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.

The size-weight illusion is well-known: if two equally heavy objects differ in size, the large one feels lighter than the small one. Most explanations for this illusion assume that because the information about the relevant attribute (weight itself) is unreliable, information about an irrelevant but correlated attribute (size) is used as well. If such reasoning is correct, one would expect that the illusion can be inverted: if size information is unreliable, weight information will be used to judge size. We explored whether such a weight-size illusion exists by asking participants to lift Styrofoam balls that were coated with glow in the dark paint. The balls (2 sizes, 3 weights) were lifted using a pulley system in complete darkness at 2 distances. Participants reported the size using free magnitude estimation. The visual size information was indeed unreliable: balls that were presented at a 20% larger distance were judged 15% smaller. Nevertheless, the judgments of size were not systematically affected by the 20% weight change (differences < 0.5%). We conclude that because the weight-size illusion does not exist, the mechanism behind the size-weight illusion is specific for judging heaviness.

In this paper, we present an overview of parameters that are of relevance for the perception of vibrotactile patterns on the back. These patterns are delivered via varying numbers of vibration motors fixed to the back rest of a chair, vests or belts. We present recent findings from the literature about vibrotactile anisotropy, timing, spacing, anchor points, resolution and intensity. From this overview, we derive recommendations that should be considered when designing a vibrotactile device for the back. The main recommendations are: 1) Use sequential stimulation for conveying spatial patterns; 2) Avoid tactors on the spine; 3) For a rectangular grid 4 × 4 tactors seems optimal; 4) Carefully consider relative horizontal and vertical spacing. We hope that this overview will raise awareness of several issues that play a role in perception and that our recommendations will provide guidance when designing vibrotactile communication devices.

To judge the contents of a box, we do not necessarily have to open it. By shaking a box we can make an estimate of its contents based on haptic and auditory information. Not much is known about the perception of properties of objects that are inside a box. In this study we investigated how accurately participants can judge the number of wooden spheres inside a small handheld box by shaking the box. This was done in a 'haptic + auditory' condition in which participants shook the box and in a subsequent 'auditory only' condition in which recorded sounds from the trials in the haptic + auditory condition were played back. In both conditions participants had to judge the number of spheres (1 to 5) inside the box. In the haptic + auditory condition participants could perform this task accurately for up to about 3 spheres, while for larger numbers they systematically underestimated the numerosity. Although participants could perform this task above chance in both conditions, accuracy was lower in the auditory condition than in the haptic + auditory condition. By actively shaking the box the number of objects inside can be judged accurately for up to 3 objects.

When picking up objects using a pinch grip, there are usually numerous places at which one could place the thumb and index finger. Yet, people seem to consistently place them at or close to the centre of mass (COM), presumably to minimize torque and therefore the required grip force. People also prefer to grasp objects by parallel surfaces and ones with higher friction coefficients (rough surfaces), to prevent the object from slipping when they lift it. Here, we examine the trade-off between friction and COM. Participants were asked to grasp and lift aluminium bars of which one end was polished and therefore smooth and the other was rough. Their finger positions were recorded to determine how they grasped the objects. The bars were oriented horizontally in the frontal plane, with the centre aligned with the participants’ body midline. The bars varied in the horizontal offset between the COM and the edge of the rough region. The offset could be 0, 1 or 2 cm. We expected participants to grasp closer to the rough area than the centre of the bar. Completely rough bars and completely smooth bars served as control conditions. The slipperiness of the surface that was grasped affected the height of the grasping points, indicating that participants adjusted their grasping behaviour to the slipperiness of the surface. However, the tendency to grasp closer to the rough area was minimal. This shows that the judged COM largely determines how an object is grasped. Friction has very limited influence.