Trial-by-trial covariation of pupil dilation, microsaccades, and visual evoked potentials during saliency processing

Salient stimuli evoke multiple orienting responses, including microsaccades, pupil dilation, and visually evoked potentials (VEPs); yet, these signals are typically studied in isolation, leaving unclear whether they covary systematically during visual orienting. We simultaneously recorded eye movements, pupil size, and EEG while participants performed a visual discrimination task with varying stimulus contrast. High-contrast stimuli elicited larger pupil dilation, smaller microsaccade amplitudes, and greater P1 and N1 amplitudes than low-contrast stimuli. Trial-by-trial analyses revealed reliable intra-individual covariation among these measures: stronger microsaccade inhibition and smaller amplitudes were associated with larger pupil dilation and more negative N1 amplitudes, and stimulus-related pupil dilation covaried with P1 amplitude. Several orienting measures were also associated with reaction time, indicating behavioral relevance. In contrast, contrast-induced modulations did not correlate across individuals, suggesting that coordination primarily reflects within-individual trial-level variability. These findings demonstrate structured trial-by-trial covariation among cortical, oculomotor, and autonomic responses during visual orienting.