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    • More recent developmental neuroimaging studies which have fu

    2018-11-09

    More recent developmental neuroimaging studies, which have further investigated both symbolic and non-symbolic numerical magnitude processing, have indicated hemispheric differences in the engagement of the parietal lobe. Specifically, in response to numerical symbols developmental changes have been particularly reported in the left IPS (e.g., Emerson and Cantlon, 2014). On the other hand, neuroimaging studies investigating the processing of non-symbolic numerical magnitudes (e.g., dot-arrays) have demonstrated an early recruitment of the right IPS in preverbal infants and young children (Cantlon et al., 2006; Hyde et al., 2010; Izard et al., 2008). These findings suggest that while the right IPS is engaged during non-symbolic numerical magnitude processing from infancy onwards, the left IPS gradually increases in its response to numerical symbols. Thus, developmental changes associated with symbolic and non-symbolic numerical magnitude processing are multifaceted and involve both changes within the parietal cortex as well as a decrease in the engagement of prefrontal regions. Such changes have been argued to reflect the increasing fluency and accuracy of processing numerical magnitude. Evidence for a continued refinement in the precision of symbolic numerical representation over developmental time is provided in behavioural studies that have found age-related changes (i.e., decrease) in the size of the behavioural distance effect (Holloway and Ansari, 2008; Moyer and Landauer, 1967). However, the precise neurocognitive mechanisms linking changes in Phos-tag Biotin activation to behavioural performance remain opaque. A key limitation of the studies comparing brain activation in children and adults is that developmental studies are extremely sensitive to age-related performance differences in reaction time and accuracy (Poldrack, 2000). As a consequence, it is notoriously difficult to control for general developmental changes in task performance, especially in tasks in which overt decisions – such as choosing the larger number of two presented numerals – are required (Göbel et al., 2004; Göbel and Rushworth, 2004). As such, it is currently unclear whether age-related brain activation differences observed in developmental imaging studies are a reflection of representational changes or a function of age-related non-numerical performance differences. A way to mitigate the confound between age and task performance is to use experimental paradigms such as functional magnetic resonance adaptation (fMR-A) that do not require overt behavioural responses (Grill-Spector et al., 2006; Grill-Spector and Malach, 2001). FMR-A has been recently used in adults to investigate the neural correlates of symbolic numerical magnitude representation in order to control for non-numerical task demands. Results of these fMR-A studies have provided compelling evidence to suggest that the IPS is modulated by numerical magnitude in the absence of response selection. This research has also demonstrated that predictors that have been widely used to characterize the nature of numerical representations modulate brain activation of the left hemisphere. Specifically, numerical ratio is frequently used to measure the processing of numerical magnitudes in behavioural and neuroimaging studies. A significant increase in brain activation can be observed in number comparison experiments when the discrimination of numerals with a large ratio (e.g., 8 vs. 9: a ratio of 0.89) is contrasted with the discrimination of numerals with a small ratio (e.g., 1 vs. 2: a ratio of 0.5). Since the discrimination of numerical magnitudes is scaled by the numerical ratio between them – smaller ratios are more easily discriminated than larger ratios – it is commonly argued that numerical ratio indicates the approximate nature of noisy and imprecise representations of numerical magnitudes in the human brain (for a review see also Nieder and Dehaene, 2009). Notebaert et al. (2011) used fMR-A to investigate the numerical ratio effect of small numbers (e.g., 3, 4, 5) and large numbers (e.g., 16, 20, 26) in adults. More specifically, the participant\'s brain response was first adapted to either a small (i.e., the Arabic digit 6) or a large (i.e., the Arabic numeral 32) numerical value. The repeated presentation of this adaptation numeral was then interspersed with the presentation of new numerals (i.e., the numerical deviants) that systematically differed in numerical ratio from the adaptation numeral 6 or 32. A whole brain analysis, using numerical ratio as a predictor, revealed that the left IPS was the only region in the brain that showed a significant ratio dependent neural signal recovery (independent of whether large numbers 16, 20, 26 or small numbers 3, 4, 5 were presented). This result not only showed that the activation of the left IPS was significantly modulated by numerical ratio but, importantly, that the brain activation of the IPS was modulated in the absence of overt response selection.