The Journal of Neuroscience, January 15, 2002, 22(2):600-610
Predictions Specify Reactive Control of Individual Digits in
Manipulation
Yukari
Ohki,
Benoni B.
Edin, and
Roland S.
Johansson
Section of Physiology, Department of Integrative Medical Biology,
Umeå University, S-901 87 Umeå, Sweden
When humans proactively manipulate objects, the applied fingertip
forces primarily depend on feedforward, predictive neural control mechanisms that depend on internal representations of the
physical properties of the objects. Here we investigate whether predictions of object properties also control fingertip forces that
subjects generate reactively. We analyzed fingertip forces reactively
supporting grasp stability in a restraining task that engaged two
fingers. Each finger contacted a plate mounted on a separate torque
motor, and, at unpredictable times, both plates were loaded
simultaneously with forces tangential to the plates or just one of the
plates was loaded. Thus, the apparatus acted as though the plates were
mechanically linked or as though they were two independent objects. In
different test series, each with a predominant behavior of the
apparatus and with interspersed catch trials, we showed that the
reactive responses clearly reflected the predominant behavior of the
apparatus. Whether subject performed the task with one hand or
bimanually, appropriate reactive fingertip forces developed when
predominantly both contact plates were loaded or just one of the plates
was loaded. When a finger was unexpectedly loaded during a catch trial,
a weak initial reactive response was triggered, but the effective force
development was delayed by ~100 msec. We conclude that the predicted
physical properties of an object not only control fingertip forces
during proactive but also in reactive manipulative tasks. Specifically,
the automatic reactive responses reflect predictions at the level of
individual digits as to the mechanical linkage of items contacted by
the fingertips in manipulation.
Key words:
manipulation; human hand; fingertip forces; internal
models; sensorimotor prediction; grasp stability; reactive
responses
Copyright © 2002 Society for Neuroscience 0270-6474/02/222600-11$05.00/0
