Serotonergic fiber staining in the inferior colliculus (IC). Center panel shows a transverse section of a typical mouse IC demonstrating SERT-positive fiber staining at 4× magnification. The three subdivisions of the IC (dorsal (ICd), external (ICe), and central (ICc)) are superimposed. Notice that the ICd as well as dorsomedial portions of the ICe and ICc regions contain darker staining, indicating a greater density of serotonergic fibers in these areas compared with more ventrolateral areas. Panels on the left and right show SERT-positive fiber staining in areas of the ICd and ICc subdivisions, respectively, at a magnification of 40×. A ‘space ball’ probe has been superimposed in each, appearing as a black 2D circle. Notice that at this plane of focus, SERT-positive fibers cross the perimeter of the space ball probe six times in the area of the ICd, but only three times in the area of the ICc.