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Background: Previous research demonstrated atypical attention in children with attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Regarding visual orienting, findings suggest a differential impairment: Atypical orienting to relatively unexpected targets in ASD, and atypical processing of alerting cues in ADHD. The locus coeruleus-norepinephrine (LC-NE) system plays an important role in exploiting alerting cues to increase attention and task performance. The present study's aim was to examine differential subcortical processes underlying visual orienting in ASD and ADHD with pupil dilation (PD) as index of LC activity. Methods: Pupil dilation (PD) progression metrics during visual orienting were calculated for task-evoked PD locked to cue, stimulus onset, and behavioral response. Group differences in PD and reaction time (RT) were compared between children with ASD without ADHD (ASD-) (N = 18), ADHD without ASD (ADHD-) (N = 28), both disorders (ASD + ADHD) (N = 14), and typically developing children (TD) (N = 31) using linear mixed models (LMM). To further explore the modulatory role of the LC-NE system group differences in the effect of task-evoked PD metrics on RT were examined exploratively. Results: ASD (+ADHD) showed slower orienting responses to relatively unexpected spatial target stimuli as compared to TD, which was accompanied by higher PD amplitudes relative to ADHDÀ and TD. In ADHDÀ, shorter cue-evoked PD latencies relative to ASDÀ, ASD + ADHD, and TD were found. Group differences in the effect of cueand stimulus-evoked PD amplitudes on RT were found in ASDÀ relative to TD. Conclusions: Study findings provide new evidence for a specific role of the LC-NE system in impaired reflexive orienting responses in ASD, and atypical visual processing of alerting cues in ADHD. Key pointsPrevious findings suggest different attention processes during visuospatial orienting task performance in ASD and ADHD. This is the first study to compare pupil dilations (PD) as an index of LC activity between ASD and ADHD during visuospatial orienting.PD during visuospatial orienting differentiated between ASD and ADHD, suggesting different subcortical processes in ASD and ADHD.

Attenuated social attention has been described as a reduced preference for social compared to geometric motion in preschoolers with autism spectrum disorder (ASD). The locus coeruleus-norpinephrine (LC-NE) system modulates sensory reactivity and is a promising underlying mechanism. LC-NE activity is indexed by a stimulus-evoked pupillary response (SEPR) and partially by a luminanceadaptation pupillary response (LAPR), which were both shown to be aberrant in ASD. We examined whether SEPR and LAPR explain an attenuated social motion preference. We applied pupillometry via video-based eye tracking in young children (18-65 months) with ASD (n = 57) and typically developing (TD) children (n = 39) during a preferential looking paradigm of competing social and geometric motion and a changing light condition paradigm. We found an attenuated social motion preference in the ASD compared to the TD group. This was accompanied by atypical pupillometry showing a smaller SEPR to social motion, a larger SEPR to geometric motion and a reduced LAPR to a dark screen. SEPR but not LAPR explained the group difference in social motion preference. An ASD diagnosis was statistically predicted by the social motion preference, while this effect was mediated by the inclusion of SEPR to geometric and social motion. Our findings suggest a decreased sensory reactivity to social and increased reactivity to non-social motion in ASD, which may concurrently contribute to an attenuated social attention. The LC-NE system is supported as a promising underlying mechanism of altered social attention in young children with ASD, while the specificity of findings remains to be addressed.