Neural control of on-line guidance of hand reaching movements.

Claude Prablanc, Michel Desmurget, Hélǹe Gréa
Progress in Brain Research. 2003-01-01; : 155-170
DOI: 10.1016/s0079-6123(03)42012-8

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https://www.bordeaux-neurocampus.fr/12300

1. Prog Brain Res. 2003;142:155-70. doi: 10.1016/S0079-6123(03)42012-8.

Prablanc C(1), Desmurget M, Gréa H.

Author information:
(1)INSERM Unité 534, 16 avenue Doyen Lépine, 69676 Bron, France.

Orienting one’s gaze towards a peripheral target is usually composed of a
hypometric primary saccade followed by a secondary ‘corrective saccade’
triggered automatically (without conscious perception) by the retinal error at
the end of the primary saccade and characterised by a short latency. Due to
visual suppression during the saccade, the artificial introduction of a random
small target jump during that short period remains undetected and triggers after
the end of the primary saccade a normal ‘corrective saccade’. As a result this
procedure simulates an error in the planning of the primary saccade. On the
other hand optimum hand pointing (trade-off between movement time and accuracy)
is considered classically to involve a natural parallel initiation of saccade
and hand response based on a poor peripheral retinal location, and a further
amendment of the hand motor response based on the retinal error provided by the
simultaneous vision of target and hand during the movement home phase. To test
the hypothesis that the retinal feedback at the end of the primary saccade is
used to update the visual target position and amend the ongoing hand motor
response, we developed a paradigm involving both an optimum hand pointing and an
undetected random target perturbation during the orienting saccade. In order to
show that the amendments were controlled by a loop comparing the perceived
target location with the dynamic hand position signal, vision of the limb was
removed at movement onset. Results showed that the movement was smoothly
monitored on-line without additional time processing demands. This functional
property of flexibility of the ongoing hand motor response, was generalized from
movement extent to movement direction. The undetectability of the perturbation
at a conscious level was not a prerequisite for motor flexibility, which was
further shown to depend on a critical phase of the limb movement beyond which
the latter was no longer amendable, even when the limb was visible. The hand
pointing flexibility was further generalised from pointing to the more complex
hand reaching and grasping process. It was shown that the flexibility of both
the transport and the grasp components were closely coupled. A careful analysis
of the data suggested the controlled variable to be the general posture of the
upper limb, reaching Bernstein’s intuitions about redundancy reduction in
skeletomotor systems with degrees of freedom in excess. A kinematics study of
the motor flexibility of reaching and grasping in a patient with a bilateral
optic ataxia favoured the idea of a posterior parietal cortex involvement in the
error processing underlying motor flexibility, reaching the same conclusions as
other recent studies using either Positron Emission Tomography or Transcranial
Magnetic Stimulation.