Fig. 1.

Singing-related neural activity distinguishes four classes of putative striatal neurons in area X. A: schematic of the avian song circuit showing the anterior forebrain pathway (AFP, —) containing the basal ganglia structure area X, and the motor pathway (···). B: the songbird area X is a striato-pallidal structure that resides in a thalamo-cortical loop. Four cell classes found in the mammalian striatum are also found in area X: medium spiny neurons, fast spiking interneurons (FS), tonically active neurons (TANs), and low-threshold spiking interneurons (LTS). C: representative histology for area X recordings. Small electrolytic lesions were made with the recording electrodes (←) to verify electrode position within area X. D, top: the mean firing rate during singing is plotted against the mean firing rate during nonsinging epochs for all neurons recorded in Area X. Bottom: histogram of nonsinging firing rates separates 2 classes of neurons: putative pallidal neurons that fired at high rates (>50 Hz), and putative striatal neurons that fired at low rates (<30 Hz). E: for all putative striatal neurons, the spike width is plotted against the mean firing rate during singing. The bottom histogram distinguishes type-1 neurons, which fired at low rates during singing (<10 Hz). Left: histogram distinguishes type-2 neurons, which had thin spike widths (<0.06 ms, see methods). Inset: spike waveforms (means ± SD, n = 50 spikes) from representative neurons of each cell class. *, thin spike of a type-2 neuron. F: for all putative striatal neurons (excluding type-1), the spikewidth is plotted against the peak firing rate during singing (99th percentile rate, see methods). Histogram of peak firing rate distinguishes an additional 2 classes of neurons: type-3 neurons that do not generate high-frequency discharge (peak rate: <600 Hz), and type-4 neurons that do (peak rate: >600 Hz). For all scatter plots, ▵ and ○, neurons recorded from subsong and plastic song birds, respectively.