Shifts of the psychometric function: Distinguishing bias from perceptual effects

Abstract

Morgan, Dillenburger, Raphael, and Solomon have shown that observers can use different response strategies when unsure of their answer, and, thus, they can voluntarily shift the location of the psychometric function estimated with the method of single stimuli (MSS; sometimes also referred to as the single-interval, two-alternative method). They wondered whether MSS could distinguish response bias from a true perceptual effect that would also shift the location of the psychometric function. We demonstrate theoretically that the inability to distinguish response bias from perceptual effects is an inherent shortcoming of MSS, although a three-response format including also an "undecided" response option may solve the problem under restrictive assumptions whose validity cannot be tested with MSS data. We also show that a proper two-alternative forced-choice (2AFC) task with the three-response format is free of all these problems so that bias and perceptual effects can easily be separated out. The use of a three-response 2AFC format is essential to eliminate a confound (response bias) in studies of perceptual effects and, hence, to eliminate a threat to the internal validity of research in this area.

Visuospatial selective attention in chickens

“…3F) is the proportion of incorrect Go responses as a fraction of the total number of all responses (Go + NoGo). Other models have incorporated lapse rates when fitting data with a 2-AUFC-like model (25). Analyzing our data with such models could result in more accurate estimates of the d′ and bias (absolute values), but would not alter our claim regarding the improvements of d′ (relative values) with cueing.…”

“…3C). The importance and advantages of incorporating such opt-out responses in lieu of the standard 2-AFC design have been highlighted previously (25). Modeling the decision-making process with a 2-AUFC model (indecision model) permitted us to demonstrate that the strategy used by the birds in performing the task was not one of maximizing rewards, but one of maintaining a constant (high) certainty of averting errors when making a Go choice (Fig.…”

“…However, in our task, they could give a NoGo (opt-out) response that was occasionally rewarded (with a probability of 0.33). Tasks that include such an opt-out choice have been referred to as "unforced choice," "free choice," or "uncertain option" tasks (23)(24)(25). As explained below, the unforced-choice design of our task allowed us to estimate the birds' confidence in their ability to localize target stimuli.…”

“…3A, biases b 1 and b 2 , thick vertical lines) that represented cutoff values for each decision (for the upper location or the lower location) along the decision axis. A similar signal detection model, called the "indecision model" with asymmetric criteria, has been proposed recently (25).…”

On the discrepant results in synchrony judgment and temporal-order judgment tasks: a quantitative model

“…We have also shown that lack of bias in TOJ tasks (i.e., ξ 0 .5) does not solve the problem. This is reminiscent of a similar contamination that has the same deleterious effect in 2AFC tasks aimed at estimating psychometric functions and thresholds in detection or discrimination experiments and that can only be surmounted by replacing the binary response format of the 2AFC task with a ternary format in which observers are additionally allowed to give an "I don't know" response García-Pérez, 2010;García-Pérez & Alcalá-Quintana, 2010a, 2010b, 2011a, 2011b, 2012bGarcía-Pérez, Alcalá-Quintana, Woods, & Peli, 2011). These results concur with those of Ulrich (1987) and van Eijk et al (2008) in advising against the use of TOJ tasks in research on perception of temporal order (see also Spence & Parise, 2010).…”