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Distinguished Member
Join Date: Apr 2006
Location: In the Superstitions
Posts: 1,452
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Vision and Shooting research--Long read
VISION AND SHOOTING
By Edward C. Godnig, O.D., FCOVD
Staff Member, PPSC
Information contained within this article will give firearm instructors and marksmanship students a better understanding of how vision significantly contributes to shooting ability and success. Appreciating the dominant role vision plays in directing and monitoring most of the skills used during shooting will prove useful in updating training methodology. The ultimate result of incorporating useful scientific models and research into a training curriculum should result in shooting performance enhancement.
A comprehensive definition of vision goes beyond the classic 20/20 sight definition. A limited concept of vision is often defined as the ability to see a sharp, clear, 20/20 or better visual acuity image. However, defining vision as a dynamic, learned process of deriving meaning and directing action from light energy establishes a scientific model to better appreciate the importance of vision for accurate and safe shooting.
Visual skills provide intelligent information to shooters concerning where targets are located, what details and characteristics constitute the target, as well as target speed and direction of movement. This type of spatial, temporal and labeling information is used to make a decision whether or not to coordinate a response to shoot the target. Understanding how visual abilities dominate the process of shooting targets accurately and quickly will provide a framework to improve firearms instruction.
An overview of the basic anatomy and physiology of how the eye responds to light to begin the visual process establishes a framework of reference. The amount and intensity of light entering the eye dictates what neurological information is sent via the optic nerve to the brain for processing and interpretation. Generally, basic vision function is divided into three levels of light intensity; daylight (photopic), twilight (mesopic) and low light, night (scotopic) vision function.
Photopic vision functions during bright light levels. Specific neuroreceptors called cones dominate the eye’s response to bright levels of light. The inner photosensitive part of the eye, called the retina, has approximately 7 million cones. Cones are concentrated in the area of the retina that corresponds to straight ahead vision. This anatomical area of the retina is called the macula, and within the macula is a depression called the fovea consisting almost entirely of cones. Cones convert light energy into neural energy sending information via the optic nerve to the brain. Reflected light from targets stimulates cones to send information to the brain about forms, shapes, textures, colors and high contrast sensitivity detection of various line forms. This information is then combined and analyzed by the brain to form an impression of the target.
From a practical perspective, only in daylight vision can very precise detail and color of a target be seen. Also, precise 3-D depth perception (stereopsis) is only possible during cone-dominated daylight viewing conditions. The highest degree of depth perception occurs when the central, straight ahead fixation point in each eye sends information to the brain in a highly coordinated fashion. During low light conditions, the cones are unable to send precise signals for the brain to process depth.
Daylight vision enables the eyes to maintain the highest degree and control of eye fixation, the ability to maintain steady and accurate eye position upon a stationary target. Also, the ability to follow a moving target (called pursuit eye movements) functions optimally during photopic viewing conditions. A different type of eye movement of looking from one separated target to another target to another target, etc. (called saccadic eye movements) function much better during bright light conditions than during low light conditions. The voluntary act of allowing the extraocular muscles of the eye to position the eye such that images fall on the retina where cone density is highest is an important component of establishing visual attention on targets.
The ability to maintain accurate focus (accommodation) on a target requires sufficient light to activate the eye focusing system. The accommodative response functions most efficiently when the target reflects sufficient light to stimulate accurate eye focus. Cones have the best ability to receive the refracted light that the lens inside the eye alters during the act of focusing clearly on a target. When light diminishes, the cone function is suppressed and the quality of the eye focusing ability declines.
Once bright light declines and darkness emerges, there is a period of light transition (seen during dusk) defined as mesopia. During mesopia there is a shift from cone domination of vision to rod domination of vision. However, during mesopic vision, both rods and cones are partially active. The 120 millions rods are located throughout the entire peripheral retina. The main functions of rods are to send visual information to the brain about movement detection, organizing spatial orientation of where targets may be located in space, and responding to low levels of light that may be present in the environment. During mesopia there is a gradual loss of color perception, gradual loss of discerning target detail, gradual loss of the ability to maintain accurate eye focus upon target, contrast sensitivity losses, and a diminishing ability to maintain accurate three dimensional depth perception. From a practical viewpoint, mesopia is complete when color perception is eliminated, and at this point, the visual system begins to function in scotopia.
When light levels fall into darkness, the human eye functions in a state of scotopia. Rod physiology does not allow for color vision nor the ability to discern detail. It is estimated that the best visual acuity during scotopia is 20/200. When you change from day vision to darkness immediately (e.g. entering a dark room during the day), the dark adaptation of cones is complete in five minutes, while full rod adaptation takes about 30 minutes. However, rods are more sensitive than cones at the seven-minute mark. Complete dark adaptation requires about 30 minutes for the rods to reach their highest level of sensitivity while in darkness.
The ability to maintain accurate eye focus upon a target is greatly reduced during scotopic vision function. Other important visual changes that accompany scotopic vision include increased awareness of peripheral light and movement, increased pupil size resulting in less depth of field, reduction in contrast sensitivity, loss of texture perspective, altered target search strategies and variability of eye focus control increases. It follows that detection of the fine details of an object of attention is greatly reduced. Unless there is added light source directed at a target, the human visual system is unable to judge accurately target characteristics such as size, shape, contour, texture and color.
Above and beyond the basic visual functions that are operational at various lighting conditions, there are specific visual changes that occur when a shooter is threatened by a dangerous situation. The Body Alarm Reaction (BAR) is the body’s response to an unexpected and sudden change in the environment, most commonly initiated during the early stages of a life threatening attack. The BAR is often associated with combat or violent encounters. The most immediate visual change in response to the BAR is that the eye focusing system (accommodation) loses it ability to maintain clear focus on targets at close distances. It is not possible during the first few seconds after entering into the BAR to clearly focus upon the front sights of a gun. A shooter’s visual focusing and attention is drawn to focus toward far distant viewing, toward infinity. This focusing change toward far distant focus is a direct result of the change from parasympathetic nervous system control to sympathetic nervous system control. This shift in the autonomic nervous system balance is responsible for changing how the crystalline lens inside the eye changes it shape and optical power. During the immediate stages of the BAR, the lens becomes less convex in shape and this results in an optical shift of focus resulting in clear focus only while viewing distant targets.
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