Natural Brain Waves Correspond with Eye Movements During Reading
Post by Lani Cupo
The takeaway
The brain’s neural activity imposes its own rhythm onto processing. A matching pattern of activity has been found in language processing, not just for speech which has a rhythm of its own, but also in reading.
What's the science?
Previous studies established that neural oscillations, or rhythmic neural activity, are involved in processing language, such as speech and sign language. However, language (either spoken or signed) contains a rhythm of its own, making it difficult to understand whether the synchrony between speech and brain oscillations arises because of the speech’s rhythm (outside in) or the rhythm of the brain (inside out). This week in The Journal of Neuroscience, Henke and colleagues provide evidence that the brain may impose its own intrinsic rhythm onto language processing using eye-tracking and electroencephalography (EEG) during reading.
How did they do it?
The authors analyzed openly available data from the Zurich Cognitive Language Processing Corpus, where twelve participants were instructed to read 300 sentences on a screen while they tracked their eye movements and brain activity (with EEG). From the eye-tracking data, the authors could study fixations, where the eye fixates on a target, and saccades, where it moves between targets. If rhythmic neural activity is relevant to reading, the authors hypothesized that the eye movements should also be rhythmic and that the cycles of eye movement data would be correlated with cycles of brain oscillations, which they assessed by examining their phase coherence. Additionally, language comprehension is thought to be broken into multi-word groups or “chunks” where words are meaningfully related to one another. The authors expected to see that the cycles of the eye movements relate to the formation of chunks; at the end of a chunk, there would be a change in the duration of a fixation. To that end, they examined if the fixation durations and the EEG signal were always at a specific point within their cycle at sentence endings.
What did they find?
The authors found the expected rhythmicity of the eye movements. Importantly, the eye movements synchronized with brain oscillations from electrodes above the visual cortex in two different frequency bands. These included the theta band, involved in visual attention and syllable parsing (important for single-word comprehension), and the delta band, involved in chunking speech into multi-word units. Because the language stimulus was written, the rhythmic activity was not likely imposed on the brain by the external stimulus, but naturally arose. Contrary to their hypothesis, the authors could not relate the changes in fixation duration to sentence endings. They suspect this is because they sampled only a single fixation per word, which might have impacted the baseline in their statistical approach. However, they could relate the cycles of oscillatory brain activity to chunking.
What's the impact?
While the study does not allow the authors to claim that the neural oscillations causally impact the fluctuations in eye movements during reading, the data provide evidence that the synchronized brain activity and eye movements may shape reading and information processing. In time, such work might be extended to reading-impaired populations, including individuals with dyslexia, to improve support for individuals who struggle to read.
