Informational masking of negative masking

Abstract

Negative masking (NM) is a ubiquitous finding in near-"threshold" psychophysics in which the detectability of a near-threshold signal improves when added to a copy of itself, i.e., a pedestal or masker. One interpretation of NM suggests that the pedestal acts as an informative cue, thereby reducing uncertainty and improving performance relative to detection in its absence. The purpose of this study was to test this hypothesis. Intensity discrimination thresholds were measured for 100-ms, 1000-Hz near-threshold tones. In the reference condition, thresholds were measured in quiet (no masker other than the pedestal). In comparison conditions, thresholds were measured in the presence of one of two additional maskers: a notched-noise masker or a random-frequency multitone masker. The additional maskers were intended to cause different amounts of uncertainty and, in turn, to differentially influence NM. The results were generally consistent with an uncertainty-based interpretation of NM: NM was found both in quiet and in notched-noise, yet it was eliminated by the multitone masker. A competing interpretation of NM based on nonlinear transduction does not account for all of the results. Profile analysis may have been a factor in performance and this suggests that NM may be attributable to, or influenced by, multiple mechanisms.

FIG. 1.

Masking functions obtained in quiet (open symbols, solid lines) and in the random condition (black symbols, dashed lines) for the three observers and the mean across observers. Error bars are standard errors of the mean.

FIG. 2.

Psychometric functions for all three observers based on fits to the adaptive data. Psychometric functions are least-squares fit to the underlying data, representing the function d' = a(ΔA/A_0,Quiet)^b. The three lines in each panel represent three conditions: detection in quiet (solid lines), intensity discrimination in quiet for 0 dB SL pedestals (dashed lines), and intensity discrimination in the random condition for 0 dB SL pedestals (dotted lines).

FIG. 3.

Masking functions obtained in quiet (open symbols, solid lines), in the notched-noise condition (gray symbols, dotted lines), and in the random condition (black symbols, dashed lines) for all three observers and the mean across observers. Error bars are standard errors of the mean.

FIG. 4.

As in Fig. 3 except that the notched-noise and random data shown here were obtained with different physical pedestal levels, chosen so as to bracket and include A_0,Masked. In quiet, A_0,Quiet=A_0,Masked and thus quiet thresholds are replotted from Fig. 3.