Figure 3.

Onset of stimulus-evoked blood-volume changes in the cortical microvasculature. Dynamic measurements by imaging fluorescence and reflected light at 565 nm are shown. A, Series of single-condition (horizontal gratings divided by blank) fluorescence images of ∼3 × 2 mm of cat visual cortex area 18. Brightening corresponds to increased fluorescence with respect to baseline. Range of grayscale used (“clipping range”): -0.12-0.55%. B, Same as A but reflection at 565 nm, instead of fluorescence. Here, the inverse color map was used (brightening corresponding to decreased reflection) to facilitate comparison with A; clipping range, -0.1-1.78%. Here and in Figures 6 and 7, the numbers on the top show the time (seconds after stimulus onset) at which each frame was acquired. The baseline image (averaged over 0.88 s before stimulus onset) was subtracted from all frames to eliminate slow, stimulus-unrelated vascular noise. The frames before 0.5 s (shown in Fig. 6) are not displayed here, because no activity pattern could be seen there.