Regarding van den Bedem, Schutte, van der Helm, and Simonsz: Mechanical properties and functional importance of pulley bands or ‘Faisseaux Tendineux’

Regarding van den Bedem, Schutte, van der Helm, and Simonsz: Mechanical properties and functional importance of pulley bands or ÔFaisseaux TendineuxÕ q In their paper entitled ''Mechanical properties and functional importance of pulley bands or ÔFaisseaux Tendi-neuxÕ, van den Bedem et al. (Vision Research 45 (2005) 2710–2714) argue that mechanical properties of connective tissues they define as ''pulley bands'' are unsuited for stabilization of rectus extraocular muscles (EOMs) (van den Bedem, Schutte, van der Helm, & Simonsz, 2005). Such claims seem likely to confuse rather than clarify the issue. van den Bedem et al.Õs quotation from Tenon describes lateral bending of the lateral rectus (LR) path. Tenon surmised that during LR contraction, fascia would inflect the LR path laterally in the manner of a pulley, as also noted by Sappey (Sappey, 1888, 2001). While magnetic resonance imaging (MRI) of living orbits confirms as physiologic such lateral inflection of horizontal rectus EOM path (De-mer, Oh, & Poukens, 2000, Demer, 2005), this is not the pulley action proposed in the active pulley hypothesis (APH), which rather is resistance to EOM sideslip (Demer, 2004, 2005; Demer et al., 2000). Studies not cited by van den Bedem et al. suggest that orbitally coupled connective tissues are responsible for rec-tus EOM resistance to sideslip, and for sharp EOM path inflections. Initial evidence for orbitally coupled pulleys was the MRI observation that posterior EOM paths are little altered by large surgical transpositions of their inser-out musculo-global connections as the basis of EOM path stability. MRI has subsequently demonstrated in secondary (Clark, Miller, & Demer, 2000) and tertiary (Kono, Clark, & Demer, 2002a) gaze positions large anteroposterior shifts in path inflections during EOM contraction and relaxation (Kono et al., 2002a). Orbitally-stabilized pulleys change EOM pulling direction as eye position changes, fundamentally influencing neural control of eye movements Half angle dependence of ocular rotational velocity axis on eye position is equivalent to ListingÕs Law (LL) (Tweed & Vilis, 1990). Neurophysio-logical observations are consistent with a mechanical basis for LL: motoneurons innervating vertical rectus and oblique EOMs do not encode LL torsion during pursuit (Gha-sia & Angelaki, 2005), while direct abducens nerve stimulation evokes saccades conforming to LL (Klier, Meng, & Angelaki, 2005). Large shifts in rectus pulley positions underlying LL were predicted by the APH (Demer et al., 2000), and confirmed by MRI (Kono et al., 2002a). van den Bedem et al. suggest that the lateral rectus pulley …