If the previous posting on Neuroscience for Human Movement led you to this posting, you know why I want to talk about the limitations of human sensorimotor system. If not, I can summarize by saying that human nervous system demonstrates its amazing ability to predict and learn while trying to solve the problems related to the limitations of our default motor control system. What do I mean by the default motor control system? Basically, it is the system that allows the sensory receptors to feed information so that the central nervous system generates movement via muscles. The information regarding the consequence of the action is then collected by, again, the sensory receptors. The illustration below describes the process I just explained.
Seems like a pretty complete system, isn't it? Then what's wrong with this system? I should be clear that "default" motor control system is not an academically defined term. I basically made it up to conveniently describe the situation where the brain hasn't learned to make up for the limitations of each individual hardware within the system. I'll clarify below as to what I mean by the limitations of our hardware with some examples.
Limitation #1 - Modality-specific sensory information
Imagine that you are standing on a bus looking out the window. Your bus is not moving but the bus parked next to your bus starts moving forward all of a sudden. In this case, you may get a weird illusion that your bus is moving backward. I am sure many of you have experienced such an illusion at least once.
Why does this illusion occur and how do we snap out of this illusion so quickly? This illusion occurs because our vision alone does not inform our brain whether what we see is due to our motion or motion of the object. The brain has to integrate the information that comes from other sensory receptors such as vestibular apparatus (detects head motion) or somatosensory receptors (detects bodily motion and joint position) to get the complete picture of what is going on. We can also appreciate that the brain has to not only integrate various sensory information but also prioritize the most useful information for the context we are in. This is often called the problem of sensory integration and re-weighting (Carver et al., 2006; Bair et al., 2007). Based on the example I talked about, the sensory integration and re-weighting sound like a solution rather than a problem. Then why is it a problem? It is because integration itself has its own limitation which I will talk about in the next posting.
Stay tuned!
Reference
Carver, S., Kiemel, T., & Jeka, J. J. (2006). Modeling the dynamics of sensory reweighting. Biological cybernetics, 95(2), 123-134.
Bair, W. N., Kiemel, T., Jeka, J. J., & Clark, J. E. (2007). Development of multisensory reweighting for posture control in children. Experimental Brain Research, 183(4), 435-446.