Readers move their eyes mindlessly using midbrain visuo-motor principles


  • Vitu Françoise
  • Adeli Hossein
  • Zelinsky Gregory

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Saccadic eye movements rapidly shift our gaze over 100,000 times daily, enabling countless tasks ranging from driving to reading. Long regarded as a window to the mind and human information processing, they are thought to be cortically/cognitively controlled movements aimed at objects/words of interest. Saccades however involve a complex cerebral network wherein the contribution of phylogenetically older sensory-motor pathways remains unclear. Here we show using a neuro-computational approach that mindless visuo-motor computations, akin to reflexive orienting responses in neonates and vertebrates with little neocortex, guide humans’ eye movements in a quintessentially cognitive task, reading. These computations occur in the superior colliculus, an ancestral midbrain structure, that integrates retinal and (sub)cortical afferent signals over retinotopically organized, and size-invariant, neuronal populations. Simply considering retinal and primary-visual-cortex afferents, which convey the distribution of luminance contrast over sentences (visual-saliency map), we find that collicular population-averaging principles capture readers’ prototypical word-based oculomotor behavior, leaving essentially rereading behavior unexplained. These principles reveal that inter-word spacing is unnecessary, explaining metadata across languages and writing systems using only print size as a predictor. Our findings demonstrate that saccades, rather than being a window into cognitive/linguistic processes, primarily reflect rudimentary visuo-motor mechanisms in the midbrain that survived brain-evolution pressure.

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