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Thom Carney, Stanley A. Klein; Mysterious temporal order effects in 2IFC experiments. Journal of Vision 2009;9(14):56. doi: 10.1167/9.14.56.
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Two interval forced choice (2IFC) is undoubtedly the most commonly used psychophysical method when unbiased threshold estimates are desired. In analyzing 2IFC data an assumption is made that interval 1 (I1) is compared to I2 and the interval with the higher perceived contrast is chosen. Nachmias (2006) and Lapid et al. (2008) raised questions about this assumption with their finding that when the standard was in the first 2IFC interval, thresholds (JND) were >30% lower than with the standard in the second interval. There are several possible explanations of asymmetries between the two intervals. 1) Inattention to one of the intervals can cause a JND bias. 2) A bias in the PSE, say due to inattention to one of the intervals, can leak into a JND bias. 3) The asymmetry in timing between the intertrial interval (ITI) and the interstimulus interval (ISI) could lead to asymmetric adaptation in the two intervals.. 4) The Nachmias' favored explanation is that rather than comparing the two intervals a comparison is made with a memorized virtual standard, and that standard is made less reliable if the test is in the first interval. We carried out a number of contrast discrimination experiments, varying the ISI and ITI with either a fixed standard or a roving standard, and with the test either placed always above the standard or on both sides. We fit separate psychometric functions to the data of each interval with care taken to ensure the staircase placed trials appropriately. Our results are not compatible with a pure version of any of the above hypotheses. Rather the data point to a combination of models being used and that combination can be different across subjects but the same for a given subject across runs. However, in general observers perceive the second interval as being higher contrast. We conclude that 2IFC can become an even more interesting method to use than previously thought, it can provide a window into the underlying processes.
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