Human visuo-spatial system

The human visuo-spatial system is what makes us perceive the world in 3D.

From a neuroscience perspective, it is roughly broken into three parts, starting with the visual cortex which receives visual information from the eyes:

1. Visual cortex (The back part of the brain): V1/V2 – retinal coordinates (image space)

2. Other parts of the visual cortex: MT / V3A – motion & depth cues (still retinal)

3. Parietal cortex (The upper part of the brain): Posterior Parietal Cortex

The human visuo-spatial system determines distance largely based on sharpness of edges of objects.

• Intraparietal sulcus

Visual system

The human visual system (HVS) is everything that enables vision in humans. It includes portions of the brain, nervous system, and eyes.

For the human visual system, the persistence of vision is typically on the order of 50 milliseconds.

It is linked to the vestibular system.

Each eye's retina is mostly a 2D image sensor. The photoreceptors in the retina measure the brightness only.

There are neurons that recognize edges in the visual cortex.

The eyes connect to the thalamus, which connects to the visual cortex.

The lenses of the eyes

Figure 1. The lenses of the eyes adjust their accommodation according to the distance of the viewed object. (Image: doc-ok.org)

Figure 2. The lens placed between the eye and object redirect the light rays, helping the eyes to focus the image. (Image: doc-ok.org)

The need for lenses in virtual reality HMDs is related to how the human eyes function. The eyes have built-in lenses that are placed behind the pupils. On the back of the eyes, there are light-sensitive receptors (rods and cones) that send the received light input signals to the brain in order to translate them into images.

It is very hard or even impossible to focus on things that are very close to the eyes. For example, if a person puts a piece of paper with text on it, one inch in front of one of the eyes, it will prove difficult to read it unless that person is extremely near-sighted.

The lenses in the eyes alter the incoming light in order to focus it on the receptors. The lenses bend according to the distance between the object being focused on and the eyes, converging the incoming light rays on the light-sensitive cells on the retina. When an object is closer to the eye’s lens, the rays coming from that object diverge at a large angle and, consequently, the lens has to adjust in order to focus the rays on the retina (Figure 1). Rays of light coming from an object at medium range will diverge less, and rays from an object infinitely far away are parallel and do not diverge. This means that as the distance between the eye and the object increases, the eye’s lens does not need to bend as much to maintain focus. This process of changing the shape of the lenses is called accommodation.

A ring of muscle around the lens - the ciliary muscle - is responsible for making the lens thicker and rounder when viewing close objects; the muscles relax, flattening the lenses when the object is at a distance. There is a limit to how much the muscles can compress the lens. This means that there is a minimum distance on which the eye can focus. Generally, it is around 4 inches for children and young adults with normal vision. This value increases with age.

Regarding virtual reality, this biological limitation affects the engineering of the VR HMDs. Since it is not possible to focus the images produce by a screen inches away from the face, lenses placed between the user’s eyes and the display are necessary in order to provide focus.