Brain and Mind - Chapter 3 Notes
Language Sensory Pathways
Two main input pathways for language: audition and vision. Touch is an input modality for Braille readers.
Visual pathways are involved in reading and sign language perception.
Auditory pathways begin in the inner ear with the cochlea, which is sensitive to air pressure variations.
The cochlea acts as a frequency analyzer, decoding different sound frequencies.
The result is sent to the primary auditory cortex in the superior temporal gyrus.
Auditory pathways pass through the medial geniculate nucleus of the thalamus.
They also connect to the inferior colliculi, mainly for posture regulation via information from the inner ear's labyrinth.
After the primary auditory cortex, sound information is further analyzed in the auditory association cortex (the remainder of the superior temporal gyrus).
Phonological processing (analysis of language-specific sounds) takes place here.
Information diffuses toward surrounding brain regions involved in retrieving word meaning (semantic memory).
Language Production: Neuropsychology
Paul Broca (1860s) described a relationship between the inferior prefrontal cortex and language production based on patient Leborgne.
Leborgne (nicknamed “Tan”) could understand speech but only produce the syllable “tan”.
Broca found a lesion in the inferior prefrontal cortex and called it the “center for the motor images of words”. Damage impairs word production.
Aphasia: speech disorders caused by brain damage, not due to basic (peripheral) sensory or motor dysfunction.
Aphasia refers to the inability to produce or comprehend speech fluently, not caused by primary motor or sensory deficits (e.g., paralysis of the mouth).
Broca’s aphasia: slow, laborious, non-fluent speech, often lacking grammatical words (function words).
Broca’s aphasics produce groups of meaningful words (content words) without clear syntax (agrammatism).
They often struggle to find the word they want to say (anomia).
Broca’s aphasia involves comprehension deficits; patients struggle to keep heard words in the correct sequence in their mind.
Broca’s aphasia is not a pure deficit but a syndrome with various sub-deficits.
Broca’s aphasia typically involves lesions in the inferior prefrontal cortex, near the lateral fissure, and in the depth of the brain under this region (white matter, basal ganglia, especially the head of the caudate nucleus).
Example of Psychophysiological Approach: Neuroimaging
Brain imaging studies investigate the activation of “Broca’s Area” (inferior prefrontal cortex) in word production and comprehension.
Broca's area is activated in many language tasks and non-language tasks.
Language-related tasks that activate Broca’s area include:
Phonological awareness tasks (detecting sounds of language within words).
Word and letter repetition.
Word generation and stem completion tasks (saying as many words as possible starting with 3 particular letters).
Grammatical tasks (gender judgment).
Syntactic tasks (judging whether a sentence is structurally acceptable).
Semantic tasks (categorization of words based on meaning).
Broca’s area has been implicated in:
Short-term memory for language.
Linking different types of linguistic information (e.g., phonology ↔ articulation).
Resolving conflict between different speech sounds or meanings.
Language Comprehension: Neuropsychology
Wernicke discovered the “center for the auditory images of words”.
Wernicke's Discovery was based on patients with impaired spoken language comprehension but fluent language production.
Comprehension deficits are associated with lesions in the posterior part of the superior temporal cortex (near the lateral fissure, historically called “Wernicke’s area”).
Productions from Wernicke’s aphasics are totally fluent.
Their speech “sounds” grammatical (structured in normal sentences with adequate function words).
However, some content words (meaningful words) are deformed (paraphasia), sometimes making them impossible to interpret.
Wernicke’s aphasics are often unaware of their deficit.
Wernicke’s aphasia is considered primarily a comprehension deficit, although production is abnormal in terms of meaningfulness.
Example of Psychophysiological Approach: ERPs
The N400 is a negative Event-Related Potential (ERP) component peaking 400ms after Stimulus Onset Time (SOT).
The N400 can be seen when participants undergoing EEG recording process the meaning of words.
The more a word is unexpected in a sentence, the larger the N400 amplitude.
Example: The N400 elicited by “tree” at the end of “I was late, I nearly missed the…” is greater than that elicited by “train”.
Although the N400 is not specific to language (it is observed for pictures and sounds), it is considered a good indicator of the search for meaning in semantic memory.
In Wernicke’s aphasics, the N400 component is often tremendously reduced or abolished.
Repetition: Conduction Aphasia
Neuropsychological studies show that some patients can repeat words without understanding them (transcortical sensory aphasia).
Information can be transferred directly from Wernicke’s to Broca’s area without necessarily “making sense” to the speaker.
This may relate to a direct connection between Wernicke’s area and Broca’s area, known as the arcuate fasciculus (a bundle of axons in the inferior parietal cortex).
Patients with normal language comprehension and fluent production (Wernicke’s and Broca’s areas functioning normally) but a selective lesion of the arcuate fasciculus should be impaired for repetition. Such patients have been observed.
Although they can repeat words without apparent problem (access to word meaning), they cannot repeat non-words (made-up words without meaning).
This syndrome is called conduction aphasia.
Deep Dysphasia
A rare syndrome where people have good understanding of spoken language and fluent production.
However, when required to write words under dictation or repeat words, they often produce semantic paraphasias (repeating or writing a closely related word).
Example: “shark” becomes “dolphin”, “tiger” becomes “lion”, “commander” becomes “captain”.
These patients are thought to be conduction aphasics with a deficit in short-term memory.
They hear the word and retrieve its meaning immediately, but when repeating or writing, they retrieve what was stored in short-term memory and sometimes fail.
All that remains is the meaning of the word they heard.
Therefore, they say or write another word that relates in meaning to the original word.
Such patients often have lesions in the inferior parietal lobe and Wernicke’s area.
Semantic Memory
Semantic memory (memory for the meaning of words, objects, people, and events) is essential for language.
The systems supporting semantic memory are probably distributed widely across the brain.
Semantic memory might be distributed virtually everywhere in the cortex, except in highly specialized regions for basic sensory analysis and motor control.
A network connecting all sensorimotor association cortices together is the only system that could retrieve and integrate all the sensory dimensions involved in encoding a concept (especially abstract concepts).
Example: Retrieving the meaning of “crème fraîche” involves:
Representations of the color white, the temperature cold, of freshness, of sourness.
Smooth contact at the end of the finger and many related concepts such as delicious, smoked salmon, pouring and whipping.
The feeling of guilt when we eat too much.
Semantic memory involves at least the auditory association cortex, the visual association cortex, and the somatosensory association cortex.
Sub-cortical structures and the limbic system implement important memory and emotional aspects of meaning.
To combine all this information into a coherent concept, we need a supramodal area that acts as an intermediary semantic hub.
That hub might be located in the anterior temporal lobe.
Conclusion
Convergence of data from experimental psychology, clinical neuropsychology, and psychophysiology provides a rich picture of brain and behavior relationships.
Simplified map of auditory language processing in the brain:
Auditory analysis of speech takes place in the primary auditory cortex.
The superior temporal cortex (Wernicke’s area) takes part in phonological processing and auditory word form recognition.
Supramodal associative cortices of the anterior temporal cortex work with modality-specific sensorimotor association regions to retrieve word meaning.
Prefrontal regions (Broca’s area) deal with production aspects of spoken language such as retrieving the combination of movement allowing word articulation, elucidating the sequence of heard words, and analyzing syntax.