Biomedical Engineering Reference
In-Depth Information
Anticipatory feedback control is responsible for the correction of the outgoing
motor commands on the basis of sensory (typically visual and proprioceptive) infor-
mation; in contrast with feedforward control, it requires an
internal forward model
of the body dynamics which combines a copy of the efferent command patterns with
the delayed reafferent signals and thus can reconstruct, in a similar way to a Kalman
filter, the actual state of the plant [16]. Also this internal model requires a process of
learning but it is conceptually simpler and computationally less critical than learning
the internal inverse model.
Indeed, the two control modalities seem to coexist in the motor system [59]. Ex-
perimental evidence suggests that both components may use some form of
internal
model
of body dynamics [9, 25]. The view of the cerebellum as a computing ma-
chinery that has competence as regards the physics of the body [12, 32, 33, 34, 42]
suggests that the cerebellar circuitry is likely to play an important role in carrying
out these tasks and hints at cerebellar syndromes as crucial pathological conditions
for understanding the role of the cerebellum in motor coordination.
17.4
The role of cerebellum in the coordination of
multiple joints
Cerebellar syndromes, also known as ataxias, form a useful case study to improve
our understanding of the mechanisms underlying sensorimotor coordination. Ataxia
is the main sign of cerebellar dysfunction. According to the classic description by
Holmes [27, 28], the term indicates multiple problems in the planning and execu-
tion of movements, including: (1) delay in movement initiation, (2) inaccuracy in
achieving a target (dysmetria), (3) inability to perform movements of constant force
and rhythm (dysdiadochokinesia), and (4) difficulty to coordinate multi-joint move-
ments. Additional cerebellar symptoms are diminished resistance to passive limb
displacement (hypotonia) and kinetic tremor (a pattern which appears at movement
onset and increases in amplitude while approaching the target).
Quantitative methods for movement analysis have a long history. (In fact, the clas-
sic Holmesian description of ataxia is based on an ingenious technique for recording
hand trajectories.) Methods based on kinematic and/or kinetic analysis of move-
ments may potentially allow one to identify more subtle aspects of movement disor-
ders as well as small changes in the degree of the involved impairments over time.
For example, kinematic measurements of single-joint arm movements in patients
with cerebellar ataxia [13] have provided a precise description of the alteration of
the temporal structure of these movements. While patients preserved the linear rela-
tionship between peak velocity and movement amplitude which is typical of normal
subjects, the speed profiles were asymmetric, with a longer deceleration phase. The
asymmetry of the speed profiles was also observed in multi-joint arm movements
[5, 60]: see
Figure 17.7,
which compares the typical patterns of normal subjects (left
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