Measurements and morphometric landmarks of the human spinal cord: A cadaveric study.

The topographical neuroanatomy of the human spinal cord (SC) is currently based on the adjacent vertebrae. This morphometric study sought to develop a dataset allowing for statistical analysis of human SC segment characteristics. Overall, 32 human SCs were dissected (18 female and 14 male donors), and individual SC segments were identified. Anterior and posterior lengths, thicknesses and widths were measured by two examiners. Statistical analyses included t-tests, as well as intraclass and Pearson's correlation coefficients. The SC length was significantly shorter in females than males. The cranial (C4) and caudal (T1/T2) limits of the cervical enlargement, along with its maximal width (C6-C7), were identified by combining widths and thicknesses of the segments. The thoracic region, T2 to T12, could be identified using segments widths and thicknesses values. The length of the lumbosacral region, from segments L2 to S5, was particularly stable, independently of SC length and sex. The lumbar enlargement was characterized by a thickness increase between the segments L2 and S1 which reached its maximum at the level of L3, L4, and L5, whereas the width was not significantly increased. From the S2 to S5 segments, width and thickness were equal, with both decreasing of 1 mm per segment. The morphometrical analysis of 32 human SCs provided a dataset allowing for statistical analysis of segmental measures with significant results. A combined approach mostly using widths and thicknesses provided landmarks of potential interest for the localization of SC segments in a clinical MRI setting.

Reward timing matters in motor learning.

Reward timing, that is, the delay after which reward is delivered following an action is known to strongly influence reinforcement learning. Here, we asked if reward timing could also modulate how people learn and consolidate new motor skills. In 60 healthy participants, we found that delaying reward delivery by a few seconds influenced motor learning. Indeed, training with a short reward delay (1 s) induced continuous improvements in performance, whereas a long reward delay (6 s) led to initially high learning rates that were followed by an early plateau in the learning curve and a lower performance at the end of training. Participants who learned the skill with a long reward delay also exhibited reduced overnight memory consolidation. Overall, our data show that reward timing affects the dynamics and consolidation of motor learning, a finding that could be exploited in future rehabilitation programs.

Reward boosts reinforcement-based motor learning.

Besides relying heavily on sensory and reinforcement feedback, motor skill learning may also depend on the level of motivation experienced during training. Yet, how motivation by reward modulates motor learning remains unclear. In 90 healthy subjects, we investigated the net effect of motivation by reward on motor learning while controlling for the sensory and reinforcement feedback received by the participants. Reward improved motor skill learning beyond performance-based reinforcement feedback. Importantly, the beneficial effect of reward involved a specific potentiation of reinforcement-related adjustments in motor commands, which concerned primarily the most relevant motor component for task success and persisted on the following day in the absence of reward. We propose that the long-lasting effects of motivation on motor learning may entail a form of associative learning resulting from the repetitive pairing of the reinforcement feedback and reward during training, a mechanism that may be exploited in future rehabilitation protocols.