<p>Masking is defined as the change in threshold energy <i>e</i><sub><b>T</b></sub>*(τ) of a test stimulus <b>T</b> induced by a masking stimulus <b>M</b> of energy <i>e</i><sub><b>M</b></sub> as a function of the relative time τ of occurrence. Masking is maximum when <b>T</b> and <b>M</b> occur simultaneously. A slight decrease in threshold for tests preceding the masking impulse by about 0.1 sec was explained as an alteration in appearance of the subsequent masking flash by a "subthreshold" test flash. Impulse-contrast threshold e<sub>T</sub><sup>*</sup>/e<sub>M</sub> was investigated for masking impulses <b>M</b> of seven different energies superimposed on five backgrounds <b>B</b>. The increases in test threshold caused by <b>M</b> and by <b>B</b> were found to be independent and a modified Weber’s law (adjusted contrast threshold Cδ*≍0.1) held approximately. This conclusion was supported in a supplementary investigation of Cδ<sup>*</sup> using a category-rating-scale method.</p><p>Impulse masking results were applied to predicting the masking peak at the onset of a long flash by treating the first 60 msec as an impulse. The lowering of thresholds of tests delayed in a long masking flash implied other detection mechanisms (e.g., temporal resolution). Theoretical predictions accounted for 94% and 97% of the variance in two relevant experiments, correctly predicting the effect of masking-flash duration and of background intensity.</p>In both steady and intermittent light, masking is attributed primarily to fast processes (time constant «1 sec) which presumably have a neural rather than a photochemical basis.
GEORGE SPERLING, "Temporal and Spatial Visual Masking. I. Masking by Impulse Flashes," J. Opt. Soc. Am. 55, 541-559 (1965)