Martin Kanowski

Audiovisual Temporal Correspondence Modulates Human Multisensory Superior Temporal Sulcus Plus Primary Sensory Cortices

The brain should integrate related but not unrelated information from different senses. Temporal patterning of inputs to different modalities may provide critical information about whether those inputs are related or not. We studied effects of temporal correspondence between auditory and visual streams on human brain activity with functional magnetic resonance imaging (fMRI). Streams of visual flashes with irregularly jittered, arrhythmic timing could appear on right or left, with or without a stream of auditory tones that coincided perfectly when present (highly unlikely by chance), were noncoincident with vision (different erratic, arrhythmic pattern with same temporal statistics), or an auditory stream appeared alone. fMRI revealed blood oxygenation level-dependent (BOLD) increases in multisensory superior temporal sulcus (mSTS), contralateral to a visual stream when coincident with an auditory stream, and BOLD decreases for noncoincidence relative to unisensory baselines. Contralateral primary visual cortex and auditory cortex were also affected by audiovisual temporal correspondence or noncorrespondence, as confirmed in individuals. Connectivity analyses indicated enhanced influence from mSTS on primary sensory areas, rather than vice versa, during audiovisual correspondence. Temporal correspondence between auditory and visual streams affects a network of both multisensory (mSTS) and sensory-specific areas in humans, including even primary visual and auditory cortex, with stronger responses for corresponding and thus related audiovisual inputs.

Eddy current correction in diffusion-weighted imaging using pairs of images acquired with opposite diffusion gradient polarity.

In echo‐planar‐based diffusion‐weighted imaging (DWI) and diffusion tensor imaging (DTI), the evaluation of diffusion parameters such as apparent diffusion coefficients and anisotropy indices is affected by image distortions that arise from residual eddy currents produced by the diffusion‐sensitizing gradients. Correction methods that coregister diffusion‐weighted and non‐diffusion‐weighted images suffer from the different contrast properties inherent in these image types. Here, a postprocessing correction scheme is introduced that makes use of the inverse characteristics of distortions generated by gradients with reversed polarity. In this approach, only diffusion‐weighted images with identical contrast are included for correction. That is, non‐diffusion‐weighted images are not needed as a reference for registration. Furthermore, the acquisition of an additional dataset with moderate diffusion‐weighting as suggested by Haselgrove and Moore (Magn Reson Med 1996;36:960–964) is not required. With phantom data it is shown that the theoretically expected symmetry of distortions is preserved in the images to a very high degree, demonstrating the practicality of the new method. Results from human brain images are also presented. Magn Reson Med 51:188–193, 2004.

Quantitation of simulated short echo time 1H human brain spectra by LCModel and AMARES.

LCModel and AMARES, two widely used quantitation tools for magnetic resonance spectroscopy (MRS) data, were employed to analyze simulated spectra similar to those typically obtained at short echo times (TEs) in the human brain at 1.5 T. The study focused mainly on the influence of signal‐to‐noise ratios (SNRs) and different linewidths on the accuracy and precision of the quantification results, and their effectiveness in accounting for the broad signal contribution of macromolecules and lipids (often called the baseline in in vivo MRS). When applied in their standard configuration (i.e., fitting a spline as a baseline for LCModel, and weighting the first data points for AMARES), both methods performed comparably but with their own characteristics. LCModel and AMARES quantitation benefited considerably from the incorporation of baseline information into the prior knowledge. However, the more accurate quantitation of the sum of glutamate and glutamine (Glx) favored the use of LCModel. Metabolite‐to‐creatine ratios estimated by LCModel with extended prior knowledge are more accurate than absolute concentrations, and are nearly independent of SNR and line broadening. Magn Reson Med 51:904–912, 2004.

