Trying to get to the bottom of this enigma, the Salk researchers relied on naturally occurring activity patterns that were recorded in living animals from a part of the brain known as the hippocampus - a structure critical for memory formation and learning. They used these recorded patterns to stimulate isolated groups of neurons and measured which signals synapses transmitted to neighboring cells and which ones they dropped.
In the past, similar studies were commonly performed at room temperature. Since scientists had found that results were often too complex to interpret, the Salk researchers recorded data in warmer conditions, slightly below body temperature. "Intuitively, I recorded at physiological temperatures instead of room temperature and that turned out to be the key," remembers Klyachko. "I found that synaptic transmission is highly temperature-dependent."
From there it was only a small step to the discovery that the two major types of synapses, excitatory and inhibitory ones, that were previously thought to always work against each other, act in concert to identify patterns carrying relevant information in an incoming signal. Stevens explained: "Synapses recognize bursts of neuronal activity and turn up their strength, acting like a switch." As a result, meaningful patterns are amplified, while stray noise disappears into some sort of "synaptic abyss."
Until now, experimental evidence for a filtering function of synapses has been elusive. "Our work is the confirmation that everybody has been waiting for," explains Klyachko. "It is the precisely-tuned filtering properties of the two major types of synapses and their collaboration that makes the information processing reliable."