How does the brain represent digits? Investigating the neural correlates of symbolic number representation using fMRI-Adaptation

How does the brain represent numerical symbols (e.g., Arabic digits)? Activity in left parietal regions correlates with symbolic number processing. Research with functional resonance imaging adaptation (fMRI-A) indicates that the intraparietal sulcus (IPS) exhibits a rebound (increase in activation) effect when a repeatedly presented number is followed by a new number. Importantly, this rebound effect is modulated by numerical ratio as well as the difference between presented numbers (distance). This ratio-dependent rebound effect could reflect a link between symbolic numerical representation and an approximate number system (ANS). In this doctoral dissertation, fMRI-A is used to investigate mechanisms underlying symbolic number representation. The first study investigates an alternative to the ANS hypothesis of symbolic number representation: could the positional relations between ordered symbols (e.g., letters, numbers) explain activity observed in the parietal cortex for number symbols? The predicted distance-dependent rebound effect is exhibited in bilateral IPS for number symbols. This effect is not found for letters (which, like numbers, can be represented using an ordered sequence – the alphabet). The contrast between numbers and letters reveals greater activity for numbers in the left inferior parietal lobule. The hypothesis that general ordinal mechanisms underpin neural parametric recovery in the IPS is not supported. What does the development of symbolic number representation look like in the brain? In the second study, I replicate Vogel et al. (2015; n=19) with a larger sample (n=45) of 6-14-year-olds. While Vogel et al. found a correlation between age and the ratio-dependent rebound effect in the left IPS, my data suggest an age-invariant, ratio-dependent rebound effect in bilateral IPS. Therefore, findings from Vogel et al. were not replicated. The final study asks: does handedness of participants play a role in the neural lateralization of symbolic number representation? Right-handers demonstrate the predicted left-lateralized rebound effect within the IPS. When left- and right-handed groups are compared, results do not suggest group differences in laterality. These findings do not support the hypothesis that handedness plays a role in neural lateralization of symbolic number processing. Results from these studies are discussed in terms of theoretical implications for symbolic numerical representations in the brain