LAUSR

Abstract Background According to the Neurodevelopmental continuum hypothesis, neurodevelopmental disorders such as ADHD, Autism and Schizophrenia, should be conceptualised as lying on a continuum of neurodevelopmental impairment, with the individual disorders reflecting different degrees of abnormal brain development and resulting functional impairment. The model also hypothesizes that these disorders lie on a gradient of severity, suggesting that they might differ to some extent quantitatively but also qualitatively. The severity of neurodevelopmental impairment is also expressed by the severity of cognitive impairment and persistence of functional impairment. The main objective of the present study was therefore to investigate commonalities and differences among the three conditions in various cognitive subdomains using standard ocular-motor paradigms. Methods Four groups of young adults were included: 21 with Early-Onset Schizophrenia (“SCZ”, 19.7±1.7, 15 males), 26 with Autism Spectrum Disorder (“ASD”, 19.7±1.9, 25 males), 28 with Attention-Deficit/Hyperactivity Disorder (“ADHD”, 19.9±1.4, 15 males), 29 typically developing participants (“TD”, 19.8±1.6, 12 males). Participants were matched on age and full-scale IQ. Eye movements were recorded binocularly at 1000 Hz using the Eye Link 1000+ system while four basic ocular-motor tasks (prosaccade (PRO), antisaccade (ANT), memory-guided saccade (MEM), fixation (FIX)) were administered in counterbalanced order. Eye movements were analysed with Data Viewer 3.2, SPSS 2.3 and MorePower 6.1. Results With respect to standard deviation of saccadic reaction times (SRTSD), ASD and ADHD did not differ from TD for PRO, ANTI, MEM. Conversely, SCZ were significantly more variable in all tasks than TD, but not compared to ASD and ADHD. Furthermore, SCZ were significantly slower than TD and ASD, but not ADHD, in PRO and ANT. Compared to TD, increased percentage of anticipatory saccades was found for MEM in ASD and ADHD, only marginally for PRO in ASD, again for all tasks in SCZ, when compared to both TD and ASD and, in less extent, to ADHD in ANT. A significantly higher percentage of direction errors during ANT was common to all clinical groups, this inhibitory deficit being significantly more severe in SCZ than ASD, but not compared to ADHD. Increased frequency of intrusive saccades during FIX was found in SCZ and ASD, and marginally in ADHD, when compared to TD. As for microsaccades, SCZ and ASD showed a higher frequency of microsaccades for FIX (condition without distractors) and PRO (preceding correct saccades), only SCZ for ANT (both preceding correct saccades and direction errors), compared to TD. Discussion Results suggest psychomotor slowing, unique for SCZ, as compared to ASD and ADHD, as indicated by increased mean RT in PRO. Conversely, increased anticipatory saccades and direction errors support the presence of inhibition deficits across groups, suggesting a common (pre-)frontal functional impairment. Of note, mean RT discriminates the clinical groups better than ISV, thus emerging as a trans-diagnostic process to the three conditions and, uniquely for SCZ as a deficit independent of the specific tasks administered here. Finally, atypical microsaccadic frequency might suggest common abnormalities in motor response preparation in SCZ and ASD. Across all tasks, participants with SCZ appear more impaired than those with ADHD and ASD, while the performance of ADHD was often in between SCZ and ASD, possibly in line with the idea of a continuum of neurodevelopmental impairment.