Susan K. Lemieux

An fMRI investigation of covertly and overtly produced mono- and multisyllabic words.

Studies suggest that the left insula may play an important role in speech motor programming. We used functional magnetic resonance imaging to investigate the role of the left insula in the production of monosyllabic or multisyllabic words during overt and covert speech conditions. The left insula did not show a BOLD response for multisyllabic words (which should require more speech motor programming) that was different from that for monosyllabic words. Left parietal lobe regions showed a greater response for multisyllabic as compared to monosyllabic words. This is consistent with clinical studies showing that left parietal lobe lesions can produce a deficit in speech programming. Despite similarities, covert and overt speech did not demonstrate the same patterns of BOLD response. The BOLD response was greater during overt speech in areas that have been shown to play an important role in speech production including left premotor cortex/BA6, left primary motor cortex, left insula, and left superior temporal gyrus.

Simultaneous MRI and PET imaging of a rat brain.

Multi-modality imaging is rapidly becoming a valuable tool in the diagnosis of disease and in the development of new drugs. Functional images produced with PET fused with anatomical structure images created by MRI will allow the correlation of form with function. Our group is developing a system to acquire MRI and PET images contemporaneously. The prototype device consists of two opposed detector heads, operating in coincidence mode. Each MRI-PET detector module consists of an array of LSO detector elements coupled through a long fibre optic light guide to a single Hamamatsu flat panel position-sensitive photomultiplier tube (PSPMT). The use of light guides allows the PSPMTs to be positioned outside the bore of a 3T MRI scanner where the magnetic field is relatively small. To test the device, simultaneous MRI and PET images of the brain of a male Sprague Dawley rat injected with FDG were successfully obtained. The images revealed no noticeable artefacts in either image set. Future work includes the construction of a full ring PET scanner, improved light guides and construction of a specialized MRI coil to permit higher quality MRI imaging.

fMRI Measures of Perceptual Filling-in in the Human Visual Cortex

Filling-in refers to the tendency of stabilized retinal stimuli to fade and become replaced by their background. This phenomenon is a good example of central brain mechanisms that can selectively add or delete information to/from the retinal input. Importantly, such cortical mechanisms may overlap with those that are used more generally in visual perception. In order to identify cortical areas that contribute to perceptual filling-in, we used functional magnetic resonance imaging to image activity in the visual cortex while subjects experienced filling-in. Nine subjects viewed an achromatic disc with slightly higher luminance than the background and indicated the presence or absence of filling-in by a keypress. The disc was placed in either the upper or lower left quadrant. Similar high-contrast stimuli were used to map out the retinotopic representation of the disc. Unexpectedly, the lower-field high-contrast stimulus produced more parietal cortex activation than the upper-field condition, indicating preferential representation of the lower field by attentional control mechanisms. During perceptual filling-in, we observed significant contralateral reductions in activation in lower-tier retinotopic areas V1 and V2. In contrast, increased activation was consistently observed in visual areas V3A and V4v, higher-level cortex in the intraparietal sulcus, posterior superior temporal sulcus, and the ventral occipital–temporal region, as well as the pulvinar. The filling-in activation pattern was remarkably similar for both the upper- and lower-field conditions. Behaviorally, filling-in was reported to be easier for the lower visual field, and filling-in periods were longer for the lower than the upper quadrant. We suggest this behavioral asymmetry may be partially due to the preferential parietal representation of the lower field. The results lead us to propose that perceptual filling-in recruits high-level control mechanisms to reconcile competing percepts, and alters the normal image-related signals at the first stages of cortical processing. Moreover, the overall pattern of activation during filling-in resembles that seen in other studies of perceptually bistable stimuli, including binocular rivalry, indicating common control mechanisms.

Retinotopic organization in children measured with fMRI.

Many measures of visual function reach adult levels by about age 5, but some visual abilities continue to develop throughout adolescence. Little is known about the underlying functional anatomy of visual cortex in human infants or children. We used fMRI to measure the retinotopic organization of visual cortex in 15 children aged 7-12 years. Overall, we obtained adult-like patterns for most children tested. We found that significant head motion accounted for poor quality maps in a few tested children who were excluded from further analysis. When the maps from 10 children were compared with those obtained from 10 adults, the magnitude of retinotopic signals in visual areas V1, V2, V3, V3A, VP, and V4v was essentially the same between children and adults. Furthermore, one measure of intra-area organization, the cortical magnification function, did not significantly differ between adults and children for V1 or V2. However, quantitative analysis of visual area size revealed some significant differences beyond V1. Adults had larger extrastriate areas (V2, V3, VP, and V4v), when measured absolutely or as a proportion of the entire cortical sheet. We found that the extent and laterality of retinotopic signals beyond these classically defined areas, in parietal and lateral occipital cortex, showed some differences between adults and children. These data serve as a useful reference for studies of higher cognitive function in pediatric populations and for studies of children with vision disorders, such as amblyopia.

