Large-scale imaging shows mushroom body activity patterns are sparse and randomly distributed
Cold Spring Harbor, NY — We know from experience that particular smells are almost inseparable in our minds with memories, some vague and others very specific.聽The smell of just-baked bread may trigger an involuntary mental journey, even if for a moment, to childhood, or to a particular day during childhood. Or it may, more diffusely, remind someone of grandma.聽How are these associations forged in the brain and how do we remember them?
A research team at 黑料吃瓜资源 (CSHL) led by Assistant Professor Glenn Turner has published results of experiments that show large populations of neurons in the brains of living fruit flies responding to a variety of odors.
The results demonstrate that a portion of the fly brain known to be important in learning and memory responds in a characteristic fashion that helps explain how an association is made between an odor and an experience—the basis of a memory.
Turner and his CSHL colleagues were able to simultaneously monitor the activity of over 100 individual neurons in a portion of the fly brain called the mushroom body (MB).聽They did this by engineering a line of flies whose MB neurons expressed a gene that made the neurons emit a fluorescent calcium signal while firing.聽They could then record and closely scrutinize the patterns.
Previous research suggested that neuronal firing in the MB was 鈥渟parse.鈥� This follows from a general rule about how animals are thought to represent and remember odors. They have an abundance of receptors that detect odors, and these are 鈥渂roadly tuned,鈥� meaning they are designed to take in and process as many odors as possible, even those that are in chemical terms very similar.聽There鈥檚 plenty of overlap in their sensitivity.
The reverse condition is thought to apply 鈥渄eeper down,鈥� as raw signals move 鈥渋nward鈥� from receptors into the brain, where they are processed.聽In the brain, it is supposed that odor-processing neurons become much more specific, and therefore a single odor evokes a much more sparse pattern of activity in the deeper layers than it does among the 鈥渨ide-band鈥� receptors that capture the raw signals. Consequently, neurons deeper in the brain have relatively little overlap in their sensitivity.聽According to Turner, 鈥渢his could be useful for forming memories accurately.鈥�
In the research Turner鈥檚 team published Aug. 17 in the Journal of Neuroscience, they used the calcium-signaling method to clearly demonstrate what the theory proposes.聽They found that fly mushroom body neurons represent odors in consistently sparse fashion (see illustration), and also that the responding neurons were distributed randomly within the MB, and showed no sign of spatial organization.
The team found that fewer than 20% of the MB neurons will glow with a calcium signal when a specific odor is presented to the fly. Surprisingly, this does not vary with the intensity of the odor, or in proportion to whether the odor is that of a very simple or complex molecule; or, indeed, whether the odorant is naturally occurring or artificial.
Another remarkable finding of Turner鈥檚 team: while the constellation of cells that fire in response to, say, the odor of an orange, does not change in a single individual from trial to trial, the constellation is always different in another individual. That is to say, the pattern is not hard-wired and shared by individuals within the species.聽 Rather, says Turner, 鈥渨e find that the pattern of firing is entirely random, although consistently sparse.鈥�
The fact that there is a consistent pattern of firing explains the utility of sparseness to an individual fly: whatever that individual鈥檚 pattern of MB neuronal firing in response to the odor of orange, that pattern will repeat and therefore is something that can be remembered.聽A remembered pattern鈥攁 memory鈥攊s a basis for action, such as homing in on the source, or moving away from it, if the association is a negative one.聽鈥淭he fact that the patterns of cells activated by different smells are randomly scattered throughout this brain area means that it’s very rare that two patterns will look alike,鈥� Turner says, and this could be important for ensuring that memories of different smells are unique.
鈥漈he lack of specialization in this important part of the fly brain surprised us,鈥� Turner says. 鈥渇or instance, we showed that natural and artificial odors are not distinguishable from the fly鈥檚 point of view. And this leads us to propose that the mushroom body is a purely associative brain center.
鈥淥ur observations suggest the MB does not have one group of cells that fire when an odor is attractive and a different set when the odor is repulsive; or one group that fires when it smells something 鈥榝ruity鈥� and another when it smells something 鈥榞rassy.鈥� It seems the MB isn鈥檛 specialized at all.聽It responds to all types of odors similarly, and the responding cells are scattered throughout the MB randomly.鈥�
Read “Making Sense of Smell,” (pdf) about the how the Turner lab and other labs at CSHL are studying olfaction to learn more about how the brain works.
Written by: Peter Tarr, Senior Science Writer | [email protected] | 516-367-8455
Funding
This work was supported by a grant from the National Institutes of Health (NIH), the Crick-Clay Fellowship from the Watson School of Biological Sciences at 黑料吃瓜资源 and a predoctoral training grant from the National Institute of General Medical Sciences, NIH.
Citation
鈥淐ellular-Resolution Population Imaging Reveals Robust Sparse Coding in the Drosophila Mushroom Body鈥� appears in The Journal of Neuroscience August 17, 2011.聽The authors are: Kyle S. Honegger, Robert A.A. Campbell and Glenn C. Turner.聽the paper can be viewed online at: doi: 10.1523/鈥婮NEUROSCI.1099-11.2011.