SPATIAL & TEMPORAL RESOLUTION of fMRI: Overview

Recently introduced functional MRI techniques provide the capability for visualizing increased neuronal activity with spatiotemporal specificity and resolution that have not been previously available using other non-invasive methods. Such a capability is essential for a system-level understanding of those human brain functions that possess unique attributes and cannot be investigated with the invasive techniques employed in animal model studies. However, the origin and limitations of the signal intensity changes detected by fMRI have not been fully investigated. Project 1 focuses on examining three issues critical to fMRI: (i) the physiological basis of fMRI signals, (ii) spatial resolution, and (iii) temporal resolution. In order to properly interpret fMRI data, we must understand the relationship between neural activity and the observed fMRI signals. In the Human Brain Project, the dynamic relationship between neural activity and the fMRI signal has been and will continue to be investigated.

For an in-depth investigation of cortical information processing, it is crucial to map the basic functional units of neuronal activity such as cortical columns. Neurons with common functional properties are often clustered into columns which span the entire cortical plate. Individual functional columns in mammals are ~0.5 mm wide, and iso-functional columns often repeat about every millimeter. However, the spatial specificity of the conventional positive BOLD signal is limited by its point spread function, which extends about 2-3 millimeters beyond the locus of neuronal activity. To overcome this limitation, the early negative BOLD signal related to increased metabolic rate was utilized. Orientation columns in the cat visual cortex obtained by early-negative BOLD fMRI are illustrated in the figure. Single-condition maps were generated by using early-negative BOLD signals during moving grating stimulation with different orientations; a composite-angle map was obtained by pixelwise vector addition of four single-orientation maps, and the resulting orientation preference at each cortical location was color-coded. Tangentially to the cortical surface the preferred orientations change smoothly. Continuity is interrupted at "orientation pinwheels", where the cortical columns for different orientations are arranged in a circular manner. This figure demonstrates that column-resolution fMRI can be obtained. With the proposed improvement in fMRI temporal resolution, sub-second temporal resolution of cortical columns can be obtained.