Figure 9. Mean area under the receiver operating characteristic curves (AUC) as a function of normalised modulation frequency (f_m), modulation depth, and sound level, for a population of PL, CT, and CS units.

The area under the ROC curve decreases with decreasing modulation depth and with increasing sound level. This corresponds to a decreasing ability of neurons to discriminate between modulated and unmodulated signals by means of spike synchrony at f_m as modulation depth decreases, and as sound level increases. Colour key as in Fig. 8. Top row, at 10 dB above pure-tone threshold. Bottom row, at 40 dB above pure-tone threshold. Left column, primary-like (PL) responses. Middle column, transient chopper (CT) responses. Right column, sustained chopper (CS) responses. Dashed line at AUC = 0.5 indicates chance performance. Dashed line at AUC = 0.75 indicates threshold for AM detection (i.e. AUC significantly different from 0.5). Error bars indicate 95% confidence intervals around the population mean.