Complexity-and-the-Nervous-System[2].ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,Complexity and Nervous System,*,Complexity and the Nervous System,Teleology,History,Structure,Codes and Computation,Complexity and Nervous System,1.Teleology,Complexity and Nervous System,Sense through many different modalities by extracting relevant patterns,Control and coordinate movements,Form memories,Construct models of the world and its dynamics,Brains functions,Complexity and Nervous System,2.History,Complexity and Nervous System,How does an evolutionary perspectivehelp explain brain complexity?,The first is based on the,concept of“evolvability.”,The second issue concerns the,relation between optimality and complexity,of brain design.,Complexity and Nervous System,1)The concept of“evolvability”,Todays species owe their existence to the ability of their ancestors to adapt and evolve.We can thus assume that,evolvability,must have given a selective advantage,to those organisms who had it in higher degree.,Complexity and Nervous System,What features favor evolvability,and do these features engender complexity?,Organismic compartmentalization,Redundancy,Linkages between regulatory processes,Component robustness,Complexity and Nervous System,Hence,the probability of,obtaining,through the vagaries of evolution,a brain,that does many things well with a single,pluripotent network must be very,low,.In contrast,the probability of evolving brains with separated subsystemssome for,controlling basic functions,such as respiration,threat detection,and nursing and others for more subtle functions such as exploratory behavior or memory of placesmust be,greater,.,Complexity and Nervous System,It is therefore,reasonable,to assume that such,indirect pressures,should lead to systems replete withs pecialized circuits,parallel pathways,and redundant mechanisms.,Complexity and Nervous System,2,)relation between optimality and complexity,of brain design.,simple designs,BUT,high efficiency,Complexity and Nervous System,Insects use very small external lenses for optics.To obtain a large field of view,insectsjuxtapose many(1000 to 30,000)lenses as well as their associated neural machinery(ommatidia).,This has,advantages,:An image is constructed from a mosaic of pixels,and no optical pinhole is needed.The eye can be small and yet cover a wide visual field.However,this,comes at a price,:Tiny lenses are more severely limited by diffraction than larger ones.,Because neighboring lenses focus light from overlapping parts of the world,two specific photoreceptors in neighboring ommatidia will catch photons from the,same source.,To exploit this feature,the axons of these two photoreceptors converge to the same secondorder neurons so as to increase efficiency.Conversely,the six photoreceptors within one ommatidium diverge to,six different second-order cells,(Fig.1A)(5).,Complexity and Nervous System,Circuit complexity in nervous systems.(A)Projections of photoreceptor axons from the retina(top)onto the lamina(bottom)of the flys neural superposition eye.?Note the organized divergence of the six main receptor axons within one retinal bundle and their precise rearrangement within six new laminar bundles.,To exploit this feature,the axons of these two photoreceptors converge to the same secondorder neurons so as to increase efficiency.Conversely,the six photoreceptors within one ommatidium diverge to,six different second-order cells,.,Complexity and Nervous System,As a consequence of this peculiar optimal design,the spatial mapping between consecutive neuronal processing stages needs to be more intertwined in a precise manner.,In,conclusion,evolution is a remarkable engineer,but imposes complexity,.,Complexity and Nervous System,3.Structure,Complexity and Nervous System,Brain complexity is reflected in the complexity of its structural makeup.,The genome of the worm,Caenorhabditis elegans,contains sequences for 80 different types of potassium-selective ion channels,90 ligand-gated receptors,and around 1000 G protein linked receptors.,Aplysia californica,a marine mollusk,expresses voltage-,dependent glutamate receptors whose actions underlie,at least partly,associative learning in mammalian cortex.,Complexity and Nervous System,Dendritic trees in,mollusks and insects,are as,profusely branched and varied as in a,primates,brain.The dynamics of the firing of a,lobsters,neurons are at least as rich as those in,mammalian,thalamus or neocortex.,Complexity and Nervous System,In,short,no brain,however small,is structurally simple.,Complexity and Nervous System,Recent theoretical work,exploring the complexity and dynamics of food-web networks in ecology and incorporating analog connection weights and nonlinear elements,might have applicability to neuroscience.,Complexity will be high,if a large number of,subassemblies,of varied sizes,can be formed,within the population.,Complexity and Nervous System,Given the nonstationary nature of neuronal activity and our limited ability to sample activity from more than a handful of neurons simultaneously,it remains an open challenge to apply this notion of complexity,to spike trains recorded from behaving animals.,Complexity and Nervous System,4.Codes and Computation,Complexity and Nervous System,The brain,most likely represents the world using neural assemblies,and,population codes could be more subtle.,Although some neurons,integrate inputs,regardless of their temporal structure,evidence,exists that the,relative timing of action potentials,matters,even allowing for combinatorial,spatiotemporal codes.,Complexity and Nervous System,One well-explored code is that found in the insect analog of the mammalian,olfactory bulb.,Complexity and Nervous System,In this system of about 1000 neurons,individual odors are represented by dynamical assemblies of about 100 neurons,so that,information useful,to downstream networks and to the animal,is decoded from both the identity,of the activated neurons,and the relative timing,of their activity.,Population,codes can thus be combinatorial in both space and time,adding several levels of complexity that are still not widely appreciated.,Complexity and Nervous System,The differences between brains and computers.,Brains:,Software and hardware are completely,interwoven,in brainsa neurons biophysical makeup is intrinsically linked to the computations it carries out(for instance,to detect temporal coincidence).,Brains,wire themselves up during development,as well as during adult life,by modifying,updating,replacing connections,and even in some circuits by generating new neurons.,While brains do indeed perform something akin to information processing,they,differ profoundly,from any existing computer in the scale of their intrinsic structural and dynamic complexity.,Computers:,Software and hardware can be easily,separated,.,Complexity and Nervous System,Complexity:A Useful Framework?,While everyone agrees that brains constitute the very embodiment of complex adaptive systems,nervous system complexity,remains hard to define,quantitatively or meaningfully.,Complexity and Nervous System,A bees brain for instance,with about,a million neurons,and stupendously intricate microcircuits,controls very elaborate behaviors.,Complexity and Nervous System,The,brain-to-body mass,ratio of sharks and rays is,very close to that of,mammals,and much greater than that of,bony fish,amphibians,or reptiles.,Complexity and Nervous System,The,size of the neocortex,in,toothed whales,normalized for its body size,ranks with that of,primates,above all other mammals.,Complexity and Nervous System,If forced,how would you,rank complexity,among these brains?,Complexity and Nervous System,For now,perhaps the most obvious thing to say about,brain function,from a“complex systems”perspective is that,continued reductionism and atomization will,probably not,on its own,lead to fundamental understanding,.,Complexity and Nervous System,Finally,what of any,possible link,between the complexity of nervous systems and those most,tantalizing of phenomena,consciousness,and,subjective experience,?,Complexity and Nervous System,Tononi and Edelman propose that the,existence and expression of consciousness is related to complexity,.,A positive correlation between complexity,as measured across large neuronal ensembles over a fraction of a second or longer,and consciousness is not implausible,.,Complexity and Nervous System,Alternatively,it is at this time,equally plausible,that,consciousness,arises out of a novel feature of,certain types of brains,some cellular or circuit property,with a unique molecular,anatomical,or,physiological signature,.,Complexity and Nervous System,Only time will tell if and how the mind arises out of“complex”brains.,Complexity and Nervous System,Thank you!,Complexity and Nervous System,