Biomedical Engineering Reference
In-Depth Information
method of constant stimuli, the levels of a certain property of the stimulus are not related
from one trial to the next, but presented randomly. Finally, the method of adjustment asks
the subject to control the level of the stimulus, instructs them to alter it until it is just
barely detectable against the background noise, or is the same as the level of another stim-
ulus. All these requirements must be taken into account when developing virtual reality
displays.
The haptic world consists primarily of objects, surfaces, and their properties, rather than
of sensory inputs. Of course some of these environmental properties are directly coded by
the sensory system and sometimes even just by a small subset of single receptors, such as
with temperature or small discontinuities on a surface [123], but the perception of many
of environmental properties is based on the integration of different sensory sources. These
integration mechanisms bear further possibilities for the development of haptic technology,
as they may allow for the substitution of one type of sensory input with another.
Whereas there has been a large body of theoretical and empirical research on visual
space perception, there is no agreed-upon definition of haptic space. Distinctions have
been made between manipulatory and ambulatory space, in which the former is within
reach of the hands, whereas the latter requires exploration by movements of the body.
Both involve haptic feedback, although to different effectors. A variety of studies have
established that the perception of manipulatory space is nonveridical although there have
been studies that have demonstrated various distortions in space perception which is,
itself, anisotropic. People, even those who are congenitally blind, regularly overestimate
the length of touched vertical lines as compared to horizontal ones [124, 125]. They also
regularly overestimate the length of radial movements (to and from the body) by about
10% compared to tangential movements [126]. As a further example, the orientation of
a line that is felt in space is greatly dependent on its position (especially tangential) with
respect to the body. Furthermore, when attempting to orient bars in parallel by means
of touch alone, deviations of up to 40% can result, especially if done bimanually [92,
93, 127]. Therefore, obliquely oriented bars are less accurately reproduced compared to
those oriented in line with the axis of the body [128, 129]. Many further distortions
in the form of optical illusions have been reported, such as that given by Mueller-Lyer
[130]. Attempts to describe these different distortions by some kind of nonveridical, but
inherently consistent haptic metrics, proved unsuccessful. For example, a metric derived
from distortions in perceived angles at different positions in space did not fit with one
derived from perceived length [131].
However, there are numerous studies relating these spatial distortions to different fac-
tors within the movements. For example, body and arm position play a role in the size
of horizontal - vertical and radial - tangential illusions [132, 133]. The latter illusions were
accounted for by different movement velocities in the radial and tangential directions and
vanished when people were obliged to assimilate the velocities of their movements in the
two different directions [134]. This corresponds to the observation that movement velocity
strongly affects estimates of the length of a line up to a factor of three (reported for veloc-
ities between 0.5 to 50 cm s
-1
[133]). Furthermore, oblique effects (in the vertical plane
parallel to the observer) seem to relate to gravitational forces [128], at least in part [129].
There are other strong illusions directly related to movement patterns. If people move
along a curved line and then attempt to estimate the shortest distance between start
and end point, the longer the movement path, the more the distance is overestimated
