Number space is made by response space: Evidence from left spatial neglect

Area of Science:

• Cognitive Neuroscience
• Numerical Cognition
• Neuropsychology

Background:

• The spatial representation of numbers on a Mental Number Line (MNL) is debated, with some theories suggesting an intrinsic left-to-right spatial component.
• Right brain damaged patients with left spatial neglect (N+) provide a unique model to study spatial cognition due to their attentional deficits.

Purpose of the Study:

• To investigate whether number magnitude representation has an intrinsic spatial component or if it is triggered by spatial coding.
• To examine the role of spatial codes in activating the Mental Number Line (MNL).

Main Methods:

• Utilized a bimanual Magnitude Comparison SNARC (Spatial Numerical Association of Response Codes) task requiring left/right spatial codes.
• Employed a uni-manual Magnitude Comparison Go/No-Go task not requiring contrasting spatial codes.
• Assessed performance in right brain damaged patients with left spatial neglect (N+).

Main Results:

• N+ patients showed a significant Reaction Time (RT) asymmetry in the SNARC task, with slower responses to smaller numbers near a reference point.
• This RT asymmetry correlated with lesions in white matter tracts, specifically the Fronto-Occipital Fasciculus.
• No significant RT asymmetry was observed in the Go/No-Go task, indicating the absence of left-to-right number organization.

Conclusions:

• The findings suggest that the Mental Number Line (MNL) is activated by the use of contrasting left/right spatial codes, not by an inherent spatial component of number magnitude.
• Number representation is not intrinsically spatial; spatial coding is necessary to organize numbers along a left-to-right axis.
• The study highlights the crucial role of spatial coding in shaping numerical cognition and the MNL.