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Project
Topographical organization of object selective areas in the visual cortex of the macaque monkey.
In primates visual information is processed in multiple areas within the cerebral cortex. Although more than 30 higher-order visual cortical areas have been identified in non-human primates, much remains to be learned about their exact boundaries, internal organization, function, and connectivity patterns. The aim of this thesis was to investigate the detailed topographic organization of extra-striate visual cortex in non-humanprimates using (functional) magnetic resonance imaging (fMRI). During the last decade macaque fMRI has become an important tool to bridge the gap between high-resolution invasive techniques typically used in monkeysand the low-resolution non-invasive human imaging modalities. However, to investigate the fine-scale organization of the cerebral cortex several technical challenges, aimed at enhancing the spatial resolution of thefMRI experiments, need to be addressed. In the first part of this thesis novel phased-array coils for macaque anatomical and functional imagingare described. Afterwards, the phased-array coil technology was appliedto investigate corticalparcellation, primarily focusing on the internaltopographical organization (i.e. retinotopy) of visual areas. Furthermore, the correspondence with other parcellation criteria based on functional, anatomical, and connectivity-related properties is also investigated.
In a first experiment we developed and characterized a receive array coil with 22 channels for (anaesthetized) macaque MRI to improve the measurement sensitivity and to reduce the required scan time, in analogy with recent advances in human MR technology. With the developed 22-channel coil significant improvements in the sensitivity and acceleration of the data acquisition were indeed obtained throughout the brain. The applications for this novel receive technology range from high-resolution anatomical and diffusion-weighted imaging to functional experiments.
In a second experiment we also increased the sensitivity in awake macaque fMRI by reducing the distance between the receive coil and the brain. An 8-channel receive array was constructed and permanently implanted within the acrylic headpost assembly on top of the skullof the monkey. In vivo measurements showed substantial improvements in the sensitivity for the implanted versus the external coils. These results suggest that a stable implanted phased-array coil can be used for awake macaque fMRI to increase the spatial resolution.
In the secondpart of this thesis phased-array coil technology was applied in functional studies. First, the extent of the retinotopic organization within inthe occipital and temporal cortex was studied and compared to the position of the face patches using fMRI. The face patches are cortical regions that respond stronger to faces than to non-face objects. They form aspecial microcosm of object processing. Our results indicated that the retinotopic organization extents far into the higher-order visual cortex. In addition, the gradual decrease in retinotopic organization occurs around the location of the posterior and middle temporal face patches. Inthe anterior-temporal and prefrontal face patches no retinotopic organization was found. These results support the hypothesis that different face patches are functionally segregated and that they obey a certain hierarchical order. The retinotopic regions were also combined in a probabilistic atlas that can be used as reference frame in future electrophysiology and fMRI studies.
In the final experiment of this thesis, the implanted array coil technology was employed to study the parcellation of the visual areas in the posterior parietal cortex with ultra-high resolution fMRI. A consistent internal retinotopic organization was revealed in areas PIP and CIP, together forming a complete visual field map cluster, and also in area LIP. An independent connectivity-based parcellation, using diffusion-weighted imaging, revealed a parcellation scheme closely resembling that found with retinotopic mapping methods. Furthermore, a consistent split between area LIP and the posterior field map cluster was found based on myelination maps. Finally, motion and shape localizers confirmed previously observed localization of functional properties within posterior parietal cortex. In conclusion, this ultra-high resolution MRI study combined for the first time 4 independent criteria for cortical parcellation in-vivo and within the same subjects. Across these techniques, we were able to consistently define the visual areas LIP, CIP, and PIP in the posterior parietal cortex.
In summary, the obtained results in this thesis illustrate the potential value of phased-array coil technology specifically designed for macaque MRI. We have shown that high-resolution macaque MRI can be used to study the properties of higher-order visual areas in detail and that multi-model MRI is capable of resolving the discrepancies in cortical parcellation acrossdifferent studies and techniques.
In a first experiment we developed and characterized a receive array coil with 22 channels for (anaesthetized) macaque MRI to improve the measurement sensitivity and to reduce the required scan time, in analogy with recent advances in human MR technology. With the developed 22-channel coil significant improvements in the sensitivity and acceleration of the data acquisition were indeed obtained throughout the brain. The applications for this novel receive technology range from high-resolution anatomical and diffusion-weighted imaging to functional experiments.
In a second experiment we also increased the sensitivity in awake macaque fMRI by reducing the distance between the receive coil and the brain. An 8-channel receive array was constructed and permanently implanted within the acrylic headpost assembly on top of the skullof the monkey. In vivo measurements showed substantial improvements in the sensitivity for the implanted versus the external coils. These results suggest that a stable implanted phased-array coil can be used for awake macaque fMRI to increase the spatial resolution.
In the secondpart of this thesis phased-array coil technology was applied in functional studies. First, the extent of the retinotopic organization within inthe occipital and temporal cortex was studied and compared to the position of the face patches using fMRI. The face patches are cortical regions that respond stronger to faces than to non-face objects. They form aspecial microcosm of object processing. Our results indicated that the retinotopic organization extents far into the higher-order visual cortex. In addition, the gradual decrease in retinotopic organization occurs around the location of the posterior and middle temporal face patches. Inthe anterior-temporal and prefrontal face patches no retinotopic organization was found. These results support the hypothesis that different face patches are functionally segregated and that they obey a certain hierarchical order. The retinotopic regions were also combined in a probabilistic atlas that can be used as reference frame in future electrophysiology and fMRI studies.
In the final experiment of this thesis, the implanted array coil technology was employed to study the parcellation of the visual areas in the posterior parietal cortex with ultra-high resolution fMRI. A consistent internal retinotopic organization was revealed in areas PIP and CIP, together forming a complete visual field map cluster, and also in area LIP. An independent connectivity-based parcellation, using diffusion-weighted imaging, revealed a parcellation scheme closely resembling that found with retinotopic mapping methods. Furthermore, a consistent split between area LIP and the posterior field map cluster was found based on myelination maps. Finally, motion and shape localizers confirmed previously observed localization of functional properties within posterior parietal cortex. In conclusion, this ultra-high resolution MRI study combined for the first time 4 independent criteria for cortical parcellation in-vivo and within the same subjects. Across these techniques, we were able to consistently define the visual areas LIP, CIP, and PIP in the posterior parietal cortex.
In summary, the obtained results in this thesis illustrate the potential value of phased-array coil technology specifically designed for macaque MRI. We have shown that high-resolution macaque MRI can be used to study the properties of higher-order visual areas in detail and that multi-model MRI is capable of resolving the discrepancies in cortical parcellation acrossdifferent studies and techniques.
Date:1 Oct 2009 → 2 Oct 2013
Keywords:Topographical organization, visual cortex, macaque monkey
Disciplines:Neurosciences, Biological and physiological psychology, Cognitive science and intelligent systems, Developmental psychology and ageing
Project type:PhD project