Richard L Gregory

The Oxford companion to the mind

Preface. List of contributors. Note to the Reader. A-Z text. Index. Illustration acknowledgements. Contributors

Vision with Isoluminant Colour Contrast: 1. A Projection Technique and Observations

An optical technique is described for projecting two-colour pictures with controlled brightness contrast, which may be set to zero—at isoluminance. Colour registration is maintained without adjustment or special setting up. It is suggested that colour- and brightness-contour registration in the visual channel is a problem which may be solved neurally by master brightness signals locking slave colour signals. The projection apparatus allows the supposed master brightness signals to be removed—at isoluminance—when contour disturbances should occur. Observations of this kind are reported.

Perceptual Illusions and Brain Models

An adequate theory of visual perception must explain how the fleeting patterns of light upon the retinas give knowledge of surrounding objects. The problem of how the brain ‘reads’ reality from images is acute, because images represent directly but few, and biologically unimportant, characteristics of objects. What matters biologically are such things as whether an object is poisonous or food, hard or soft, heavy or light, sharp or blunt, friend or foe. These are not properties of images. The owner of the eye cannot eat or be eaten by its images, and yet his life depends upon interpreting them in terms of quite different characteristics of objects. It follows that eyes are of little biological value unless there is an adequate brain to interpret their images; which raises the evolutionary problem : how did the eye‒brain combination arise? (cf. Gregory 1967a). To read reality from images is to solve a problem : a running set of very difficult problems throughout active life. Errors are illusions. Certain situations present special difficulty, giving rise to systematic errors : can these serve as clues to how the brain generally solves the problem of what objects are represented by which images? Illusions can occur in any of the sense modalities, and they can cross the senses. A powerful illusion crosses from the seen size of an object to its apparent weight, as judged by lifting. Small objects feel up to fifty percent heavier than larger objects of the same scale weight. Thus weight is evidently not judged simply by the input from the arm but also by prior expectation set by the previous handling of weights. When the density is unexpectedly great or small we suffer a corresponding illusion of weight.

Border Locking and the Café Wall Illusion

The Café Wall illusion (seen on the tiles of a local café) is a Münsterberg chequerboard figure, but with horizontal parallel lines which may have any luminance separating the rows of displaced squares. These (the ‘mortar’ lines) display marked wedge distortion which is especially affected by: contrast of the squares (‘tiles’); width of the ‘mortar’ lines, and their luminance which must not be significantly higher than that of the light squares or lower than that of the dark squares for distortion to occur. An experiment is described from which quantitative data have been obtained by varying these parameters. It is suggested that contiguous regions of different luminance (and contiguous colour regions) are normally held in spatial register by locking from common luminance boundaries. The Café Wall illusion is attributed to this border locking producing inappropriate contour shifts from neighbouring regions of contrasting luminance when separated by narrow gaps of neutral luminance. Further implications on the border-locking notion are discussed.

Fusion and Rivalry of Illusory Contours

The general question is raised: “Are visual contours given directly from striate-cortex feature-detector activity?‘’ Phenomena of ‘subjective’ or ‘cognitive’ contours are examined to challenge this view, on the ground that contours can be extrapolations across low-probability gaps. The contours may be curved and may have poor ‘gestalt’ qualities—so ‘gestalt closure’ is not appropriate, but may be a sub-class of these phenomena. It is suggested that these illusory contours (and brightness differences) are generated by perceptually postulated masking objects—these being part of perceptual ‘scene analysis strategy’, since strong evidence for nearer objects is provided by improbable gaps. Experiments are reported, in which each eye is given a different ‘cognitive’ contour figure such that there are disparate but illusory contours. It is found that these are fused to give three-dimensional illusory. surfaces bowing in front of the display. Masking objects must be in front of gaps; what happens here with reversal of stereo depth? Switching the eyes often gives rivalry of the illusory contours when masking is incompatible with the stereo depth. Implications for normal stereo vision are discussed.

Illusion-Destruction by Appropriate Scaling

The inappropriate constancy scaling theory of visual distortion illusions is tested by optically projecting typical models giving these figures by perspective. Appropriate or inappropriate stereoscopic disparities are then added–with the prediction that when perspective and stereo are geometrically correct the distortion should vanish. This is confirmed with measurements for the Müller-Lyer illusion and by observation of several other classical examples. It is suggested that much previous work has investigated ‘end stop’ conditions, given by angles too extreme to be generated as perspective. Conditions for appropriate scaling, giving zero or small distortions, are found to be critical but readily attainable.

The after-effect of seen motion: The role of retinal stimulation and of eye movements

An experiment is described in which movement after-effects are noted, following presentation of moving stripes under various conditions of eye movement. After-effects only occur when the retinal image moves systematically across the retina, though movement may be observed when this is not the case. The after-effects are due to specifically retinal stimulation, not to perception of movement pev se.

The medawar lecture 2001 knowledge for vision : vision for knowledge

An evolutionary development of perception is suggested—from passive reception to active perception to explicit conception—earlier stages being largely retained and incorporated in later species. A key is innate and then individually learned knowledge, giving meaning to sensory signals. Inappropriate or misapplied knowledge produces rich cognitive phenomena of illusions, revealing normally hidden processes of vision, tentatively classified here in a ‘peeriodic table’. Phenomena of physiology are distinguished from phenomena of general rules and specific object knowledge. It is concluded that vision uses implicit knowledge, and provides knowledge for intelligent behaviour and for explicit conceptual understanding including science.

Visual dissociations of movement, position, and stereo depth: some phenomenal phenomena.

Helmholtz (1867) described as “irradiation” the apparently greater size of a white compared with a dark square, or disc or whatever of the same physical size. The illusory size difference is reversed at low contrasts (Weale, 1974). It is also known that rapid increases in brightness gives apparent movement (gamma movement), though there is no agreed explanation for either phenomenon. When narrow bordering stripes are added, further systematic phenomena occur. With intensity modulation of an edge-striped grey rectangle, which has a dark stripe on the left side and a light stripe on the right (which is similar to figures used by Stuart Anstis and Brian Rogers), the entire figure shifts, with reversed motion when the background luminance is modulated. By presenting a pair of such figures, mirror reversed one to each eye and fused stereoscopically, the question may be asked: Do these illusory shifts produce stereo depth? The answer is surprising: stereo is produced-but at the cross-over with luminance of the central grey rectangle with the background the depth change is opposite to that given by normal, non-illusory, opposed lateral shifts. We interpret this anomalous stereo depth as a switch of which edges of the stripes are fused, with the change of relative contrast of the edges of the dark and light stripes as the figure-background contrast is changed. Measures of static shift, lateral movement, and stereo depth, give somewhat different functions. These are considered in terms of different signalled positions, stereo depth, and movement. This study brings out the importance, for explaining such perceptual anomalies, of distinguishing between neural signal channel characteristics and which stimulus features from the display are selected and accepted for perception. Although conceptually clearly distinct these are all too easily confused in psycho-physical experiments.

Mechanisms of Perception

The studies of biological and machine visual perception are becoming happily integrated so that each provides data and concepts for the other; though it remains unclear just how far practical machine perception will be similar to brain processes of biological perception.