4.2. Pointing accuracy
A systematic bias of pointing orientation in clockwise direction
was found when using the trackball. The bias is due to the operation
of the trackball by means of rotational movements of the wrist.
Leftwards- and rightwards-pointing movements are achieved via
radial and ulnar abductions of the wrist. Human anatomy limits the
range of radial and ulnar abductions, whereas radial abductions
cover a smaller range than ulnar abductions. According to Hussain
et al. (2016), the range of movement (ROM) for the radial abduction
is about 1/4th to 1/3rd of the ROM for the ulnar abduction. When
visual feedback is absent, the range of the abduction of the wrist
affects the amplitude of such movements. Therefore, participants
tend to execute radial abductions with smaller amplitudes than
ulnar abductions. Considering that the trackball is operated with
the right hand, the effect of movement range causes an overshoot
for rightwards-pointing orientations and an undershoot for
leftwards-pointing orientations, resulting in the found clockwise
bias of pointing. A clockwise bias, albeit a smaller one, was also
found for up and down pointing orientations, indicating that
pointing to up and down orientations include rightwards and
Fig. 5. Average correct direction response rate (CDRR) for the orthogonal (left hand bars) and diagonal (right hand bars) orientations. Results are for tolerance levels of