What is the appropriate control for the tilt illusion?

Area of Science:

• Visual perception
• Psychophysics
• Optical illusions

Background:

• The measurement of tilt illusions using parallel matching to an acute angle's test arm is common.
• A standard control condition involves matching the test arm without the inducing arm present.
• Previous research suggests this control may be inappropriate due to a confounding depth-based illusory effect.

Purpose of the Study:

• To provide direct evidence for the inappropriateness of the standard control condition in tilt illusion measurements.
• To investigate alternative control conditions for accurately measuring tilt illusions.
• To determine the most suitable stimulus configuration for a universal control condition.

Main Methods:

• Directly testing the influence of a depth-based illusory effect in the pretest control condition.
• Comparing the results of the proposed inappropriate control with alternative conditions.
• Evaluating different inducing-test-line angles for their efficacy as control stimuli.

Main Results:

• Direct evidence confirms the presence of a depth-based illusory effect in the standard pretest control.
• This confounding effect is not present in the test conditions, validating previous indirect findings.
• No single stimulus configuration serves as an appropriate pretest control for all tilt illusion stimuli.

Conclusions:

• The standard control condition for measuring tilt illusions by parallel matching is indeed inappropriate.
• A control condition where the test and inducing lines intersect at 90 degrees appears to be the most appropriate for all inducing-test-line angle displays.
• Future research should adopt this 90-degree intersection as a more accurate control to avoid confounding depth-based illusions.