The intensity-difference limen for Gaussian-enveloped stimuli as a function of level: Tones and broadband noise

Van Schijndel et al. [J. Acoust. Soc. Am. 105, 3425-3435 (1999)] have propo sed that the internal excitation evoked by an auditory stimulus is segmente d into "windows" according to the stimulus spectrum and stimulus length. Th is "multiple looks" model accounts for the mid-duration hump they observed in plots of intensity-difference limens (DLs) versus pip duration for Gauss ian-shaped 1- and 4-kHz tones, an effect replicated by Baer et al. [J. Acou st. Soc. Am. 106, 1907-1916 (1999)]. However, van Schijndel et al. and Baer et al. used few levels. A greater number of levels were used by Nizami (19 99) for Gaussian-shaped 2-kHz tone-pips whose equivalent rectangular durati on (D) was 1.25 ms. The DLs show the mid-level hump known for clicks [Raab and Taub, J. Acoust. Soc. Am. 46, 965-968 (1969)]. At some duration this pa ttern must become the "near-miss to Weber's law." To determine this duratio n, as well as the level-dependence of the mid-duration hump, DLs were estab lished for Gaussian-shaped 2-kHz tone-pips of D=1.25, 2.51, and 10.03 ms at levels of 30-90 dB SPL. The across-subject average DLs for the tone-pips r ise up at mid-levels for D=1.25 and D=2.51 ms. The DLs for D=2.51 ms are la rger, creating the mid-duration hump. At all durations, the new DLs are sma ller at high levels than at low levels, consistent with the near-miss to We ber's law. DLs were also obtained here for Gaussian-shaped broadband-noise pips of D=0.63, 1.25, 2.51, 5.02, and 10.03 ms. The DLs for the noise-pip s how a add-level hump for all pip durations. The noise-pip DLs decrease as t he pip lengthens, such that the plot of DL versus log duration shows a line ar decline, with no mid-duration hump. Analysis of variance reveals that th e mid-level hump coexists with the classical patterns of level-dependence, perhaps reflecting the existence of two level-encoding mechanisms, one that depends on firing-rates counted over single neurons and which is responsib le for the classical patterns, and one that depends on the initial coordina ted burst of neuronal spikes caused by rapid ramping, and which presumably causes the mid-level hump. (C) 2001 Acoustical Society of America.