brain

Scientific American July 2011 - Page 36 - The limits of intelligence - the laws of physics may create limits that prevent the human brain from continuing to evolve into a more powerful thinking machine.

• Neurons are limited by their nature, and the statistically noisy chemical exchanges
• "Information, noise and energy are inextricably linked by a connection that exists at the thermodynamic level," says Simon Laughlin, University of Cambridge theoretical neuroscientist
• Vijay Balasubramanian, U. of Pennsylvania physicist who studies neural coding of information, says he has never seen the idea of a thermodynamic limit on neuron-based intelligence even considered in science fiction, let alone in real science
• Increasing brain size - hits a limit of slowness and energy-hunger
• Increasing brain wiring - takes up space
• Making neurons thinner - again has thermodynamic limits similar to transistors in computer chips - also noise issues
• Relative size of brains
  • Cow brain more than 100 times larger than a mouse, but no smarter
    • Larger body seems to require more work to process input from more tactile nerves, larger retinas, and controlling more muscle fibers
• Eugene Dubois - Dutch anatomist who discovered skull of "homo erectus" in Java in 1892 - tried to estimate intelligence by size of fossil skulls
• the encephalization quotient - compares species brain mass with what is predicted based on body mass
  • Humans - 7.5x
  • Bottlenose dolphins - 5.3
  • Monkeys as high as 4.8
  • Oxen - 0.5
• Neurons in larger brains dont have to fire as often because there are more pathways
• Human brain is 2% of body weight, but takes 20% of calories. Newborn brains use 65% of calories
  • Cortex - 80% of energy consumption is communications
• "Cortical gray matter neurons are working with axons that are pretty close to the physical limit" - Simon Laughlin
• Gustav Adolph Guldberg - in 1880's described 5 hour process to extract intact whale brains
• Larger brains have larger nerve cells, that pack less densely, with longer axons, and thicker to maintain speed (thick nerve axons carry faster signals)
  • larger brains divide into more distinctive areas, corresponding with functions, and varieties of reason
• Mark Changizi, 2AI Labs, Boise Idaho, theoretical neurobiologist claims specialization is done to compensate for connectivity problems in larger brains.
• Jan Karbowski, Polish Academy of Science, computational neuroscientist - "Somehow brains have to optimize several parameters simultaneously, and there must be trade-offs. If you want to improve one thing, you screw up something else."
• Larger brains spend more time in wiring (white matter) than in grey matter (computations)
• Jon H. Kaas, Vanderbilt University neuroscientist
  • Primates pack their brains better, the neurons dont enlarge so much as brain increases in size. Rodents only increase by 60% for similar size increase.
  • Human - 100 billion neurons in 1.4KG of brain - equivalent scaling of rat brain would be 45kg
    • Large rodents dont seem to be smarter
• Gerhard Roth, Urusula Dicke, University of Bremen in Germany - "The only tight correlation with intelligence is in the number of neurons in the cortex, plus the speed of neuronal activity"
• Brightest people seem to have the quickest lines of communication between their brain areas
  • magnetoencephalographic studies show those with the most direct communications and fastest neural chatter have the best working memory (ability to hold several numbers in memory at once)
• Unreliability of proteins used to generate neural electrical pulse are unreliable
  • According to Laughlin - "When axons get down to 150 to 200 nanometers in diameter, they become impossibly noisy"
    • Brains smallest axons may generate 6 false signals a second, but a little bit smaller, and it would be 100 per second
    • For comparison - current transistors are 22 nanometers, and it is challenging to uniformly dope silicon. At 10 nm, random presence or absence of boron will cause unpredictable operation
• Brains of widely different animals such as honeybee, octopus, crow and intelligent mammals do not look alike - but the circuitry for tasks such as vision, smell, nabigation, and episodic memory are "all have absolutely the same basic arrangement."

Resources:

• Evolution of the brain and intelligence - by Gerhard Roth and Ursula Dicke in Trends in Cognitive Sciences, vol 9, no. 5 pg 250-257, may 2005
• Cellular scaling rules for primate brains - Proceedings of National Academy of Sciences, USA, Vol 104, no 9, pages 3562-3567, Feb 27, 2007
• Efficiency of functional brain networks and intellectual performance - MArtjin P. van den Heuvel, Cornelis J. Stam, Rene S. Kahn, Hilleke E. Hulshoff Pol - Journal of Neuroscience, Vol 29, No. 23, Pages 7619-7624, June 10, 2009