Figure 9.

Rostrocaudal organization of HVC neural connectivity as demonstrated by focal injection of DiO. Four coronal tissue sections (A–D) selected at 240 μm increments span the rostrocaudal axis of HVC and are shown under fluorescent illumination. The Nissl-defined borders of HVC (obtained after counterstaining of each section) are indicated by a white dashed line. The horizontal offset in the images accommodates the medial shift that occurs between rostral and caudal poles of HVC (see Fig. 1). In this bird, a centrally situated dye injection near caudal HVC (C) produces retrograde labeling within the lateral half of caudal HVC (D) and within the medial half of rostral HVC (A), indicating a pattern of retrograde labeling aligned to the rostrocaudal axis of HVC. Note that DiO is a hydrophobic tracer and it therefore produced more focal injection sites compared with fast blue (see Fig. 8C). Overexposure of the DiO injection site (C) ensured capture of labeling lateral and medial to the injection site; such labeling was limited in extent compared with the labeling present in caudal and rostral HVC. D, Dorsal; M, medial.