Top-down and bottom-up neuromodulation over two different visual illusions

For decades visual illusions have been used as a tool to investigate the properties of the visual system. In parallel, the neural bases of visual illusions have been investigated. Two principal methods have been used: the modification of perception in brain damage patients and the recording of brain activity induced by illusion perception. In general terms, the neuropsychological and imaging techniques suggest that the perception of illusions takes place at low level in the human brain. Here we use a non-invasive brain stimulation technique (tDCS, transcranial Direct Current Stimulation) to verify the bottom-up arising and the top-down modulation of a visuo-spatial illusion (Brentano illusion, BI) and of a brightness illusion (Glare effect, GE). Two groups of healthy subjects were tested with BI and GE while receiving cathodal or sham tDCS to the right or left posterior parietal cortex (PPC) or to the right or left occipital cortex (OC). Results show different effects for the two illusions. For BI, there is an increase of the illusory effect after cathodal tDCS of the right PPC and a decrease after the stimulation of the left PPC. No changes were found for OC stimulations. On the contrary GE was not modulated neither by PPC nor by OC stimulations. These evidences uphold the involvement of attention and its hemispheric asymmetry in modulating the perceptual processing underlying the BI, mediated by PPC and not by OC. GE as a pure luminance gradient illusion does not seem to be modulated by tDCS.