SIMILAR MULLER-LYER EFFECTS FROM OPERANT AND COMPARISON RESPONSE-MODES

A one-sided (monopole) Version of the Muller-Lyer was used to assess t he effect of response mode on the metric distortion associated with th e illusion. Two different response modes were tested for comparability . The Operant Mode required the judgment of stimulus span to be indica ted by marking a dot on the test page. The Comparison Mode required ju dgment of equality of two adjoining prepositioned spans. The perceptua l effects with the comparison procedure are known to consist of undere stimation of spans which are bounded by concave fin-sets and overestim ation of spans which are bounded by convex fin-sets. The question is w hether the motor demands of the operant task produce an additional sou rce of metric bias. Analysis indicate a very high correlation of judgm ents for the two modes of testing. Also, data with either mode of resp onding closely fit a linear model of the effect, and the model provide s comparable index values for the concave and convex versions. These r esults should mitigate concern about potential motor bias from the ope rant method and encourage its use as a more efficient procedure for as sessing metric distortion.