Development of selectivity for natural sounds in the songbird auditory forebrain

In adult songbirds, auditory neurons in the primary auditory forebrain region of field L and a secondary auditory forebrain region of caudal mesopallium (CM) are highly responsive to natural sounds, such as conspecific song. Because these nuclei are involved in sensory representations of songs, we investigated how their function changes during development. We recorded neural responses to conspecific and tutor song and acoustically matched synthetic sounds in field L and lateral CM (CLM) of urethane-anesthetized juvenile male zebra finches of approximately 35 days of age. At this age, juvenile songbirds are memorizing the songs of their adult tutors but do not yet sing mature song. They are also starting to recognize songs of individual conspecifics. Compared with adult auditory forebrain neurons, juvenile neurons in field L were on average less responsive to auditory stimuli and exhibited less selectivity for natural sounds compared with the synthetic sounds. This developmental effect was more pronounced in the secondary subregions of L1 and L3 than in the primary thalamo-recipient subregion L2 of field L. CLM showed adultlike selectivity for natural sounds. Also, we did not find any evidence of memory for the tutor song in either field L or CLM. We note that the neural development of selective responses to conspecific song in the secondary subregions of field L is correlated with the emergence of individual song preference around 35 days of age. Therefore we suggest that the emergence of natural sound selectivity in field L could be important for the behavioral development of song recognition.