Laterality - Laterality and Speech

Laterality and Speech

Cerebral dominance 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 (Annett, 1991). 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 (1979), the exposed cortex was mapped revealing the same cortical sites were activated in 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 hand movements for language communication, it was proposed that skilled hand motions and speech require sequences of action over time. In deaf patients suffering from 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 (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. 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 pseudo-words (Shtyrov, Pihko, and Pulvermuller, 2005). 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 (Imaizumi, Koichi, Kiritani, Hosoi & Tonoike, 1998).

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 (Hull & Vaid, 2006).