Binocular coordination of saccades: development, aging and cerebral substrate

Fine binocular coordination of saccades is essential for clear binocular vision, whatever the viewing distance. When saccading between targets at far distance (for example 400 cm), the saccade starts with visual axes almost parallel (small vergence angle); coordination of saccade allows the angle to be maintained. At near distance (for example 40 cm), the eyes converge; yet the saccades should move the eyes by the same amount in the same direction so that the initial angle of convergence is again maintained. By which mechanisms the central nervous system (CNS) succeeds in such coordination is a controversial issue since early debates between Hering (1868) and Helmholtz (1856-1866). According to Hering, binocular coordination is based on neuroanatomic connections, allowing the same innervation to be sent to conjugate muscles so that the eyes move as a single organ. According to Helmholtz, neuroanatomic connections would be separate for each eye and thus binocular coordination of movements could be reached only via training and binocular visual experience. Hering’s thinking dominated both basic and clinical research. Electrophysiological evidence in favour of Hering’s law of equal innervation has been presented in monkeys: burst saccade neurons project to motoneurons innervating the lateral, external rectus of one eye and also project to interneurons, that is, the neural relay to the motoneurons innervating the contralateral medial rectus (for a review see Leigh & Zee, 2006). For vertical saccades, Moschovakis, Scudder and Highstein (1990) provided evidence for an even more robust structural basis allowing equal innervation of vertical recti of both eyes. Furthermore, a clinical study (Kommerell, Olivier & Theopold, 1976) showed that spontaneous adaptation to unilateral oculomotor paresis obeys Hering’s law. Indeed, the two patients studied used the paretic eye to fixate, because it had better acuity than the non paretic eye. The size of the saccades became appropriate for the paretic eye while the non paretic eye made hypermetric saccades. The adaptative increase of the gain of saccade command was equal for both eyes.