The eyes of some individuals with profound visual impairment often appear to drift or wander, sometimes settling into an upward gaze. This phenomenon is not universal, nor is it a conscious effort, but a complex physiological response to the absence of visual data. To understand why this occurs, we must examine how the nervous system ordinarily uses sight to anchor the eyes in place. The upward drift results from the brain losing a constant, stabilizing input, revealing two underlying mechanisms of the oculomotor system.
The Role of Visual Input in Stabilizing Gaze
In sighted individuals, the eyes are held steady by a continuous, involuntary feedback loop known as visual fixation. This system relies heavily on the fovea, the small central area of the retina responsible for sharp, detailed vision. The brain constantly monitors the image projected onto the fovea, issuing minute, corrective signals to the extraocular muscles to ensure the image remains stable.
When a person looks at a stationary object, the visual system prevents movement of the image away from the fovea. This process involves numerous reflexes, including tiny, rapid eye movements called microsaccades. Without this continuous visual input, as occurs in blindness, the eyes lose their primary anchor. This loss of visual feedback disrupts the neural network responsible for maintaining a stable gaze position, allowing the eyes to drift.
The stability of the eye position depends on a neural integrator, a circuit in the brainstem that converts the velocity signals of eye movements into a position signal for the muscles. When visual input is removed, this neural integrator is deprived of the sensory feedback required to optimize its performance. The resulting instability often manifests as a slow, uncontrolled drift of the eyes, a condition called gaze instability.
The Mechanism Causing Upward Eye Drift
The tendency for the eyes to drift specifically upward can be explained by two physiological factors related to the mechanics and reflexes of the eye muscles. The first factor is the natural resting tension, or tonus, of the extraocular muscles. The superior rectus muscle, which primarily elevates the eye, tends to have a slightly stronger resting tone than the opposing inferior rectus muscle.
When the brain is not actively exerting control to fixate the gaze, this slight imbalance in muscle tone causes the eye to default to an elevated position. The eye’s tendency to drift upward in the absence of visual fixation is the physical manifestation of this muscle balance.
The second factor relates to protective reflexes, such as Bell’s phenomenon. This is a normal, defensive reflex where the eyeballs automatically roll upward and slightly outward when the eyelids close or when there is a threat to the eye. In individuals with profound visual loss, active control over eye position is diminished or absent. This lack of engagement allows the default, reflexive position—the upward roll—to dominate the resting state of the eyes.
Factors Influencing Eye Position and Movement
The pattern of eye movement in a visually impaired person is not uniform and is influenced by several variables. The age at which vision loss occurred is a significant factor. Individuals who lose their sight later in life often retain a more stable eye position, or a less pronounced drift, because their oculomotor system was fully developed with years of visual feedback. Conversely, those who are congenitally blind or lose vision early in childhood may exhibit a more dramatic instability because the visual system never fully developed the necessary neural pathways for fixation.
The underlying cause of blindness also dictates the type of eye movement observed. Conditions affecting the optic nerve or retina, which remove the sensory input, often lead to the wandering gaze or upward drift.
Damage to the brain’s visual processing centers or motor control pathways can result in different patterns. These may include nystagmus, an involuntary, rapid, and rhythmic movement of the eyes, or strabismus, a misalignment of the eyes. The upward gaze is primarily a passive consequence of the nervous system’s response to the absence of visual data, rather than a universal symptom of sightlessness.