Correlated individual differences in conditions used to measure psychophysical suppression and signal enhancement

Two-tone suppression and notched-noise signal enhancement (overshoot) were measured at 1000 Hz using standard procedures in 40 normal-hearing, naive adults, counterbalanced for sex, listening ear, and testing order. The threshold range across subjects for the same condition often exceeded 30 dB. In the suppression conditions, for some subjects the 20-ms forward-masked signal was as much as 15 dB easier to detect when the total masker (1000 Hz) and tonal suppressor (1150 Hz) were presented together, than when the masker was presented alone, indicating suppression. For other subjects, however, presenting the masker and suppressor together increased thresholds by as much as 13 dB, indicating additional masking. On average, adding the suppressor to the masker increased threshold by 0.6 dB. In the enhancement conditions, for every subject the 20-ms signal was harder to detect when it was presented at the onset of a 500-ms notched-noise masker (notch width: 1000 Hz) than when signal onset was delayed by 400 ms, and was hardest to detect when the signal and masker were gated on and off together for 20 ms. The average thresholds were 7 and 19 dB higher for the two conditions in which the signal was presented at masker onset than for the condition in which the signal was presented after a delay, indicating signal enhancement. The thresholds in all of the listening conditions appeared to be primarily determined by a single factor. However, it is not clear whether the best designation for this factor would be sharpness of frequency tuning or across-channel inhibitory strength.