Smooth at one end and rough at the other: influence of object texture on grasping behaviour.

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.

Voluntary control and the dynamics of perceptual bi-stability.

Voluntary control and conscious perception seem to be related: when we are confronted with ambiguous images we are in some cases and to some extent able to voluntarily select a percept. However, to date voluntary control has not been used in neurophysiological studies on the correlates of conscious perception, presumably because the dynamic of perceptual reversals was not suitable. We exposed the visual system to four ambiguous stimuli that instigate bi-stable perception: slant rivalry, orthogonal grating rivalry, house-face rivalry, and Necker cube rivalry. In the preceding companion paper [van Ee, R. (2005). Dynamics of perceptual bi-stability for stereoscopic slant rivalry and a comparison with grating, house-face, and Necker cube rivalry. Vision Research] we focussed on the temporal dynamics of the perceptual reversals. Here we examined the role of voluntary control in the dynamics of perceptual reversals. We asked subjects to attempt to hold percepts and to speed-up the perceptual reversals. The investigations across the four stimuli revealed qualitative similarities concerning the influence of voluntary control on the temporal dynamics of perceptual reversals. We also found differences. In comparison to the other rivalry stimuli, slant rivalry exhibits: (1) relatively long percept durations; (2) a relatively clear role of voluntary control in modifying the percept durations. We advocate that these aspects, alongside with its metrical (quantitative) aspects, potentially make slant rivalry an interesting tool in studying the neural underpinnings of visual awareness.