Action Primes or Not? Jax et al. (2007) vs. Cant et al. (2005)

With three experiments done in an object avoidance paradigm, Jax et al. (2007) reported that reaching paths were biased (or primed) by preceding trials in terms of path curvature, and argues that the lack of such priming effect in Cant et al. (2005) as well as Garofeanu et al. (2004) was due to the usage of the dependent variable RT , a less sensitive measure to the action priming. Jax et al. went on to argue that, contrary to the short-live motor representations of the dorsal stream proposed by Goodale and Milner, the dorsal stream of visual system DOES have memory about "the length of the axis orthogonal to the direct movement path".

There are, however, several reasons why Jax et al. did not observe action priming. First, Jax's participant reached for virtual targets instead of real ones. Reaching in virtual reality without actual sensorimotor feedback via contacting the real object may involve relatively more ventral stream, memory-based processes. Secondly, Jax et al. based their arguments on reaching path in the face of obstacles, while Cant et al. and Garofeanu et al. tested RTs in grasping. Perhaps these two types of actions inherently resort on different neural computations. For example, grasping requires computing not only the path of reaching but also the orientation and the size of the object. Because the latter is more complicated than the former, it might be more difficult to maintain the motor plan for grasping. Finally, whereas Cant et al. used a "cue-target" paradigm in which an action cue is presented temporally closer to the target to be grasped, Jax et al. used a "target-target" paradigm in which there are longer durations between reaches. The longer temporal separation may drive the participants in Jax et al. rely on memory and expectation, which is more prone to previous performed actions.

In sum, I don't think Jax et al. effectively offer evidence for the existence of action priming, at least not for actions resemble those would be carried out in real life.