Vibrations on the back of a person can convey information about direction through sequentially switching on two vibration motors. For perception of direction the oblique effect can occur, meaning that perception of cardinal directions is more precise than perception of oblique directions. We investigated the role of the positioning of the vibrations with respect to the spine. In the first condition all vibration motors were placed in a circle around the spine ('Circle' condition) and direction was conveyed by switching on vibration motors on opposite sides of the circle. In the second condition the vibrations were placed in two semi-circles of which the centers were on the left and right sides of the back ('Semi-circle' condition). We found that participants showed larger deviations as well as a larger spread for oblique directions than for cardinal directions in both conditions. This indicates that the oblique effect occurred. Therefore, the oblique effect can occur irregardless of the positioning of the vibration motors with respect to the spine. Both deviations and spread were larger in the 'Semi-circle' condition than in the 'Circle' condition suggesting an advantage for centering motors around the spine, although this might have been influenced by the distance between vibrations.

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We investigate the ability of human observers to judge the direction of illumination from image texture. Photographs of 61 real surfaces were used, taken from the Columbia-Utrecht Reflectance and Texture (Curet) database (http://www.cs.columbia.edu/CAVE/curet). All samples were normally viewed but obliquely illuminated, the elevation of the source being 22.5°, 45.0°, or 67.5°. The illumination was with a collimated, parallel beam. Stimuli were presented in random orientation, and observers had to judge both the elevation and the azimuth of the source. Observers judged the azimuth within approximately 15°, except for the fact that they committed random (with approximately 50% probability) sign flips (180° flips). Connected with this finding is the fact that observers judged the illumination to be from above rather than below in the overwhelming majority of cases, despite the fact that each case occurred with equal probability. The elevation of the illumination can be judged to some extent but is not far above chance level. The data are in good agreement with a simple model that bases the estimate of illumination direction on the second-order statistics of local luminance gradients. This locates the locus of the probable mechanism very early in the visual stream.

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Image texture due to 3D surface corrugations ("3D texture") differs fundamentally from image texture due to pigmentation variations of a smooth surface ("flat texture"). Flat texture yields no information concerning the way the surface is illuminated, whereas 3D texture is highly dependent upon the illumination. Question: Is the human observer able to perceive the illumination direction from 3D texture? Method: In order to address this question we extracted over sixty 3D textures due to natural materials from the Curet database. Fiducial illumination directions were 22.5°, 45° and 67.5° from the normal direction, all viewing directions were normal. The samples were clipped to circular discs and were randomly rotated in the picture plane. Observers indicated perceived illumination direction by way of an indicator panel showing a hemispherical boss on a plane, viewed normally to the plane. The simulated illumination direction was under the observer's control. Five observers viewed all samples in six different orientations. Result: We find that the azimuth of the illumination direction can be perceived quite veridically except for the fact that 180° confusions are frequent. The elevation cannot be perceived with great certainty (deviations up to 45° occurring frequently) though the increase of perceived elevation with true elevation is significant. We find a number of interesting variations on this pattern that depend critically upon the type of texture.@en

In four experiments, we tested whether haptic comparison of curvature ranging from -4/m to +4/m is qualitatively the same for static and for dynamic touch. In Experiments 1 and 3, we tested whether static and dynamic curvature discrimination are based on height differences, attitude (slope) differences, curvature differences, or a combination of these geometrical variables. It was found that both static and dynamic haptic curvature discrimination are based on attitude differences. In Experiments 2 and 4, we tested whether this mechanism leads to errors in the comparison of stimuli with different lengths for static and dynamic touch, respectively. If the judgments are based on attitude differences, subjects will make systematic errors in these comparisons. In both experiments, we found that subjects compared the curvatures of strips of the same length veridically, whereas they made systematic errors if they were required to compare the curvatures of strips of different lengths. Longer stimuli were judged to be more curved than shorter stimuli with the same curvature. We conclude that similar mechanisms underlie static and dynamic haptic curvature comparison. Moreover, additional data comparison showed that static and dynamic curvature comparison is not only qualitatively, but also quantitatively similar.

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We investigated whether haptic comparison of the curvature of strips is influenced by the tilt (the average slope relative to the horizontal) of the curved strips. This particular stimulus manipulation was chosen to decide between two broad ways in which dynamic curvature comparison might be done: on the basis of the attitude (slope) differences over the surfaces or on the basis of the attitude (slope) differences between successively presented surfaces. For this purpose we conducted matching and discrimination experiments in which strips of constant curvature 9 cm long were touched dynamically and the tilts of the strips were varied. The results of the matching experiment showed some influence of tilt: the strips were judged to be slightly more curved if the tilt was nonzero than if it was zero. The results of the discrimination experiment did not show an effect of tilt. Because the attitude (slope) differences over the surfaces were independent of the tilt it is concluded that curvature comparison is based primarily on attitude differences over the curved surfaces.@en

An investigation was undertaken into whether haptic comparison of curvature and of shape is influenced by the length/width ratio of the hand. For this purpose three experiments were conducted to test the curvature matching of curved strips (experiment 1), the matching of cylindrically curved hand-sized surfaces (experiment 2), and the shape discrimination of elliptically curved hand-sized surfaces (experiment 3). The orientation of the stimuli with respect to the fingers was varied. The results of the two matching experiments showed that a given curvature is judged to be more curved when touched along the fingers than when touched across the fingers. The phenomenal flatness along and across the fingers was found to be different and subject dependent. The results of the shape-discrimination experiment showed that the orientation of ellipsoidal surfaces influences the judgments of the shapes of these surfaces. This influence could be predicted on the basis of results of the second matching experiment. It is concluded that similar mechanisms underlie the (anisotropic) perception of curvature and shape. For the major part the trends in the results can be explained by the length/width ratio of the hand and the phenomenal flatnesses.@en

Static haptic discrimination of the curvature of convex, concave, or straight 20-cm-long strips was investigated for nine placements on the hand. In one condition, the strips were touched with the palmar side of the hand, and in the other condition, with the dorsal side. The influence of the lengths of the strips, and thus of contact lengths, was also investigated. For all placements, discrimination was poorer in the dorsal than in the palmar condition, owing to poorer cutaneous resolution on the dorsal side of the hand (the kinesthetic stimulation was the same in both conditions). Thus cutaneous stimulation is important. In both experiments, performance appeared to depend primarily on contact length. Moreover, the discrimination thresholds for all different placements and contact lengths followed the same trend. We conclude that in these experiments the effective stimulus for the discrimination of curved strips is the total difference of local surface attitude - that is, the slope difference over the far ends of the stimulus.@en