4.4.1.1 Experimental Conditions
Two experimental conditions were used in this experiment: the
glove and the Fball. A pilot study showed that the best performances
with both conditions were achieved when the control display ratio
(control gain) was 1. In such cases, subjects can take the advantage
of the direct mapping nature of the 6 DOF isotonic position controls.
4.4.1.2 Experimental Task
A docking task was used for this experiment. This task was very
similar to those of Experiments 1 and 2, except for the initial
cursor and target positions. In Experiments 1 and 2, the target
stayed in the centre of the 3D space while the cursor randomly
appeared in one of the four pre-set arbitrary locations and orientations.
In this experiment, at the beginning of each experimental trial
the cursor appeared instead in the centre of the 3D space while
the target randomly appeared in one of five pre-set arbitrary
locations and orientations. During practice sessions, the target
appeared in completely uncontrolled random locations and orientations.
This exchange of the target and the cursor locations was designed
to increase the level of task difficulty, an issue identified
from Experiment 2 results. When the target was kept in the centre
of the 3D display, the final fine positioning stage of the docking
task was always carried out around in the same location. Subjects
might bring the cursor to the same location (typically in front
of the target) and then push it to the target. This means that
regardless of the initial cursor locations, the final stage of
the docking task remained essentially the same, making it very
easy to learn. In contrast, when the target is randomly assigned
to one of the multiple locations, subjects were forced to use
a different set of control movements for each target location,
hence facing an increased level of difficulty.
4.4.1.3 Experimental Design
A within-subjects design was used in this experiment in consideration
of efficiency. Each subject was tested with both of the two conditions,
the glove and the Fball, on the same day. According to the results
in Experiment 1, subjects' performance with the isotonic position
control started to stabilise after 20 minutes of practice. In
this experiment, each condition was given about 25 minutes of
exposure, which was composed of five tests and some practice trials
between tests. Each test consisted of two identical blocks of
trials. Each block had 5 trials with 5 distinctive initial target
locations in random order. Test 1 started after a short demonstration
and two warm-up trials. Test 2, Test 3, Test 4 and Test 5 started
5, 10, 15, and 20 minutes after the beginning of Test 1 respectively.
Including the demonstration and warm-up trials, each condition
of the experiment took approximately 25 minutes. Subjects were
alternatively assigned to one of the two experiment orders: glove
first (GB) and Fball first (BG). After completing the first condition,
a short break was given to the subject before proceeding to the
second condition.
4.4.1.4 Subjects
Twelve paid volunteers who had not participated in Experiments
1, 2 and 3 were recruited. Two of them failed to pass the screening
task due to weak stereopsis (using the Baush and Lomb Orthorator).
The remaining 10 participated in the complete experiment. Their
ages ranged from 22 to 33, with median 29. Eight of the subjects
were right handed and two were left handed. Subjects were asked
to use their dominant hand in using both input devices (Fball
or glove).
4.4.2 Within-Subjects Analysis of the Overall Results
As in the analyses of the earlier experiments, log transformation was applied to the completion time data to meet the residual distribution requirement in ANOVA for the following statistical analysis.
Figure 4.4 shows the subjects' mean trial completion times in each of the five tests. On average, task completion times were shorter for the Fball than for the glove in each of the five tests. This performance difference remained consistent over different learning phases. Repeated measures analysis (Table A3.4.1, Appendix 3) showed that overall performance with the two devices was significantly different: F(1, 8) = 26.554, p < 0.005.
With both modes, subjects significantly improved their performance over the five learning phases: F(4, 32) = 34.04, p < 0.0001. The performance difference between the two modes was independent of learning phase, as indicated by the absence of a significant interaction: Device x Phase: F(4, 32) < 1, p > 0.5. Other significant factors included Block: F(1,8) = 26.44, p < 0.001. As said earlier, each test consisted of two blocks of trials. Completion times in the second block were significantly shorter than for the first block, due to the learning effect.
The presentation order of the two modes was not significant: F(1,8)
= 2.2, p > 0.1 but Order x Device interaction was significant:
F(1,8) = 22.587, p < 0.005. This could imply an asymmetrical
skill transfer due to within-subjects design (Poulton, 1969, 1974).
Note that the Order x Device x Phase interaction is also significant,
indicating that the transfer effect varies with learning phase.
Two approaches were taken to test if possible asymmetrical skill
transfer has caused the performance difference between the Fball
and the glove. First, a between-subjects analysis (4.4.3) has
been carried out. Second, as we have seen, the performance difference
between conditions in test 5 is very similar to early tests but
asymmetrical skill transfer is least likely to be still in effect
after 4 tests and 20 minutes of practice with the second device.
A within subject analysis (4.4.4) was therefore conducted with
data collected from the last test.
4.4.3 Between-Subjects Analysis
In order to remove the possibility that the results were due to asymmetrical skill transfer, a between subjects analysis was carried out with only the data for the first device used by each subject. Subjects were divided into two groups. Members of the Fball group were the subjects who were tested with the Fball first and the glove later. Their data with the glove were discarded for the between subject analysis. Similarly, the Fball data were discarded for the group who tested the glove first. This approach is expected to be much less sensitive than the within subject analysis in last sub-section, given the small number of subjects in each group. Figure 4.5 shows the results after discarding half of data. Table A3.4.2 shows the results of the repeated measure variance analysis of this between subject design. The major conclusions with regard to the Fball versus the glove were consistent with the early analysis, i.e., completion time with the Fball was significantly shorter than the glove: F(1,8) = 3.6, p < 0.05. Learning did not reduce this difference, as the Phase x Device interaction was insignificant: F(4, 32) < 1, p > 0.5.
