Monocular depth perception is the ability to perceive the distance and depth of objects using only one eye. It is a skill that humans and many animals use to navigate their environment and avoid obstacles. Monocular depth perception relies on various visual cues that provide information about the relative position and size of objects in the scene. Some of these cues are:
- Relative size: This cue is based on the assumption that objects of the same type tend to have similar sizes in reality. Therefore, if two objects of the same type appear different in size on the retina, the smaller one is perceived as farther away than the larger one. For example, if you see two cars on the road, one closer and one farther, you can estimate their distance based on their apparent size. According to Healthline, relative size is a monocular cue that gives you the ability to measure how far away something is by judging how big or small it is.
- Interposition: This cue is based on the fact that objects that are closer to the observer tend to block or occlude the view of objects that are farther away. Therefore, if one object partially covers another object, the covered object is perceived as more distant than the covering object. For example, if you see a tree blocking the view of a building, you can infer that the tree is closer than the building. Interposition is also known as occlusion or overlap.
- Linear perspective: This cue is based on the observation that parallel lines seem to converge or meet at a vanishing point as they recede into the distance. Therefore, if two objects are aligned along parallel lines, the object that is closer to the vanishing point is perceived as farther away than the object that is farther from it. For example, if you look at a railroad track, you can estimate the distance of different points along it based on their proximity to the vanishing point.
- Aerial perspective: This cue is based on the effect of atmospheric conditions on the appearance of objects in the distance. Because light scatters as it passes through air, especially moist or polluted air, objects that are farther away tend to look hazier, less saturated, and less contrasted than objects that are closer. Therefore, if two objects have similar shapes and colors, but one looks more faded and blurred than the other, the faded object is perceived as more distant than the clear object.
- Texture gradient: This cue is based on the observation that textured surfaces tend to appear finer and smoother as they recede into the distance. Therefore, if two objects have similar textures, but one looks more coarse and detailed than the other, the coarse object is perceived as closer than the smooth object. For example, if you look at a grassy field, you can estimate the distance of different parts of it based on their texture.
- Shading: This cue is based on the effect of light and shadow on the appearance of objects in three-dimensional space. Because light sources tend to illuminate objects from a certain direction, objects that are closer to the light source tend to have brighter and sharper highlights and shadows than objects that are farther away. Therefore, if two objects have similar shapes and colors, but one looks more shaded and contoured than the other, the shaded object is perceived as closer than the flat object.
- Motion parallax: This cue is based on the relative motion of objects in relation to an observer’s movement. When an observer moves, objects that are closer tend to move faster and in opposite direction to the observer’s motion, while objects that are farther away tend to move slower and in same direction as the observer’s motion. Therefore, if two objects move differently when you move your head or body, you can estimate their distance based on their motion parallax.
- Accommodation: This cue is based on the adjustment of the lens of the eye to focus on different distances. When we try to focus on near objects, our eye muscles contract and make our lens thicker and more curved. When we try to focus on far objects, our eye muscles relax and make our lens thinner and flatter. The kinesthetic sensations of these muscle movements provide feedback for judging distance.
These are some of the most common monocular cues of depth perception that we use every day. However, there are other cues that are not related to monocular depth perception, such as:
- Binocular disparity: This cue is based on the difference between the images projected on each retina due to their horizontal separation (about 6 cm). Because each eye sees a slightly different view of the same scene, there is a disparity or mismatch between them. The brain uses this disparity to calculate depth by comparing and fusing the two images into one. Binocular disparity is also known as stereopsis or stereoscopic vision.
- Convergence: This cue is based on the inward rotation of both eyes toward a center point when we focus on near objects. The angle of convergence depends on the distance of the object from the eyes. The brain uses this angle to estimate depth by sensing the tension of the eye muscles that control convergence.
These two cues are binocular cues, meaning that they require visual input from both eyes for depth perception. They are more effective and accurate than monocular cues, especially for judging short distances. However, they are not available when we use only one eye, such as when we close or cover one eye, or when we have a vision impairment in one eye.
Therefore, the answer to the question “which of the following is not related to monocular depth perception?” is either binocular disparity or convergence, depending on the options given. These two cues are related to binocular depth perception, not monocular depth perception.
