Biomedical Engineering Reference
In-Depth Information
Superior Colliculus
Superior Colliculus
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5°
10°
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40°
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-40°
-20°
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(a)
(b)
FIGURE 13.58
A detailed view of the retinotopic mapping over of the superior colliculus for a (a) 20
move-
ment and a (b) 2
movement. Notice the locus of points for the 2
movement is smaller than that for the 20
move-
ment, which implies that fewer neurons are firing for the smaller movement. The movement fields within
the superior colliculus also reflect that the number of neurons firing for saccades less than 7
is smaller than those
firing for saccades greater than 7
. For saccades above 7
, the movement field is approximately constant.
The cerebellum is responsible for the coordination of movement and is composed of a
cortex of gray matter, internal white matter, and three pairs of deep nuclei: fastigial nucleus
(FN), the interposed and globose nucleus, and dentate nucleus. The deep cerebellar nuclei
and the vestibular nuclei transmit the entire output of the cerebellum. Output of the cerebel-
lar cortex is carried through Purkinje cells. Purkinje cells send their axons to the deep cere-
bellar nuclei and have an inhibitory effect on these nuclei. The cerebellum is involved with
both eye and head movements, and both tonic and phasic activities are reported in the
cerebellum. The cerebellum is not directly responsible for the initiation or execution of a
saccade but contributes to saccade precision. Sites within the cerebellum important for the
control of eye movements include the oculomotor vermis, FN, and the flocculus. Consistent
with the operation of the cerebellum for other movement activities, the cerebellum is postu-
lated here to act as the coordinator for a saccade and as a precise gating mechanism.
The cerebellum is included in the saccade generator as a time-optimal gating element,
using three active sites during a saccade: the vermis, FN, and flocculus. The vermis is
concerned with the absolute starting position of a saccade in the movement field and cor-
rects control signals for initial eye position. Using proprioceptors in the oculomotor muscles
and an internal eye position reference, the vermis is aware of the current position of the eye.
The vermis is also aware of the signals (dynamic motor error) used to generate the saccade
via the connection with the Nucleus Reticularis Tegmenti Pontis (NRTP) and the SC.
With regard to the oculomotor system, the cerebellum has inputs from SC, lateral genic-
ulate nucleus (LGN), oculomotor muscle proprioceptors, and striate cortex via NRTP. The
cerebellum sends inputs to the NRTP, LLBN, EBN, VN, thalamus, and SC. The oculomotor
vermis and fastigial nuclei are important in the control of saccade amplitude, and the floc-
culus, perihypoglossal nuclei of the rostral medulla, and possibly the pontine and mesence-
phalic reticular formation are thought to form the integrator within the cerebellum.
One important function of the flocculus may be to increase the time constant of the neural
integrator for saccades starting at locations different from primary position.
