The marvel of visual space: the somatic sensation that support visual perception
The main topic in the study of the perception of visual space
In the world we can see, a “space” with breadth, height, and depth extends around us. Presumably there are few who would dispute that this kind of space arises on the basis of an image formed on our retinas by light entering our eyes, or, in other words, on the basis of our visual sensation. If the space we see were nothing more than an image projected on our retinas, however, we ought to have a more imperfect experience of space, like looking at a scene in a photograph. So how is it that we come to have such a rich experience of space from the weak images on our retinas? Answering this question has been a focus shared by those who investigate visual space from the birth of psychology to today.
Put this way, it may seem like we are talking about very complicated and difficult to understand phenomena. Without our noticing it, however, we are surrounded by phenomena related to this question in our daily lives. Here I would like to present the circumstances that sparked my interest in this topic in the form of two simple thought experiments.
Experiment 1: Picture a stick standing vertically (parallel to your body) against a completely white background. When the stick is tilted to the left or right without your body moving at all, you have the experience “the stick moved.” Now try angling your neck and tilting your head left and right. In this case you do not have the experience “the stick moved.”
Experiment 2: Next I would like you to picture the image of a stick against a white background displayed on a TV. Now imagine the case in which only the stick on the TV screen moves, and the case in which the TV itself tilts left and right. If you focus only on the movement of the stick and ignore that of the TV, in both cases you have the experience “the stick moved.” It is only when you perceive whether or not the TV moved that you experience the first case as “the stick moved” but not the second.
The importance of the sensations that ascertain the movement of the body
The space that emerges through visual sensation is the same as that projected on the TV. Just like when the movement of the TV was ignored in Experiment 2, in Experiment 1 there is no difference between moving the stick and moving your head when it comes to the change in the retinal image. On the other hand, the space we actually see is clearly one in which we ascertain the movement of the TV. This is why we don’t experience “the stick moved” when we tilt our heads. As a result, while our visual space depends heavily on our visual sensation, it can be posited that it arises through a mechanism that is underpinned by sensations of the movement of the physical body equipped with this sense of sight.
What are the sensations of the movement of the body? They are the proprioception that detects changes in the joints and muscles of the body, the skin sensation that detects pressure on the body’s surface, and the vestibular sensation that detects gravity acting on the body. Through the workings of these sense organs we are able to determine the posture of our body and know how our body is moving even when our eyes are closed. And the fact that when our eyes are open and our body moves we are able to experience objects in the images on our retinas as stationary even though the image itself changes can be assumed to result from movement on the retina being adjusted in accordance with the movement of the body through the workings of these sense organs. This being the case, I would like to consider the following issues that arise on the basis of this assumption.
The study of our perception of visual space going forward
If we encounter a situation in which these sense organs fail to function correctly, the normal visual space we ordinarily experience will presumably not be maintained. For example, in the case of the virtual reality (VR) that has begun to be used in video games, specialized devices allow us to experience visuals that move in accordance with our bodies by detecting information such as our position, acceleration, and gravity. This does not include, however, information about our joints and muscles or the pressure applied to our skin. As another example, in a weightless environment like outer space we cannot obtain information from the vestibular sense, and the function of perceiving the burden of gravity possessed by proprioception and skin sensation also becomes useless. In these situations, how much does the visual space we perceive differ from the one we normally experience? If it does differ, what information is most effective in helping us obtain a normal visual space? To answer these questions we must clarify the relationship between visual sensation and somatic sensation, and this research will continue going forward.