field is critical for prolonged in vivo magnetic resonance spectroscopy (MRS) acquisitions, especially for difference spectroscopy. This study was focused on the implementation and optimization of a field-frequency lock (FFL) on a whole body spectrometer, to correct the main field drift during localized proton MRS of the human brain. The FFL was achieved through a negative feed-back applied in real time on the Z0 shim coil current, after calculation of the frequency shift from a reference signal. This signal was obtained from the whole head with a small flip angle acquisition interleaved with the PRESS acquisition of interest. To avoid propagation of the important short-term time-correlated fluctuations of the head water frequency (mainly due to respiratory motion) onto Z0 correction, the sampling rate of the reference frequency and the smoothing window for the Z0 correction were carefully optimized. Thus, an effective FFL was demonstrated in vivo with no significant increase of the short-term variance of the water frequency.