The Microvascular System of the Striate and Extrastriate Visual Cortex of the Macaque

Abstract

In functional neuroimaging, neurovascular coupling is used to generate maps of hemodynamic changes that are assumed to be surrogates of regional neural activation. The aim of this study was to characterize the microvascular system of the primate cortex as a basis for understanding the constraints imposed on a region's hemodynamic response by the vascular architecture, density, as well as area- and layer-specific variations. In the macaque visual cortex, an array of anatomical techniques has been applied, including corrosion casts, immunohistochemistry, and cytochrome oxidase (COX) staining. Detailed measurements of regional vascular length density, volume fraction, and surface density revealed a similar vascularization in different visual areas. Whereas the lower cortical layers showed a positive correlation between the vascular and cell density, this relationship was very weak in the upper layers. Synapse density values taken from the literature also displayed a very moderate correlation with the vascular density. However, the vascular density was strongly correlated with the steady-state metabolic demand as measured by COX activity. This observation suggests that although the number of neurons and synapses determines an upper bound on an area's integrative capacity, its vascularization reflects the neural activity of those subpopulations that represent a "default" mode of brain steady state.

Cerebral Blood Flow Modeling in Primate Cortex

“…The two thirds-one third ratio of penetrating arteries-veinules is also visible on this figure in which 24 penetrating veins have been identified among a total of 62 penetrating vessels (proportion of arteries equal to 0.61). Comparing this proportion of arteries to the 0.61 found in Weber et al (2008) on 249 measurements did not show any statistically significant difference (P = 0.999), so our finding in the marmoset is perfectly consistent with parallel estimates in the macaque. A similar conclusion (unless less significant (P = 0.2) data are considered) is reached when our results are compared with human data (142 arteries for 200 identified penetrating vessels, i.e., a proportion 0.71 of arteries (Lauwers, 2007)).…”

Cortical Depth-Dependent Temporal Dynamics of the BOLD Response in the Human Brain

“…The feeding arterioles enter the cortex perpendicularly from the pial surface and are spatially matched to a cortical unit (whisker barrel, Woolsey et al, 1996). From there they feed into capillaries, the highest density of which is located in layer IV (Bell and Ball, 1985;Weber et al, 2008). Next, blood drains to postcapillary venules oriented toward the surface.…”

Simulation study of brain blood flow regulation by intra-cortical arterioles in an anatomically accurate large human vascular network. Part II: Flow variations induced by global or localized modifications of arteriolar diameters

“…In fact, it is recognized that GE-EPI is sensitive to local susceptibility gradients (Ogawa et al, 1992) which originate predominantly near veins (Uludağ et al, 2009). Note that, as underlined by many authors (Harrison et al, 2002;Harel et al, 2006;Weber et al, 2008), the vascular PSF is closely linked to the minimum spatial scale for the vascular control of blood flow changes. In this context, our results demonstrate that the regulation of blood flow at the level of individual intra-cortical arterioles, without any active or passive modifications of vessel diameter at capillary level, is sufficient to explain the submillimeter PSF experimentally measured by ASL fMRI at 9.4 T. Moreover, our results theoretically confirm the need to develop sequences yielding high micro-vasculature weighting of the fMRI signal, excluding the contribution of intra-cortical veins, especially in the context of ultra-high magnetic fields (Yacoub et al, 2008).…”

“…There is increasing recognition that, by its structural implication in neuro-vascular coupling, underlying vascular architecture is a key player in hemodynamically based functional imaging techniques (including fMRI and PET) (Harrison et al, 2002;d'Esposito et al, 2003;Logothetis and Wandell, 2004;Lauwers et al, 2008;Weber et al, 2008). In particular, the spatial resolution and specificity of such techniques are bound to the density of blood capillary vasculature and blood flow regulating structures (Harel et al, 2006).…”