Contrast discrimination by the methods of adjustment and two-alternative forced choice

Abstract

Fifty years after the advent of signal-detection theory, some visual scientists still adhere to the notion of a high, fixed threshold. Indeed, experienced observers are able to deliver consistent settings of thresholds using the method of adjustment, but those settings are different from the thresholds obtained with two-alternative forced choice (2AFC) procedures. This article illustrates the difference by reference to four existing studies of contrast discrimination. All four sets of data show clear subthreshold facilitation, but the method of adjustment produces a rectilinear characteristic that corresponds to the use of different criteria over different ranges of contrast, whereas 2AFC procedures produce a curvilinear characteristic with generally lower thresholds. This article proposes a theoretical explanation for the difference.

Near perfect visual compensation for atmospheric color distortions

“…Despite the advantages of the classic paired comparison methodology, 29 the method of adjustment was preferred. Under the conditions of the experiment, good resolution with a 2AFC methodology can only be obtained with an enormous and unpractical number of comparison trials 30…”

Mobile app Aston contrast sensitivity test

“…These higher threshold values seen with the app charts may be a result of using the method of adjustment (patients were asked to find the height of the vertical bars). This is analogous with an adjustment‐based descending trial, which increases threshold measures in psychophysical experiments compared to 2‐AFC methods. While the CSV‐1000 paper chart displays a definitive peak threshold at 6.0 cpd with no foil in place, the app Aston contrast sensitivity charts shows similar sensitivity at 1.5 and 3.0 cpd before thresholds reduce at higher spatial frequencies.…”

Efeitos da estimulação transcraniana por corrente contínua sobre o processamento visual básico

“…Assim, é possível que o efeito encontrado com o método do ajuste reflita na verdade um efeito da ETCC sobre o critério de resposta dos participantes (Laming et al 2013, Turner et al 1994. Este é mais um exemplo de que iniciativas de replicação devem ser estimuladas na pesquisa em ETCC e que a escolha dos métodos de avaliação dos efeitos da ETCC é um passo crítico do delineamento destas pesquisa e deve ser sempre enfatizado.…”