Language in the left hemisphere

Here is the posting mentioned in the last post. A recent paper (Harvey M. Sussman; Why the Left Hemisphere Is Dominant for Speech Production: Connecting the Dots; Biolinguistics Vol 9 Dec 2015), deals with the nature of language processing in the left hemisphere and why it is that in right-handed people with split brains only the left cortex can talk although both sides can listen. There is a lot of interesting information in this paper (especially for someone like me who is left-handed and dyslexic). He has a number of ‘dots’ and he connects them.

Dot 1 is infant babbling. The first language-like sounds babies make are coos and these have a very vowel-like quality. Soon they babble consonant-vowel combinations in repetitions. By noting the asymmetry of the mouth it can be shown that babbling comes from the left hemisphere, non-babbling noises from both, and smiles from the right hemisphere. A speech sound map is being created by the baby and it is formed at the dorsal pathway’s projection in the frontal left articulatory network.

Dot 2 is the primacy of the syllable. Syllables are the unit of prosodic events. A person’s native language syllable constraints are the origin of the types of errors that happen in second language pronunciation. Also syllables are the units of transfer in language play. Early speech sound networks are organized in syllable units (vowel and associated consonants) in the left hemisphere of right-handers.

Dot 3 is the inability for the right hemisphere to talk in split brain people. When language tasks are directed at the right hemisphere the stimulus exposure must be longer (greater than 150 msec) than when directed to the left. The right hemisphere can comprehend language but does not evoke a sound image from seen objects and words although the meaning of the objects and words is understood by that hemisphere. The right hemisphere cannot recognize if two words rhyme from seeing illustations of the words. So the left hemisphere (in right-handers) has the only language neural network with sound images. This network serves as the neural source for generating speech, therefore in a split brain only the left side can speak.

Dot 4 deals with the problems of DAS, Development Apraxia of Speech. I am going to skip this.

Dot 5 is the understanding of speech errors. The ‘slot-segment’ hypothesis is based on analysis of speech errors. Two thirds of errors are the type where phonemes are substituted, omitted, transposed or added. The picture is of a two-tiered neural ‘map’ with syllable slots serially ordered as one tier, and an independent network of consonant sounds in the other tier. The tiers are connected together. The vowel is the heart of the syllable in the nucleus slot. Forms are built around it with consonants (CV, CVC, CCV etc.). Spoonerisms are restricted to consonants exchanging with consonants and vowels exchanging with vowels; and, exchanges occurring between the same syllable positions – first with first, last with last etc.

Dot 6 is Hawkin’s model, “the neo-cortex uses stored memories to produce behaviors.” Motor memories are used sequentially and operate in an auto-associative way. Each memory elicits the next in order (think how hard it is to do things backwards). Motor commands would be produced in a serial order, based on syllables – learned articulatory behaviors linked to sound equivalents.

Dot 7 is experiments that showed representations of sounds in human language at the neural level. For example there is a representation of a generic ‘b’ sound, as well as representations of various actual ‘b’s that differ from one another. This is why we can clearly hear a ‘b’ but have difficulty identifying a ‘b’ when the sound pattern is graphed.

Here is the abstract:

Evidence from seemingly disparate areas of speech/language research is reviewed to form a unified theoretical account for why the left hemisphere is specialized for speech production. Research findings from studies investigating hemispheric lateralization of infant babbling, the primacy of the syllable in phonological structure, rhyming performance in split-brain patients, rhyming ability and phonetic categorization in children diagnosed with developmental apraxia of speech, rules governing exchange errors in spoonerisms, organizational principles of neocortical control of learned motor behaviors, and multi-electrode recordings of human neuronal responses to speech sounds are described and common threads highlighted. It is suggested that the emergence, in developmental neurogenesis, of a hard-wired, syllabically-organized, neural substrate representing the phonemic sound elements of one’s language, particularly the vocalic nucleus, is the crucial factor underlying the left hemisphere’s dominance for speech production.

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