Intrinsic Optical Imaging (IOS). Physical base: light аabsorption and light reflection, optic features of the brain tissue. Optical correlations of the oxygenation and deoxygenation. Signal strength, biological and non ц biological optic noise.

Onje of the most effective strategy of imaging functional architecture is based on the slow intrinsic changes in the optical properties of active brain tissue, permitting visualization of active cortical regions at a spatial resolution greater than 50 μm. This can be accomplished without some of the problems associated with the use of extrinsic probes. The sources for these activity-dependent intrinsic signals include either changes in physical properties of the tissue itself which affect light scattering and/or changes in the absorption, fluorescence or other optical properties of intrinsic molecules having significant absorption or fluorescence. Recently become possible to use optical detection of intrinsic signals for the imaging of the functional architecture of the cortex.

Optical imaging of functional maps in vivo (Grinvald et al, 2001, above). The setup: images are taken of the exposed cortex of the animal, which is sealed in an oil-filled chamber. The cortex is illuminated with light of 605nm wavelength. The images are acquired with the camera, during which time the animal is stimulated with visual-, auditory, electrical or any other type of stimuli. The acquired images are digitized by a computer controlling the entire experiment. The signal to noise of the functional maps is improved by averaging several stimulus sessions. Functional maps are subsequently analyzed, and are displayed on a color monitor. a color coded ocular dominance map is shown here. To determine the quality of the maps during the imaging sessions, the data can be sent to a second computer for detailed, quasi on-line analysis.

Although to date most optical imaging studies have been done in the visual cortex, this is by no means the only sensory system which can be studied using this method. Indeed, this methodology has also proven useful for investigating functional architecture in the somatosensory cortex of the rat, monkey, guinea pig, gerbil, chinchilla. Certain outstanding questions cannot be explored by performing acute experiments but require long term chronic recordings. Particularly important, with regard to the feasibility of chronic optical imaging, was the finding that cortical maps can be obtained through the intact or a thinned dura and even through a thinned bone. These results were achieved using near infra-red light, which penetrates the tissue considerably better than light of a shorter wavelength.

In order to optimally image functional maps in the neocortex, and to interpret these maps properly, it is crucial to understand the mechanisms underlying the intrinsic signals, and particularly their relation to the electrical activity of neurons. аAlthough the intrinsic signal has different components which originate from different sources, it has been shown that functional maps obtained at different wavelengths are very similar. Therefore, it appears that all of these components can be used for functional mapping albeit with a different signal to noise ratio and different spatial resolution. The main conclusion regarding the origin of the intrinsic signal was that following sensory stimulation there is an initial increase in the concentration of deoxy hemoglobin, due to increased

oxygen consumption. This increase is referred to as Уthe initial dipФ by the f-MRI community. It is followed by a larger decrease, due to large but delayed changes in blood flow, supplying highly oxygenated blood to the activated cortical area.It has been shown that the amplitude of the differential intrinsic signals is well correlated with spike rates in the neocortex.

Typical optical signals in a rodent model of optical responses (above) demonstrate different spatial/temporal patterns depending on wavelength of imaging. Optical responses to a 2-second C-1 whisker stimulation over the rodent somatosensory cortex are displayed at 550, 610, and 850 nm. The value 550 nm is an isobestic point of absorption for oxy- and deoxyhemoglobin. Therefore, optical responses at this wavelength are believed to emphasize changes in total hemoglobin. The response at 550 nm is typically high intensity and monophasic. Occasionally, following the decrease in reflectance, an increase in reflectance may be observed overlying the arterial vessels. The monophasic time course is shown in the graph at the bottom of the figure. Overlying on the 0-second image is a schematic representation of the rodent somatosensory barrel cortex for comparison. At 610 nm, oxyhemoglobin absorption is negligible compared with that of deoxyhemoglobin. Responses at this wavelength are therefore believed to represent changes in deoxyhemoglobin. Following stimulation, there is initially a focal increase in absorption (at 1 second, interpreted as an increase in deoxyhemoglobin) followed by a more wide-spread decrease in absorption (3.5-6.5 seconds, interpreted as a decrease in deoxyhemoglobin concentration). The second phase is related to the blood oxygen level-dependent fMR imaging signal. The biphasic time course of optical responses at 610 nm is shown in the graph at the bottom of the figure. At 850 nm, neither isoform of hemoglobin absorbs much light. The signal is instead believed to originate from light scattering due to blood volume changes. This signal, like that at 550 nm, is monophasic but significantly less intense. The monophasic temporal profile is highlighted by the monophasic graph shown for 850 nm at the bottom of the figure. Time courses were calculated by determining the mean reflectance change within a statistically defined region of interest, using methods described by Blood, et al. Time courses were normalized to one for comparison purposes. The height of the graphs, therefore, does not reflect the magnitude of the reflectance changes. Rather, the graphs have been shown to demonstrate the overall temporal profile. The optical signals and time courses were derived from averaging 12 trials in a single animal. Stim = stimulation.