Building on Broca and Dax
- Since Broca’s discovery, we’ve learned more about brain organization for language, primarily in the left hemisphere.
A Simplified Language Circuit
- Spoken language enters auditory cortex in both hemispheres.
- On the left, it’s processed further in:
- Ventral temporal lobe
- Temporoparietal junction (TPJ) - integrates sensory information
Dorsal vs. Ventral Pathways
- Information from TPJ travels to the frontal lobe through two pathways:
- Dorsal pathway (superior): responsible for speech production.
- Ventral pathway (inferior): responsible for speech comprehension.
Understanding Speech
- Sounds are analyzed on different timescales (phonemes, syllables, etc.).
- The ventral pathway organizes these sounds and creates an interpretation.
- This interpretation is sent to the TPJ, which acts as a “lexical interface” attaching meaning (semantics) to the sounds.
Visual and Somatosensory Input
- People who are deaf use visual (sign language) or somatosensory (braille) input.
- This sensory information also goes to the TPJ for semantic interpretation.
- A case study of a woman who lost braille reading after a visual cortex stroke demonstrates this shared pathway.
Language vs. Articulation Disorders
- Damage to Broca’s area (dorsal pathway) affects sign language production as well as speech, indicating a language disorder, not an articulation issue.
Wernicke’s Aphasia
- Lesions near the temporal lobe cause Wernicke’s aphasia, characterized by fluent speech but impaired comprehension.
- Difficulty understanding complex sentences is a hallmark.
Why Link Sensory and Motor Areas?
- We learn language by hearing it, so comprehension is crucial for developing speech production.
- We constantly learn new words, requiring the link between sensory input and motor output.
- People who lose hearing often experience speech degradation, highlighting the ongoing feedback loop between production and comprehension.
Conclusion
- This simplified circuit offers a basic understanding of language processing.
- Aphasia is a devastating condition, but recovery is possible after a stroke.