An Orientation Dependent Size Illusion Is Underpinned by Processing in the Extrastriate Visual Area, LO1

Area of Science:

• Neuroscience
• Visual Perception
• Computational Neuroscience

Background:

• The Helmholtz squares illusion demonstrates a discrepancy between physical size and perceived size based on orientation.
• Understanding the neural basis of visual illusions, like the Helmholtz squares illusion, is crucial for mapping visual processing pathways.
• The role of early visual areas versus extrastriate areas in generating perceptual illusions remains an active area of research.

Purpose of the Study:

• To investigate whether BOLD responses in V1 underpin the Helmholtz squares illusion.
• To identify extrastriate visual areas responsible for the Helmholtz squares illusion using transcranial magnetic stimulation (TMS).
• To determine the specific role of V1, LO1, and LO2 in processing orientation and size perception within this illusion.

Main Methods:

• Functional magnetic resonance imaging (fMRI) was used to measure BOLD responses in V1 to Helmholtz squares with differing orientations.
• Transcranial magnetic stimulation (TMS) was applied to V1, LO1, and LO2 to disrupt neural activity during perceptual judgments of the Helmholtz squares illusion.
• Perceptual judgments of aspect ratio were compared between conditions with and without TMS, and across different stimulated brain areas.

Main Results:

• fMRI data indicated that V1 activity correlated with the physical size of the squares, not their perceived size.
• TMS applied to V1 did not alter the strength of the Helmholtz squares illusion.
• Disruption of area LO1, but not LO2, significantly weakened the illusion, suggesting LO1's role in processing orientation-based size perception.

Conclusions:

• Neural activity in V1 does not account for the Helmholtz squares illusion; it processes physical dimensions.
• The extrastriate visual area LO1 plays a significant role in generating the Helmholtz squares illusion.
• These findings highlight the contribution of specific extrastriate areas in constructing illusory visual percepts of size.