Chapter 2. Prehension

102 THE PHASES OF PREHENSION
same task; however, placing the bar from this posture would be even
more awkward. Rosenbaum and colleagues then asked subjects to
grasp the horizontal bar and either place it vertically on the table or into
a suspended holder. They noted that, without restricting the subjects
initial grasp posture, 6 of the 12 subjects twirled the bar so that they
could comfortably grasp and place the bar. In a third experiment, re-
strictions were placed on the subjects, urging them to ‘grab the bar in
the middle, holding it like a tennis racket.’ In this experiment, some
of the tasks required subjects to perform two sequence moves (e.g.,
grasp bar and place its black side on the table, then place its white end
in the suspended holder). Of the sixteen combinations of one and two
sequence moves, subjects consistently used postures that reduced
awkwardness at the end of the first placement, even in the tests where
two moves had to be made in sequence. In this case, it appears that
planning is occurring for only the first part of the task, not the second
part. This agrees with the observation of Henry and Rogers (1960)
that later parts of a movement sequence can be programmed after
movement has been initiated.
A B
C D
Figure 4.16 Rosenbaum et al. (1990) horizontal grasp experiment.
The arrow is the z-axis perpendicular to the palm; this shows the
orientation of the palm. (a) Two colored bar is grasped overhand.
(b) Bar is placed on the table with its grey side down. (c) Bar is
grasped underhand. (d) Bar is placed with its black side down.
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