Principles of perceptual grouping: implications for image-guided surgery

The laws and principles which predict how perceptual qualities can be extracted from the most elementary visual signals were discovered by the Gestalt psychologists (e.g., Wertheimer, 1923; Metzger, 1930, translated and re-edited by Spillmann in 2009 and 2012, respectively). Their seminal work has inspired visual science ever since, and has led to exciting discoveries which have confirmed the Gestalt idea that the human brain would have an astonishing capacity for selecting and combining critical visual signals to generate output representations for decision making and action. This capacity of selection and integration enables the perception of form and space, and the correct estimation of relative positions, trajectories, and distances of objects represented in planar images. The Gestalt laws and principles were initially aimed at answering a single all-encompassing question: " Why does the world look the way it does. " They have subsequently been made operational in experimental studies (for an illustration of ongoing research see the international METHUSALEM project, coordinated by Johan Wagemans, at www.gestaltrevision.be) aimed at deepening our insights into the ways in which specific characteristics and qualities of visual configurations may determine perceptual organization and behavior at various levels of processing. Perceptual organization directly determines the ability of human observers to assess (1) which parts of an image belong together to form a unified visual object or shape, and (2) which parts should be nearer and which further away from the observer if the represented objects were seen in the real world. This opinion paper argues that the Gestalt principle of Pragnanz and the Gestalt law of good continuation address specific problems of perceptual organization with critical implications for visual interface design, and the design of image-guided surgery platforms in particular. The principle of Pragnanz relates to the general Gestalt postulate that objects in the visual field will produce the simplest and most complete perceptual solution possible under the conditions given. The Gestalt laws of perceptual organization, of which the law of good continuation is a particular example, describe the conditions under which specific perceptual solutions (groupings) are likely to occur. The question of how planar image structures are grouped into perceptual representations of figure and ground is one of the study grounds the Gestalt laws have been designed for. Figure-ground representation is a perceptual solution that enables the observer to assess which objects in the image would be likely to be nearer and which objects would be likely to be further away in a real world configuration. It is mediated by specific image cues to shape and to relative distance, involving local signals of contrast and orientation to fill in specific regions of an image and thereby enabling the perception of surfaces. The associated perceptual sensations of local contrast enhancement make visual objects in the image appear to stand in front of other objects represented in the same plane.