Fig. 2.

Morphological and physiological characterization reveals that song-selective LMAN neurons are projection neurons. A, Camera lucida reconstructions (objective magnification, 63×, left, and 10×,right) of two song-selective LMAN neurons show that these cells are spiny projection neurons with bifurcated axons projecting toward RA and area X and local collaterals within LMAN. Because of the great distance between LMAN and RA (>4 mm), axons were rarely completely filled to RA. Reconstructions are in the sagittal plane; rostral is to the left; dorsal isup. B, Five traces(top) are the responses of a song-selective LMAN projection neuron to injected currents (bottom). Note the high spontaneous synaptic activity and gradual spike onset and sharp afterhyperpolarization characteristic of these neurons.C, The highly linear relationship of firing frequency to current injection amplitude (n = 49 cells) is shown. D, Instantaneous firing frequency is plotted as a function of spike interval number, showing slight accommodation (1 sec pulses; +600 and +800 pA; n = 49 cells;dotted lines are linear fits for each current amplitude). E, Examples of a LMAN projection neuron's selective response to BOS versus reversed BOS are shown. The 10traces at the top are raw intracellular current-clamp records of responses to repeated song stimuli [depicted as an oscillogram in the lowesttrace, with syllables delineated by lettersa–d(i, introductory notes)]. The plotimmediately below the raw traces is an action potential PSTH (25 msec bin width), below which is the median-filtered average V_m showing underlying subthreshold responses (see Materials and Methods).