8-1 neuropsych
Module Overview17.html
Language and Communication
Language Development and Impairment
Language encompasses all forms of human communication, including the following:
- Expressing and understanding the spoken word
- Reading and writing
- Processing of nonverbal communications, such as prosody or body movements
Language is symbolic in that words, phrases, and gestures represent ideas and things. Many other animals, such as whales, bees, and chimpanzees, use various forms of communication. However, humans have developed the most complex form of language in the animal kingdom.
A key concept with respect to the neuroanatomy of language is lateralization. The brain is separated into two hemispheres. The corpus callosum is a collection of axons (white matter) that connects the left and right hemispheres. It allows for interhemispheric communication within the brain. The dominant hemisphere is identified as most important for language (or language dominant). About 70% of the population is left-hemisphere dominant, 10% right-hemisphere dominant, and 20% bilateral (Morgan & Ricker, 2008). However, the language-specific areas on the dominant side are mirrored on the nondominant side by nonverbal functions. For example, in a left-dominant person, Broca’s area in the left frontal lobe is responsible for speech production. The same area on the right frontal lobe is responsible for the tone and prosody of speech.
Language development begins early. Infants can discriminate between speech sound and babble by about four months (Meyers, 2010). Children begin to speak their first words at around age one. Language develops very rapidly after two years. Noam Chomsky theorized that all humans have an innate language acquisition device that naturally allows the learning of language. Language develops naturally and without formal schooling. Thus, it is a primary biological mechanism. In contrast, reading and writing are secondary abilities that must be taught more formally.
A key concept in understanding language development is a critical period (or sensitive period) when language development is most prominent. Children will have great difficulty learning any language fully after that period passes (about age seven in humans). One proposed mechanism to explain the critical period is that humans are born with and develop an overabundance of synaptic connections. The excess connections begin to be severed (synaptic pruning) early in infancy. This severing increases the efficiency of neural communication. For example, think about how cutting off half the computer connections on an internet service can increase bandwidth. Thus, when a child does not use those connections, the neural substrate for a specific ability is lost as those synaptic connections are severed, while used connections are reinforced. Also, developmental language disabilities are seen throughout childhood; these are distinct from acquired disabilities that reflect a loss of previous function due to neurological insult.
Key speech-related brain areas are Broca’s area (responsible for speech production) and Wernicke’s area (responsible for speech comprehension). The circuit connecting the two areas subcortically is known as the arcuate fasciculus. Acquired damage (such as from a stroke) to any of these areas can cause aphasia (“without speech”). Aphasias are classified by a hierarchy of three abilities: fluency, comprehension, and repetition (Blumenfeld, 2010). Broca’s aphasia describes intact comprehension but impaired speech production. In contrast, Wernicke’s aphasia reflects intact fluency but impaired comprehension. The deficit is called global aphasia when fluency, comprehension, and repetition are all impaired. Transcortical and conduction aphasias result from subcortical damage as opposed to damage of the cortex seen in Broca’s and Wernicke’s aphasias. Acquired reading and writing disorders are specific types of aphasias, known as alexia and agraphia, acquired conditions whose developmental versions are dyslexia and dysgraphia.
Language is an important ability that is related to different development phases. In fact, language can be considered a key foundational cognitive ability. When language impairment is present, assessment of other cognitive domains cannot reliably be undertaken. For example, a person may perform in the severely impaired range on a memory test due to an inability to comprehend the instructions. Thus, it is not reflective of frank memory problems. Language is tied to developing higher cognitive functions, such as memory and reasoning.
References
Blumenfeld, H. (2010). Neuroanatomy through clinical cases (2nd ed.). Sinauer Associates.
Morgan, J. E., & Ricker, J. H. (2008). Textbook of clinical neuropsychology. Taylor & Francis Group.
Myers, D. G. (2010). Psychology (9th ed.). Worth Publishers.