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Two-interval forced choice: This is another method, elaborating the forced-choice method, that we use frequently. In this method, there are two stimuli presented (two "intervals"). They can be visual stimuli presented side-by-side, or they can be visual or auditory (or other) stimuli presented one after the other. One of the stimuli is a blank (noise-alone) and the other is a signal+noise stimuli (with an intensity that is typically randomly varied from trial to trial).
Two-alternative forced choice (2AFC) (and the variant Two-interval forced choice (2IFC)) Task is a psychophysical method, developed by Gustav Theodor Fechner, for eliciting responses from a person about his or her experiences of a stimulus. Specifically, the 2AFC experimental design is commonly used to test speed and accuracy of choices between two alternatives given a timed interval. The task is an established controlled measure of choice and is widely used to test a range of choice behaviors in animals and in humans.
Perception & Psychophysics, 70, 1151-1164. [pdf] Yeshurun, Y. , Carrasco, M. & Maloney, L. (2008) Bias and Sensitivity in two-interval forced choice procedures: Tests of the difference model. Vision Research, 48, 1837-1851.
^ Miller et al. , 2002. "Nonparametric relationships between single-interval and two-interval forced-choice tasks in the theory of signal detectability". Journal of Mathematical Psychology archive. 46:4; 383 - 417. Available from: http://portal. acm. org/citation.
We estimated the tone level necessary for 94% correct detections with a two-interval forced-choice procedure in which we adaptively adjusted the tone level in each 30-trial block by using the maximum-likelihood method (26). Tone thresholds for individual listeners are based on either one or the mean of two estimates for the long tone and on the mean of two or three estimates for the short tone.
[pdf] Yeshurun, Y. , Carrasco, M. & Maloney, L. T. (2008), Bias and sensitivity in two-interval forced-choice procedures, Vision Research, 48, 1837-1851 [pdf] Gepshtein, S. , Elder, J. & Maloney, L. T. (2008), Perceptual organization and neural computation, Journal of Vision, 8(7):i, 1-4. [pdf] Fulvio, J.