If horizontal strabismus angle is explicitly encoded in abducens nucleus by separate populations of right eye and left eye neurons, one might expect to find numerous monocular neurons near both the horizontal and vertical lines in
Figure 4. For a strong majority of the neurons in both strabismic monkeys, however, the slope was larger for the ipsilateral eye (data points above the dashed unity line), as is typical for abducens neurons in normal monkeys.
18 Particularly noteworthy is the fact that 6 of 24 neurons in monkey ET1 had significantly negative slopes for the contralateral eye. This indicates that, for a given position of the ipsilateral eye, these neurons discharged at a higher rate if the contralateral eye was deviated further in the neurons' off-direction. For monkey ET1 this equated to a less pronounced esotropia. One neuron had a negative slope for the ipsilateral eye. For monkey XT1, 1 of 34 cells showed a significantly negative slope for the contralateral eye. Based on the resulting distributions of eye position coefficients, the neurons were classified as follows: conjugate (16/58 neurons, see
Fig. 5A for bootstrap distributions for one example neuron in this category), monocular ipsilateral (18/58 neurons,
Fig. 5B), monocular contralateral (2/58 neurons), binocular with ipsilateral preference (12/58 neurons), binocular with contralateral preference (2/58 neurons), and opposite where the slopes for the two eyes were in opposite directions (8/58 neurons,
Fig. 5C). For monkey ET1, the mean slope was 0.58 for the contralateral eye and 3.96 for the ipsilateral eye (
Table, part A). The mean
R2 was 0.63. For monkey XT1 the mean slope was 1.70 for the contralateral eye and 3.75 for the ipsilateral eye. The mean
R2 was 0.75. It must be emphasized that while these correlational analyses and classifications cannot distinguish between abducens motoneurons and internuclear neurons,
47 they do indicate which eye's motion is coded by a given neuron.