Emerging auditory response interactions to harmonic complexes in field L of the zebra finch.

Auditory responses of single units to harmonic complexes were studied in field L of the adult male zebra finch (Poephila guttata) to understand the origin of song selectivity in HVc. One of the attributes in this song selectivity is a large increase in response interactions when tones are presented simultaneously. Single units in the subdivisions of field L-L1, L2 and L3, were examined to see where these response interactions to harmonic complexes begin to develop along the auditory pathway towards HVc. Results showed relatively simple stereotyped response in L2 and widely varying ones in L1 and L3. The responses of some neurons in L1 and L3 to signals composed of the simultaneous sum of two harmonic complexes significantly differed from the linear sum of their responses to individual harmonic complexes whereas the neurons of L2 showed relatively weak response interactions. These neurons in L1 and L3 exhibited strong response interactions to harmonic complexes comparable to those of song-selective neurons in HVc. Thus, L1 and L3 are suggested to have emerging selective response properties and to provide auditory inputs relevant to the song selective HVc neurons.