EFFECTS OF FORWARD MASKING ON INTENSITY DISCRIMINATION, FREQUENCY DISCRIMINATION, AND THE DETECTION OF TONES IN NOISE

A series of experiments investigated the deterioration in intensity discrimination at intermediate levels caused by preceding the brief sinusoidal stimuli by an intense narrow-band noise [Zeng et al., Hear. Res. 55, 223-230 (1991)]. Experiment 1 showed that this deterioration acted in a roughly additive manner with the previously reported midlevel deterioration for the intensity discrimination of brief 6500-Hz tones in quiet. The experiment also showed that a smaller, although still substantial, deterioration occurred for the detection of brief tones in bursts of synchronous noise. In experiment 2, the masker could occur either before the signal (louder tone) or standard (softer tone), the choice being selected at random on a trial-by-trial basis, and with the Weber fraction tracked for each condition separately using two interleaved, adaptive procedures. Weber fractions were lower when the masker preceded the signal than when it preceded the standard, arguing against a major role for adaptation in causing the midlevel deterioration. Instead, it is suggested that the deterioration shared a common mechanism with the phenomenon of ''loudness enhancement'' [R. J. Irwin and J. J. Zwislocki, Percept. Psychophys. 10, 189-192 (1971); Galambos et al., J. Acoust. Soc. Am. 52, 1127-1130 (1972)]. The third experiment showed that Weber fractions did not depend greatly on whether the forward masker occurred only in interval one or only in interval two of a two-interval trial. A fourth experiment investigated the effect of a forward masker on frequency difference limens (DLs). When the masker was presented in both intervals of each trial, there was only a moderate increase in frequency DLs which, in contrast to the elevation in Weber fractions for intensity discrimination, was independent of signal level. Presenting the masker before only the lower frequency tone of a pair to be discriminated usually yielded lower DLs than presenting it only before the higher frequency tone, but this effect was less consistent than for intensity discrimination. The results of all experiments are discussed with respect to the various ways in which an intense narrow-band masker can affect the perception of subsequent brief sounds.