The tilted Hermann grid illusion: 'illusory spots' versus 'phantom bands'.

Area of Science:

• Visual perception
• Neuroscience
• Psychophysics

Background:

• The Hermann grid illusion, characterized by dark illusory spots at the intersections of a white grid on a black background, is a well-known phenomenon.
• Previous research suggested that minor alterations, such as changing straight bars to curved ones, could completely eliminate these illusory spots.

Purpose of the Study:

• To investigate whether the Hermann grid illusion is an all-or-none effect concerning bar curvature.
• To explore the relationship between bar curvature and the manifestation of illusory spots and phantom bands.
• To determine if different types of visual illusions can coexist within the same stimulus.

Main Methods:

• Systematic transformation of straight and curved Hermann grids into rhombic Motokawa grids.
• Gradual tilting of the horizontal bars in the grid patterns.
• Observation and documentation of the resulting visual illusions, including illusory spots and phantom bands.

Main Results:

• The study demonstrated that the elimination of illusory spots is not an all-or-none effect; the visual system exhibits tolerance to a degree of bar tilt or curviness.
• A continuum of illusions was observed: initially, only dark illusory spots were present, followed by a combination of dark spots and phantom bands, and finally, only dark phantom bands.
• These findings indicate that both illusory spots and phantom bands can coexist within a single grid pattern, depending on the degree of bar distortion.

Conclusions:

• The Hermann grid illusion is not abolished by small figural changes but rather modulated by the degree of bar curvature.
• Two distinct types of visual illusions, illusory spots and phantom bands, can emerge and coexist in response to variations in grid geometry.
• This research provides new insights into the plasticity and complexity of the human visual system's processing of spatial information.