Fig. 6.

In HVc, X-projecting and RA-projecting neurons as well as interneurons are song selective, displaying enhanced action potential firing to forward over reverse BOS playback. For each cell type, the PSTHs (spikes/bin) and median-filtered averaged membrane potential traces[ $\overline{V}$_m (mV)] are shown. These three cells were collected from a single bird; responses are to 20 iterations each of the forward and reverse BOS playback, shown as oscillograms at thebottom of the figure. For the X-projecting neuron, slight membrane hyperpolarization and highly phasic subthreshold depolarizations were evoked by forward BOS playback, with only minor subthreshold depolarizations accompanying reverse BOS playback. For the RA-projecting neuron, both forward and reverse BOS playback elicited subthreshold depolarizations, although only forward BOS evoked a significantly elevated action potential response. For the interneuron, only forward BOS playback elicited subthreshold or suprathreshold responses. The X-projecting neuron and interneuron were identified morphologically, whereas the RA-projecting cell was identified on the basis of its intrinsic electrophysiological properties.