In this blog post, we take an interesting look at how binocular cues and monocular cues work together to enable us to perceive the three-dimensional world.
If humans originally had only one eye, would we be able to perceive the world in three dimensions? Three-dimensional perception refers to the process of recognizing the distance to an object and understanding the world in three dimensions. Stereoscopic perception is achieved by obtaining various cues from visual information entering the eyes, which can be divided into binocular cues and monocular cues. Binocular cues are obtained through the cooperation of both eyes, and a typical example is similar images with a difference in position (parallax) sent from both eyes. Monocular cues can be obtained with one eye, and humans can perceive the world in three dimensions based on previous experience using only monocular cues. Although the images formed on the retina are two-dimensional, stereoscopic perception is achieved through various clues that allow us to recognize the differences in depth between the images.
Since the two eyes of a human being are located slightly differently, the same object appears at slightly different angles to each eye. This is called binocular disparity, and the brain analyzes these subtle differences to calculate the distance to the object. For example, if you hold your finger in front of your nose and close one eye and then the other, you will see that the position of your finger changes significantly. This is a case of large binocular disparity, which appears more pronounced with objects that are closer.
On the other hand, monocular cues are information that can be obtained with one eye. Humans can perceive the world in three dimensions through previous experience and learning using only monocular cues. For example, when the same object appears in our field of vision at different sizes, we perceive the side with the larger visual angle as being closer. The relative size of objects is a typical monocular cue. Another monocular cue is “linear perspective.” We perceive that the narrow side of parallel lines created by a road or railroad tracks extending forward is farther away than the wide side.
Another monocular cue, “slope,” is obtained from the continuous change in size of objects that appear simultaneously in the field of view when the same objects are distributed collectively on a surface. For example, when we look at flowers blooming in a field, the flowers in the foreground appear larger and gradually become smaller toward the background, and this visual cue easily creates a sense of perspective.
In some cases, monocular cues can be obtained from movement. “Motion parallax” is the apparent speed of stationary objects as seen by an observer in motion, which provides clues about the relative distance to the objects. For example, when looking out the window of a moving train, trees that are close by appear to pass by quickly, while mountains in the distance appear to be almost stationary. This phenomenon occurs because distant objects occupy a smaller portion of the field of view, while nearby objects occupy a larger portion.
Animals can also use monocular cues to perceive depth. In particular, animals with eyes on the left and right sides of their heads cannot use binocular cues because there is almost no overlap between the fields of view of their eyes. In such cases, monocular cues play a decisive role in depth perception. For example, some birds determine the distance to a stationary object based on how much it appears to move when they move their heads from side to side. The use of monocular cues has been observed in various animals, demonstrating that stereoscopic perception plays an important role in survival.
In conclusion, if humans had only one eye, stereoscopic perception would be difficult. The information provided by binocular cues is very important for perceiving the world in three dimensions. However, stereoscopic perception is possible to a certain extent even with monocular cues alone, and can be supplemented by previous experience and learning. It is interesting that not only humans but also many animals use these monocular cues to perceive the world in three dimensions. The complex mechanism of stereoscopic perception allows us to understand the depth of the world and provides us with a richer visual experience.
