In Greenwood [J. Acoust. Soc. Am. 33, 484–502 (1961a)] the ratio of masked signal threshold to masker level (S/M) decreased about 4 dB at a masker level of about 50 dB SL, the ‘transition’ level, when noise bands were subcritical but not when supercritical. Schlauch et al. [J. Acoust. Soc. Am. 71, S73 (1982)] report a related result. A pilot study [Greenwood, Harvard Psychoacoustic Lab. Status Report 37, 8–9 (1961)] in which pure tones masked identical tones in-phase showed a larger change in S/M. Detailed tone-tone growth-of-masking curves from over a dozen subjects in 1967–1969, and in 1960, are reported here. A transition in slope, of variable abruptness, often begins to occur at about 50 dB SL, dropping S/M ratio by 6 to 8 dB or more [Rabinowitz et al., J. Acoust. Soc. Am. 35, 1053 (1976)]; the curves sometimes possess two segments, sometimes are simply convex. All have overall slopes less than 1.0, known also as the ‘near miss’. Consistent with other results [Zwicker, Acustica 6, 365–396 (1956); Viemeister, J. Acoust. Soc. Am. 51, 1265–1296 (1972); Moore and Raab, J. Acoust. Soc. Am. 55, 1049–1060 (1974)], addition of low-level wide-band and high-pass noise was found to counteract the change in S/M, i.e., to raise the high-level section of the growth-of-masking curve. However, the ability of narrow ‘band-pass’ noise to exert this effect was greatest when added at a frequency ratio (band/masking-tone) of 1.3 to 1.5, which seems more closely to link the effects of added noise to the effects of increasing a masking band from sub- to supercritical width (above). Interpretation of the decrease in DL with level begins by noting that the ‘transition’ level correlates approximately with the level at which a primary unit population excited by a given pure tone begins rapidly to expand basally. Underlying this, the basalward shift of a tone's displacement envelope peak accelerates at about the same level [Rhode, J. Acoust. Soc. Am. 49, 1218–1231 (1971); Sellick et al., J. Acoust. Soc. Am. 72, 131–141 (1982)]. Given the hypothesis that the DL may (normally) be determined by detection of the shift in the basal-edge of the tone's neural excitation zone, explanations of changes in the size of the DL - with level and with addition of high-side noise-must be cast first in terms of cochlear physics, in order to account for the level change needed for a tone to dominate more territory, i.e., to recruit more basal area and receptors at the expense of other components. Then, second, neuro-anatomical and physiological explanation is needed to specify(a) how the mechanically determined edge, between zones of dominance, is preserved at central synapses by the differences in neural discharge on either side of, and at, the edge (in rate, phase-locked timing, and/or variance) and(b) what determines the size of the minimal edge-shift needed for detection. In short, primary units recruited by a tone change their firing patterns — including positive or negative rate change, but the edge-shift itself may be what ‘registers’ in the CNS to determine an intensitive DL, as when a spot of light, with diameter increasing and edges shifting on the retina, is perceived to expand.