Researchers within the field of psychology are interested in various higher order cognitive functions exhibited within human behavior. However, one area that is highly researched is that of language, or the way in which human beings communicate with one another. This communication can take place and be processed and produced either audibly or visually (Gazzaniga, Irvy, & Mangun, 2018). This function within humans is supported via various brain structures making up the perisylvian language network of the left hemisphere. This network includes brain areas such as portions of the posterior superior temporal gyrus (Wernicke’s Area), anterior and lateral temporal cortex, inferior parietal lobe, inferior frontal cortex, and the insular cortex. Additionally, language comprehension is upported via processes in the right hemisphere such as the temporal sulcus and right prefrontal cortex, middle gyrus and the posterior cingulate (Gazzaniga, Irvy, & Mangun, 2018).
Along with physical brain structures, researchers also believe that the function of language comprehension is supported by a type of mental storage system that includes various aspects of language called the mental lexicon. This suspected mental lexicon stores information including word meanings (semantic info), how words can be combined to form sentences (syntactic info) and details of different forms and sound patterns of words (Gazzaniga, Irvy, & Mangun, 2018). With this storage system, the human brain can quickly process word-form activations based off of intended input or output (lexical access), select best matches from the storage device to match input or output (lexical selection), and integrate words into full sentences in order to develop coherent communication in the form of language, or lexical integration (Gazzaniga, Irvy, & Mangun, 2018).
Although researchers can learn much about brain structures involved in language by studying fMRI’s from healthy brains, they often learn invaluable data by studying those with deficits in their language abilities, or aphasia. Aphasia is a term describing the abnormality of language comprehension or production due to lesions in various parts of the language system of the brain (Gazzaniga, Irvy, & Mangun, 2018). Depending on where the lesion is located and which systems they are affecting, researchers have classified aphasia into many different categories. Two forms of aphasia that can be present within an individual are Wernicke’s Aphasia and Conduction Aphasia.
Within Wernicke’s Aphasia, patients often experience deficits in their ability to understand spoken or written language. Although they fail to comprehend incoming language, these patients often do not struggle speaking with normal fluency. In some cases, these individuals may be able to communicate, but they are unaware that what they say does not always make sense to the recipient (Gazzaniga, Irvy, & Mangun, 2018). In addition to nonsensical output, or errors in lexical selection of words, these individuals also exhibit other types of lexical output errors. For example, they may also produce unintended syllables, phrases and words, a condition known as paraphasic errors (Drago & Foster, 2011). Wernicke’s Aphasia is often categorized with lesions within specific areas of the brain. For example, lesions within the superior temporal lobe, auditory association cortex, and the posterior superior temporal gyrus. Additionally, damage to the white matter within the temporal lobe or supramarginal gyrus is noted in some cases that result in Wernicke’s Aphasia (Drago & Foster, 2011).
On the other hand, those who suffer form Conduction Aphasia suffer from a different list of issues. Conduction Aphasia will likely not stop someone from speaking fluently, but, like Wernicke’s Aphasia, they may experience paraphasic errors, or the unintentional use of syllables and extra phonemes within speech production (Drago & Foster, 2011). Additionally, this specific aphasia is associated with repeated phonological attempts to communicate a specific word, poor sentence repetition, and difficulty encoding incoming written or spoken language (lexical access and selection) (Cocks et al., 2011). Those who suffer from this condition may also experience issues comprehending information, assigning labels to specific items, and experience complications with their short-term memory system (Drago & Foster, 2011). Just as was the case in Wernicke’s Aphasia, Conduction Aphasia is associated with damage noted within specific brain areas. For example, those with this condition typically exhibit lesions within the left temporal cortex, left parietal cortex, and the left planum temporal region. Along with lesions, typically these patients show decreased amounts of white matter in the supramarginal gyrus and left parietal cortex (Drago & Foster, 2011).