This is a hypothesis that recently came to my mind but I was not able to quickly prove or reject it with a Google search.
=> Body-brain mass is highly correlated on a logarithmic scale
=> Bigger bodies have a higher inertia than smaller ones
=> Bigger brains have potentially more neurons and on average longer conduction delays
My question is: Is one reason for the correlation between body- and brain mass that more neurons and longer conduction delays are needed to computationally incorporate the inertia of mass during motor sequences (which results in temporally extended sequences?) More neurons, for example, could be needed to implement a finer acceleration and deceleration of body parts. Longer conduction delays could contribute to a better timing in learning motor sequences in bigger bodies.
If this is true, this relationship would point to a very general principle of learning and encoding motor sequences that would require an evolutionary perspective to understand certain brain functions.
And yes, of course I know that there are other factors as well determining the body-brain mass ratio :)
Hi Martin: There is a rich literature on brain to body mass ratios with various formulations used in an attempt to predict relative cognitive ability across species while accounting variances in body mass. These include the encephalization quotient [Jerison, 1973] and the neocortex ratio (see Byrne and Corp, 2004). For example, the neocortex ratio is used to compare the ratio of the size of the neocortex to the rest of the brain. This approach assumes that the denominator (i.e., the rest of the brain) reflects general sensory-motor trafficking and provides a standard for assessing excess cognitive processing. However, in an interesting paper, Deaner et al. (2007) found that overall brain size and not the encephalization quotient better predicts cognitive ability across non-human primates.
Below is a link to the paper. The article will likely lead you to other research that addresses your question.
Reference:
http://www.karger.com/Article/Pdf/102973
Brain Behav Evol 2007;70;115–124
Hello Brian,
thank you for your response and the references. These are very interesting articles. However it turns out to be very difficult to find something specific about brain to body mass ratio and inertia. Only (Berridge 1990) seems to touch this topic as noted here
http://books.google.de/books?id=4RGgYgPU7L0C&pg=PA170&lpg=PA170&dq=body+brain+allometry+inertia&source=bl&ots=mzphATpFiF&sig=7gGtL0ANIZQespUEYjwQArkZVWo&hl=de&sa=X&ei=6uDvUfbSHuPL4ASg3IDADw&redir_esc=y#v=onepage&q=body%20brain%20allometry%20inertia&f=false
But given the rich literature about body-brain mass ratio and other aspects of behaviour I conclude that inertia has possibly very little influence on brain mass.
Hi Martin,
Yes, you are correct that there is not much in the literature regarding body-brain mass ratio in regards to processing of inertia; however, I tend think of inertial load as being lumped in with overall sensorimotor processing and trafficking. You might want to also consider the role of the spinal cord in processing sensory/proprioceptive stimuli independently of the brain. That is the spinal neurons receive sensory input, such as gait, and can independently process and send motor signals in anticipation of the inertial movement. This would free up the brain (reduce size) and the brain would only be involved in interpreting the action after the fact.
For more on this concept, see the book chapter lined below entitled:
"Sensory Information Monitored and Processed by the Spinal Cord"
in Spinal Cord Medicine: Principles and Practice. Lin VW, Cardenas DD, Cutter NC, et al., editors. New York: Demos Medical Publishing; 2003.
Pubmed URL: http://www.ncbi.nlm.nih.gov/books/NBK9085/
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