Figure 2.

Uva_HVC neuron spikes are negatively correlated with spikes in HVC_I neurons and bursts in RA neurons. a, Extracellular record of an Uva_HVC–HVC_I pair recorded during sleep. b, IFR functions of the neurons in a. Periods of high Uva_HVC firing (top, thick horizontal bars) are coincident with periods of reduced HVC_I bursting (bottom). Uva_HVC bursts >100 Hz have been truncated for better visibility of low firing rates. c, The cross-covariance function of this neuron pair (top) and the average covariance function of n = 7 Uva_HVC–HVC_I pairs in three birds (bottom) both exhibit a broad dip extending up to 3 s of Uva_HVC spikes and sharp positive and negative peaks close to zero time lag (arrows). To not smear over the sharp peaks, almost no smoothing was applied (4-ms-wide Gaussian). d, e, Same as b and c, but for RA instead of HVC_I neurons (n = 8 Uva_HVC–RA pairs). RA single spikes have been removed for the computation of covariance functions (see Materials and Methods). Smoothing was performed with a 20-ms-wide Gaussian.