Effects of changes in absolute signal level on psychophysical tuning curves in quiet and noise in patas monkeys.

Forward masking psychophysical tuning curves (PTCs) were measured in patas monkeys (Erythrocebus patas) at 2, 4, and 8 kHz at signal levels of 10, 30, and 60 dB SL in quiet, and at 10 dB above masked threshold in two levels of wideband noise. Absolute signal levels with masking approximated those at 30 and 60 dB SL in quiet. Results in quiet agree with those reported in the literature, demonstrating broadening of the PTC as signal level is increased. The PTCs measured in noise also demonstrated a similar broadening, or loss of selectivity, at higher SPLs. These later findings differ from those of a previous study [D.M. Green, B.R. Shelton, M.C. Picardi, and E.R. Hafter, J. Acoust. Soc. Am. 69, 1758-1762 (1981)] which used maskers to control the broadened excitation pattern in humans at levels of up to 34 dB above threshold. Differences in findings might be attributed to higher SPLs used in the present study. The data taken in noise backgrounds are not consistent with explanations for broadening based on an increase in the width of excitation patterns, but instead support the suggestion that the filter itself is nonlinear. Moreover, comparisons of PTCs in quiet and noise suggest that "off-frequency" listening acts at any given measurement level to artificially sharpen PTCs.