Vascular regulation at sub millimeter range. Sources of intrinsic signals for high resolution optical imaging.

The idea that electrical activity is coupled to microvascular changes is not new. Already a century ago, Roy and Sherrington (1890) postulated that "the brain possesses an intrinsic mechanism by which its vascular supply can be varied locally in correspondence with local variations of functional activity". Later studies have indeed demonstrated that there is a strong coupling between neuronal activity, local metabolic activity, and blood flow (Kety et aI., 1955; Lassen and Ingvar 1961; Sokoloff, 1977; Raichle et aI., 1983; Fox et aI., 1986). Modern neuroimaging techniques have used signals that originate from metabolic activity or vascular response because the monitoring and precise localization of such signals sources is often better than those which originate directly from electrical activity. In order to optimally and faithfully image functional maps in the neocortex it is crucial to understand the mechanisms underlying the intrinsic signals and their relationship to the electrical activity of neurons. Recent experiments have shown that there are at least three components that contribute to the intrinsic optical signals (Frostig et aI., 1990). These components are related to activity-dependent changes in the oxygen saturation level of hemoglobin, blood volume and light scattering of the tissue. Although the different components of the intrinsic signal originate from different sources and exhibit different action spectra it has been shown that similar functional maps were obtained when different illumination wavelengths were used. Therefore, it appears that all components can be used for functional mapping. (Frostig et aI., 1990)