Neuromodulation of the perception of an illusion of length

The Müller-Lyer figure is a visual illusion in which two identical horizontal lines are perceived as different in their physical length because of the presence of fins (inducers) at the line ends. The Brentano illusion combines the two configurations of the Müller-Lyer illusion in one figure on the horizontal planum. In healthy subjects this configuration generates a displacement of the subjective midpoint of the horizontal line. In brain-damaged patients, with unilateral spatial neglect (USN+) without hemianopia (H-), the Brentano illusion is preserved, with illusory effects even enlarged in the contralesional hemispace. This finding suggests an inhibitory modulation of the top-down mechanisms on the bottom-up processing at the basis of such illusory phenomenon, likely mediated by the right posterior parietal cortex (PPC), which represents the main neural substrate of neglect. In order to verify the hypothesis that the attentional system can modulate the Brentano illusion and that this modulation is lateralized, a group of healthy subjects was tested with the Brentano Hemianopia Test (BHT) while receiving cathodal or sham transcranial Direct Current Stimulation (tDCS) to the right or left PPC. The results show an increase of the illusory effect after cathodal tDCS of the right PPC and a decrease after the stimulation of the left PPC. This evidence uphold the involvement of attention and its hemispheric asymmetry in modulating the perceptual processing underlying the Brentano illusion of length.