Error signals in the parvocellular division of the red nucleus, not the magnocellular division, drive adaptation in reaching.

We examined the distinctive roles of the parvocellular (RNp) and magnocellular (RNm) divisions of the red nucleus in motor control and learning. RNp and RNm neurons encoded information on motor error (ME) during the ongoing movement and/or visually apparent error (AE) after the movement ended. Microstimulation of RNp, but not of RNm, induced a gradual increase in error in the direction opposite to the preferred direction of AE and ME. Interestingly, RNm neurons, in particular, encoded information on AE from the preceding reaching movement before the next movement took place. These results indicate that the RNp provides visual and ME signals that drive trial-by-trial adaptation through modifications of the cerebellum via the inferior olivary nucleus. Meanwhile, the RNm may participate in online motor corrections based on ME signals or in pre-movement adjustments based on the previous AE, processes that are distinct from those of RNp.