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September 05, 2009

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Cities work much like brains, study finds

Sept. 5, 2009
Courtesy Rens­se­laer Pol­y­tech­nic In­sti­tute
and World Science staff

Ci­ties are or­gan­ized much like brains, and evolve in a si­m­i­lar way, ac­cord­ing to a new stu­dy.

Just as ad­vanced brains re­quire a ro­bust net­work of cell con­nec­tions to achieve more com­plex thought, large cit­ies need ad­vanced high­ways and trans­porta­t­ion sys­tems to al­low larg­er and more pro­duc­tive popula­t­ions.

Ci­ties are or­gan­ized much like brains, and evolve in a si­m­i­lar way, ac­cord­ing to a new stu­dy. (Im­age: Rens­se­laer/Mark Chan­gi­zi)
The new re­search points to si­m­i­lar­i­ties in how larg­er brains and cit­ies deal with the prob­lem of main­tain­ing enough in­ter­con­nect­ed­ness.

En­vi­ron­men­tal pres­sures have “guided the ev­o­lu­tion of mam­ma­li­an brains through­out time, just as politi­cians and en­trepreneurs have in­di­rectly shaped the or­gan­iz­a­tion of cit­ies large and small,” said Mark Chan­gi­zi, a neuro­bi­ol­o­gist at Rens­se­laer Pol­y­tech­nic In­sti­tute in Rho­de Is­land, who led the stu­dy. 

“It seems both of these ‘in­vis­i­ble hands’ have ar­rived at a si­m­i­lar con­clu­sion: brains and cit­ies, as they grow larg­er, have to be si­m­i­larly densely in­ter­con­nect­ed to work well.

As brains grow more com­plex from one spe­cies to the next, they change in struc­ture and or­gan­iz­a­tion in or­der to achieve the right in­ter­con­nect­ed­ness. One could­n’t simply grow a double-sized dog brain, for ex­am­ple, and ex­pect it to work like a hu­man brain. This is be­cause, among oth­er things, a hu­man brain does­n’t merely have more cells; it al­so has, for each cell, more con­nec­tions to oth­er cells.

Much the same can be said of cit­ies, Chan­g­i­zi said. For in­stance, al­though Chi­ca­go is three times as big as Se­at­tle, one could­n’t put to­geth­er three Se­at­tles and ex­pect the re­sult to have the same in­ter­con­nect­ed­ness and ef­fi­cien­cy as Chi­ca­go. There would be too many high­ways with too few ex­its and too-narrow lanes. 

Changizi in­ves­t­i­gated how such infrastruc­tures scale up with re­spect to the size of brains and cit­ies. 

As both struc­tures evolve, he found, im­por­tant fea­tures grow at si­m­i­lar rates.

One such fea­ture is the num­ber of “con­nec­tors,” which in cit­ies, are high­ways, and in brains, are a type of closely-in­ter­con­nect­ed cell called py­ram­i­dal neu­rons. 

Anoth­er such fea­ture is the num­ber of ter­mi­nal points for these con­nec­tors: that is, high­way ex­its in cit­ies, and “sy­naps­es” in the brain. Synapses are points where ex­ten­sions of nerve cells branch off and make con­tact with oth­er nerve cells.

Both con­nec­tors and ter­mi­nals, Chan­g­izi ex­plained, be­come more nu­mer­ous at spe­cif­ic rates with re­spect to the ar­ea of the city, and ar­ea of the brain sur­face. The sur­face lay­er of the brain is the loca­t­ion of the all-im­por­tant ne­o­cor­tex, a brain struc­ture as­so­ci­at­ed with ad­vanced cog­ni­tive ca­pa­bil­i­ties in mam­mals.

For in­stance, Changizi said, as ar­ea in­creases, the num­ber of “con­nec­tors” con­sist­ently rises in pro­por­tion to that ar­ea, to the pow­er of three fourths. For you math buffs, that means the fourth root of the cu­be of the ar­ea.

For the num­ber of “ter­mi­nal points,” on the oth­er hand, one would re­place that three fourths with nine eighths.

These de­tails are less im­por­tant than the over­all point: that “when scal­ing up in size and func­tion, both cit­ies and brains seem to fol­low sim­i­lar” laws, Chan­g­izi said. “They have to ef­fi­ciently main­tain a fixed lev­el of con­nect­ed­ness, in­de­pend­ent of the phys­i­cal size of the brain or city.”

The find­ings are pub­lished this week in the re­search jour­nal Com­plex­ity.

* * *

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Cities are organized like brains, and evolve in a similar way, according to a new study. Just as advanced brains require a robust network of cell connections to achieve more complex thought, large cities need advanced highways and transportation systems to allow larger and more productive populations. The new research points to similarities in how larger brains and cities deal with the problem of maintaining enough interconnectedness. Environmental pressures have “guided the evolution of mammalian brains throughout time, just as politicians and entrepreneurs have indirectly shaped the organization of cities large and small,” said Mark Changizi, a neurobiologist at Rensselaer Rensselaer Polytechnic Institute in Rhode Island, who led the study. “It seems both of these ‘invisible hands’ have arrived at a similar conclusion: brains and cities, as they grow larger, have to be similarly densely interconnected” to work well. As brains grow more complex from one species to the next, they change in structure and organization in order to achieve the right interconnectedness. One couldn’t simply grow a double-sized dog brain, for example, and expect it to work like a human brain. This is because, among other things, a human brain doesn’t merely have more cells; it also has, for each cell, more connections to other cells. Much the same can be said of cities, Changizi said. For instance, although Chicago is three times as big as Seattle, one couldn’t put together three Seattles and expect the result to have the same interconnectedness and efficiency as Chicago. There would be too many highways with too few exits and too-narrow lanes. Changizi investigated how such infrastructures scale up with respect to the size of brains and cities. As both structures evolve, he found, important features grow at similar rates. One such feature is the number of “connectors,” which in cities, are highways, and in brains, are a type of closely-interconnected cells called pyramidal neurons. Another such feature is the number of terminal points for these connectors: that is, highway exits in cities, and “synapses” in the brain. Synapses are points where extensions of nerve cells branch off and make contact with other nerve cells. Both connectors and terminals, Changizi explained, become more numerous at specific rates with respect to the area of the city, and area of the brain surface. The surface layer of the brain is the location of the all-important neocortex, a brain structure associated with advanced cognitive capabilities in mammals. For instance, Changizi said, as area increases, the number of “connectors” consistently rises in proportion to that area, to the power of three fourths. For you math buffs, that means the fourth root of the cube of the area. For the number of “terminal points,” on the other hand, one would replace that three fourths with nine eighths. These details are less important than the overall point: that “when scaling up in size and function, both cities and brains seem to follow similar” laws, Changizi said. “They have to efficiently maintain a fixed level of connectedness, independent of the physical size of the brain or city.” The findings are published this week in the journal Complexity.