Reassessment of activity-related optical signals in somatosensory cortex by an algorithm with wavelength-dependent path length.

Incorporating the wavelength dependence of the scattering effect into a simple linear multicomponent analysis of intrinsic optical signals, we have reexamined the change in the hemoglobin (Hb) concentration and the origins of intrinsic signals in somatosensory cortex evoked with electrical stimulation of the hind limb (5 Hz, 2 s) of anesthetized rat. The concept of the analysis was to separate the effect of light scattering involved in the observed optical signals into two factors, light attenuation and modification of Hb absorption as a result of the wavelength dependence of the optical path length. This dependency was experimentally assessed with a tissue-simulating phantom whose absorption spectra were nearly identical to those of cerebral tissue through a thinned skull window in vivo. Using those phantom spectra, we carried out a curve fitting of the reflection spectra from the rat somatosensory cortex activated with an electrical stimulation of hind limb (5 Hz, 2 s). Oxygenated Hb slightly decreased at 0.5-1.5 s after an onset of the stimulus followed by an increase, which peaked at 4 s. Deoxygenated Hb increased at 1.0-1.5 s followed by a large late decrease. We again confirmed an early increase in the concentration of deoxygenated Hb in the rat somatosensory cortex after stimulation of the hind limb.