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

Forward masking PTCs were measured in patas monkeys (Erythrocebus patas) at 500 Hz and 2, 4, and 8 kHz and signal levels of 10, 30, and 60 dB SL in quiet, and at 10 dB above masked threshold in two levels of wideband noise. Noise levels were sufficient to increase masked test‐tone thresholds to levels that approximated the absolute signal levels tested at 30 and 60 dB SL in quiet. The background masker was used to control for “off‐frequency” listening, which can act to broaden PTCs at higher signal levels, by maintaining a constant signal‐to‐noise ratio with increases in SPL. Results in quiet are in agreement with those reported in the human literature demonstrating broadening of the PTC as signal level is increased. The PTCs measured in noise also demonstrate a similar broadening, or loss of selectivity, at higher SPLs. These findings are not in agreement with a previous study [Green et al., J. Acoust. Soc. Am. 69, 1758–1762 (1981)] that attempted to control off‐frequency listening with background maskers at different signal intensities in humans. Differences in findings might be explained as differences in acoustic or experimental parameters between studies. [Research supported by NIH Grant No. NS 05785.]