2AFC methods are frequently used to assess the influence of a secondary stimulus parameter (e.g. stimulus noise) on the percept of a primary stimulus dimension (e.g. motion direction). In this case, test and reference not only differ along the primary stimulus dimension, but also in their value of the secondary stimulus parameter. As a result, even a thorough instruction does not guarantee that a subject's choice is exclusively based on the perceived differences along the primary stimulus dimension. This can lead to an incorrect characterization of perceptual biases and thresholds.
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