Research ReportEye orientation during static tilts and its relationship to spontaneous head pitch in the laboratory mouse
Introduction
The use of the laboratory mouse in ocular motor research is growing rapidly due to widespread interest in applying the tools of molecular genetics to the study of ocular motor circuits and to the recent adaptation of accurate eye movement recording techniques for use in these tiny animals (Stahl, 2004b, Stahl, 2008). To date, most studies of murine ocular motility have focused on the eye movements induced by dynamic stimulation of the semicircular canals (the angular vestibulo-ocular reflex, aVOR), retina (the optokinetic reflex, OKR) or macular (otolith) organs (Andreescu et al., 2005, Killian and Baker, 2002). In contrast, eye movements induced by static stimulation of the macular organs have received less attention. The macular organs induce the compensatory maculo-ocular reflexes (MOR) in response to either maintained tilts (tiltMOR) or linear translations (transMOR) (Paige and Tomko, 1991). While transMORs are weak in mammals lacking a fovea (or homologous retinal structure) such as rabbits and rats (Baarsma and Collewijn, 1975, Hess and Dieringer, 1991), their tiltMORs are strong (Maruta et al., 2001, Van der Hoeve and De Kleijn, 1917). Descriptions of the mouse response to sustained tilts published to date indicate that the mouse tiltMOR is similarly robust, but these descriptions were either semi-quantitative (Harrod and Baker, 2003) or limited to assessments of eye position relative to an arbitrary zero position and for rotation about one axis (roll) over a relatively restricted range (Andreescu et al., 2005). It is possible using video oculography, however, to determine eye positions in more absolute terms, i.e., as deviations with respect to the earth-horizontal and to the mid-sagittal plane of the animal (Stahl, 2004a).
The control of head pitch and the MOR are interrelated. Both eye position and head orientation are influenced by the macular organs via the tiltMOR and the vestibulo-collic reflexes, respectively. At the same time, the mechanisms that control head position determine the rest position of the eye because head orientation determines the orientation of the macular organs and thus the output of the tiltMOR. This effect of head position on eye position has a very practical significance for ocular motor research in which eye movements are recorded with the head fixed in place; the selection of the pitch in which the head is fixed determines, in any animal with a robust tiltMOR, the rest position of the eyes, and this may in turn influence the geometry of compensatory eye movements. To date, there have been only limited data available regarding the natural pitch orientation of the mouse head. Vidal and colleagues (Vidal et al., 2004) x-rayed mice as they trotted on a treadmill and concluded that the horizontal semicircular canals are carried at an angle pitched up from the earth-horizontal by 15.4°. However, the study does not frame its results in terms of the lambda–bregma axis, the bony reference point more commonly used to determine pitch angle where stereotactic apparatus are involved. Moreover, it is unclear to what extent these data – obtained from vestibular-deficient mutants and C3H controls – are applicable to the C57BL/6 mouse — the strain more commonly used for studies of “normal” mice or as the genetic background for many mutant strains.
The purpose of this study was to characterize the tiltMOR during pitch and roll over the entire 360° range, determining the eye angles relative to the mid-sagittal and earth-horizontal planes, rather than as angles relative to an arbitrary zero. Additionally we determined the pitch of the head during forward ambulation, expressing this pitch in terms of the orientation of the lambda–bregma axis with respect to the earth-horizontal. By obtaining the tiltMOR and head orientation data from the same animals we were able to determine where the animals “place themselves” on the relationship between eye position and pitch angle and so to approximate the vertical position of the eyes during ambulation. A preliminary report has been published (Oommen and Stahl, 2007).
Section snippets
Pitch tilts
Figs. 1A and C plot the horizontal eye position (relative to the mid-sagittal plane) and vertical eye position (relative to the earth-horizontal plane) versus the angle of the lambda–bregma axis with respect to earth-horizontal (∠L–B) in response to tilts about the pitch axis. (See Experimental procedures for definitions of all angles abbreviated in Results.) Separate curves are shown for eye positions measured in the light and dark. The dashed line indicates the predicted horizontal and
Discussion
The experiments demonstrated that the tiltMOR in the mouse is robust, producing sustained deviations of the eye of approximately 50% of the pitch or roll tilt angle (i.e., gains of approximately 0.5) over the central ± 30° of tilt. The 0.5 gain values take into account the fact that the optic axes of the mouse (and thus the reference frame for our 2D video oculography) are significantly deviated with respect to both the pitch and roll stimulus axes. The uncorrected sensitivity and gain to roll
Experimental procedures
The animal experiments were approved by the Institutional Animal Care and Use Committee at Case Western Reserve University and followed the guidelines of the National Institutes of Health on the use and care of laboratory animals. C57BL/6 mice were obtained from The Jackson Laboratory (Bar Harbor, ME). Ten animals were studied, aged 5–10 months at the time of the experiments. Animals were prepared for eye movement recording by surgically implanting an acrylic head fixation pedestal as
Acknowledgments
The authors wish to thank Robert James for technical assistance in the experiments and Igor Kofman for machining components of the recording apparatus. BS Oommen was supported by a Crile Summer Research Fellowship. JS Stahl was supported by EY13370 and the Department of Veterans Affairs.
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