Fixation duration on natural scenes is explained by memory encoding not processing demand.

Before each of around 200,000 eye movements we make each day, the brain decides how long to fixate before shifting gaze to new information. Here we investigate this process using a large-scale scene-viewing experiment (4,080 natural scenes, five participants) that combines magnetoencephalography, eye tracking and a semantic captioning task. Using multivariate analysis of magnetoencephalography source-space patterns, behavioral analyses and artificial neural network (ANN) modeling, we show that longer fixations do not reflect prolonged visual processing but relate to downstream memory encoding. First, temporal variability of ventral stream representational dynamics did not explain variability in fixation duration. Second, fixation durations were anticorrelated with ANN-estimated patch classification difficulty. Third, fixation durations correlate positively with ANN-predicted patch memorability and caption-inclusion and co-occur with increased theta-gamma phase-amplitude coupling, particularly in frontal and hippocampal regions. These results indicate that eye-movement timing decisions are shaped by memory-encoding demands rather than by perceptual processing limits.