Masking is defined as the change in threshold energy eT*(τ) of a test stimulus T induced by a masking stimulus M of energy eM as a function of the relative time τ of occurrence. Masking is maximum when T and M 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 eT*/eM was investigated for masking impulses M of seven different energies superimposed on five backgrounds B. The increases in test threshold caused by M and by 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* using a category-rating-scale method.
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.
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.
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