Investigation of muscle lipid metabolism by localized one- and two-dimensional MRS techniques using a clinical 3T MRI/MRS scanner

To demonstrate the feasibility of estimating the relative intra‐ and extramyocellular lipid (IMCL and EMCL) pool magnitudes and calculating the degree of lipid unsaturation within soleus muscle using single‐voxel localized one‐ and two‐dimensional (1D and 2D) MR spectroscopy (MRS).

Cerebellar volume in humans related to magnitude of classical conditioning

Neural circuits in the cerebellum are essential for eyeblink classical conditioning, and hippocampal activation is also present during acquisition. Anatomical (volumetric) brain MRI, delay eyeblink conditioning and neuropsychological tests were administered to eight healthy older subjects. The correlation between cerebellar volume (corrected for total cerebral volume) and conditioned response percentage was 0.81 (p < 0.02), but neither hippocampal nor total cerebral volume correlated with conditioning or any neuropsychological test scores. There was no relationship between age and cerebellar volume, but the correlation between hippocampal volume and age was −0.80 (p < 0.02). These volumetric results add to the increasing evidence in humans demonstrating a relationship between the integrity of the cerebellum and eyeblink classical conditioning.

Implementation and validation of localized constant-time correlated spectroscopy (LCT-COSY) on a clinical 3T MRI scanner for investigation of muscle metabolism.

To implement and evaluate a novel single‐volume two‐dimensional localized constant‐time‐based correlated spectroscopy (2D LCT‐COSY) sequence on a clinical 3T MR scanner. This sequence exhibits homonuclear decoupling along the F1 dimension, leading to improved spectral resolution compared to that of non‐constant‐time localized correlated spectroscopy (L–COSY).

Cholesterol Increases Ventricular Volume in a Rabbit Model of Alzheimer's Disease

One of the hallmarks of Alzheimers disease is a significant increase in ventricular volume. To date we and others have shown that a cholesterol-fed rabbit model of Alzheimers disease displays as many as fourteen different pathological markers of Alzheimers disease including amyloid-β accumulation, thioflavin-S staining, blood brain barrier breach, microglia activation, cerebrovasculature changes, and alterations in learning and memory. Using structural magnetic resonance imaging at 3T, we now report that cholesterol-fed rabbits also show a significant increase in ventricular volume following 10 weeks on a diet of 2% cholesterol. The increase in volume is attributable in large part to increases in the size of the third ventricle. These changes are accompanied by significant increases in the number of amyloid-β immuno-positive cells in the cortex and hippocampus. Increases in the number of amyloid-β neurons in the cortex also occurred with the addition of 0.24 ppm copper to the drinking water. Together with a list of other pathological markers, the current results add further validity to the value of the cholesterol-fed rabbit as a non-transgenic animal model of Alzheimers disease.

Anterior Medial Temporal Lobe Activation during Encoding of Words: FMRI Methods to Optimize Sensitivity.

The existence of a rostrocaudal gradient of medial temporal lobe (MTL) activation during memory encoding has historically received support from positron emission tomography studies, but less so from functional MRI (FMRI) studies. More recently, FMRI studies have demonstrated that characteristics of the stimuli can affect the location of activation seen in the MTL when those stimuli are encoded. The current study tested the hypothesis that MTL activation during memory encoding is related to the modality of stimulus presentation. Subjects encoded auditorily or visually presented words in an FMRI novelty paradigm. Imaging and analysis parameters were optimized to minimize susceptibility artifact in the anterior MTL. Greater activation was observed in the anterior than posterior MTL for both modalities of stimulus presentation. The results indicate that anterior MTL activation occurred during encoding, independent of stimulus modality and provide support for the hypothesis that verbal-semantic memory processing occurs in anterior MTL. The authors suggest that technical factors are critical for observing the rostrocaudal gradient in MTL memory activation.

A new atomic beam polarized ion source for the Triangle Universities Nuclear Laboratory: overview, operating experience, and performance

Abstract A newly constructed source of polarized H ± and D ± ions is described. Atomic H or D beams from a dissociator with a cooled nozzle enter a system of two sextupole magnets and several radio-frequency transitions where they are focused and polarized. They enter a downstream electron-cyclotron-resonance-heated plasma ionizer from which positive ions are extracted. When negative ions are desired, they may be produced from the positive beam by charge-exchange in cesium vapor. Emerging beams are intense, have good polarization, low energy spread, and good optical quality. Descriptions are included for all major systems and for diagnostic procedures used to optimize both the intensity and the polarization of the output H ± or D ± beams obtained. Typical operating experience, performance figures, and a description of routine maintenance procedures are given.