The different effects of aspect ratios of letters on the letter-row tilt illusion in staircase and non-staircase stimuli

The letter-row tilt illusion is the illusion that the row is perceived to be tilted, when a set of letters is repeated in a physically horizontal or vertical row. We quantitatively examined the effects of aspect ratios of letters on the letter-row tilt illusion in horizontal letter-rows with or without a staircase structure of horizontal line segments. In Experiment 1, the results quantitatively showed that the illusion significantly occurred in letter-rows with and without the staircase structure. In Experiment 2, the results showed that the amount of illusion in letter-rows with the staircase structure increased as relative and absolute lengths of the horizontal line segments increased. In Experiment 3, the results showed that the amount of illusion in letter-rows without the staircase structure had different tendencies in aspect ratios from that with the staircase structure. The present study suggested that different mechanisms were responsible for the letter-row tilt illusions with and without the staircase structure.