How atypical is atypical language dominance

Atypical, right-hemisphere language dominance is poorly understood. It is often observed in patients with brain reorganization due to lesions early in life. It can also be encountered in seemingly normal individuals. We compared the patterns of neural language activation in 7 individuals with left- and 7 with right-hemisphere language dominance, none of whom had any evidence of brain lesions. We speculated that incongruencies in the activation patterns in atypical, right-hemisphere language dominance could indicate a reorganized neural language system after undetected early brain damage. Functional magnetic resonance imaging analysis of brain activation during phonetic word generation demonstrated (1). no increased activation in the subdominant hemisphere in right compared to left language dominance, (2). a similar variability in the pattern of activation in both groups, and (3). a mirror reverse pattern of activation in right- compared to left-hemisphere dominant subjects. These findings support the view that in individuals with an unrevealing medical history right-hemispheric dominance constitutes a natural rather than an abortive variant of language lateralization.

Contemporary ultrasound systems allow high-resolution transcranial imaging of small echogenic deep intracranial structures similarly as MRI: a phantom study.

Transcranial sonography (TCS) of small deep brain structures, such as substantia nigra and brainstem raphe, is increasingly used for assessment of neurodegenerative disorders. Still, there are reservations against TCS because of the smallness of evaluated structures and constraints on image resolution that is discussed to be lower compared to magnetic resonance imaging (MRI). To evaluate two different-generation TCS systems in visualizing fine intracranial structures, we studied image resolution on a phantom consisting of 0.80 mm x 1.05 mm regular meshwork of nylon threads embedded in a wet, gel-filled ex vivo human skull. Imaging was performed with a former-generation and a present-day clinical ultrasound system and for comparison with MRI. In axial direction of insonation both TCS systems resolved 0.80-mm and 1.05-mm thread-to-thread distance at depths between 55 and 120 mm using transmission frequencies > or =2.5 MHz. The meshwork, however, was recognizable as such only with the contemporary TCS system at depths between 60 and 85 mm due to its higher lateral resolution. MRI resolved the meshwork if image resolution was chosen sufficiently high but not if realistic clinical conditions were applied with its trade-offs between image SNR, resolution, total scan time, and unavoidable head motion during the latter. Hence, if the requirements for optimal TCS image resolution are fulfilled, i.e. sufficient acoustic bone window, increased echogenicity of target structure and its localization in a distance of maximum +/-15 mm from midsagittal plane, findings suggest that contemporary TCS systems achieve higher image resolution of intracranial structures in comparison not only to former-generation systems, but also to MRI under clinical conditions.

Plasticity and stability of the visual system in human achiasma.

The absence of the optic chiasm is an extraordinary and extreme abnormality in the nervous system. The abnormality produces highly atypical functional responses in the cortex, including overlapping hemifield representations and bilateral population receptive fields in both striate and extrastriate visual cortex. Even in the presence of these large functional abnormalities, the effect on visual perception and daily life is not easily detected. Here, we demonstrate that in two achiasmic humans the gross topography of the geniculostriate and occipital callosal connections remains largely unaltered. We conclude that visual function is preserved by reorganization of intracortical connections instead of large-scale reorganizations of the visual cortex. Thus, developmental mechanisms of local wiring within cortical maps compensate for the improper gross wiring to preserve function in human achiasma.

Retinotopic mapping of the human visual cortex at a magnetic field strength of 7 T

OBJECTIVE fMRI-based retinotopic mapping data obtained at a magnetic field strength of 7T are evaluated and compared to 3T acquisitions. METHODS With established techniques retinotopic mapping data were obtained in four subjects for 25 slices parallel to the calcarine sulcus at 7 and 3T for three voxel sizes (2.5(3), 1.4(3), and 1.1(3)mm(3)) and in two subjects for 49 slices at 7T for 2.5(3)mm(3) voxels. The data were projected to the flattened representation of T1 weighted images acquired at 3T. RESULTS The obtained retinotopic maps allowed for the identification of visual areas in the occipito-parietal cortex. The mean coherence increased with magnetic field strength and with voxel size. At 7T, the occipital cortex could be sampled with high sensitivity in a short single session at high resolution. Alternatively, at lower resolution simultaneous mapping of a great expanse of occipito-parietal cortex was possible. CONCLUSION Retinotopic mapping at 7T aids a detailed description of the visual areas. Here, recent findings of multiple stimulus-driven retinotopic maps along the intraparietal sulcus are supported. SIGNIFICANCE Retinotopic mapping at 7T opens the possibility to detail our understanding of the cortical visual field representations in general and of their plasticity in visual system pathologies.

