Chapter 2. Prehension

Chapter 5 - Movement Before Contact 165
spatial variability was no greater on perturbed trials than on control
trials, reminiscent of Haggard and Wing’s (1991) suggestion that
compensatory response return/place the hand to the spatial track
required by that opposition space being set up.
Paulignan, Jeannerod, MacKenzie, & Marteniuk (1991) discussed
that the longer time for corrections to perturbations of intrinsic object
properties like object shape and size reflected the underlying central,
cortical mechanisms for visuomotor processing of intrinsic object
properties and controlling distal movements of the hands and fingers
(see Jeannerod, 1986). In contrast, the more rapid adjustments to
direction and distance reflect the pathways for visuomotor processing
related to reaching, which may have a shorter time constant. In this
regard the spinal reaching circuits discussed earlier have been
implicated in visually guided target reaching in cats (Alstermark et al.,
1990). Regardless, the requirements for the hand are still driving the
arm, and the two must be coordinated in time and space to set up
opposition space. We suggest as a working hypothesis that the goal
driving the trajectories is the alignment of the grasping surface patches
of the hand with the seen opposition vector, given the opposition
space parameterization appropriate for the task.
In the direction perturbation studies of Paulignan, MacKenzie,
Marteniuk, and Jeannerod (1991), recall that the illuminated dowel
switched positions at movement onset. In these studies, there was an
apparent dissociation between the raDid motor corrections associated
with this Derturbation, and subjects’ subjective awareness of the
occurrence of the target switching (Paulignan et al., 1990). That is,
subjects reported perceiving that a dowel switched locations when the
hand was almost at the first illuminated dowel, some time after the
initial adjustments (at about 100 ms) in limb transport had been made.
To obtain objective measures of both the motor response to the
perturbation, and subject’s awareness, Castiello, Paulignan and
Jeannerod (1991) replicated Paulignan et al. (1990) and asked subjects
to signal the time at which they became aware of dowel displacement
by a single utterance (Tah!). In contrast to the rapid limb response to
perturbation (first acceleration at 107 ms), the vocal response
indicating awareness of perturbation occurred 420 ms following object
displacement, or more than 300 ms after the onset of the limb transport
correction. They performed appropriate control experiments, showing
that the dual task requirements did not interfere with one another, i.e.,
the dual task values were similar to those obtained from single task
control experiments. Similarly, Castiello and Jeannerod ( 199 1)
compared manual adjustments to perturbations of object size with