Readers move their eyes mindlessly using midbrain visuo-motor principles

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 1 and human information processing 2 , they are thought to be cortically/cognitively controlled movements aimed at objects/words of interest 3–10 . Saccades however involve a complex cerebral network 11–13 wherein the contribution of phylogenetically older sensory-motor pathways 14–15 remains unclear. Here we show using a neuro-computational approach 16 that mindless visuo-motor computations, akin to reflexive orienting responses 17 in neonates 18–19 and vertebrates with little neocortex 15,20 , guide humans’ eye movements in a quintessentially cognitive task, reading. These computations occur in the superior colliculus, an ancestral midbrain structure 15 , that integrates retinal and (sub)cortical afferent signals 13 over retinotopically organized, and size-invariant, neuronal populations 21 . Simply considering retinal and primary-visual-cortex afferents, which convey the distribution of luminance contrast over sentences (visual-saliency map 22 ), we find that collicular population-averaging principles capture readers’ prototypical word-based oculomotor behavior 2 , leaving essentially rereading behavior unexplained. These principles reveal that inter-word spacing is unnecessary 23–24 , explaining metadata across languages and writing systems using only print size as a predictor 25–26 . 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 27 .