Body relative direction

A right-handed Cartesian coordinate system, illustrating the x (right-left), y (forward-backward) and z (up-down) axes relative to a human being.

The most common relative directions are left, right, forward(s), backward(s), up, and down. No absolute direction corresponds to any of the relative directions. This is a consequence of the translational invariance of the laws of physics: nature, loosely speaking, behaves the same no matter what direction one moves. As demonstrated by the Michelson-Morley null result, there is no absolute inertial frame of reference. There are definite relationships between the relative directions, however. Left and right, forward and backward, and up and down are three pairs of complementary directions, each pair orthogonal to both of the others. Relative directions are also known as egocentric coordinates.

Cultures not using relative directions

Use of relative directions for reference is almost universal among human cultures. However, exceptions do exist. The Australian Aboriginal people the Guugu Yimithirr for instance, in their language have no words denoting the egocentric directions, but instead exclusively refer to cardinal directions, even when describing small-scale spaces. For instance, if they want someone to move over on the car seat to make room, they would say "move a bit to the east." To tell someone where exactly they left something in their house, they would say, "I left it on the southern edge of the western table." Or they would warn a person to "look out for that big ant just north of your foot." Other peoples from Polynesia to Mexico and from Namibia to Bali similarly have predominantly "geographic languages".

Nautical terminology

Bow, aft, port, and starboard are nautical terms that convey an impersonal relative direction in the context of the moving frame of persons aboard a ship.

Geometry of natural environment

If someone climbs a rope one meter up, they will have moved negative one meter down. Furthermore, they will not have moved left or right at all, nor forward or backward. One must mind the geometry of the environment when using relative direction to express motion, however. For example, if someone continues walking forward until they have almost circumnavigated the Earth, they will expend much effort only to move backward slightly. This is because Earth is (near-)spherical.

The right-hand rule is one common way to relate the three principal directions. For many years a fundamental question in physics was whether a left-hand rule would be equivalent. Many natural structures, including human bodies, follow a certain handedness, but it was widely assumed that nature did not distinguish the two possibilities. This changed with the discovery of parity violations in particle physics. If a sample of cobalt-60 atoms is magnetized so that they spin counterclockwise around some axis, the beta radiation resulting from their nuclear decay will be preferentially directed opposite that axis. Since counterclockwise may be defined in terms of up, forward, and right, this experiment unambiguously differentiates left from right using only natural elements: If they were reversed, or the atoms spun clockwise, the radiation would follow the spin axis instead of being opposite to it.

Tradition and acculturation

Since definitions of left and right based on the geometry of the natural environment are unwieldy, in practice, the meaning of relative direction words is conveyed through tradition, acculturation, education, and direct reference. One common definition of up and down uses gravity and the planet Earth as a frame of reference. Since there is a very noticeable force of gravity acting between the Earth and any other nearby object, down is defined as that direction which an object moves in reference to the Earth when the object is allowed to fall freely. Up is then defined as the opposite direction of down. Another common definition uses a human body, standing upright, as a frame of reference. In that case, up is defined as the direction from feet to head, perpendicular to the surface of the Earth. In most cases, up is a directionally oriented position generally opposite to that of the pull of gravity.

Forward and backward may be defined by referring to an object or person's motion. Forward is defined as the direction in which the object is moving. Backward is then defined as the opposite direction to forward. Alternately, 'forward' may be the direction pointed by the observer's nose, defining 'backward' as the direction from the nose to the sagittal border in the observer's skull. With respect to a ship 'forward' would indicate the relative position of any object lying in the direction the ship is pointing. Objects on ships can be named from their location, for example 'Forward Fire Pump' or 'Aft Fire Pump'.

The terms left and right are derived from the fact that some 90% of the population use the hand of the same side of their body for writing and other manual tasks. Through cognate of the word right in the meaning correct or good, the side with the hand most used was defined as the right side. The word "left" comes from the Old English lyft, meaning weak. This dichotomy can also be seen in the words "dextrous" and "sinister," from the Latin words for right and left. Given significant distance from the equator, one can figure which hand is which using a clock, a compass, and the sun. Facing the sun, in the northern hemisphere, before noon, the compass points to the left hand. After noon, it points to the right. The opposite is true of the southern hemisphere. In most people, the heart is to the left of the body's center line and the liver is to the right.

