Neurons in the primate superior colliculus coding for arm movements in gaze-related coordinates
V. Stuphorn, E. Bauswin & K.-P. Hoffmann
Journal of Neurophysiology, 83:1283-1299, 2000
In the intermediate and deep layers of the superior colliculus (SC), a well-established oculomotor structure, a
substantial population of cells is involved in the control of arm movements. To examine the reference frame of
these neurons, we recorded in two rhesus monkeys (Macaca mulatta) the discharges of 331 neurons in the SC and the
underlying mesencephalic reticular formation while monkeys reached to the same target location during different
gaze orientations. was collected on. For 65 reach related cells with sufficient data and for simultaneously recorded
electromyograms (EMGs) of 11 arm muscles we calculated an ANOVA (factors: target position, gaze angle) and a gaze
dependency (GD) index. EMGs and the activity of many (60 %) of the reach-related neurons were not influenced by
the target representation on the retina or eye position. We refer to these as 'gaze-independent' reach neurons.
For 40 %, however, the GD fell outside the range of the muscle modulation and the ANOVA showed a significant influence
of gaze. These 'gaze-related' reach neurons discharge only when the monkey reaches for targets having specific
coordinates in relation to the gaze axis, i.e. for targets in a gaze-related 'reach movement field' (RMF). Neuronal
activity was not modulated by the specific path of the arm movement, the muscle pattern that is necessary for its
realization or the arm that was used for the reach. In each SC we found gaze-related neurons with RMFs both in
the contralateral and in the ipsilateral hemifield. The topographical organization of the gaze-related reach neurons
in the SC could not be matched with the well-known visual and oculomotor maps. Gaze-related neurons were more modulated
in their strength of activity with different directions of arm movements than were gaze-independent reach neurons.
Gaze-related reach neurons were recorded at a median depth of 2.03 mm below SC surface in the intermediate layers,
where they overlap with saccade-related burst neurons (median depth: 1.55 mm). Most of the gaze-independent reach
cells were found in a median depth of 4.01 mm below the SC surface in the deep layers and in the underlying MRF.
The gaze-related reach neurons operating in a gaze-centered coordinate system could signal either the desired target
position with respect to gaze direction or the motor error between gaze axis and reach target. The gaze-independent
reach neurons, possibly operating in a shoulder- or arm -centered reference frame, might carry signals closer to
motor output. Together these two types of reach neurons add evidence to our hypothesis that the SC is involved
in the sensorimotor transformation for eye-hand coordination in primates.
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