GLOBAL SYNCHRONOUS RESPONSE TO AUTOGENOUS SONG IN ZEBRA FINCH HVC

1. The spatial distribution of neuronal responses to autogenous song ( AS) was investigated in the HVc of urethan-anesthetized adult male zeb ra finches (Taeniopygia guttata). In seven birds, penetrations covered the entire mediolateral, rostrocaudal, or dorsoventral extents of HVc . In an eighth, control bird penetrations were made near to but outsid e of HVc. Reconstruction of recording sites from histological material indicated a good correspondence between sites that exhibited stronger responses to AS than to tone or noise bursts, and sites that were wit hin HVc. 2. Within each experimental bird but not in the control, mult iple-unit responses to AS were similar across the entire spatial exten t of HVc (up to 1.3 mm). For each experimental bird, the strongest res ponses occurred within a narrow range of times. The middle of this ran ge of times is called the time of maximum synchronization (TMS). Acros s birds, 34-75% of recording sites exhibited the same TMS. With the us e of a criterion of >33% of sites exhibiting their strongest responses at the TMS, the temporal scatter around the TMS varied between 6 and 138 ms across individuals. In six of the seven experimental birds, the position of the TMS was not affected by changing the window of integr ation from 10 to 150 ms. In two experimental birds, short windows of i ntegration tended to emphasize beginning portions of the song. In one case this effect was sufficiently strong to change the TMS for short w indows of integration. 3. Each TMS was associated with a syllable of m aximum synchronization (SMS). The positions of the SMS varied consider ably across birds. In four birds the SMS was one of the syllables of t he first motif( a motif is a temporal sequence of syllables that can b e repeated greater than or equal to 1 times to form a song), in two bi rds the SMS was the introductory note of song, and in one bird the SMS was the second syllable of the last (3rd) motif. Syllables of the sam e type as the SMS but occurring in other motifs typically elicited muc h weaker responses, in many cases weaker than other syllables in those motifs. Syllables that elicited strong responses in non-SMS motifs di d not necessarily elicit strong responses in the SMS motif, even if th ey preceded the SMS. There were no apparent acoustical features of the SMS or the preceding syllable that could account for the global synch ronous response to song. 4. A measure of the aggregate response of HVc was derived by summing multiple-unit data across all recording sites of each bird. The aggregate response exhibited phasic excitation at sy llable onsets and offsets. Additionally, a relaxation process after th e end of AS was observed with a long inhibitory component. The duratio n of the relaxation process (up to 6 s) varied approximately one order of magnitude across individuals. Features of the motif dependency of the response to the SMS were related to the duration and relative stre ngth of the relaxation process. 5. The single units that were isolated from multiple units using waveform clustering techniques accounted fo r approximately one-third of the total spikes in response to AS, with high spontaneous rate neurons (greater than or equal to 2.5 spikes/s) contributing the greatest number of spikes. Typically, the SMSs derive d for ''responsive'' single units, residual multiple units, and the ov erall multiple-unit response were in agreement. 6. The auditory repres entation of song in the zebra finch HVc is apparently distributed and redundant. These data are discussed in relation to the anatomic organi zation of HVc's afferents and efferents, and in relation to the organi zation of HVc as a motor structure. The global pattern of excitation a nd inhibition in response to AS is strongly temporally modulated. A te mporally encoded representation may provide a useful reference framewo rk for error detection, either in relationship to auditory feedback or in relationship to detection of conspecific songs.