Comparative neurophysiological studies of hearing and vocalizations in songbirds

1.Acoustic neurons in the avian cochlear nucleus are tonotopically arranged according to the sites of innervation on the basilar membrane by the connecting primary auditory fibers. 2.The thresholds of single units vary distinctly with their characteristic frequencies The distribution of unit thresholds matches extremely well with the behavioral audibility curve. 3.The numbers of units in the nucleus also vary with their CF's. The greatest number of units occurs within the frequency range in which auditory thresholds are lowest. Smaller numbers of units represent the frequency ranges higher or lower than the most sensitive region. 4.The upper range of single unit CF's shows distinct interspecific variation which is correlated with the differences in the range of vocal frequencies. The upper limit of CF may be correlated with the bird's size. 5.The lowest thresholds of single auditory neurons tend to fall uniformly between −60 and −80 DB (re. 1 dyne per cm2) among different species. The differences between the highest and lowest unit thresholds are also homogeneous (40–50 DB) among different species. 6.Most songbirds do not seem to produce frequencies below 1 KHz but can hear them. 7.The popular belief that songbirds can hear frequencies inaudible to man is not supported by reliable evidence