Being blind in one eye, a condition known as monocular vision, presents a sensory experience that is far from seeing “blackness” on the affected side. Instead, the brain seamlessly fills in the missing visual information, resulting in a continuous field of view that simply lacks input from one side. This adaptation means a person does not perceive a dark hole or a blank spot. The immediate visual world remains intact, though the underlying mechanics of processing and perceiving that world change significantly.
The Visual Field and What Is Not Seen
The most direct consequence of monocular vision is a substantial reduction in the total field of view. A person with two functioning eyes typically has a horizontal field of view approaching 180 degrees or more. Losing vision in one eye physically removes the lateral vision on that side, creating a larger, permanent blind spot on the side of the non-seeing eye.
The brain actively suppresses the lack of input from the non-seeing eye, which is a neural mechanism that prevents the constant, distracting sensation of a missing image. Functionally, this restricted visual field means objects approaching from the blind side may be missed entirely, especially in crowded environments. To compensate, individuals often develop a habit of frequently turning their head toward the non-seeing side to scan the environment.
The Impact on Depth Perception
A major functional difference in monocular vision is the loss of stereopsis, the primary mechanism for highly accurate depth perception. Stereopsis relies on binocular disparity, the slight difference between the images captured by the two eyes. Since the eyes are separated, the brain fuses these two offset images to calculate precise distance and depth. With only one eye, this cue is eliminated, removing the ability to judge distances accurately, particularly in the near-field up to about twenty feet.
Consequently, simple tasks requiring fine motor control and depth judgment can become challenging initially. These difficulties include misjudging the distance when reaching to grasp an object, pouring liquid into a cup, or navigating stairs. The degradation of stereopsis can be profound, affecting discrimination accuracy at walking distances.
The loss of stereopsis is not a loss of all depth perception, but it removes the most efficient method the brain uses for this purpose. The initial adjustment period can be frustrating as the brain attempts to recalibrate its visual-motor coordination.
How the Brain Compensates for Monocular Vision
The brain is highly adaptable, and over time, it learns to utilize a collection of monocular cues to simulate depth perception and distance judgment. These cues are available to a single eye and are the same ones artists use to create a sense of depth on a two-dimensional canvas.
Monocular Cues Used for Depth
- Relative size, where the brain judges an object’s distance based on its known size.
- Motion parallax, which involves using movement to gauge distance.
- Interposition, where an object blocking the view of another is interpreted as being closer.
- Linear perspective, where parallel lines seem to converge in the distance.
- Light and shadow, which provide information about the three-dimensional shape of an object.
Through conscious effort and natural adaptation, the brain integrates these remaining cues to form a workable, though less precise, sense of depth. Most individuals with monocular vision successfully adapt and are able to perform daily activities without significant long-term impairment.