Abstract
How are number symbols (e.g., Arabic digits) represented in the brain? Functional resonance imaging adaptation (fMRI‐A) research has indicated that the intraparietal sulcus (IPS) exhibits a decrease in activation with the repeated presentation of the same number, that is followed by a rebound effect with the presentation of a new number. This rebound effect is modulated by the numerical ratio or difference between presented numbers. It has been suggested that this ratio‐dependent rebound effect is reflective of a link between the symbolic numerical representation system and an approximate magnitude system. Experiment 1 used fMRI‐A to investigate an alternative hypothesis: that the rebound effect observed in the IPS is related to the ordinal relationships between symbols (e.g., 3 comes before 4; C after B). In Experiment 1, adult participants exhibited the predicted distance‐dependent parametric rebound effect bilaterally in the IPS for number symbols during a number adaptation task, however, the same effect was not found anywhere in the brain in response to letters. When numbers were contrasted with letters (numbers > letters), the left intraparietal lobule remained significant. Experiment 2 demonstrated that letter stimuli used in Experiment 1 generated a behavioral distance effect during an active ordinality task, despite the lack of a neural distance effect using fMRI‐A. The current study does not support the hypothesis that general ordinal mechanisms underpin the neural parametric recovery effect in the IPS in response to number symbols. Additional research is needed to further our understanding of mechanisms underlying symbolic numerical representation in the brain.
Highlights
Number symbols (e.g., Arabic numerals) are a relatively recent human invention, it is unlikely that evolution has adapted the human brain to process and represent numbers symbolically (Núñez, 2017)
Accuracy for each task was examined and participants who scored below three SDs from the mean accuracy on that task were not included in analyses involving that task
What mechanisms underlie the distance-dependent parametric rebound effect that has been reproduced across different studies following adaptation to numerical symbols? What can this effect tell us about symbolic number representation? It is often hypothesized that the symbolic number system is mapped onto an approximate nonsymbolic magnitude system, and that the parametric effect is a signature of this analog system
Summary
Number symbols (e.g., Arabic numerals) are a relatively recent human invention, it is unlikely that evolution has adapted the human brain to process and represent numbers symbolically (Núñez, 2017). Adult fMRI research has repeatedly shown that the activity in the parietal cortex is correlated with tasks that involve the processing of numerical symbols (e.g., number comparison). The research has converged upon areas in the parietal lobe such as the IPS and SPL as key neural regions for the processing of numerical stimuli. Even though a parametric effect for letters was not obtained at the neural level with a passive task, based on previous research we would still expect letters to generate a behavioral distance effect (Van Opstal, Gevers, De Moor, & Verguts, 2008). The NDE was attributed to processes related to response selection, as opposed to a specific numerical process (Van Opstal et al, 2008)
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