The dimensionality of behavior after brain injury: Neither dogma nor artefact

A growing literature examines the dimensionality of behavioral deficits in neurological populations using principal component analysis and related methods. Two opposing views have emerged. On one side, behavioral variability after brain injury (e.g., stroke, brain tumors, neurodegeneration) appears intrinsically low-dimensional, often captured by a few components. On the other side, this apparent simplicity has been attributed to lesion anatomy, task impurity, and methodological biases that may underestimate true dimensionality. Here both perspectives are summarized into a domain-dependent reconciliation. Cognitive performance measured with standard neuropsychological batteries may appear low-dimensional partly because tasks share common demands (e.g., attention, language, executive control), and because clinical severity induces correlated deficits that diminish with milder impairment. In contrast, motor behavior may be genuinely low-dimensional, reflecting biomechanical synergies. Thus, dimensionality is not a fixed property but depends on two interacting factors: disease severity and behavioral domain, offering a more nuanced framework for interpreting post-injury cognition.