ERP bootstrap results. Significant ERP amplitude difference at all electrodes (vertical axis). Gray areas represent non-significant ERP amplitude differences. Significant average ERP amplitude differences (μV) are represented in an increasing gradient from blue to red (jet64). Bootstrap tests revealed significant amplitude differences between (a) voices vs. birdsongs, (b) voices vs. environmental sounds, and (c) voices vs. both birdsongs and environmental sounds (conjunction of (a) and (b)).