SLP528 Neurology
Guide to neurology for clinicians and students in speech-language pathology
The Brain and Language
The brain is made up of 60 BILLION synapses! Think of a synapse as the place where information passes from one neuron (brain cell) to another. Synapses can be thought about as microcircuits. Microcircuits make up local microcircuits. When local microcircuits communicate with each other in the same lobe, this is a local circuit. Interregional circuits are when there is communication between two different lobes. Let’s look at the following example:
The supramarginal gyrus is an area of the brain in the parietal lobe important for writing. The angular gyrus is an area of the brain in the parietal lobe important for reading. When these two areas communicate, this is a local circuit. Broca’s area is an area in the frontal lobe important for speech production. The angular gyrus may need to communicate with Broca’s area in order to read something aloud. This is an interregional circuit. The behavioral system is the goal of that area of the brain (e.g. writing or reading in this example).
Neural connectivity
How is the brain encoding or learning a word?
**Neurons that fire together wire together!**
There are several connectivity patterns of neuron communication:
Synaptic divergence: one neuron excites several neurons
Synaptic convergence: many neurons excite one neuron
Presynaptic inhibition: when a couple of neurons result in inhibiting an activity
Barlow (1972) came up with the idea that local circuits code percepts (things you perceive) by storing them in the connections of neurons.
Parallel Distributive Processing
In the above picture, the circles represent neurons and the lines represent the connections between neurons. The thicker the line = the stronger the connection. Our knowledge and memory of words are stored in neuronal CONNECTIONS, and NOT the neurons themselves!
When we read or hear a familiar word, this utilizes an autoassociator network. These are the connections that are already formed in our brain, so they help us understand what we are reading or hearing. When we need to learn a new word, we must activate a pattern associator network. This is when we need to create a new pattern in our brains by studying and memorizing that new word!
LANGUAGE THEORIES
The semantic network theory is a theory on word processing that attempts to explain how our brains activate words. When we read or hear a word, our brains activate semantic and phonological neighbors. A semantic word is a word that is contextually related; a phonological neighbor is a word that is phonologically related. For example: if the target word is “run” semantically related words may include: walk, fast, and sneakers. Phonologically related words may include: fun, sun, and bun.
Wernicke-Lichtheim (1885)’s House Model
This old House Model from Wernicke-Lichtheim was used to describe what type of aphasia (language disorder) would occur in a patient. The numbered lines represent points of possible damage in the brain.
In this model of language, Wernicke and Geschwind attempt to explain the areas involved in comprehending, speaking, and reading. This model is also now outdated as speech and language are very complex processes!
Levelt’s WEAVER++ Model (1999)
Levelt (1999) created a model to describe the mental representations people activate before speaking. These steps include:
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Conceptualization: choosing the idea you want to express
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Lexical selection: choosing the word(s) to express that idea
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Morphological encoding: choosing the correct form of the word
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Phonological encoding: how will you say the word?
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Phonemic encoding: your plan of articulators
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Articulation: using your vocal tract to produce speech!
While all models may not necessarily be correct (for example—a competing theory after WEAVER++ suggested that these steps occurred simultaneously, not discretely), they help clinicians think about which area a client may have a particular deficit in.