SCAN is the coolest!

@washuneurology.bsky.social

The advancements in brainstem imaging will make it possible to examine a host of clinically relevent structures like the eriaqueductal gray and substantia nigra in humans, something that has been very hard to do well previously

This work was supported by a lot of people including @drdamienfair.bsky.social @ndosenbach.bsky.social @roselynechauvin.bsky.social @gordonneuro.bsky.social @jarodroland.bsky.social @forrestwhiting.bsky.social @nasiametoki.bsky.social

We have a lot of supplemental information, including tests against partial volume effects that I really encourage people to check out. Functional imaging now allows for single voxel level high quality results, something that I thought was impossible when I started my post-doc.

We speculate that the human red nucleus is well positioned to implement goal-directed behavior by integrating behavioral valence (through salience network) with action plans (through action-mode network) instead of serving a pure motor-effector function.

Additionally, networks, a well-recognized dominant model of cortical organization, also appear to be an organizing principle as far down as the brainstem. This nicely highlights that networks aren’t really ‘cortical’ they are a system of whole brain organization.

Big takeaway: the brainstem is nowhere near as evolutionarily conserved as many believe. This contradicts a lot of neuroscience training (certainly mine). In fact, the brainstem has undergone very active selection over evolution. For more reading please see Joan Baizer tinyurl.com/mry5tvac

We next assumed that averaging over the red nucleus must have obscured motor-effector regions within the structure, so we tested if any voxels had primary connectivity to any of the 3 motor-effector networks. Shockingly, basically no voxels are preferentially motor-effector connected (triangles).

So where is the red nucleus functional connectivity within the precentral gyrus, something that you would predict from tract tracing? It is specific to @gordonneuro.bsky.social recently discovered somato-cognitive action network (SCAN) and not motor-effector regions.

Looking at task data from the HCP, we found that cues to indicate upcoming movement had a large effect in the red nucleus, larger than actual movement. Motor cues also greatly activate the action-mode network. Additionally, task analysis shows the red nucleus responds to rewarding stimuli.

Shockingly, red nucleus connectivity was virtually absent with motor effector networks (le-SMN, ue-SMN, and f-SMN). Instead, connectivity was largest to the salience network (involved in motivated behavior) and the action-mode network (a.k.a. cingulo-opercular network; involved in action).

If the red nucleus is a motor structure, it should 1) be connected to motor-effector networks (somatomotor hand/foot/mouth) and 2) respond to movement. To test this, we used high SNR data with multi-echo ICA (tinyurl.com/55vr747p; example noise component below) and group-averaged task/rest datasets

However, perspective pieces and a lot of literature still seemed to assume the red nucleus had to do something related to the control of muscles (either directly or indirectly) in the service of movement, making it a motor nucleus.

As walking changed from quadrupedal to bipedal, so has the red nucleus. The magnocellular red nucleus (projects to the spinal cord; shaded) has shrunken! But another division, the parvocellular red nucleus (shown white) expands tinyurl.com/2ctttmmw

Based on offhand commends during my graduate training, I remembered that the red nucleus was supposed to be involved in walking, because its emergence in evolution coincided with the use of limbs for walking (and also navigating shallow waters).

The red nucleus is a slightly pinkish structure (hence the name ‘red’) in the midbrain of the brainstem. Fortunately, it is easily visible on standard structural MRI images, especially T2 weighted.