Effect of creatine supplementation on metabolite levels in ALS motor cortices

Mitochondrial pathology is an early observation in motor neurons and skeletal muscle of patients with amyotrophic lateral sclerosis (ALS). To clarify the relevance of this finding, we determined the effects of a 1-month oral administration of creatine on (1)H NMR-visible metabolites in the motor cortices of 15 controls and 15 patients with sporadic ALS, most of whom had mitochondrial pathology in skeletal muscle. In the motor cortex of the ALS group the N-acetylaspartate (NAA)/creatine (Cr(t)) metabolite ratio was lower than in our control group, indicating NAA loss. Upon creatine supplementation we observed in the controls a decline in the NAA/Cr(t), NAA/choline (Cho), glutamate + glutamine (Glx)/Cr(t), and Glx/Cho metabolite ratios. In contrast, in the ALS patient group the NAA/Cr(t) and the NAA/Cho metabolite ratios remained unchanged, while the Glx/Cr(t) and Glx/Cho metabolite ratios decreased. These data are compatible with the interpretation that creatine supplementation causes an increase in the diminished NAA levels in ALS motor cortex as well as an increase of choline levels in both ALS and control motor cortices. Because NAA is synthesized by mitochondria in an energy-dependent manner and the NAA/Cho metabolite ratios in the ALS motor cortices were found to be correlated to the degree of mitochondrial pathology in ALS skeletal muscle, our results can be explained by a deficiency of enzymes of mitochondrial respiratory chain in the ALS motor cortex which might affect motor neuron survival.

Determining the hemispheric dominance of spatial attention: a comparison between fTCD and fMRI.

Human brain mapping allows the systematic assessment of interindividual differences in functional brain anatomy. Functional transcranial Doppler sonography (fTCD) is an imaging tool that allows for fast and mobile assessment of hemispheric lateralization of task‐related brain activation. It is ideal to screen large cohorts of subjects. The goal of the present study was to investigate whether fTCD and functional magnetic resonance imaging (fMRI) determine hemispheric lateralization of brain activation related to visuospatial attention concordantly. Used together, fMRI and fTCD may then open up a wide range of potential applications in neuroscience. Fifteen subjects were examined both with fTCD and fMRI while they judged accuracy of line bisections (Landmark task). For fTCD, the maximal mean difference in stimulus‐related relative cerebral blood flow velocity changes in the left and right middle cerebral arteries was assessed as the lateralization index LIfTCD. For fMRI, two approaches were used to determine hemispheric dominance. First, we measured brain activity as the extent of the activated region, i.e., the number of activated voxels above a statistical threshold. Second, we calculated the magnitude of the fMRI signal change between the activation and the control task within a region of interest. Results of fTCD and fMRI were concordant in every single case. Therefore, scanning large cohorts with fTCD for hemispheric dominance during Landmark task will provide results consistent with fMRI. FMRI can then be used for in depth assessment of the specific patterns of activation. Hum Brain Mapp 23:168–180, 2004.

Endoscopic eye tracking system for fMRI

Here we introduce a new video-based real-time eye tracking system suitable for functional magnetic resonance imaging (fMRI) application. The described system monitors the subjects eye, which is illuminated with infrared light, directly at the headcoil using an endoscopic fibre optical system. This endoscopic technique assures reliable, easy-to-use and fast adjustment. It requires only a minimal amount of equipment at the headcoil and inside the examination room. Moreover, the short distance between the image acquisition optics and the eye provides high spatial tracking resolution. Interference from physiological head movement is effectively reduced by simultaneous tracking of both eye and head movements.