Left-right confusion

Left-right confusion is the difficulty some people have in distinguishing the difference between the directions left and right. According to research by John R. Clarke (Drexel University) it afflicts about 15% of the population. These people can usually normally perform daily activities such as driving according to signs and navigating according to a map, but will often take a wrong turn when told to turn left or right and may have difficulties performing actions that require precise understanding of directional commands, such as ballroom dancing.

Most professionals accept the definition of Dyslexia as given by the International Dyslexia Association, which the organization changes as information develops, and so the site must be accessed for the most recent definition. Dyslexia is a major cause of right/left confusion, but only in a minor subset of cases. Indivduals suffering from the problem often do not know that they have it, as it appears normal to them. The defect may appear when faced with learning to drive, in geometry when being told to move a figure in space to the right or left, trying to tell time on a clock face, or working one on one with a tutor. Certain types of mathematical probelms demonstrate right/left confusion, such as setting up proportions, as the student does not understand what side of an equation figures should appear. As per major Neurology Texts used by physicans, the sections on Dyslexia available for download at a price on the internet, and edited by the heads of Ivy league medical schools, describe left/right confusion in the presence of a minority of Dyslexics. These individuals have the most severe form of the disorder. They have the typical difficulty with reading, now universaly accepted as the inability to correlate graphemes (or strings of letter representations) with phenomes (sounds). This is a more severe problem in English than almost any other language, as it has the largest number of graphemes (over 1000) to represent approximately 40 phenomes or sounds. The now established knowlege that Dyslexics have their language processing centers in the non-dominent hemisphere has been established by the NIMH with modern imaging techniqes, and is the underlying deficit in Dyslexia.

Many dyslexics also have their writing ability impaired due to failure of rapid automatic writing, which is an ability that is usually accomplished in early years in school. The International Dyslexia Association openly encourages the reproduction of documentation of how to test for typical dyslexia, available over the internet, and includes handwriting tests. Most current Achievement tests include tests for rapid automatic writing, even before this was known to be a predictor of written expression disabilities. The inability to write normally and without effort, often accompanied by letter reversals, is called dysgraphia. There is arguement in professional circles as to whether this is a form of right/left confusion as well, or simply delayed writing development.

When basic math calculations such as simple addition, multiplication, and subtraction and left right confusion are added, the syndrome cannot be distingushed from a brain tumor, or in rare cases, a specific stroke, or other neurologic event that interferes with communication between the dominent and non-dominent hemispheres. In the case where a person of any age, has the features of dyslexia, and has been observed to have right/left confusion, they require a brain scan to rule out a medical cause. This is called Gerstmann Syndrome, when there is a medical cause or Developmental Gerstmann Syndrome when it is the Dyslexic cause, when described in Neurological and Neurosurgical Communities. Without the aspect of right/left confusion, brain scans would not be ordered.

Right/Left confusion is a very important sign of Dyslexia, and should not overly concern parents of youngsters. But it should be documented, and then the child should be seen by a pediatric neurologist, as educational testing does not address this finding. In adults where it is a new development, it is a medical emergency. In addition, since most Dyslexics are affected in only reading and writing, when a child has difficulty in reading, writing and math it may be incorrectly assumed that the child has poor abilities in general. Such children are often quite gifted in non-verbal abilities, and should have non-verbal IQ tests. Large numbers of Dyslexics have ADHD as well, and are inaccurately assumed to have all their problems attributed to this disorder. Thorough evaluation by a specialist is often required, as early intervention can do much to correct thier difficulties. Finally, it is important to note that the symptoms of Dyslexia can vary from day to day, and under stress. It is one of the most common learning disabilities, and yet most underdiagnosed. Excellent information is available online from the International Dyslexia Association and Council on Exception Children. == See also ==

• Anatomical terms of location
• Cardinal direction
• List of international common standards
• Port and starboard
• Rotation
• Cerebral hemisphere
• Windward and leeward