Impact of chiasma opticum malformations on the organization of the human ventral visual cortex.

Congenital malformations of the optic chiasm, such as enhanced and reduced crossing of the optic nerve fibers, are evident in albinism and achiasma, respectively. In early visual cortex the resulting additional visual input from the ipsilateral visual hemifield is superimposed onto the normal retinotopic representation of the contralateral visual field, which is likely due to conservative geniculo-striate projections. Counterintuitively, this organization in early visual cortex does not have profound consequences on visual function. Here we ask, whether higher stages of visual processing provide a correction to the abnormal representation allowing for largely normal perception. To this end we assessed the organization patterns of early and ventral visual cortex in five albinotic, one achiasmic, and five control participants. In albinism and achiasma the mirror-symmetrical superposition of the ipsilateral and contalateral visual fields was evident not only in early visual cortex, but also in the higher areas of the ventral processing stream. Specifically, in the visual areas VO1/2 and PHC1/2 no differences in the extent, the degree of superposition, and the magnitude of the responses were evident in comparison to the early visual areas. Consequently, the highly atypical organization of the primary visual cortex was propagated downstream to highly specialized processing stages in an undiminished and unchanged manner. This indicates largely unaltered cortico-cortical connections in both types of misrouting, i.e., enhanced and reduced crossing of the optic nerves. It is concluded that main aspects of visual function are preserved despite sizable representation abnormalities in the ventral visual processing stream.

Ultra-high field MRI for primate imaging using the travelling-wave concept.

OBJECT: Ultra-high field (UHF) neuroimaging is usually conducted with volume transmit (Tx) and phased array receive (Rx) coils, both tightly enclosing the object. The travelling-wave (TW) concept allows a remote excitation offering more flexible experimental setups. To investigate the feasibility of primate MRI in horizontal UHF MRI, we first compared the distribution of the electromagnetic fields in an oil phantom and then verified the concept with an in vivo experiment. MATERIALS AND METHODS: In the phantom experiments an in-house circularly polarized hybrid birdcage coil and a self-developed patch antenna were used for Tx and an eight-element phased array antenna for Rx. B1+ fields were calculated and measured for both approaches. For in vivo experiments the Rx part was replaced with an optimized three-element phased array head coil. The SAR was calculated using field simulation. RESULTS: In the phantom the field distribution was homogenous in a central volume of interest of about 10 cm diameter. The TW concept showed a slightly better homogeneity. Examination of a female crab-eating macaque led to homogeneous high-contrast images with a good delineation of anatomical details. CONCLUSION: The TW concept opens up a new approach for MRI of medium-sized animals in horizontal UHF scanners.

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.

Robust and fast whole brain mapping of the RF transmit field B1 at 7T.

In-vivo whole brain mapping of the radio frequency transmit field B(1) (+) is a key aspect of recent method developments in ultra high field MRI. We present an optimized method for fast and robust in-vivo whole-brain B(1) (+) mapping at 7T. The method is based on the acquisition of stimulated and spin echo 3D EPI images and was originally developed at 3T. We further optimized the method for use at 7T. Our optimization significantly improved the robustness of the method against large B(1) (+) deviations and off-resonance effects present at 7T. The mean accuracy and precision of the optimized method across the brain was high with a bias less than 2.6 percent unit (p.u.) and random error less than 0.7 p.u. respectively.

High resolution fMRI of subcortical regions during visual erotic stimulation at 7 T.

OBJECT: Involvement of distinct subcortical structures during sexual arousal was shown in animals and functional imaging studies gave coarse evidence for a similar organisation in humans. In contrast to previous imaging studies at lower field strengths, we tried to investigate activation in distinguishable subcortical structures at high spatial resolution during a short stimulating paradigm to further account for potential effects of attenuation or adaptation. MATERIALS AND METHODS: Seven healthy subjects were investigated using functional magnetic resonance imaging (fMRI) on a 7 T scanner. High resolution EPI images of 1.4 x 1.4 mm2 inplane resolution were acquired in a single functional session of 13.6 minutes. During the session erotic and non-erotic pictures were presented in an event-related design. RESULTS: In the unsmoothed data with preserved high spatial resolution significant effects were detected in relevant structures, including anterior caudate and mediodorsal thalamus. These effects were restricted to subcortical target structures and their anatomical boundaries. CONCLUSION: This study demonstrates that fMRI at high fields provides an ideal tool to investigate functional anatomy of subcortical structures. Due to an increased signal-to-noise ratio, functional scans of short duration can be acquired at high resolution without the need for further spatial smoothing.