Chapter 2. Prehension

Chapter 5 - Movement Before Contact 125
The amplitude of movement, A, is an extrinsic object property and the
width of the target or target tolerance, W, is an intrinsic object
property. Using a WATSMART system (Northern Digital, Waterloo),
MacKenzie et al. (1987) measured the MT of the tip of the stylus, its
time to peak resultant velocity, and the percentage of movement time
after peak resultant velocity. When plotting MT against ID, a linear
relationship is seen, replicating Fitts’ Law. There was a differential
effect of target size and amplitude on these parameters. As seen in
Figure 5.5a, in plotting ID vs the time before the peak resultant ve-
locity (the acceleration time or phase), there was a noticeable effect of
amplitude. For each target size, the acceleration time increased as the
ID increased (primarily due to amplitude). For each amplitude, there
was no effect of target size on the acceleration time. In Figure 5Sb,
the data are normalized in order to examine the percentage of MT after
peak resultant velocity (the deceleration phase). For each amplitude,
this measure increased as the ID increased. That is, as the diameter of
the targets became smaller, the percent of time spent in the deceleration
phase of the movement increased. The value of peak velocity was
scaled to the amplitude of movement; i.e., as the amplitude of
movement increased, so did the value of peak velocity, although the
relative timing of acceleration and deceleration components of the
movement remained invariant for a given target size.
These results indicate that the resultant velocity profile is not sym-
metrical. Fitts’ Law states that the MT will increase as the target size
decreases; here, it can be seen that the reason that the MT increases is
because of a relatively longer deceleration phase for smaller target di-
ameters. The results show that the time spent in the deceleration phase
was predicted by ID as well or better than MT. This was not the case
for acceleration time. Only movement amplitude affected the time to
peak velocity. Thus, amplitude and target size effects were disasso-
ciable in that the shape of the tangential velocity profile was a function
of target size (accuracy), and the peak speed along the path of the tra-
jectories was scaled according to movement amplitude.
MacKenzie et al. (1987) found a systematic lengthening of the de-
celeration phase of the tangential velocity profile with decreases in tar-
get size, and operationally defined a lengthening of the deceleration
phase as a ‘precision effect’. In contrasting our asymmetric velocity
profiles with the shape invariance identified earlier by Atkeson and
Hollerbach (1985), we noted that their subjects made unrestrained
pointing movements in the dark without making contact with target
surfaces. With full vision and room illumination, our subjects
contacted target plates; our findings showed that impact velocity at