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    • Moreover while previous developmental neuroimaging studies h

    2018-10-25

    Moreover, while previous developmental neuroimaging studies have found a shift from prefrontal regions in children to parietal regions in adults (Ansari et al., 2005; Cantlon et al., 2009; Holloway and Ansari, 2010; Kaufmann et al., 2005, 2006), the present study did not reveal a negative correlation between chronological age and signal recovery in the prefrontal cortex. While the absence of an effect is difficult to interpret, this null result may nevertheless indicate that previously observed frontal activations in number comparison tasks may not be related to developmental changes of symbolic numerical magnitude representation per se or the mapping between numerical magnitudes and their symbolic referents. Rather, in view of the present findings, it is plausible that regions of the prefrontal cortex are related to domain general processes such as selecting the correct stimuli for response execution. In the present study no response selection was required and no negative correlation between age and activation was found in prefrontal regions of the brain. This might suggest that age-related changes in the parietal cortex are specific to processing numerical symbols and symbolic numerical magnitude representation. Age-related changes in the prefrontal cortex however may be associated with age-related changes in response-selection and task performance that may not be number-specific. Lastly, the results of the present study highlight the importance of using adequate designs in order to reduce and to eliminate confounding variables in developmental imaging studies (Poldrack, 2000). The striking specificity of phenylephrine hydrochloride activation observed in the present study, but also in the studies of Notebaert et al. (2011) and Holloway et al. (2012), demonstrates the additional value that can be gained when confounding variables such as response selection are mitigated. For instance, hemispheric differences in brain activation of the left and right IPS have been largely inconsistent in previous developmental studies (Ansari et al., 2005; Holloway and Ansari, 2010). This inconsistency might be due to the fact that most developmental imaging studies have used coarse group comparisons (i.e., comparing groups of children with wide age ranges to groups of adults) in addition to active numerical paradigms, which in combination may have washed out consistent hemispheric differences. Controlling for these variables may in fact draw a clearer picture of how the left and right IPS interact in order to generate symbolic numerical magnitude representation. The present study provides the first clear evidence that age-related changes in the neural representation of symbolic numerical magnitude are largely restricted to the left hemisphere. Taken together, the results of the present fMR-A study in children demonstrate a protracted and dynamic development of how the brain represents the meaning of numerical symbols between 6 and 14 years of age. Specifically, while the right IPS is modulated by the semantics of Arabic numerals at all ages (from 6 onwards), activation in the left IPS increases as a function of chronological age. This finding suggests an age-related developmental specialization of the left IPS for representing symbolic numerical magnitude. This age-related trajectory in brain activation may be reflective of an increase in fluency/acuity with which numerical magnitude is processed in the child\'s brain, pointing towards a more precise and richer representation as children age. This finding is largely consistent with behavioural findings that have demonstrated developmental changes in the fluidity with which symbolic numerical magnitudes are processed (Holloway and Ansari, 2008, 2009; Moyer and Landauer, 1967). In contrast to previous studies, which have investigated age-related changes underlying the processing of symbolic numerical magnitudes, the present study did not reveal a significant decrease in prefrontal brain activation as a function of numerical ratio and age. This may suggest that aforementioned findings of an association between the prefrontal cortex and age-related changes in numerical magnitude processing are unrelated to the process of constructing a semantic representation of symbolic numerical magnitude. As such, the findings of the present study open up a new set of interesting questions that could shed more light on how the human brain constructs symbolic numerical magnitude representation over developmental time.