In conclusion, the between-subjects analysis performed by dropping
half of the experimental data still reached the conclusion that
the Fball outperformed the glove.
4.4.4 Performance Analysis of the Final Test
This subsection analyses the performance in the last test in each condition, using the within-subject, repeated measure method. Table A3.4.3 in Appendix 3 summarises the results of this analysis which again confirmed the conclusions drawn from previous two approaches: completion time was significantly shorter with the Fball than with the glove: F(1, 8) = 15.8, p < 0.005. Furthermore, neither Order of presentation, nor Order x Device interaction was significant, meaning that any possible skill transfer effect between conditions did not significantly affect performance in the last test.
On the basis of all the above three methods of analysis, we can
therefore confidently conclude that the Fball outperformed the
glove in the experiment.
4.4.5 The Effect of Clutching vs. Muscle Groups
From a practical point of view, the above analyses have concluded that the Fball is a more efficient device than the Glove. However, from a more theoretical point of view, the cause of the performance differences is still not clear. As discussed earlier in this chapter, the Fball differs from the glove in two major aspects: the use of finger joints and the absence of a clutch. With the glove, when the subject reaches an awkward posture, he/she has to disengage the glove (by releasing the button under the fingers), restore the hand/arm to a more comfortable posture and re-engage (by closing the fingers around) the manipulated object. This re-clutching process takes time to complete (from the moment of disengaging to the moment of re-engaging). Subject usually make 1 to 3 clutches/declutches in each trial. This could be the sole cause of the performance difference in above analyses, leaving the effect of using finger joints/muscle groups unknown.
This issue had been considered during the design stage of the experiment, however. The accumulated re-clutching time was recorded during the experiment. In the following analysis, the re-clutching times are subtracted from the trial completion times for the glove condition. The net score is labelled as C-R Time. Note that the C-R Times is a biased measure against the Fball condition for two reasons. First, with the Fball, the re-clutching process still exists, although not as explicitly. From time to time subjects had to move the fingers to different parts of the ball surface to make further rotation. This effort (and time) was not taken into account by the C-R time, since no explicit re-clutching time could be measured. Second, during the re-clutching time with the glove, subjects were not necessarily idle but probably instead engaged in mentally making decisions about what to do next. It is known that mental rotation takes up a certain amount of time (Shepard and Metzler, 1971) . This time may overlap with the re-clutching time in the glove condition and is therefore reduced in the C-R Time measure. Nonetheless, C-R Time serves as a conservative measure to test if the use of fingers was really advantageous. If the Fball still outperformed the glove as measured by C-R Time, the advantage of using fine joints must exist. However the converse may not be true.
Figure 4.6 shows the performance differences between the Fball and the glove as measured by C-R Time. As can be seen, the mean completion times with the Fball were still shorter than the mean C-R Times with the glove. Table A3.4.4 in Appendix 3 shows the results of repeated measure variance analysis of C-R Times collected in Test 5 (Last phase of experiment). The difference between completion time with the Fball and the C-R time with the glove is still significant: F(1,8) = 5.324, p < 0.05. Neither the order of presentation nor its interaction with device was statistically significant, suggesting that this difference was not caused by asymmetrical skill transfer.
The analysis with C-R Time, which exclude the effects of the clutch,
therefore further supports the conclusion that the use of different
muscle groups is indeed one cause of the superior performance
of the Fball as compared to the glove.
4.4.6 Subjective Evaluation
Upon completing the experimental trials, subjects rated each of the devices on a continuous scale ranging from -2 to +2 (-2: very low, -1: low, 0: OK, 1: high, 2: very high). Of the 10 subjects, 6 rated the Fball higher than the glove. The other 4 subjects rated the glove higher than the Fball. Subjects were encouraged to jot down comments on features about which they felt strongly. Seven subjects felt that the cord with the Fball got in the way. Three subjects did not like the wrist rotations imposed by the glove. Two subjects wrote that the Fball was less natural than the glove. One subject particularly liked the clutch function with the glove. One subject reported fatigue with both devices.
Figure 4.7 Subjective Ratings of Fball vs. Glove;, Upward arrows indicate that the Fball was preferred
Figure 4.7 shows the ratings each subject gave to the Fball and the glove. On average, the Fball received higher ratings than the glove (mean value: 0.78 vs. 0.60, 0 is OK, 1 is high), but this difference was not statistically significant: F < 1, p > 0.5.
The ambiguous subjective ratings are in obvious contrast with task performance measures. In the last test, all subjects, except subject A, had a shorter task completion time with the Fball than with the glove (Figure 4.8). Subject A reported fatigue during his last Fball test. (This was also the last test in his entire session). His second last test with the Fball had a much shorter mean completion time (6.98 second).
One possible reason for the disparity between the performance
measures and the subjective evaluations could be that the subjective
preferences were strongly affected by some salient features of
the devices. In this experiment, the Fball cord could be such
a feature. Although most of the subjects were able to overcome
the inconvenience caused by the cord and indeed performed better
with the Fball than with the glove, 4 of them still rated the
Fball lower than the glove, possibly due to the interference of
the Fball cord. One other reason could be that some subjects felt
more "natural" with the glove. This is discussed further
in next subsection.
