7.1 Oculomotor and Monocular Depth Cues

The image created on the retina is two dimensional and yet we can perceive a three-dimensional world.  Spend a moment looking at the visual scene in Figure 7.1.

 

The image shown in Figure 7.1 is complex, with multiple components including the house, fountain cascade, trees and the landscape beyond the house leading to the horizon. But how do we know how all the components fit together? For example, how do we know which trees are in front of the house and which are behind it or whether the trees in the distance are far away or just small? We can interpret the scene using various types of information (or cues) to estimate whether what we are looking at is far away or close to us. For example, when we look at something close to us, we contract some of our extraocular muscles so that our eyes converge or turn inwards, such that the image still falls on each fovea (see Figure 7.2). Also, we need to contract the ciliary muscle to change the shape of the lens so that what we are looking at is in focus. Therefore, we get important occulomotor depth cues from the eye muscles when we look at near objects.

We also have a number of depth cues that can be used with one eye. These are called monocular depth cues. These include:

1. Relative Height: Things at a distance look like their bases higher than things that are close. The height of the base of the trees increases as the trees look further away in Figure 7.3.
2. Relative Size: Objects farther away from other objects are smaller (Figure 7.3). The trees at the end of the path look smaller than those that appear close to us.
3. Occlusion: We assume that when one object covers another, it is because it is in front of it. Figure 7.4 shows how we use occlusion cues to tell which toy is closer to us and which is further away.
4. Linear perspective: Parallel lines converge as they move further away. This is illustrated in the railroad track in Figure 7.5. the fountain in Figure 7.1 and the path in Figure 7.3.
5. Shadows: Figure 7.6 shows how the position of shadows can make an object look convex or concave.
6. Texture Gradient: Textures look finer in the distance (see Figure 7.7 – but notice what other monocular cues are present in this picture).
7. Atmospheric Perspective: Things that are far away look hazy or out of focus. We can see this with the hills behind the house in Figure 7.1, the toys in the back of the line in Figure 7.4, and the hills in Figure 7.5.