Fig. 6.

There was a positive correlation in our castrates between amount of singing (songs per hour) and number of³H-labeled HVC neurons per day of ³H-thymidine injection. Neuronal identity was established by the manner in which neuronal nuclei stained with cresyl violet. The three panels show the correlation between amount of singing;A, total number of³H-labeled HVC neurons (n= 18); B, number of ³H-labeled HVC neurons that were not backfilled from RA (mostly interneurons?);C, number of ³H-labeled HVC neurons backfilled from RA (projection neurons). Ten of the 18 birds inA received FG injections and are the ones shown inB and C.