Fig. 4.

Changes in blood volume predict size of initial dip. (A) Molar extinction coefficient for HbR and HbO. Vertical dashed lines indicate isosbestic points. Shaded regions indicate: ranges where HbR absorbs more strongly than HbO. (B) Predicted response time courses at wavelengths moving from isosbestic to oxymetric marked in A, showing progression from a monophasic signal at the isosbestic 584 nm to a biphasic signal at 600 nm. (Note: The predicted signal time courses here are for the pure spectral wavelengths indicated; while qualitatively similar to our measured imaging signals they are quantitatively different because our LED sources had finite bandwidths giving the corresponding admixture of responses. All quantitative calculations using our LED sources accounted for this finite bandwidth; see Methods). (C) The predicted ratio of the size of the initial dip relative to the rebound across wavelength. Note an explosive increase in this ratio toward isosbestic points. (D) Predicted spatial profiles at 3 s poststimulus onset normalized to maximum response amplitude. Wavelengths same as in B. Note broader spreads at wavelengths showing a rebound. (Inset) Not normalized for amplitude.