Fig. 5. Activation of the valvula cerebelli following a 10°C temperature drop as determined by blood oxygenation level-dependent (BOLD)-weighted and cerebral blood volume (CBV)-weighted functional magnetic resonance images (fMRI). (A) T₂-weighted high-resolution spin echo image showing the location of the corpus cerebelli (cc), the valvula cerebelli (vc), and the tectum opticum (to). (B) Average of the color-coded percentual BOLD difference maps within the first 5 min after the onset of the temperature drop overlaid on the high-resolution image. The valvula cerebelli, but not the tectum opticum shows a reduced BOLD signal intensity (blue). This fish (but not the other two examined) also demonstrated pronounced early BOLD signal increase (red) in the corpus cerebellum. (C) Biphasic change of BOLD signal intensity measured in the valvula cerebelli indicates activation followed by inactivation of this brain region following the temperature drop. (D) Transient decrease of CBV signal reveals increased blood supply to the valvula cerebelli, demonstrating that the decrease of the BOLD signal is caused by increased cellular activity. Scale bar, 2.5 mm (A,B). Values are means ± s.d. (N=3).