Imaging of intrinsic optical signals in primate cortex during epileptiform activity.
Localized increases in neuronal activity are known to alter the distribution and oxygen content of blood within the surrounding brain tissue. In the neocortex, these activity-evoked hemodynamic changes are predominantly mediated through the dilation of the microscopic pial arterioles that lie on the surface of the brain, nearest to the site of activation. Since hemoglobin absorbs light throughout the visible and near-infrared spectrum, optical microscopy combined with computer imaging techniques can be used to map the patterns of hemodynamic changes associated with neuronal activity. Examples of optical imaging data are provided here to demonstrate four points. First, depending on the optical wavelength chosen for illumination of the cortex, different spatial and temporal patterns of optical changes are elicited by similar stimuli yielding distinctly different types of physiological information. Second, by selecting the appropriate wavelengths, it is possible to generate maps from optical-imaging data that represent changes predominately due to either blood volume (at 535 nm) or blood oxygenation (at 660 nm). Third, "negative" optical signals are negative only relative to a given optical wavelength, and appear to be associated with more intense types of neuronal activation. Fourth, optical imaging is a useful technique for studying neocortical seizure activity in animal models, with the caveat that species-specific differences in cortical size and vascularization patterns may be important to consider in the interpretation of optical imaging data.
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