Four sets of data were obtained. The initial set was obtained under light adapted conditions using 60-second carotid occlusions. Simultaneous measurements of RBF and ChBF were made in one subgroup of animals, and BOLD measurements were made in another subgroup of animals. The second data set was obtained in separate subgroups for RBF and ChBF, and for retinal BOLD (RBOLD) under light adapted conditions using carotid occlusion durations of 15, 30, 60, or 90 seconds. RBF also was tested with a 6-second occlusion; however, this was deemed too short a duration for the RBOLD temporal resolution, so it was not attempted. Because choroidal reactive hyperemia was negligible in the first two sets, the remaining sets focused on retinal responses. The third data set was obtained in separate subgroups for RBF and RBOLD with the animals dark adapted, light adapted, and during flickering light stimulation using a carotid occlusion duration of 60 seconds. To determine if a shorter duration carotid occlusion (i.e., with a less than maximal reactive hyperemia) would reveal an effect of flicker stimulation, the fourth and final data set was obtained under light adapted conditions and during flickering light stimulation using an occlusion duration of 15 seconds. The animal numbers for each test are indicated in the Figures.
For each animal in the optical imaging and MRI studies, approximetely 2 to 3 trials were collected under each light condition for each occlusion duration. An approximately 5- to 20-minute break was given between trials, depending on the length of occlusion durations and animal stability. Before each trial under the dark condition, the animals were dark adapted for 40 minutes. The three light conditions and the lengths of occlusion duration were randomized. For the RBF measurements, the light source for the flicker condition was a white LED light with an intensity of 1.35 × 10
8 cd/m
2, pointed at the eye from a distance of approximately 25 cm with an angle of approximately 45° to the center axis of the rat body. For the RBOLD measurements, the light source for the flicker and constant light conditions was a white LED light guided through a 2-mm fiberoptic cable bundle,
19,28 where the maximum luminance at the end of the fiberoptic cable bundle was 3.54 × 10
4 cd/m
2. The end of the cable bundle was mounted on the coil and pointed to the equator of eye. During the constant light condition, the LED light was set at half of the maximum level. During the flicker condition, the LED light was set at maximum level. The flicker frequency was 10 Hz.