Slide Slam K3
Periodic Eye Movements During Reading—of Words and Chunks?
Lena Henke1, Ashley G. Lewis2, Lars Meyer1,3; 1Max Planck Institute for Human Cognitive and Brain Sciences, 2Radboud University, Donders Institute for Brain, Cognition and Behaviour, 3Clinic for Phoniatrics and Pedaudiology, University Hospital Münster
Current auditory neuroscience emphasizes the role of neural oscillations for speech tracking (Poeppel & Teng, 2020). Recently, the complimentary role of intrinsic rhythms in internal linguistic processing has entered focus (Meyer et al., 2020). If indeed intrinsic rhythms are critical to linguistic processing, they should also emerge for input that does not contain any rhythmic structure, which might otherwise disguise intrinsic activity. One good modality to study this is reading—words occur in space without imposing a specific tracking rate on the reader. Eye movements are guided by linguistic knowledge in combination with attention and oculomotor constraints (Kliegl et al., 2006). Rhythmic eye movements during reading might provide insight into the degree to which internal linguistic processing exhibits intrinsic rhythms, and the extent to which there are specific intrinsic rates regulating different internally generated linguistic units (e.g., words versus phrases). We analyzed the Ghent Eye-Tracking Corpus (Cop et al., 2017) of eye-tracking data from 14 participants during naturalistic reading of a novel. In a first step, we analyzed the power spectrum of saccade onset times. In line with previous research (Gagl et al., 2019), we hypothesized periodicity in the theta-band range (i.e., 4–8 Hz), previously linked to attentional cycles in vision and syllabic tracking in audition (e.g., VanRullen, 2018). Power spectra were calculated and tested against surrogates to establish above-chance saccade rates; spectral variance was used as effect size. We replicated previous observations of a 4–5-Hz rhythm, confirming that saccadic information uptake during reading is rhythmic. In a second step, we tested for larger temporal units, consistent with a proposed role of delta-band oscillations (i.e., 0.5–4 Hz) in linguistic chunking (e.g., Henke & Meyer, 2021). Inspired by previously reported wrap-up effects at the end of clauses and sentences (Tiffin-Richards & Schroeder, 2018), we hypothesized that processing should slow down at the end of larger temporal units. To test for periodic fixation slowdowns, we computed a time series of differences between fixation durations for subsequent words—effectively highlighting slowdowns from word to word. Comparison of fixation duration spectra with surrogates indicated a prominent peak in the delta band (i.e., < 1 Hz), suggesting that fixations indeed slow down in a periodic fashion at a frequency within the delta band. Together, these results show that eye movements during naturalistic reading contain hierarchical rhythmic structure akin to neural oscillations during speech tracking. Moreover, this rhythmicity appears to be intrinsic rather than externally imposed. We suggest that the saccadic rhythm may reflect cycles of optimal sensitivity for sampling new words, whereas fluctuations in fixation duration may index an intrinsic chunking mechanism that integrates words at a rate corresponding to larger linguistic units.