An illustration of the stimulus paradigm and the construction of frequency-level receptive field (FLRF). A) The probe alone paradigm was used to construct the control FLRF, and the forward masking paradigm (masker + probe) was used to construct the masked FLRF with preceding noise (masker). B) The left panel represents the control FLRF, and the right panel represents the masked FLRF under preceding noise conditions. The line curve in the right panel is the boundary of the control FLRF for comparison. Each short vertical line in the left and right panels represents a frequency versus level combination that elicited at least one spike for two presentations of probe stimuli. MT, minimum threshold; CF, characteristic frequency; BW, bandwidth of the FLRF. See texts in methods for details.