Smells, great and gross, reshape the brain
The study focused on locusts because their nervous systems are smaller, and thus likely to reveal their secrets sooner than those of their vertebrate counterparts. (Credit: Steven Nichols / Flickr)
CALTECH (US) — Based on a new study with locusts, researchers better understand how the brain adapts to remember new and specific smells.
The California Institute of Technology (Caltech) study focuses on a key feature of human and animal brains—that they are adaptive. They are able to change their structure and function based on input from the environment and on the potential associations, or consequences, of that input.
“Although these results were obtained from experiments with insects, the components of the mechanism exist also in vertebrate, including mammalian, brains which means that what we describe may be of wide applicability,” says Stijn Cassenaer, senior research fellow in brain circuitry at Caltech and lead author of a paper—published in the journal Nature —that outlines the findings.
The study focused on insects because their nervous systems are smaller, and thus likely to reveal their secrets sooner than those of their vertebrate counterparts.
To home in on sensory memories, the researchers concentrated on olfaction, or the sense of smell. When a person encounters a favorite food or the perfume of a loved one, she will typically experience a recall, usually positive, based on the memories evoked by those smells.
Such a recall—to a smell, sound, taste, or any other sensory stimulus—is evidence of “associative” learning, says Gilles Laurent, a former professor of biology at Caltech and senior author of the study, as learning often means assigning a value, such as beneficial or not, to inputs that were until then neutral.
The original, neutral stimulus acquires significance as a result of being paired, or associated, with a reinforcing reward or punishment—in this case, the pleasant emotion recalled by a smell.
“When we learn that a particular sensory stimulus predicts a reward, there is general agreement that this knowledge is stored by changing the connections between particular neurons,” explains Cassenaer.
The problem, however, is that the biological signals that represent value (positive or negative) are broadcast nonspecifically throughout the brain. How then, are they assigned specifically to particular connections, so that a certain sensory input, until then neutral, acquires its new, predictive value?
“In this study, we carried out experiments to investigate how the brain identifies exactly which connections, out of an enormously large number of possibilities, should be changed to store the memory of a specific association.”
To get a closer look at these connections, Cassenaer and Laurent—who is now director at the Max Planck Institute for Brain Research in Germany—measured neural activity in an area of the locust brain where olfactory memories are thought to be stored.
They found that what allows the brain to identify which synapses should be modified, and thus where the nonspecific reward signal should act, is a very transient synchronization between pairs of connected neurons.
“When pairs of connected neurons fire in quick succession, the strength of their connection can be altered. This phenomenon, called spike-timing dependent plasticity, has been known for many years. What is new, however, is recognizing that it also makes these connections sensitive to an internal signal released in response to a reward,” says Cassenaer.
“If no reward is encountered, the cells’ sensitivity fades. However, if the sensory stimulus is followed by a reward within a certain time window, then these connections are the only ones altered by the internal reward signal. All other connections remain unaffected.”
Laurent says that the molecular underpinnings of this phenomenon, as well as the process by which the stored memories are later read out, are an area of much-needed exploration.
“We are currently developing the necessary tools to examine this with sufficient specificity, which will allow us to evaluate animals’ behavior as they learn,” says Cassenaer.
The study was funded by the Lawrence Hanson Chair at Caltech, the National Institutes on Deafness and other Communication Disorders, Caltech’s Broad Fellows Program, the Office of Naval Research, and the Max Planck Society.
Now researchers from Hungary and Sweden claim to have solved the mystery.
The stripes, they say, came about to keep away blood-sucking flies.
They report in the Journal of Experimental Biology that this pattern of narrow stripes makes zebras "unattractive" to the flies.
They key to this effect is in how the striped patterns reflect light.
"We started off studying horses with black, brown or white coats," explained Susanne Akesson from Lund University, a member of the international research team that carried out the study.
"We found that in the black and brown horses, we get horizontally polarised light." This effect made the dark-coloured horses very attractive to flies.
It means that the light that bounces off the horse's dark coat - and travels in waves to the eyes of a hungry fly - moves along a horizontal plane, like a snake slithering along with its body flat to the floor.
Dr Akesson and her colleagues found that horseflies, or tabanids, were very attracted by these "flat" waves of light.
"From a white coat, you get unpolarised light [reflected]," she explained. Unpolarised light waves travel along any and every plane, and are much less attractive to flies. As a result, white-coated horses are much less troubled by horseflies than their dark-coloured relatives.
Having discovered the flies' preference for dark coats, the team then became interested in zebras. They wanted to know what kind of light would bounce off the striped body of a zebra, and how this would affect the biting flies that are a horse's most irritating enemy.
"We created an experimental set-up where we painted the different patterns onto boards," Dr Akesson told BBC Nature.
Coloured images revealed how light was polarised as it bounced off a zebra's coat
She and her colleagues placed a blackboard, a whiteboard, and several boards with stripes of varying widths into one of the fields of a horse farm in rural Hungary.
"We put insect glue on the boards and counted the number of flies that each one attracted," she explained.
The striped board that was the closest match to the actual pattern of a zebra's coat attracted by far the fewest flies, "even less than the white boards that were reflecting unpolarised light," Dr Akesson said.
"That was a surprise because, in a striped pattern, you still have these dark areas that are reflecting horizontally polarised light.
"But the narrower (and more zebra-like) the stripes, the less attractive they were to the flies."
To test horseflies' reaction to a more realistic 3-D target, the team put four life-size "sticky horse models " into the field - one brown, one black, one white and one black-and-white striped, like a zebra.
The researchers collected the trapped flies every two days, and found that the zebra-striped horse model attracted the fewest.
Horseflies prevent the animals they bite from grazing, as well as carrying blood-borne diseases
Prof Matthew Cobb, an evolutionary biologist from the University of Manchester pointed out that the experiment was "rigorous and fascinating" but did not exclude the other hypotheses about the origin of zebras' stripes.
"Above all, for this explanation to be true, the authors would have to show that tabanid fly bites are a major selection pressure on zebras, but not on horses and donkeys found elsewhere in the world... none of which are stripy," he told BBC Nature.
"[They] recognise this in their study, and my hunch is that there is not a single explanation and that many factors are involved in the zebra's stripes.