Decrement of the Brentano Muller-Lyer illusion as a function of inspection time

Two experiments are reported in which the decline or decrement in the magnitude of the Brentano Muller-Lyer illusion was measured. Observers made a pre-test judgment and, after a variable intervening time period, a post-test judgment of illusion magnitude. In experiment 1, the intervening time periods were 1, 2, and 3 min during which time the independent groups of observers allocated to each of the three time periods either systematically scanned the Brentano figure (inspection conditions) or waited until the intervening period had elapsed (no-inspection conditions). Experiment 2, which included an additional 5 min intervening time period, evaluated a response-bias explanation for the results of the inspection conditions of experiment 1. Taken together, the findings of the two experiments indicate that sheer inspection of the Brentano figure produces illusion decrement. However, illusion decrement was independent of the duration of the inspection period, with equivalent amounts of decrement occurring across the range of viewing times examined in the two experiments. The pattern of these results suggests that theories of Muller-Lyer decrement must incorporate a factor attributable to, or correlated with, inspection time, whose effect in reducing illusion magnitude is confined mainly to the first 1 or 2 min of active visual inspection of the Brentano illusion figure.