On Symmetry, Illusory Contours and Visual Perception Introduction to Computational and Biological Vision 2007 “The perceptual system and the world it lives in cannot be separated. They need each other in order to be complete, in order to make sense. The perceptual system has a relationship with the world it lives in; thus the system itself is a particular embodiment of that relationship.” – Yiannis Aloimonos in “Active Perception”

Symmetry “invariance under a specified group of transformations” Widely present in nature Most animals have bilateral symmetry Its detection seems to be fundamental to survival Its detection is pre-attentive Many types of symmetry are observed in nature, specially rotation and reflection symmetries; and while natural symmetry isn’t perfect, usually there is an overall symmetric form that can be related to most of its elements. Symmetry is an excellent method of breaking camouflage.

Illusory Contours Happens in nature due to poor conditions of illumination and the transformation from 3D to 2D in the retina Animals (monkeys, cats, owls and bees ) studied had shown ability to perceive illusory contours as if they were real borders with 40% of the orientation-cells responding to it. Modal shape completion occurs when the borders are perceived, despite of being inexistent in the visual stimulus, as being above the stimulus by a brightness enhancement of the completed shape.

Visual Illusions the illusory contours couldn’t be induced separately by the parts of the picture and the whole configuration has emergent properties, supporting the holistic approach to visual perception. by studying visual illusions we may understand how the visual perception is ‘built’ by the brain, which neural mechanisms are used to complete this task.

Neglect “Neglect is a neurological condition under which patients with unilateral (often right) parietal lesion fail to consciously perceive stimulus shown in the contra-lateral (left) visual field.” - Driver & Mattingley, 1998. Hemineglect or unilateral neglect is the most common attention impairment disorder and has many neurological symptoms. It is associated with injuries in the “where” visual path, many times following a stroke, in general in the right parietal lobe. This condition does not involve damage to early visual areas nor processes, nor involves sensory damage: the neglected stimulus, though not consciously reported, undergoes early visual processing15. Actually, neglect affects also the capacity of describing from memory14, thus it isn’t assumed to be a sensory-perceptual failure, but rather (high level) imagery, perception and attention deficit.

Symptoms fail to detect objects on the contralateral side (opposite) to their injury fail to notice or hear people standing in their left side eat just what is in the right side of his plate shave just the right side of his face hold his head slightly inclined to the right claim that his left limbs are somebody else’s read just the second part of a word

Diagnosis

Experiment One: What do you see? A new way of diagnosis?

Experiment Two: Line Bisection Vuilleumier and Landis, 1997 Driver, Baylis and Rafal 1992 this fact may have great influence in the designing of novel techniques of image segmentation, highlighting the importance of top-down mechanisms.

Experiment Three: The “Symmetry-Completion” effect Neglect patients tend to copy/draw just half of a symmetrical object, although they don’t report “seeing” just one half of it. Is it an indication that this “symmetrical completion” takes place? Secondly, it shows that the brain compensates for the hemineglect condition by using symmetry perception mechanisms. This finding would help the development of new rehabilitee tools for the condition.

Conclusions/Future Research Do Neglect patients perceive the illusions? A new diagnosis tool Is Contour Completion really pre-attentive? Better understanding of perceptive processes Symmetry detection x contour completion