Automatic numerical-spatial association in synaesthesia: An fMRI investigation

Area of Science:

• Neuroscience
• Cognitive Psychology
• Neuroimaging

Background:

• The mental number line (MNL) describes spatial quantity representation in humans.
• Neural correlates of the MNL are primarily located in the posterior parietal cortex, particularly the intraparietal sulcus (IPS).
• Number-space synaesthesia involves explicit, idiosyncratic, and stable visualization of numbers in spatial locations.

Observation:

• This study investigated the neural basis of numerical-spatial associations using functional magnetic resonance imaging (fMRI).
• A single synaesthete (MkM) with a distinct MNL was compared to 15 non-synaesthetes.
• Participants performed a task comparing physical number size, ignoring numerical value.

Findings:

• The numerical distance effect (NDE) significantly activated the left and right IPS, bilateral supramarginal gyrus, and left angular gyrus exclusively in MkM.
• These activations occurred even when numerical values were task-irrelevant.
• The size congruity effect did not yield significant differential findings between groups.

Implications:

• The findings support a crucial role for the parietal cortex in automatic spatial and quantity coding within number-space synaesthesia.
• This research sheds light on the shared neural mechanisms underlying numerical-spatial associations.
• It highlights the utility of studying synaesthesia to understand fundamental cognitive processes.