Optical distortion occurs instantly and unconsciously

Small aberrations are produced by the cells of the primary visual cortex, distorting optical information even before the higher levels of the brain have begun to process it.

New experiments show that the tilt illusion, in which perfectly vertical lines appear slanted because they appear in front of an oriented background, occurs at an unconscious level. The brain cannot always correct for the aberration, or even recognise it as an illusion.

Read more at ARC Centre of Excellence in Vision Science