Fig. 5.

Irregular firing in spiking networks with graded persistent activity. a, Experimentally measured irregular firing (coefficients of variation of inter-spike intervals, CV_isi, higher than 1) during persistent activity in a delayed-saccade task. Adapted from [16]. b, Structure of network of spiking neurons with negative-derivative feedback. c–k, Network response to a brief (100 ms) stimulus applied at time 0. c–e, Raster plots illustrating irregular persistent firing of 50 example excitatory neurons. f–h, Instantaneous, population-averaged activity of excitatory neurons, computed within time bins of 1 ms (gray) or 10 ms (black). i–k, Balance between population-averaged excitation and inhibition following offset of external input. l–n, Histogram of CV_isi of active excitatory neurons during the persistent firing. Note that, for activity with strong input, a small subset of neurons fire regularly at high rate and exhibit low CV_isi (n). This reflects that the heterogeneity resulting from our simple assumption of completely randomly connected networks can result in excess positive feedback in some clusters of neurons.