SIGNAL-FREQUENCY UNCERTAINTY IN SPECTRAL-SHAPE DISCRIMINATION - PSYCHOMETRIC FUNCTIONS

Psychometric functions were measured for spectral-shape discrimination (profile analysis) with the signal frequency either fixed, what we ca ll the signal-known condition, or randomly varied, what we call the si gnal-unknown condition. The functions were obtained using an adaptive, up-down procedure. In the signal-unknown condition, independent track s for each signal frequency were interleaved within the same block of trials. The mean slope of the psychometric function [k in log d'= k lo g Delta L + C, where Delta L = 20 log(1 + Delta p/p)] was 1.13 for the signal-known conditions and 1.19 for the signal-unknown condition. Th e mean signal-to-standard ratio at threshold (Pc = 79.4%) obtained in the signal-unknown condition was 3 to 4 dB higher than that obtained i n the signal-known conditions. The psychometric functions of the ideal observer were derived for both the signal-known and signal-unknown co nditions. A comparison of the measured and derived psychometric functi ons suggests that the internal noise component that is statistically i ndependent across frequency channels contributes minimally to the tota l (internal and external) noise sources that are involved in the decis ion process.