Biomedical Engineering Reference
In-Depth Information
than when only one was used. According to the authors, this indirect measure presents
the advantage of indicating that, as in vision, the effect of symmetry is primitive and
incidental in so far that it appears in the initial stages of perceptual encoding and without
voluntary research.
Thus, the facilitating effect of symmetry depends on the availability of spatial reference
systems. It should be noted, however, that Ballasteros
et al
. [85] obtained facilitation in
the two-handed condition only with open figures and not with closed figures, which gave
rise to unclear results.
Orientation
- the spatial orientation of a stimulus is always relative to a reference axis,
so the vertical and horizontal axes always correspond to the direction of gravity and the
visual horizon, respectively, and so form a reference frame in 2D space, where all other
orientations are said to be oblique.
Regardless of the age of a person, their perception of orientation is more accurate
in vision than it is in haptics, and this accuracy improves with time. In haptic orienta-
tion perception, systematic anisotropy (a better perception of vertical and horizontal than
oblique orientations) in vision is not always observed. This 'oblique effect' [87] is present
in all ages for most visual tasks. It becomes more evident when the difference is made
between the performances observed for vertical and horizontal orientations (which most
often do not differ and are grouped together) and those for oblique orientations (also
grouped together when they do not differ). Because it is a question of difference, this
oblique effect may appear whatever the overall accuracy of responses. For example, in a
reproduction task, the same subjects make average errors in vision and tactile sensation of
1 and 3.8
◦
respectively for the vertical orientation and of 3.6 and 5.7
◦
for oblique orienta-
tions [88]. In vision, the processes responsible for the systematic presence of the oblique
effect are influenced by multiple factors which operate on different anatomo-functional
levels of the visual system according to the nature of the task [89, 90]. In the haptic
modality, as will be seen, the existence of an oblique effect
per se
is debated.
Line Parallelism
- Kappers [91, 92] and Kappers and Koenderink [93] investigated the
haptic perception of spatial relations in both horizontal and mid-sagittal planes. In a sim-
ple task, where blindfolded participants were asked to rotate a test bar in such a way that
it felt parallel (in physical space) to a reference bar, huge systematic subject-dependent
deviations were found. Observers systematically produced deviations of up to 90
◦
, thus
setting the two bars perpendicular! This effect was very robust and remained so even after
visual inspection and various methods of feedback. Unimanual and bimanual, and also
pointing and co-linearity measurements yielded similar results. These results confirmed
that haptic space is not Euclidean, contrary to common expectation. The current hypoth-
esis of the authors of these studies is that the deviations reflect the use of a reference
frame that is a weighted average of egocentric and allocentric reference frames. Cuijpers,
Kappers, and Koenderink [94, 95] performed similar parallelity and collinearity experi-
ments in vision. That visual space is not Euclidean was already established early in the
previous century, but these new studies provided extra evidence. For example, bars at eye
height separated by a visual angle of 60
◦
have (depending on the participant) to differ by
20
◦
in orientation in order to be perceived as being parallel, which defies conventional
explanations.
