The scent of a skunk, the familiar face of a friend, or the sound of shattering glass become part of a code that makes each of these stimuli familiar and recognizable almost immediately with each subsequent experience.
Glenn Turner, an associate professor at Cold Spring Harbor Laboratory, explores how one of the research world’s favorite test subjects, the fruit fly, develops a memory for smells.
“We can use a bunch of different techniques to understand how neurons in a memory center respond to different smells,” he said. “One of the things we found is that those neurons have really specific responses.”
Neurons, or the wiring cells that carry signals and affect everything from telling muscles to move to processing thoughts or activities, have such a highly specific signature that Turner can “easily tell the difference between when a fly smells a grapefruit and when it smells a lime.”
There is not, however, a universal signal in the brain of every fly, rat, monkey, or dolphin for certain senses. Each creature has a unique response to a sensory cue.
Turner said this is akin to a human’s response to the odor of durian, a fruit that smells like sweat socks. To those people who eat the fruit, the smell is appealing because they like the taste. Many people, however, react to the scent in the same way they would if they walked into an airless locker room after an overtime basketball game.
Thomas Clandinin, an associate professor of neurobiology at Stanford School of Medicine, who has known Turner for over two decades, described Turner’s work as “highly innovative.” Turner is “widely credited with being one of the people who made it possible to monitor neural activity in the fly, an approach that has really revolutionized fly neuroscience.”
Clandinin credits Turner with being “very good at teaching people how to use his approaches, magnifying the community impact.”
Recently, Turner conducted experiments in which he looked at the patterns of activity in the memory center of the fly’s brain to see if he could tell the difference between the neural signature of bananas and of apples. The goal was to see if the patterns were stimulus-specific so that they could account for the accuracy with which flies form memories.
The perceptual whole is “greater than the sum of the parts,” Turner said.
Fluorescence microscopy makes a protein in a cell increase in brightness whenever it is active. He used that technique to express individual neurons in a region of the fly’s brain called the mushroom body, to see what components of a smell cause the fly to react and interpret that smell.
“We were able to show that different individual synaptic sites have different tuning properties,” Turner said. “The only time the mushroom body signaled to its downstream partners was when several inputs were active at the same time.”
Flies have several input sites that are waiting for a smell to come along that they can process and understand. If the majority of the parts of a smell are consistent, the fly will process the chemicals in the air as that scent. If not, however, the fly may not react.
Another step in his research, he said, could be to determine what neurological changes occur in response to some learned connection. If, for example, a fly often smells the noxious fumes of paint thinner when he eats, he might process that signal in a way that’s different from a fly that is looking for a more conventional smell in the search for food.
One of the reasons the fly is an appealing subject for these studies is that there are mutant versions that don’t form memories. By studying these mutants, researchers like Turner can compare the neurological circuit of a fly that recognizes the smell of a banana from one that can’t.
Turner and his wife Lorraine Lew, a nutritionist who works at a renal dialysis clinic, got married last November. Turner enjoys hiking behind the Cold Spring Harbor Laboratory library and biking.
As for his research, Turner hopes his basic understanding of the way a fly interprets the information around it will have applications to other creatures.
“I would hope that one day, we will get a more concrete understanding of learning and memory,” he said.