Spatial Orientation: The Basics, Part I

 

By Dick Leland, President of NASTARSM Center

Flying high-performance aircraft or launching into space strongly affects your sense of spatial orientation. Huge amounts of information are continuously sensed by the visual system, the vestibular system, the somatosensory system (sometimes referred to as “seat of the pants”), and the auditory system, and then processed by the brain.

The knowledge of where we are and what we are doing relative to our surroundings — our spatial orientation and situational awareness — comes from perceived elements of this information that are processed by both the conscious and the preconscious areas of our brain. This series will take a look at these systems — what they do for us and problems that may occur during flight.

The Visual System

People receive approximately 70% of their total information input via the visual system. The visual system can be subdivided into the focal visual system (central 6 degrees of the visual field) and the ambient visual field (the rest).

Focal vision is the portion of the visual field used for object recognition as well as distance and depth perception. Focal visual images are projected into the fovea (the center of the retina); therefore, visual acuity is highest in the focal visual cone. Focal vision is a consciously-processed orientation input and is therefore considered our most reliable orientational input.

Daytime or cone vision is referred to as photopic vision. Cone cells are very fast-acting and able to perceive color, but they require a large amount of light — ranging from full daylight to full moonlight — to be stimulated. When illumination levels fall below full moonlight, the cone cells are not stimulated and the night blind spot occurs. The night blind spot roughly corresponds to the focal visual cone, a central 5–10 degree cone in the visual field. Since the visual fields of binocular vision overlap, the night blind spot is not always readily apparent; however, in low-light conditions, it exists for each eye.

Ambient (or peripheral) vision comprises the remainder of the approximately 170-degree visual field. It is responsible for general orientation and is particularly sensitive to flat planes and motion stimuli. Visual acuity is poor in the ambient visual field, ranging from about 20/80 at 7 degrees off-center to about 20/200 at the extreme periphery of the field. Ambient visual inputs are preconsciously processed.

When the reception of incoming stimuli moves from the fovea to the periphery of the retina, where ambient vision takes place, the number of cone cells decreases and the number of rod cells increases. The rod cells are very sensitive to light but do not perceive color. During dark adaptation, the photochemical rodopsin (sometimes referred to as visual purple) must be produced and built up for the rods to function. Therefore, the rods are very slow-acting compared to the cones. Because rodopsin, necessary for vision in low light, is destroyed by bright light, pilots should avoid exposure to bright light prior to and during night flying.

In certain circumstances your visual system can fool you. During object recognition, a seen object is compared with thousands of preconsciously stored mental images until a match occurs. If the current viewing conditions are similar to the stored image conditions, accurate object recognition occurs. Focal visual illusions result when viewing conditions differ from those of the stored image. Examples of this phenomenon include approach and landing illusions due to differing runway widths, lengths, and slopes; distance and depth perception illusions; and altered perceptions due to darkness or weather phenomena.

Next Month: The Vestibular System
 
 

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Next Month: Characteristics of Spatial Disorientation