Hemispheric contributions to sentence comprehension
The brain's classic language response to word predictability is actually a combination of distinct processing styles from the left and right hemispheres.
Source
Two sides of meaning: the scalp-recorded n400 reflects distinct contributions from the cerebral hemispheres
What they did
Researchers recorded brain activity from 24 right-handed native English speakers as they read 300 sentences. The final word of each sentence was presented quickly to either the left or right visual field to bias processing to a single brain hemisphere. The researchers manipulated how predictable the final word was, covering a range from 0 to 100 percent cloze probability.
What they found
The overall size of the language-related brain response was similar across both visual fields, shifting by about 4 microvolts across the full range of word predictability. However, the hemispheres processed moderately predictable words differently, with the left hemisphere facilitating word processing even with weak contextual clues, while the right hemisphere required highly predictable contexts to show facilitation. When averaged together, these separate hemispheric patterns merged into the classic graded response seen in typical reading.
The limits
What it doesn't show
This research relied on a small sample of 24 participants, all of whom were right-handed and between 18 and 21 years old, meaning these patterns might not apply to left-handed individuals or older adults. Additionally, visual half-field presentation is an indirect way to bias processing, so this study cannot directly prove how the hemispheres interact under normal, central reading conditions. Finally, because the study focused solely on visual reading, it does not confirm whether auditory language comprehension relies on the same bihemispheric division of labor.
Key terms
- N400
- A negative-going brain wave that peaks shortly after a person encounters a meaningful stimulus, reflecting the effort required to process its meaning.
- visual half-field presentation
- An experimental technique that presents visual stimuli to either the left or right side of a person's gaze to initially direct the information to the opposite brain hemisphere.
- cloze probability
- The likelihood that a group of people will complete a sentence frame with a specific word, used to measure how predictable that word is in context.
- hemispheric specialization
- The tendency for certain cognitive functions, such as basic grammar or emotional processing, to be handled more efficiently by one side of the brain than the other.
- event-related potential
- A measured brain response that is the direct result of a specific sensory, cognitive, or motor event, tracked using electroencephalography.
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Quiz yourself
How does the visual half-field presentation technique bias hemispheric processing in healthy individuals?
Common questions
Why does presenting words to one side of the screen target a specific hemisphere?
Our visual pathways are crossed, meaning that everything we see in our left visual field is initially processed by the right hemisphere of the brain, and everything in the right visual field goes to the left hemisphere.
How does this study change our understanding of the classic N400 brain wave?
Traditionally, the N400 was thought of as a single, uniform brain response. This study suggests it is actually a composite signal, combining the distinct semantic processing strategies of both the left and right hemispheres.
What are the different strategies used by each hemisphere?
The left hemisphere acts like a predictor, helping us process words even when contextual clues are weak or moderate. The right hemisphere is more conservative, waiting to facilitate processing until the context is highly predictable.
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