Editing Laterality (section)

=== Speech ===
[[Lateralization of brain function|Cerebral dominance]] or [[Functional specialization (brain)|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 [[Cerebral hemisphere|left hemisphere]]. Research comparing the effects of [[Brain abscess|lesions]] in the two hemispheres, [[split-brain]] patients, and [[Dichotic listening|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 localization|sound cues]] was noted (Annett, 1991). This has real world implication, since very fine [[Speech perception|acoustic discriminations]] are needed to comprehend and produce [[Speech processing|speech signals]]. In an [[Electrical brain stimulation|electrical stimulation]] demonstration performed by Ojemann and Mateer (1979), the exposed [[Cerebral cortex|cortex]] was [[Brain mapping|mapped]] revealing the same cortical sites were activated in [[Phonemic contrast|phoneme discrimination]] and mouth movement sequences (Annett, 1991).

As suggested by Kimura (1975, 1982), left hemisphere speech lateralization might be based upon a preference for movement sequences as demonstrated by [[American Sign Language]] (ASL) studies. Since ASL requires intricate [[Sign language|hand movements for language communication]], it was proposed that skilled hand motions and speech require sequences of action over time. In [[Deafness|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 [[Aphasia|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 (Annett, 1991).

One theory, known as the acoustic laterality theory, the physical properties of certain speech sounds are what determine laterality to the left hemisphere. [[Plosive|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 test|dichotic listening studies]]. [[Magnetic resonance imaging|Magnetic imaging]] results from this study showed greater left hemisphere activation when actual words were presented as opposed to [[pseudoword]]s.<ref>{{cite journal | vauthors = Shtyrov Y, Pihko E, Pulvermüller F | title = Determinants of dominance: is language laterality explained by physical or linguistic features of speech? | journal = NeuroImage | volume = 27 | issue = 1 | pages = 37–47| date = 2005 | pmid = 16023039  | doi = 10.1016/j.neuroimage.2005.02.003 }}</ref> Two important aspects of [[speech recognition]] are [[Phonetics|phonetic cues]], such as format patterning, and [[Prosody (linguistics)|prosody]] cues, such as [[Intonation (linguistics)|intonation]], [[Accent (sociolinguistics)|accent]], and emotional state of the speaker (Imaizumi, Koichi, Kiritani, Hosoi & Tonoike, 1998).

In a study done with both [[Monolingualism|monolinguals]] and [[Multilingualism|bilinguals]], which took into account language experience, [[Second-language acquisition|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 (Hull & Vaid, 2006).