Summary: A new registered fMRI study from Western University finds that the brain represents symbolic numbers in the same brain region for left- and right-handed people. The results challenge the idea that handwriting handedness determines which hemisphere processes numbers.
Source: University of Western Ontario
Left-handed and right-handed people may hold pens differently, but a recent study from Western University shows that number processing in the brain converges on the same neural site regardless of which hand they favor. These findings help clarify how symbolic numbers are represented in the brain and may inform future support for children who struggle with number and math learning.
In the registered report titled “Does writing handedness affect neural representation of symbolic number? An fMRI Adaptation Study,” researchers led by Celia Goffin and senior author Daniel Ansari examined whether the brain’s representation of symbolic numbers differs between people who write with their right hand and those who write with their left hand.
BrainsCAN-supported researchers Celia Goffin, H. Moriah Sokolowski, Michael Slipenkyj and Daniel Ansari used functional magnetic resonance imaging (fMRI) to measure neural responses when participants passively viewed Arabic digits. The goal was to test whether habitual handwriting practice on one side of the body could shape which hemisphere of the brain specializes in processing symbolic numbers.
Historically, most cognitive neuroscience studies of number processing have been conducted with right-handed participants, since roughly 90 percent of the population is right-handed. That left unanswered whether left-handers might recruit different brain regions—possibly the opposite hemisphere—to represent numeric symbols. This study deliberately included both left- and right-handed participants to address that gap.
Previous work has implicated the intraparietal sulcus (IPS), a region in the parietal lobe, as central to mapping symbols like Arabic digits onto quantity. In right-handed groups, symbolic number processing typically produces stronger activation in the left IPS. The Western team hypothesized that if handwriting experience drives lateralization, left-handed writers might show the mirror pattern—greater right IPS activation for symbolic numbers.
Using carefully pre-registered methods and fMRI adaptation measures, the researchers compared brain activation patterns across groups. While right-handed participants reproduced the expected left-lateralized response within the IPS, left-handed individuals did not show a systematic shift to right-hemisphere dominance at the group level. Statistical comparisons between groups revealed no reliable regions where left- and right-handers diverged in symbolic number activation.
“We expected that writing experience might push number representations to the side of the brain associated with fine motor habits, but the data did not support that,” said Daniel Ansari. “Handwriting preference does not appear to determine which hemisphere holds symbolic number representations.”
The study’s findings suggest that the left-lateralized processing of symbolic numbers is not explained by handwriting experience alone. As Goffin explained, children often learn numbers by tracing and writing numerals repeatedly, and the researchers tested whether that sensorimotor practice might anchor number symbols to the hemisphere used for skilled hand movements. Their results indicate that other developmental, genetic, or neural organization factors are more likely to underlie why symbolic numbers commonly activate the left IPS.
Beyond its specific conclusion, the paper is notable for being a registered report. In the registered report model, researchers submit their hypotheses, design, and analysis plan for peer review before data collection begins. This approach improves transparency, reduces publication bias, and strengthens confidence in null and positive findings alike.
Source:
University of Western Ontario
Media Contacts:
Maggie Maclellan – University of Western Ontario
Image Source:
The image is in the public domain.
Original Research: Closed access
“Does writing handedness affect neural representation of symbolic number? An fMRI Adaptation Study”. Celia Goffin, H. Moriah Sokolowski, Michael Slipenkyj, Daniel Ansari. Cortex. doi: 10.1016/j.cortex.2019.07.017
Abstract
Does writing handedness affect neural representation of symbolic number? An fMRI Adaptation Study
How the brain represents numerical symbols such as Arabic digits is a central question in numerical cognition. Many studies implicate left parietal regions, particularly the intraparietal sulcus (IPS), in symbolic number processing. Given the predominance of right-handed participants in neuroimaging research, one hypothesis is that handedness for writing could contribute to this left-lateralization. To test this idea, the present fMRI study compared brain responses to symbolic numbers in groups of right- and left-handed participants during passive viewing. At the whole-brain level, right-handers showed the expected left-lateralized IPS response. Left-handers exhibited some instances of reversed lateralization in the IPS, but group comparisons did not reveal statistically significant differences. Region-of-interest analyses computing laterality indices for each participant likewise failed to show reliable group effects. Overall, these results do not support the hypothesis that handwriting hand preference determines the hemisphere that processes symbolic numbers. Further work is required to identify the developmental and neural factors that produce consistent left-lateralization for symbolic number processing.