The task used in the MID study engaged similar brain regions during reward anticipation as have previously been seen in adults and adolescents in very similar versions of the same task. Thus, we can eliminate the possibility that this null result with psychopathology was linked to fMRI measurement failure or developmental factors related to the developmental trajectory of structures representing reward anticipation. The possibility of measurement error in terms of psychopathology at age 9-10 could be considered, but is unlikely in our view. Psychotic-like experiences were measured with a new scale developed especially for this study, which has been shown to perform well in this sample \cite{Karcher2018}. Depression was measured with the K-SADS, the gold standard in this age group. Given that in middle adolescence, and in adulthood, striatal reward anticipation has been shown to be deficient in  patients with schizophrenia and clinical or subthrehsold depression \cite{Juckel2006}\cite{Hägele2015}\cite{Arrondo2015}(Papanastasiou 2018)\cite{Stringaris2015}, it appears that clear associations between psychopathology and brain reward anticipation signatures only emerge during adolescence.  The ABCD study provides a platform within which this prediction can be tested, as the study protocol is that repeat measures of fMRI and psychopathology will be acquired in the coming years. 
The significance of psychotic-like experiences in the general population is nuanced \cite{Murray2012}. Psychotic-like experiences at the age of around 10 years are relatively common. For example, around 10% of children at this age answer yes to the question "do you heard voices when there is no-one around." , and only a minority of children with psychotic-like experiences at this age will go on to develop a psychotic illness (\cite{Kelleher2016,Kelleher2012a}). Thus the presence of such symptoms should not be seen as, in isolation, marking the presence of mental illness or of specific schizophrenia spectrum risk. However, the presence of such symptoms does indicate a raised risk (compared to children who do not express psychotic like experiences) of future mental disorder, including schizophrenia spectrum disorder, of future anxiety and depressive disorders, and of suicidal thoughts and behaviours. The frequency of endorsement of psychotic like expeirences declines over the adolescent period, but the strength of the association with mental disorder strengthens in this period for those in whom psychotic experiences persist. Thus, as age 10-11, an endorsement of psychotic like experiences items on interview is less likely to be representative of mental disorder than a similar endorsement in adolescence  \cite{Kelleher2012}, and the persistence of psychotic-like experiences over time has been shown to be associated with a higher risk of mental disorder than isolated measurements \cite{Kelleher2016,Kelleher2012a}. Previous studies have suggested links between reward processing (and other fMRI task) activation and psychopathology in children and adolescents than the current sample \cite{Stringaris2015}(\cite{Jacobson_2010,Papanastasiou_2018}. This leads to the following prediction: children who persist in endorsing psychotic-like experiences at multiple time-points over the coming years will be more prone to develop schizophrenia spectrum or depressive illnesses than other short members, and this persistence of psychopathology, and emergence of new psychopathology in follow-ups, may be associated with  (or possibly predicted by) concomitant deficits in brain activation in the MID task. This prediction can be tested in follow-up waves of the ABCD study.
The study has a number of limitations. We used various exclusion criteria including diagnosis of ADHD, which affects several hundred children in the cohort. ADHD has been associated with abnormal brain reward processing previously as well as various psychiatric symptoms such as depressive symptoms and psychotic symptoms; furthermore ADHD is commonly treated with stimulant medication that may alter brain activation during reward anticipation, making results challenging to interpret. Thus, excluding ADHD is helpful in reducing the chances of finding spurious associations between various domains of psychopathology and altered brain reward anticipation signals; however, it reduces the sample size, thus reducing statistical power, and it makes the sample less representative of the general population. Additional children had to be excluded for other reasons, such as artefacts, movement in the scanner or non-engagement with the task. The study data were collected on multiple MRI scanners, which is likely an additional source of noise and variance. We try to compensate for this by taking scanner site into account in statistical analyses. Acquiring data at a single site using a single scanner could be preferable, in terms of reducing sources of variability; however, a major advantage of a multi-site study is the potential for much larger sample size.  There are many measures of potential interest that can be generated from the MID task. We calculated reward anticipation by contrasting anticipation of a large reward versus no reward was it is an effective and reliable contrast in eliciting ventral striatal and medial prefrontal reward anticipation associated activation. We used this same contrast in our previous MID study \cite{Arrondo2015} nd as did Jia et al (2015) in IMAGEN, he largest MID study previously \cite{Jia2016}. Many other related contrasts could have been used simply to probe reward anticipation, and the task data could also or alternatively be used to examine anticipation of punishment, brain responses to reward or punishment receipt or omission of expected rewards and/or punishments; numerous connectivity analyses are also possible. Whilst these alternative analyses would be of considerable interest, each additional analysis increases the liklihood of type I error unless rigorous correction for multiple comparison is made, in which case the likelihood of type II error is increased. It is possible that a voxel-wise or vertex-wise analysis could be more sensitive than the approach we used, of regionally averaged scores. However, we note that the regionally averaged scores revealed highly significant task activation in the expected areas of striatal subdivisions, thalamus and medial frontal cortex, supporting their validity.