Mean (symbols) and standard deviation (error bars) of the sound pressure level at threshold for a sinusoidal target. Individual data are shown with open symbols and group mean data with filled symbols on the far right. The top panel shows thresholds for the 8 Hz target. The bottom panel shows thresholds for the 64 Hz target in quiet (circles) and in the presence of an 8 Hz sinusoid, presented at an SL of 9 dB (diamonds). Colored boxes in the bottom panel quantify the amount of masking. Green boxes indicate positive masking (i.e., an increase in the SPL) and red boxes negative masking (i.e., a decrease in the SPL).