Design and implementation of embedded 8-channel receive-only arrays for whole-brain MRI and fMRI of conscious awake marmosets.

PURPOSE: The common marmoset (Callithrix jacchus) is a New World primate of increasing interest to neuroscience and in translational brain research. The present work describes the design and implementation of individualized 8-channel receive-only radiofrequency (RF) coil arrays that provide whole-brain coverage and allow anatomical and functional MRI experiments in conscious, awake marmosets. METHODS: The coil arrays were designed with their elements embedded inside individualized restraint helmets. The size, geometry, and arrangement of the coil elements were optimized to allow whole-brain coverage. Coil-to-coil decoupling was achieved by a combination of geometric decoupling and low input impedance preamplifiers. The performance of the embedded arrays was compared against that of one 8-channel receive-only array built to fit the external surface of the helmets. RESULTS: Three individualized helmets with embedded coil arrays were built for three marmosets. Whole-brain coverage was achieved with high sensitivity extending over the entire cortex. Visual stimulation of conscious awake marmosets elicited robust BOLD fMRI responses in both primary and higher order visual areas of the occipitotemporal cortex. CONCLUSION: The high sensitivity provided by embedded receive-only coil arrays allows both anatomical and functional MRI data to be obtained with high spatial resolution in conscious, awake marmosets. Magn Reson Med 78:387-398, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Functional MRI of visual responses in the awake, behaving marmoset.

The visual brain is composed of interconnected subcortical and cortical structures that receive and process image information originating in the retina. The visual system of nonhuman primates, in particular macaques, has been studied in great detail in order to elucidate principles of human sensation and perception. The common marmoset (Callithrix jacchus) is a small New World monkey of growing interest as a primate model for neuroscience. Marmosets have advantages over macaques because of their small size, lissencephalic cortex, and growing potential for viral and genetic manipulations. Previous anatomical studies and electrophysiological recordings in anesthetized marmosets have shown that this species' cortical visual hierarchy closely resembles that of other primates, including humans. Until now, however, there have been no attempts to systematically study visual responses throughout the marmoset brain using fMRI. Here we show that awake marmosets readily learn to carry out a simple visual task inside the bore of an MRI scanner during functional mapping experiments. Functional scanning at 500 µm in-plane resolution in a 30 cm horizontal bore at 7 T revealed robust positive blood oxygenation level-dependent (BOLD) fMRI responses to visual stimuli throughout visual cortex and associated subcortical areas. Nonvisual sensory areas showed negative contrasts to visual stimuli compared to the fixation dot only baseline. Structured images of objects and faces led to stronger responses than scrambled control images at stages beyond early visual cortex. Our study establishes fMRI mapping of visual responses in awake, behaving marmosets as a straightforward and valuable tool for assessing the functional organization of the primate brain at high resolution.

Functional mapping of face-selective regions in the extrastriate visual cortex of the marmoset.

The cerebral cortex of humans and macaques has specialized regions for processing faces and other visual stimulus categories. It is unknown whether a similar functional organization exists in New World monkeys, such as the common marmoset (Callithrix jacchus), a species of growing interest as a primate model in neuroscience. To address this question, we measured selective neural responses in the brain of four awake marmosets trained to fix their gaze upon images of faces, bodies, objects, and control patterns. In two of the subjects, we measured high gamma-range field potentials from electrocorticography arrays implanted over a large portion of the occipital and inferotemporal cortex. In the other two subjects, we measured BOLD fMRI responses across the entire brain. Both techniques revealed robust, regionally specific patterns of category-selective neural responses. We report that at least six face-selective patches mark the occipitotemporal pathway of the marmoset, with the most anterior patches showing the strongest preference for faces over other stimuli. The similar appearance of these patches to previous findings in macaques and humans, including their apparent arrangement in two parallel pathways, suggests that core elements of the face processing network were present in the common anthropoid primate ancestor living ∼35 million years ago. The findings also identify the marmoset as a viable animal model system for studying specialized neural mechanisms related to high-level social visual perception in humans.

Dendritic spine density, morphology, and fibrillar actin content surrounding amyloid-ß plaques in a mouse model of amyloid-ß deposition.

Dendritic spines are the site of most excitatory synapses, the loss of which correlates with cognitive impairment in patients with Alzheimer disease. Substantial evidence indicates that amyloid-ß (Aß) peptide, either insoluble fibrillar Aß deposited into plaques or soluble nonfibrillar Aß species, can cause spine loss but the concurrent contributions of fibrillar Aß and nonfibrillar Aß to spine loss has not been previously assessed. We used multiple-label immunohistochemistry to measure spine density, size, and F-actin content surrounding plaques in the cerebral cortex in the PSAPP mouse model of Aß deposition. Our approach allowed us to measure fibrillar Aß plaque content and an index of nonfibrillar Aß species concurrently. We found that spine density was reduced within 6 µm of the plaque perimeter, remaining spines were more compact, and F-actin content per spine was increased. Measures of fibrillar Aß plaque content were associated with reduced spine density near plaques, whereas measures of nonfibrillar Aß species were associated with reduced spine density and size but not altered F-actin content. These findings suggest that strategies to preserve dendritic spines in AD patients may need to address both nonfibrillar and fibrillar forms of Aß and that nonfibrillar Aß may exert spine toxicity through pathways not mediated by depolymerization of F-actin.