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Andrew J. Zele, Dingcai Cao, Joel Pokorny; Lets get physical: Metrics for suprathreshold tasks. Journal of Vision 2005;5(12):98. doi: 10.1167/5.12.98.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To compare reaction time (RT) data expressed in physical, contrast units and psychophysical, threshold units. We explored a parametric data set for rod and cone reaction time (Cao et al, this meeting). Methods: An 8-channel photostimulator provided independent control of rod and cone stimulation. Threshold versus Radiance functions were measured for rod increments and decrements, favoring ON- and OFF-pathways respectively. Suprathreshold RTs were measured as a function of incremental and decremental Weber contrast. Results: At low light levels, rod thresholds for decremental stimuli were lower than for incremental stimuli. At 0.02 Td, RTs to decrements were faster than to increments when specified in Weber contrast, but slower when expressed in threshold units. Conclusions: Here we show that the choice of metric can reverse the interpretation of a physiological process in the presence of a differential threshold sensitivity between stimulus conditions. Quantification of suprathreshold stimulus magnitude in psychophysical threshold units introduces a bias in the direction of the mechanism with poorer threshold sensitivity. Without prior knowledge that the system under study is governed by the same rules at threshold and suprathreshold levels, physical contrast offers a more robust metric for specifying suprathreshold stimuli.
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