Bayesian evidence for the neural dissociation between finger and hand imitation skills

For limb apraxia-a heterogeneous disorder of higher motor cognition following stroke-an enduring debate has arisen regarding the existence of dissociating neural correlates for finger and hand gestures in the left hemisphere. We re-assessed this question asking whether previous attempts analyzing pooled samples of patients with deficits in only one and patients with deficits in both imitation types might have led to systematically biased results. We conducted frequentist and Bayesian, voxelwise, and regionwise lesion symptom mappings on (i) the full sample (N = 96) in which all patients with hand and/or finger imitation apraxia as well as without apraxia were included and (ii) three sub-samples, which excluded those patients from the full sample showing isolated hand imitation deficits, isolated finger imitation deficits or shared (finger and hand) imitation deficits. Anatomical analyses revealed a cortical dissociation of finger imitation deficits (located more anteriorly) and of hand imitation deficits (located more posteriorly). The presence of patients with shared deficits did, indeed, dilute associations that appeared stronger in the respective isolated samples. Also, brain regions truly associated with hand imitation deficits showed a positive bias for finger imitation deficits, when the sample contained patients with shared deficits. In addition, our frequentist parameters uncovered that some of our Bayesian evidence supported reverse associations (damage protecting from rather than increasing the deficit). Anatomo-behavioral analyses that analyze patients with shared (hand and finger) and isolated (hand or finger) imitation deficits together in one sample do, indeed, lead to undesirable biases. This explains why some earlier studies failed to detect the apparent neural dissociation between hand and finger imitation deficits.