Order effects in two-alternative forced-choice tasks invalidate adaptive threshold estimates
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Order effects in two-alternative forced-choice tasks invalidate adaptive threshold estimates Miguel A. García-Pérez 1 & Rocío Alcalá-Quintana 1
# The Psychonomic Society, Inc. 2020
Abstract Adaptive psychophysical methods are widely used for the quick estimation of percentage points (thresholds) on psychometric functions for two-alternative forced-choice (2AFC) tasks. The use of adaptive methods is supported by numerous simulation studies documenting their performance, which have shown that thresholds can be reasonably estimated with them when their founding assumptions hold. One of these assumptions is that the psychometric function is invariant, but empirical evidence is mounting that human performance in 2AFC tasks needs to be described by two different psychometric functions, one that holds when the test stimulus is presented first in the 2AFC trial and a different one that holds when the test is presented second. The same holds when presentations are instead simultaneous at two spatial locations rather than sequential. We re-evaluated the performance of adaptive methods in the presence of these order effects via simulation studies and an empirical study with human observers. The simulation study showed that thresholds are severely overestimated by adaptive methods in these conditions, and the empirical study corroborated these findings. These results question the validity of threshold estimates obtained with adaptive methods that incorrectly assume the psychometric function to be invariant with presentation order. Alternative ways in which thresholds can be accurately estimated in the presence of order effects are discussed. Keywords Psychophysics . Psychometric function . Adaptive methods . Threshold . Indecision model
One of the oldest-known and most intriguing regularities of psychophysical performance is what Fechner (1966, p. 75–77) called the “constant error”, by which the outcome of the perceptual comparison of two stimuli varies with their order of presentation. In Fechner’s studies, the size of the constant error reportedly varied across sessions for the same observers and he declared their general occurrence to be “totally unexpected” (p. 76). The cause of constant errors was unknown and early psychophysical studies on heaviness discrimination were always conducted with the standard weight lifted first in each comparison (e.g., Urban, 1908; Fernberger, 1913, 1914a, b, 1916, 1920, 1921). Constant errors arise in sequential presentations (e.g., lifting one weight and then the other) and in simultaneous presentations when these are possible (e.g., Electronic supplementary material The online version of this article (https://doi.org/10.3758/s13428-020-01384-6) contains supplementary material, which is available to authorized users. * Miguel A. García-Pérez [email protected] 1
Departamento de Metodología, Facultad de Psicología, Universidad Complutense, Campus de Somosaguas, 28223 Madrid, Spain
displaying two visual stimuli side by side) but here we will use the language of sequential presentations w
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