Firing rate during singing (filled circles) was higher than that during awake non-singing state, while the firing rate during sleep (crosses) was lower.

A, Rapid modulation of firing rate was quantified by calculating ratio of a given ISI to the mean of the preceding 5 ISIs. Vertical ticks indicate schematic spike train. B, Comparison of proportion of higher ISI changes (ISI change >2, indicative of firing rate deceleration) between different behavioral states across population (24 units).

A–D, An example of pallidal unit recorded from a juvenile bird (bird #1, 61 days post-hatch). A, Activity during singing. Song (oscillogram, period of singing indicated by black bars) and smoothed firing rate (15 ms gaussian window) are shown. Spectrogram of song (2 second epoch, indicated by gray shade labeled ‘ii’) is shown at the top. Typical spiking activity during non-singing and singing periods (2 second epochs, indicated by gray shades labeled ‘i’ and ‘ii'’, respectively) are shown in raster plots below. B, Activity of the same unit during sleep. Simultaneously recorded EEG signal is shown below. Typical spiking activity during tonic and phasic epochs (2 second, indicated by gray shades) are shown by raster plots. C, Power spectral densities of 2 second period of EEG during awake (gray, 3 representative traces) and sleep (black, 3 representative traces) periods. Note stronger power of low-frequency band (<10 Hz) during sleep periods. D, Unit cluster at initiation of recording and 5 hours later. Black dots are a sorted single-unit. Noise cluster is encompassed with a dotted circle. Insets indicate spike waveforms. Scale bars; 0.3 mV, 1 ms. E–H, Another example of pallidal unit (bird #2, 59 days post-hatch). Data are shown as in A–D. Despite sleep EEG waveforms appear different in B and F, power spectral densities for both periods exhibited strong power at low-frequency band, suggesting that those sleep periods can be regarded as slow wave sleep. Audio files (Audio S1, S2) are available as Supporting Information.

A, top; ISI distributions of a pallidal unit for awake non-singing and singing period (dotted and solid lines, respectively). bottom; ISI distribution of the same unit for sleep period. Note evident bimodal distribution. The ISI distribution for singing state was generated from 87.1 seconds of singing. The ISI distributions for both awake non-singing and sleep states were generated from 120 seconds of each behavioral state. Data from the same unit appeared in Fig. 2A–D. B, Another example of ISI distributions for different behavioral states. Data are shown as in A. The ISI distribution for singing state was generated from 65.7 seconds of singing, and those for both awake non-singing and sleep states were generated from120 seconds of each state. Data from the same unit appeared in Fig. 2E–H. C, Comparisons of ISI peak times between awake and sleep state. ISI peak for singing vs. short ISI peaks for sleep (filled circles). ISI peak for awake non-singing vs. long ISI peaks for sleep (open circles). D, Population summary of proportion of shorter ISI (ISI <4 ms) for different behavioral states. E, Comparison of ISI CV (coefficient of variation) between different behavioral states across population. Data from 24 units in C–E.

A, An example of pallidal unit activity during playback of bird's own song. Black bar indicates period of song playback. Expanded spectrogram of the bird's own song is shown at the top. B, Activity of the same unit during playback of tutor song (black bar), plotted as in panel A. Data from the same unit appeared in Fig. 2A–D. Audio files of the bird's own song (Audio S3) and tutor song (Audio S4) are available as Supporting Information.