138 4 Haptics as a Substitute for Vision

cussed below, is that they sometimes require non-natural exploratory procedures that, at least initially, decrease their potential usefulness.

4.2 Vision and Haptics Compared

It can be expected to be an advantage when haptics replaces vision, that the two senses have important properties in common. Here are examples of tasks that both senses can perform, even if haptics usually needs more time for exploration (according to the different procedures mentioned in the above paragraph):

•Find edges separating 3D surfaces.

•Locate objects in relation to observer in near-space (within arms’ reach).

•Perceive the size of not too large objects that are possible to explore.

•Perceive the form of not too complicated and large objects.

•Perceive the texture of surfaces (sometimes haptics is better than vision).

Tasks where haptics lags much behind vision or cannot perform the task at all:

•Provide an overview of a scene.

•Perceive 3D space beyond arm’s reach.

•Perceive colours.

•Perceive edges in a 2D picture (with no embossment).

Tasks where vision lags behind haptics:

•Perceive the weight of objects (even if vision has capacity to do this in some contexts where the objects are handled by other people; see Runeson and Frykholm 1981 and Jansson 1993).

•Perceive the hardness of surfaces.

•Perceive the temperature of surfaces.

The most important difference is probably the capacity of vision to provide a practically immediate overview of a scene. There are often severe problems in getting an overview of a scene haptically, which may be a laborious and timeconsuming task using successive exploratory investigations of the scene. Nevertheless there are also possibilities to identify objects by a “haptic glance”, that is a short contact with the object, especially when the observer has hypotheses about what object to expect and the identification is based on local properties such as texture (Klatzky and Lederman 1995).

The difficulty to get en overview from haptics and its lower discriminable capacity has to be considered when constructing graphs. Grids are often used in graphs to facilitate exact reading of values. In tactile graphs grids may be used when the graph is simple, but should be avoided when several lines have to be discriminated because of the risk of grid lines to be perceived as data lines (Lederman and Campbell 1982).