Differential activity of particular EOM compartments has been demonstrated repeatedly by MRI during specific ocular rotations,
8–10,13 and has emerged to be very complex. Horizontal rectus EOMs exhibit differential compartmental behavior during ocular counter-rolling
8 and asymmetric convergence.
9 Significantly greater contraction occurs in LRi but not in LRs in the excycloducting orbit than in the incycloducting orbit during ocular counter-rolling.
8 The MRs exhibits greater contractility than inferior compartment (MRi) during convergence.
9 Not only the horizontal rectus EOMs but also the IR and SO demonstrate differential compartmental contractility during vertical eye movements.
10,13 During vertical fusional vergence induced in healthy volunteers by monocular base-up prism viewing, IRm primarily contracts ipsilateral and relaxes contralateral to the prism despite absence of significant contractile changes in the lateral compartment (IRl).
10 Ipsilateral LRs, SOm, and contralateral SOl are also differentially active during vertical fusional vergence, although neither MR compartment demonstrates differential activation during this behavior.
10 During infraduction, IRm contracts more than IRl, and selective contractile changes in MRs and SOl also contribute, although LR exhibits little differential compartmental behavior.
13 In the context of the foregoing complexity and ubiquity of differential EOM compartmental function, it is actually surprising that in the present study of unilateral SO palsy, rectus EOMs did not exhibit differences in compartmental volumes. Ipsilesional IR and LR were hypertrophic in both of their individual compartments, whereas MR exhibited no size changes in either compartment. Although contralesional SR hypertrophy seemed to be significant only in the putative medial compartment, the magnitude of ePPV difference between the palsied and fellow eyes appeared greater in the lateral compartment. It is not clear if this apparent compartmental asymmetry in SR is real or an artifact, because it has so far been impossible to confirm by anatomic tracing any evidence of selective intramuscular innervation in this muscle.
7 Thus, the only unifying theme that has emerged from functional studies of compartmentalization is that differential EOM compartmental function is complex, may involve nearly any one or several EOMs during most normal ocular motor behaviors, and may occur ipsilateral or contralateral to a palsied SO. The main implication of the foregoing studies is that it would be particularly hazardous to draw inferences about functions of specific individual EOMs based on conventional examination of version or vergence eye movements; multiple combinations of differential function in compartments of multiple different EOMs might well be able to produce the same externally observed eye movements. Such overdetermination of ocular motor behavior by a plethora of EOM compartments might also contribute to the heterogeneity of clinical presentations of clinical forms of strabismus.