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August 08, 2011

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Brain’s map of space may fall flat when it comes to height

Aug. 8, 2011
Courtesy of University College London
and World Science staff

An­i­mal’s brains are only roughly aware of how high up they are in space, mean­ing that in terms of al­ti­tude the brain’s “map” of space is sur­pris­ingly flat, ac­cord­ing to new re­search.

In a study pub­lished on­line Aug. 7 in the jour­nal Na­ture Neu­ro­sci­ence, sci­en­tists stud­ied cells in or near a part of the brain called the hip­po­cam­pus, which forms the brain’s map of space. The goal was to see wheth­er these cells were ac­ti­vat­ed when rats climbed up­wards.

The study looked at two types of cells known to be in­volved in the brain’s rep­re­senta­t­ion of space: grid cells, which meas­ure dis­tance, and place cells, which in­di­cate loca­t­ion. Sci­en­tists found that only place cells were sen­si­tive to the an­i­mal mov­ing up­wards in al­ti­tude, and even then only weakly so.

“The im­plica­t­ion is that our in­ter­nal sense of space is ac­tu­ally rath­er flat – we are very sen­si­tive to where we are in hor­i­zon­tal space but only vaguely aware of how high we are,” said Kate Jef­fery of Uni­vers­ity Col­lege Lon­don, lead au­thor of the stu­dy. “This find­ing is sur­pris­ing and it has im­plica­t­ions for situa­t­ions in which peo­ple have to move freely in all three di­men­sions – di­vers, pi­lots and as­tro­nauts, for ex­am­ple. It al­so raises the ques­tion – if our map of space is flat, then how do we nav­i­gate through com­plex en­vi­ron­ments so ef­fec­tive­ly?”

How the hip­po­cam­pus makes its map of space is fairly well un­der­stood for flat en­vi­ron­ments. But adding a third di­men­sion to the two hor­i­zon­tal ones makes things much more com­pli­cat­ed, and it is not clear how – or even if – the brain can en­code this. The sci­en­tists stud­ied neu­rons, or brain cells, known as grid cells. These be­come ac­tive per­i­od­ic­ally and at very reg­u­lar dis­tances as an­i­mals walk around, form­ing a grid-like struc­ture of ac­ti­vity hot-spots. Pre­vi­ous work has found that grid cells are largely con­cerned with mark­ing out dis­tances.

In the stu­dy, rats walked not just on flat ground but al­so on pegs on a climb­ing wall, or else on a spir­al stair­case, so that the rats moved not only hor­i­zon­tally but al­so ver­tic­ally. In­ter­est­ing­, the grid cells still kept track of hor­i­zon­tal dis­tance but did­n’t meas­ure out ver­tical ones. It seems as if grid cells do not “know” how high they are, the re­search­ers said.

The sci­en­tists al­so ex­am­ined an­oth­er type of neu­ron known as place cells. Place cells, found in the hip­po­cam­pus, pro­duce sin­gle ac­ti­vity hotspots in the en­vi­ron­ment and seem to func­tion to en­code spe­cif­ic places. These neu­rons were only weakly sen­si­tive to height too – but they did show some re­spon­sive­ness, sug­gest­ing they re­ceived in­forma­t­ion about height from some oth­er, pos­sibly non-spe­cif­ic, source.

“It looks like the brain’s knowl­edge of height in space is not as de­tailed as its in­forma­t­ion about hor­i­zon­tal dis­tance, which is very spe­cif­ic,” Jef­fery said. “It’s per­haps akin to know­ing that you are “very high” ver­sus “a lit­tle bit high” rath­er than know­ing ex­act height.”

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Animal’s brains are only roughly aware of how high up they are in space, meaning that in terms of altitude the brain’s “map” of space is surprisingly flat, according to new research. In a study published online Aug. 7 in the journal Nature Neuroscience, scientists studied cells in or near a part of the brain called the hippocampus, which forms the brain’s map of space. The goal was to see whether these cells were activated when rats climbed upwards. The study looked at two types of cells known to be involved in the brain’s representation of space: grid cells, which measure distance, and place cells, which indicate location. Scientists found that only place cells were sensitive to the animal moving upwards in altitude, and even then only weakly so. “The implication is that our internal sense of space is actually rather flat – we are very sensitive to where we are in horizontal space but only vaguely aware of how high we are,” said Kate Jeffery of University College London, lead author of the study. “This finding is surprising and it has implications for situations in which people have to move freely in all three dimensions – divers, pilots and astronauts, for example. It also raises the question – if our map of space is flat, then how do we navigate through complex environments so effectively?” How the hippocampus makes its map of space is fairly well understood for flat environments. But adding a third dimension to the two horizontal ones makes things very much more complicated for a map, and it is not clear how – or even if – the brain can encode this. The scientists studied neurons, or brain cells, known as grid cells. These become active periodically and at very regular distances as animals walk around, forming a grid-like structure of activity hot-spots. Previous work has found that grid cells are largely concerned with marking out distances. In the study, rats walked not just on flat ground but also on pegs on a climbing wall, or else on a spiral staircase, so that the rats moved not only horizontally but also vertically. Interestingly, the grid cells still kept track of horizontal distance but didn’t measure out vertical ones. It seems as if grid cells do not “know” how high they are, the researchers said. The scientists also examined another type of neuron known as place cells. Place cells, found in the hippocampus, produce single activity hotspots in the environment and seem to function to encode specific places. These neurons were only weakly sensitive to height too – but they did show some responsiveness, suggesting they received information about height from some other, possibly non-specific, source. “It looks like the brain’s knowledge of height in space is not as detailed as its information about horizontal distance, which is very specific,” Jeffery said. “It’s perhaps akin to knowing that you are “very high” versus “a little bit high” rather than knowing exact height.”