Neuroanatomy of Language PDF

The Neuroanatomy of Language Dr Adhil Bhagwandin HUB1014S  Brief summary Language is the mental faculty that we use to communicate. It associates sounds and symbols with meaningful concepts and enables us to describe our external environment and abstract t...

The Neuroanatomy of Language Dr Adhil Bhagwandin HUB1014S  Brief summary Language is the mental faculty that we use to communicate. It associates sounds and symbols with meaningful concepts and enables us to describe our external environment and abstract thoughts. Requires the interaction of memory with sensory input and motor output systems.  Types of memory required for language 1. Phonological - the sounds of words 2. Orthographic - the spellings of words 3. Semantic - our knowledge of the world Sensory inputs to these memories 1. Auditory - for spoken words, environmental sounds and music 2. Visual - for written words, objects, faces and signs 3. Tactile - for braille reading  Motor Output for Language Enables the expression of concepts via articulation, writing, signing or drawing. Can be self-generated - in response to internally generated thought Or can be stimulus driven - e.g. in response to written or heard words. The 19th Century Neurological Model of Language for the spoken word: In 1861, Paul Broca reported a post-mortem study of a patient that had been impaired at articulating language. This patient had damage that encompassed the left inferior frontal gyrus. Thus, this region, known as Broca’s area, was associated with motor images of speech.  Continued… In 1874, Wernicke reported a post-mortem study of a patient who had impaired speech comprehension. There was damage to the left posterior superior temporal gyrus. This region, known as Wernicke’s area, was associated with the auditory images of speech.  Continued… Wernicke developed this further to state that if damage occurred to the white matter connecting Broca’s and Wernicke’s areas occurred (this white matter being the arcuate fasciculus), patients would have intact speech comprehension and production, but could not repeat what was heard. This type of “disconnection” syndrome, called “conduction aphasia” was first described by Lichtheim in 1885. Commonly known as BWL model Thus, the neurological model is largely based on observations from patients with lesions in specific regions. So, to repeat speech that was heard, the pictured pathway was developed: From primary auditory cortex, the neural information was passed to Wernicke’s area, then to Broca’s area via the arcuate fasciculus, and onto motor cortex to generate speech. The 19th Century Neurological Model of Language for the written word: Dejerine, in 1891 and 1892, distinguished 2 main alexic syndromes: 1. alexia with agraphia - patients with a deficit in reading (alexia) and writing (agraphia), associated with damage to the left angular gyrus. Thus, left angular gyrus associated to memories of visual word forms. 2. alexia without agraphia - reading deficit, but no writing deficit. Associated with lesions of the left occipital lobe and the splenium of the corpus callosum. Is thought to arise from a disconnection of the left angular gyrus from the visual cortex. So, for the written word that was read, the pictured pathway was developed: From visual cortex, the neural information was passed to the Angular gyrus, then to Wernicke’s area, then to Broca’s area via the arcuate fasciculus, and onto motor cortex to generate speech.  Problem with these models: where is the “concept” centre? There is no description of a concept centre where the meaning of words are held. Visual and Auditory comprehension deficits are typically associated with large lesions of Wernicke’s area, the left middle and inferior temporal gyri and the angular and supramarginal gyri of the left parietal lobe. Thus, language comprehension clearly involves regions outside these “classic” models. 20th Century Cognitive Models of Language Due to difficulties with the lesion-based models of language, in the past century there has been an emphasis on the complexity of linguistic functions rather than the neurological underpinnings. These are based on behavioural data alone, and some very sophisticated models have been developed.  One example of this: Aoccdrnig to a rscheeahcr at an Elingsh uinervtisy, it deosn't mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer is at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae we do not raed ervey lteter by it slef but the wrod as a wlohe.  An Example of a Cognitive Model of Word Processing Lexicon: 1. The vocabulary of a person or language; 2. The complete set of meaningful in a language NB: there are 4 lexicons An Example of a Connectionist Model of Word Processing: Neuroimaging and the Neural Basis of Language: the last 10 years. In the last 10 years non-invasive neuroimaging as revolutionised the understanding of the neural basis of language. Can examine the entire brain for language related areas in the normal person. 2 sources: positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These techniques measure neural activity by local specific changes in blood flow. Activation of the brain when repeating a heard word: Repeat aloud > rest Activation of brain when reading aloud: L R Read aloud > rest Comparison to 19th Century Model In some cases very similar - e.g. activation of auditory cortex, Wernicke’s area, Broca’s area, and motor cortex when repeating a heard word. However, reading aloud doesn’t activate the angular gyrus, rather, the left posterior inferior temporal gyrus. Also, most activations are bi-lateral compared with the left-lateralised language system of the 19th century model.  Hearing Words: Activation: 1. Ant. and post. superior temporal gyrus Basically, primary auditory cortex and 2. Dorsal surface of the superior temporal gyrus auditory association cortex Reading Silently: Activation: 1. Left post. inferior temporal gyrus 2. Left post. superior temporal gyrus 3. Cerebellum  Phonological Retrieval: Naming Wernicke’s area consistently activated by word repetition and not consistently activated by reading. Left post. inferior temporal gyrus consistently activated by reading and not consistently activated by heard word repetition. Regions Common to All Naming (or Phonological Retrieval):  The Meaning of Words – (1) the meaning of heard words: (2) The meaning of read words: (3) The meaning of read words that are objects or famous people: (4) A semantic decision about a read word: e.g. is it a living item? Functional Semantics: reading the names of objects Biographical Semantics: reading the names of famous people Reading a Sentence: Linking semantics: associating a target word with 1 of 2 choices on the basis of meaning relative to physical size. Putting all this data together: 1st step - hear or see words - involves the superior temporal gyrus (heard words) and the posterior fusiform and lingual gyri (written words). Step 1: Acquiring the words 2nd step - Semantics - the meanings of words: (a) phonological processing of speech (posterior superior temporal sulcus) (b) semantic decisions (extrasylvian temporo-parietal) (c) retrieving the name (posterior inferior temporal/mid fusiform gyri). Step 2: Understanding the words  3rd step - articulatory planning, preparing the brain to activate the regions of the brain that activate phonological output (frontal operculum and the anterior insula cortex). Step 3: Articulatory Planning  4th step - Motor output and hearing your spoken word (sensori-motor cortex and the superior temporal gyrus).  Language processing is a “distributed” system, with its different parts found in adjacent regions of the brain - several different cortical areas. The language system is far more complex than what is given in most textbooks.