Laterality
The term laterality refers to the preference most humans show for one side of their body over the other. Examples include left-handedness/right-handedness and left/right-footedness; it may also refer to the primary use of the left or right hemisphere in the brain. It may also apply to animals or plants. The majority of tests have been conducted on humans, specifically to determine the effects on language.
Human
Most humans are right-handed. Many are also right-sided in general. The reasons for this are not fully understood, but it is thought that because the left cerebral hemisphere of the brain controls the right side of the body, the right side is generally stronger; it is suggested that the left cerebral hemisphere is dominant over the right in most humans because in 90–92% of all humans, the left hemisphere is the language hemisphere.Human cultures are predominantly right-handed, and so the right-sided trend may be socially as well as biologically enforced. This is quite apparent from a quick survey of languages. The English word left comes from the Anglo-Saxon word lyft, which means 'weak' or 'useless'. Similarly, the French word for left, gauche, is also used to mean 'awkward' or 'tactless', and sinistra, the Latin word from which the English word sinister was derived, means 'left'. Similarly, in many cultures the word for right also means 'correct'. The English word right comes from the Anglo-Saxon word riht, which also means 'straight' or 'correct'.
This linguistic and social bias is not restricted to European cultures: for example, Chinese characters are designed for right-handers to write, and no significant left-handed culture has ever been found in the world.
When a person is forced to use the hand opposite of the hand that they would naturally use, this is known as forced laterality, or more specifically forced dextrality. A study done by the Department of Neurology at Keele University, North Staffordshire Royal Infirmary suggests that forced dextrality may be part of the reason that the percentage of left-handed people decreases with the higher age groups, both because the effects of pressures toward right-handedness are cumulative over time and because the prevalence of such pressure is decreasing, such that fewer members of younger generations face any such pressure to begin with.
Ambidexterity is when a person has approximately equal skill with both hands and/or both sides of the body. True ambidexterity is very rare. Although a small number of people can write competently with both hands and use both sides of their body well, even these people usually show preference for one side of their body over the other. However, this preference is not necessarily consistent for all activities. Some people may, for instance, use their right hand for writing, and their left hand for playing racket sports and eating.
Also, it is not uncommon that people preferring to use the right hand prefer to use the left leg, e.g. when using a shovel, kicking a ball, or operating control pedals. In many cases, this may be because they are disposed for left-handedness but have been trained for right-handedness, which is usually attached to learning and behavioural disorders. In the sport of cricket, some players may find that they are more comfortable bowling with their left or right hand, but batting with the other hand.
Approximate statistics, complied in 1981, are given below:
- Favoring right hand: 88.2%
- Favoring right foot: 81.0%
- Favoring right eye: 71.1%
- Favoring right ear: 59.1%
- Same hand and foot: 84%
- Same ear and eye: 61.8%
Different expressions
; Board footedness: The stance in a boardsport is not necessarily the same as the normal-footedness of the person. In skateboarding and other board sports, a "goofy footed" stance is one with the right foot leading. A stance with the left foot forward is called "regular" or "normal" stance.; Jump and spin: Direction of rotation in figure skating jumps and spins is not necessarily the same as the footedness or the handedness of each person. A skater can jump and spin counter-clockwise, yet be left-footed and left-handed.
; Ocular dominance: The eye preferred when binocular vision is not possible, as through a keyhole or monocular microscope.
Speech
or specialization has been studied in relation to a variety of human functions. With speech in particular, many studies have been used as evidence that it is generally localized in the left hemisphere. Research comparing the effects of lesions in the two hemispheres, split-brain patients, and perceptual asymmetries have aided in the knowledge of speech lateralization. In one particular study, the left hemisphere's sensitivity to differences in rapidly changing sound cues was noted. This has real world implication, since very fine acoustic discriminations are needed to comprehend and produce speech signals. In an electrical stimulation demonstration performed by Ojemann and Mateer, the exposed cortex was mapped revealing the same cortical sites were activated in phoneme discrimination and mouth movement sequences.As suggested by Kimura, left hemisphere speech lateralization might be based upon a preference for movement sequences as demonstrated by American Sign Language studies. Since ASL requires intricate hand movements for language communication, it was proposed that skilled hand motions and speech require sequences of action over time. In deaf patients with a left hemispheric stroke and damage, noticeable losses in their abilities to sign were noted. These cases were compared to studies of normal speakers with dysphasias located at lesioned areas similar to the deaf patients. In the same study, deaf patients with right hemispheric lesions did not display any significant loss of signing nor any decreased capacity for motor sequencing.
One theory, known as the acoustic laterality theory, the physical properties of certain speech sounds are what determine laterality to the left hemisphere. Stop consonants, for example t, p, or k, leave a defined silent period at the end of words that can easily be distinguished. This theory postulates that changing sounds such as these are preferentially processed by the left hemisphere. As a result of the right ear being responsible for transmission to sounds to the left hemisphere, it is capable of perceiving these sounds with rapid changes. This right ear advantage in hearing and speech laterality was evidenced in dichotic listening studies. Magnetic imaging results from this study showed greater left hemisphere activation when actual words were presented as opposed to pseudowords. Two important aspects of speech recognition are phonetic cues, such as format patterning, and prosody cues, such as intonation, accent, and emotional state of the speaker.
In a study done with both monolinguals and bilinguals, which took into account language experience, second language proficiency, and onset of bilingualism among other variables, researchers were able to demonstrate left hemispheric dominance. In addition, bilinguals that began speaking a second language early in life demonstrated bilateral hemispheric involvement. The findings of this study were able to predict differing patterns of cerebral language lateralization in adulthood.
In other animals
It has been shown that cerebral lateralization is a widespread phenomenon in the animal kingdom. Functional and structural differences between left and right brain hemispheres can be found in many other vertebrates and also in invertebrates.It has been proposed that negative, withdrawal-associated emotions are processed predominantly by the right hemisphere, whereas the left hemisphere is largely responsible for processing positive, approach-related emotions. This has been called the "laterality-valence hypothesis".
One sub-set of laterality in animals is limb dominance. Preferential limb use for specific tasks has been shown in species including chimpanzees, mice, bats, wallabies, parrots, chickens and toads.
Another form of laterality is hemispheric dominance for processing conspecific vocalizations, reported for chimpanzees, sea lions, dogs, zebra finches and Bengalese finches.