Vision is the most important sensory organ of orientation. There are, in fact, two distinct visual systems. First is the ‘Focal’ (central) vision. This is concerned with recognition and identification of an object and in general answers the question of “what”. This comprises of the central 30° of the visual cone. Focal vision is responsible for discerning the fine details of the object. Information processed by focal vision is ordinarily well represented in consciousness and is critically related to parameters such as stimulus energy and refractive error. Second is the ‘Ambient’ (peripheral) vision which sub-serves spatial localisation and orientation; and is generally concerned with the question “where”. Ambient vision is mediated by relatively large stimulus patterns so that it typically involves stimulation of the peripheral visual field and relatively coarse details.
An important aspect of the two modes of visual processing is that they can be dissociated. Such dissociation can easily be demonstrated by considering the fact that one can walk while reading. Although attention is concentrated in the central visual field to read with focal vision, maintenance of body posture, locomotion and avoidance of most obstacles is readily accomplished with little or no conscious awareness by ambient vision. Various functions of vision can be divided into these two modes. During driving for example, the steering of the vehicle is an ambient function, which is sub-served by stimuli impinging on the peripheral visual field. On the other hand recognition of obstacle such as pedestrians, animals, potholes etc. is a focal function involving the focal (central) visual field. During daylight both modes of processing function normally and at their maximum capacities. However with reduced illumination e.g. at night, the efficiency of the focal mode is sharply reduced. The net result is that unexpected hazards are recognised late. Under the same conditions, the ambient mode continues operating under daytime efficiency levels, for there is no reduction in the ability to steer the vehicle.
Cues for Visual Orientation. Various cues are used for visual orientation. Cues used by ambient vision are the position and motion cues. These are largely subconscious and difficult to describe. On the other hand, central vision uses cues are learnt consciously. These cues are required to judge the depth and distance of a particular object by an individual. Some of these cues require binocular vision, while for others monocular vision is adequate.
Binocular cues. There are three binocular cues for judgment of depth and distance. These include:-
- (a) Convergence. This is inward rotation of the eyeballs and resulting direction of gaze depending on whether the object is near or far. This is limited to a distance of 6m, hence is not very useful for orientation during flying.
- (b) Accommodation. Accommodation is focusing of an image by changing the curvature of the eyes’ lens. This can also be used to gaze distance only up to 6m and is not very useful during flying.
- (c) Stereopsis. This is the visual appreciation of 3-dimensional space that results from a fusion of slightly dissimilar retinal images of an object. This can provide orientation information till a maximum distance of about 200m only. A pilot may occasionally use this during landing.
Monocular cues. Monocular cues useful in aviation are:-
- (a) Shape and Size Constancy. The comparative shape and size of known objects is used to judge distances.
- (b) Motion Parallax. The image of the nearer object moves more than that of the farther object.
- (c) Interposition. Partial obstruction of farther objects by nearer ones.
- (d) Texture or Gradient. The apparent loss of detail with distance.
- (e) Perspective.
- (i) Linear Perspective. The convergence of parallel lines at a distance.
- (ii) Illumination perspective. The tendency to perceive a light source above the object. This implies that lighted objects are interpreted to be on top and darker or shaded parts below.
- (iii) Aerial perspective. Distance objects appear bluish and hazy.
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