Biomedical subjects

Mark Augath

Publications and source records attributed to Mark Augath.

4 recordsLinked to original sources

PubMed2002

Ultra high-resolution fMRI in monkeys with implanted RF coils.

Spatiotemporally resolved functional MRI (fMRI) in animals can reveal how wide-spread neural networks are organized and accompanying electrophysiological recordings can show how small neural assemblies contribute to this organization. Here we present a novel technique that yields high-resolution structural and functional images of the monkey brain with small, tissue-compatible, intraosteally implantable radiofrequency coils. Voxel sizes as small as 0.0113 microl with high signal-to-noise and contrast-to-noise ratios were obtained, revealing both structural and functional cortical architecture in great detail. Up to a certain point, contrast sensitivity increased with decreasing voxel size, probably because of the decreased partial volume effects. Spatial specificity was demonstrated by the lamina-specific activation in experiments comparing responses to moving and flickering stimuli. The implications of this technique for combined fMRI/electrophysiology experiments and its limitations in terms of spatial coverage are discussed.

Animals↗

PubMed2002

Magnetic resonance imaging of neuronal connections in the macaque monkey.

Recently, an MRI-detectable, neuronal tract-tracing method in living animals was introduced that exploits the anterograde transport of manganese (Mn2+). We present the results of experiments simultaneously tracing manganese chloride and wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) to evaluate the specificity of the former by tracing the neuronal connections of the basal ganglia of the monkey. Mn2+ and WGA-HRP yielded remarkably similar and highly specific projection patterns. By showing the sequential transport of Mn2+ from striatum to pallidum-substantia nigra and then to thalamus, we demonstrated MRI visualization of transport across at least one synapse in the CNS of the primate. Transsynaptic tract tracing in living primates will allow chronic studies of development and plasticity and provide valuable anatomical information for fMRI and electrophysiological experiments in primates.

Animals↗

PubMed2002

Three-dimensional shape representation in monkey cortex.

Using fMRI in anesthetized monkeys, this study investigates how the primate visual system constructs representations of three-dimensional (3D) shape from a variety of cues. Computer-generated 3D objects defined by shading, random dots, texture elements, or silhouettes were presented either statically or dynamically (rotating). Results suggest that 3D shape representations are highly localized, although widely distributed, in occipital, temporal, parietal, and frontal cortices and may involve common brain regions regardless of shape cue. This distributed network of areas cuts across both "what" and "where" processing streams, reflecting multiple uses for 3D shape representation in perception, recognition, and action.

Animals↗

PubMed2002

The effect of image scrambling on visual cortical BOLD activity in the anesthetized monkey.

We have investigated BOLD signal changes associated with scrambling natural images into different numbers of segments in visually modulated regions of the macaque monkey (macacca mulatta) brain. For 10 degrees x 10 degrees images, we observed that BOLD activity in primary visual cortex (V1) increased with scrambling, and then dramatically dropped for very highly scrambled images (128 x 128 segments). In extrastriate visual areas, BOLD signal levels did not distinguish between natural images and scrambled images, except that as in V1 very highly scrambled images led to a drop in BOLD activity. Finally in the superior temporal sulcus region and inferior temporal cortex, BOLD activity decreased systematically with scrambling. Our results are consistent with the view that the BOLD signal might reflect average activation of local orienation detectors in V1, and sensitivity to more global object representations in higher visual areas. In addition, we quantify the effects of scrambling on the Fourier amplitude spectrum of the images. This analysis shows that scrambling causes substantial changes to the spatial frequency content of images. This suggests that low-level accounts for reduced BOLD activation in higher visual areas cannot be completely ruled out based on scrambling data.

Animals↗