In principle, the eyes can be rotated about all three possible axes of rotation in three-dimensional space, within certain limits. Parallel eye movements of both eyes, with identical rotations with respect to the axis of rotation and number of degrees, are called conjugate eye movements. They usually occur unconsciously and arise, for example, as gaze-following movements when the distance to the moving object remains constant. Rapid changes of gaze, so-called cascades, also correspond to conjugate eye movements.
What are conjugate eye movements?
Parallel eye movements of both eyes, with identical rotations with respect to the axis of rotation and number of degrees, are called conjugate eye movements. In principle, the eyes can move independently to view objects in focus and as a congruent image with depth of field with both eyes at different distances from the observer. However, these independent movements are only possible within very narrow limits, especially when they involve the vertical axis. Normally, our two eyes move exactly parallel. This is true for slow and also for fast, saccadic eye movements, as well as for unconscious microsaccades that occur during fixation of a static object in order to supply the cone-shaped color sensors in the fovea centralis, the zone of sharpest color vision in the retina, with constantly changing light impressions. Parallel eye movements that occur in both eyes in exactly the same axis of rotation and the same number of degrees are called conjugate. With the exception of conscious squinting, which can be achieved by a voluntary, non-parallel twisting of the two eyes around the vertical axis, resulting in two images slightly displaced from each other, all conscious eye movements are conjugate. Also the constantly running unconscious eye movements are conjugate eye movements. Only during gaze tracking of moving objects with changing distance to the eyes the conjugate eye movements are superimposed by combined vergences in the vertical and the transverse axis, because the two visual axes have to tilt against each other with changing distances of the fixated object to bring both images to coincidence. In unconscious conjugate eye movements, with a very narrow range of motion, the longitudinal axis (Y axis) is also used for torsional movements of the eyes. The pair of muscles responsible for this cannot be addressed voluntarily – at least not without training.
Function and task
Unconscious conjugate eye movements have an unusually high utility value for humans in everyday situations and an additional passive safety value. Sharp contrast and color vision is limited to the tiny area of the fovea, the zone of sharpest vision. The fovea has an extent of about 1 degree on the retina, while the total field of view is about 100 degrees. While S, M, and L cones, each optimized for a range of wavelengths, located close together in the fovea allow sharp color vision, the so-called rods, which allow only blurred, monochromatic, vision, are mainly concentrated outside the fovea. However, the rods are extremely sensitive to light (twilight vision) and especially sensitive to movement. As soon as a moving object is noticed in the peripheral field of vision, the eyes unconsciously turn – almost abruptly – in the direction of the object in order to inspect it more closely with the fovea. The jumping gaze turn in the direction of the object, called saccade, occurs in a very fast conjugate eye movement. This has the advantage that the object, after detection by the fovea, can be observed in color with depth of field without delay. This means that the visual centers of the brain perform a tremendous feat. They strongly relieve the consciousness by reflexively guiding the eyes to an object for the purpose of fixation. The time-consuming need to consciously realign the eyes is eliminated. The ability may have originally developed within evolution for the purpose of early detection of predators or prey. But modern humans also need the ability to move safely in heavy traffic, for example. Unconscious conjugate eye movements in the microscale are also of great importance for fixations of immobile objects.To prevent local adaptation during fixation of a nonchanging object, which would cause the object to “disappear” due to fatigue of the photoreceptors, unconscious microsaccades of about 5 to 50 arcmin take place 2 to 3 times per second. The microsaccades occur completely unconsciously and are also performed as conjugate eye movements.
Diseases and complaints
Voluntary and involuntary conjugate
Eye movements are complex processes that require the functioning of the individual components of the eyes, the nervous connection of the actuating muscles with the corresponding brain centers, the interconnection of proprioceptive messages of the eye positions, and intact eye reflexes. Even an interconnection with the auditory centers must be given, because in case of a strong noise the eyes reflexively turn towards the noise in order to visually detect the object that supposedly caused the noise, if possible. Motility disorders of the eyes can be caused by diseases or impairments in the six actuating muscles of the eyes, by impairments of the cranial nerves involved (cranial nerves III, IV, VI) or by diseases of the brainstem or cerebellum. The best known motility disorders are strabismus, which may be acquired or due to gene mutations. In supranuclear gaze palsies, there is a lesion of the gaze centers in the brain. Gaze palsies impede ocular motility and completely preclude the possibility of conjugate eye movements. An autoimmune disease, endocrine orbitopathy, often occurs in association with thyroid disease. The disease results in a noticeable protrusion of the eyes and raised eyelids. In advanced stages, eye movements are impaired as the stellate muscles are attacked by the immune system. Transient motility disorders may also result from alcohol consumption or the use of other drugs with neurotoxic effects.
