The relation between sensation level and frequency discrimination under conditions of informational masking

Abstract

Previous studies of resolution of the frequency, intensity, and duration of individual elements of word-length tonal sequences have shown major degrading effects of trial-to-trial stimulus uncertainty. The same degree of stimulus uncertainty was recently found to cause elevations of detection thresholds of 40–50 dB above the levels expected from temporal masking [Watson, Kelly, and Mellen, J. Acoust. Soc. Am. 66, S83 (1979)]. We referred to the effect of stimulus uncertainty on the threshold of individual elements of patterns as informational masking. In the current experiment, we investigated the relation between the amount of informational masking and frequency discrimination for the same target tones. Detection and discrimination trials were interleaved in an adaptive psycho-physical sequence to measure observers' detection thresholds and frequency difference limens for a 45-ms, 1688-Hz target tone, presented 90 ms after the onset of a 10-tone pattern. Context (non-target) tones varied between 25–45, 35–55, or 65–85 dB SPL in level, 300–3000 Hz in frequency, and 25–105 ms in duration. Target-tone levels on frequency discrimination trials were 35, 45, 55, or 75 dB. Informational masking was found to increase 10 dB for each 10-dB increase in context level, essentially as does energy masking, although the source of the two effects is clearly different. At all negative sensation levels, the threshold values of ΔF were those required to shift target-tone frequency beyond the limits of the pattern bandwidth. Frequency discrimination improved systematically as current “effective” sensation level increased from 0.0 to 40–50 dB, yielding a range of just-detectable values of Δf from over 1000 to about 20 Hz. The relation between sensation level and frequency discrimination is thus quite similar for informational and energy masking. [Work supported by NIH.]