Decrement of the Brentano Müller-Lyer Illusion as a Function of Inspection Time

Two experiments are reported in which the decline or decrement in the magnitude of the Brentano Müller-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 Müller-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.

‘Reversed’ Illusion with Three-Dimensional Müller-Lyer Shapes

The purpose of this study was to determine whether the Müller-Lyer illusion is produced by a mechanism which uses information defined in the retinal coordinates, or by a mechanism taking into account the three-dimensional (3-D) shape of the illusion figure. The classical Müller-Lyer figure could not be used to address this question since it is two-dimensional. Three-dimensional Müller-Lyer figures were created to see if the illusion they produce is correlated with the shape of the projected retinal image, or with the shape of these figures defined in a 3-D coordinate frame. In the experiments retinal image shape was juxtaposed against the 3-D shape of the illusion displays. For some displays the direction in which the fins pointed, relative to the shafts, in the 3-D frame was the ‘opposite’ of the direction in which they pointed in the retinal images. For such displays, the illusion predicted on the basis of the 3-D structure was the opposite of that predicted on the basis of retinal image shapes. For another 3-D display the fins were oriented such that each projected a single straight line in the retinal image, thus the typical retinal image (< >, > <) was replaced by straight lines (‖, ‖). For all the displays the observed illusion was consistent with how the fins were oriented relative to the shaft in the 3-D coordinate frame, ie with the 3-D shape of the illusion displays. The retinal image shape appeared to play little, if any, role. One conclusion that emerges is that the specific retinal image shape projected by the classical line-drawn pattern is neither necessary nor sufficient for producing the illusion. The present findings are inconsistent with two well known theories of the Müller-Lyer illusion: inappropriate constancy scaling and selective filtering.

Contextual Effects in the Parallel Lines Illusion: Some Implications for Assimilation Theory

32 undergraduates participated in each of two experiments designed to determine how structural manipulations of the parallel lines figure affect the resulting illusion. Collectively and for the range of figures studied, the data show (1) progressive elongations of the contextual lines of the illusion figure first increase, then decrease, the apparent length of the judged line; (2) the misestimation of the length of the judged line diminishes as the separation between it and shorter contextual lines increases; and (3) variations in contour lightness significantly affect the illusion when the contextual lines are shorter than and at any distance from the judged line or when the contextual lines are slightly longer than and at an intermediate distance from the judged line. These data indicate that size-assimilation, as well as size-contrast processes, contributes to the parallel lines illusion and that the assimilation process must be mediated by both sensory and cognitive activity in the visual system.

The Role of the Second Oblique in the Poggendorff Illusion

It has been observed previously in the classic Poggendorff illusion figure that the end of the oblique line which abuts onto the vertical parallel appears more misaligned than its free end. Various theories suggest that angular distortions are largest at the point of abutment. Two experiments are reported which, however, suggest that, compared to displays in which a single dot is aligned with an oblique line, the introduction of a second, parallel oblique segment does not increase errors more at the abutting tip than at the free end, but, rather, reduces them more at the free end. These results not only cast doubt on the relative importance of angular effects alone in the classic Poggendorff display, but also emphasise the complexity of that figure as a tool in the investigation of visual processing.

Intermanual Transfer of Practice Decrements with a Haptic Illusion

Sixteen subjects were given 8 pre-practice, 80 practice, and 8 post-practice trials with the inverted T illusion figure. Judgements were made haptically with one hand being used on pre- and post-practice trials and the other hand on practice trials. Mean illusion diminished in magnitude over the practice trials. Inter-manual transfer of the practice decrement was found, but it was partial rather than complete. The illusion after practice was less than before practice, but it was greater than the error found on the last block of practice trials. The bearing of these results on the different theories of illusions is considered.