A large-scale horizontal-vertical illusion produced with small objects separated in depth

Area of Science:

• Visual perception
• Psychophysics
• Geometric illusions

Background:

• The large-scale horizontal-vertical illusion (HVI) is a significant visual distortion, typically 15%-20%.
• Previous research linked HVI to the presence of a ground plane, influencing size perception.
• Understanding HVI's basis is crucial for visual science and virtual environment design.

Purpose of the Study:

• To investigate the underlying mechanisms of the large-scale horizontal-vertical illusion (HVI).
• To determine the role of object distance and depth separation in HVI magnitude.
• To explore the relationship between HVI, ground plane orientation, and visual coding of spatial information.

Main Methods:

• Two experiments with a total of 81 participants were conducted in a virtual environment.
• Participants matched vertical rod sizes to horizontal extents, manipulating object size, distance, and depth separation.
• Measurements involved adjusting horizontal gaps or rods to determine perceived size and HVI.

Main Results:

• Larger/farther objects produced a greater HVI (∼13%) compared to smaller/nearer objects (∼7%).
• The horizontal gap measurement method was not the sole cause of the larger bias observed.
• Separating the comparison rod in depth from the vertical rod induced a substantial HVI (17%), irrespective of object size.

Conclusions:

• Depth separation, not solely the ground plane, is a key factor in generating large-scale HVIs.
• The findings suggest HVI may relate to differential visual coding of elevation and azimuth, influenced by depth cues.
• This research refines our understanding of HVI and its dependence on spatial context in visual perception.