Entrainment of spiny neurons into antidromic stimulation is robust and time dependent. Entrainment of spiny neurons into antidromic stimulation is robust and time dependent. A, Top, Raster plot of antidromic stimulation at 10 s intervals. During the first antidromic stimulus (10 pulses at 20 Hz, 0.2 ms duration, 18 μA), 37% of all fluo-4-loaded cells fired together (first arrow). Fewer neurons were recruited upon repeated stimulation of the same stimulus (responsive neurons in red). Bottom, Peaks of synchronization show that ∼50 ± 6% of the initially activated neurons become unresponsive after the fifth stimulus (p < 0.001; n = 7), demonstrating cumulative inhibition at brief time intervals. B, Top, Raster plot of antidromic stimulation at 120 s intervals. During the first antidromic stimuli (10 pulses at 20 Hz, 0.2 ms duration, 18 μA), numerous neurons fire together being ∼37% of all fluo-4-loaded cells (first arrow). Upon repeated stimulation, almost the same neurons (85 ± 5%) are recruited each time. Bottom, Histogram shows peaks of synchronization with a maintained number of recruited neurons through time. C, Contour map depicting the field of view of responsive cells. Red circles represent cells that respond to antidromic stimulation. D, Correlation matrix of active cells showing that antidromic stimulation robustly recruits the same set of projection neurons. Experiments performed in rats. Violeta G. López-Huerta et al. J. Neurosci. 2013;33:4964-4975 ©2013 by Society for Neuroscience