Author:

Keywords:

RETINOTOPY, MACAQUE, FUNCTIONAL MRI

Abstract:

The patterns of functional organization occurring within the macaque brain can be too small to be resolved with conventional monkey fMRI paradigms. Hence, resolving the functional architecture of such structures requires higher spatial resolution then currently achievable with ‘conventional’ field MRI scanners and receive coil designs. Importantly, an increase in the signal to noise ratio (SNR) within the tissue of interest is required for higher spatial resolution imaging due to the inverse relationship between voxel size and SNR. While previous studies have employed higher field strengths to achieve the required increases in SNR, higher field fMRI is expensive and can cause increased susceptibility artifacts rendering reliable high-resolution functional imaging throughout the brain more difficult. Implanting receive coils in close proximity to the monkey brain (Logothetis 2002) is an alternative and economically more efficient approach to improve image SNR. However, the use of single coils precludes acquiring high resolution images of the whole brain. We here attempted to improve the spatial resolution of fMRI measurements without SNR-loss in awake behaving monkeys at 3 Tesla by permanently embedding an 8-channel phased-array receive coils in the headpost of monkeys (Janssens et al. 2012). The close proximity of the coil array relative to the brain allowed for increased SNR throughout the brain. In addition, the fixed position of these coils offered increased consistency of the SNR profiles across the brain during fMRI experiments. As proof-of-principle for ultra-high resolution fMRI in awake monkeys, we conducted phase-encoded retinotopic mapping experiments with complex natural and moving shapes (dynamic faces and walking persons). During fMRI acquisition (with ~ brain-wide coverage and 0.125 mm3 isotropic voxels) monkeys performed a passive fixation task while either rotating clockwise/counterclockwise wedges or expanding/contracting rings (24 degrees in diameter) where presented in the background. This approach confirmed the retinopic organization of early visual areas and significantly expanded retinotopic descriptions of small regions within the IPS, STS and arcuate sulcus. In addition, by reliably measuring functional activity at such a high spatial resolution in the awake-behaving primate these studies pave the way for further experiments probing the function of increasingly smaller functional units in the monkey brain.