Chapter 2. Prehension

166 THE PHASES OF PREHENSION
vocal responses indicating awareness of shifts in object size, and
found latencies of about 424 ms, close to the vocal latencies of 420 ms
reported by Castiello et al. (1991) for awareness of shifts in object
direction. This similarity in times, combined with other research, led
Castiello, Paulignan and Jeannerod (1991) to suggest the invariant
time delay reflected a time consuming process for access to awareness
of a visual event. They suggested that neural activity for processing
information must be ongoing for a significant amount of time before it
can give rise to conscious experience.
Almost all of the kinematic studies of prehension reviewed to this
point have used naturally occurring or experimenter-defined pad op-
position (precision grasp). We saw in Chapter 2 the extensive classi-
fications of grasp types, and suggested these could be captured as pad,
palm and side opposition. We consider briefly now studies which
have examined different grasp types, then the underlying neural sub-
strates for visuomotor integration with different oppositions.
5.4.3 Grasp types
The type of grasp posture used could potentially affect the kine-
matic components of reaching and grasping. We saw in Chapter 2 the
variety of postures which could be adopted to match the task-specific
object properties. In an important size perturbation study, Castiello,
Bennett and Paulignan (1992) repeated the Paulignan, Jeannerod,
MacKenzie, and Marteniuk (1991) size perturbation experiment, but
encouraged subiects to adopt a natural grip appropriate to the diameter
of the dowels. Thus subjects grasped 1.5 cm diameter objects with
pad opposition using the index finger and the thumb (precision grip),
or they used palm opposition (whole hand prehensile grip), with all
fingers wrapped around the 6 cm dowels. The smaller dowel was
nested inside, and protruded above the larger one. Visual
perturbations of object size required a change in the distal program for
hand configuration, and therefore a selection of a different opposition
type. The responses to the perturbation showed that the first kinematic
landmark for adjustments to perturbation was the time to peak
deceleration (290 ms after perturbation of size), which occurred earlier
on perturbed than on control trials. Further, time to peak deceleration
was significantly earlier with perturbations from large to small objects
(requiring palm to pad opposition) than vice versa. Contrast this first
kinematic landmark of adjustment to perturbation with the apparent
interruption in the acceleration, i.e., earlier time to and lower value of
peak acceleration, for perturbations of direction reported by Paulignan