Felicia Tassone¹ (), Elisabeth Rounis^2,3,4, Fatemeh Geranmayeh², Matthew Banger², Alison McGregor², Erez Freud¹; ¹York University, ²Imperial College London, ³University of Cambridge, ⁴Chelsea and Westminster NHS Foundation Trust

Limb apraxia is a neurobehavioural consequence of stroke, characterized by impaired skilled action that cannot be attributed to basic motor impairments. It has traditionally been thought to selectively disrupt higher-level action representations while preserving basic visuomotor control. However, this view is supported largely by peak-derived metrics, such as maximum grip aperture (MGA), which may overlook potential deficits in the fine-grained spatiotemporal structure of movements. To address this gap, we assessed 17 patients with clinically-confirmed apraxia and 19 age-matched controls on an Efron-block width-scaling task (four objects, 5mm increments) involving reach-to-grasp movements (Experiment 1). We also examined 25 younger and 25 older healthy adults to determine whether any observed deficits were stroke-specific rather than age-related (Experiment 2). Consistent with previous findings, apraxic patients successfully scaled their MGA with object size, similar to controls. However, patients exhibited significantly slower movements and prolonged time to MGA. To assess the spatiotemporal structure of the movements, Dynamic Time Warping (DTW) was used to measure the similarity between trajectory pairs across object sizes. The observed similarity matrix was then correlated with an ideal object size–based model. Although both patients and controls exhibited strong correspondence with the size model, apraxic patients showed significantly weaker correspondence compared to controls, even when using their ipsilesional hands. Bayesian analysis of Experiment 2 supported the null hypothesis, revealing that young and older adults showed equivalent object size–based trajectory organization patterns. Furthermore, both healthy age groups demonstrated stronger correspondence with the size model than apraxic patients’ ipsilesional hands. These findings uncover novel deficits in apraxic reach-to-grasp behaviour. While object-related aperture scaling remains intact, the spatiotemporal organization of trajectories is disrupted: a deficit that is distinct from healthy aging, and an impairment previously under-characterized in apraxia.