"The main achievement was the fact that we used the inhibition of the inhibitory neurons" to stimulate the memory patterns, says physicist Eshel Ben-Jacob, senior author of a paper on the findings published in the May issue of Physical Review E. "We probably made [the cell culture] trigger the collective mode of activity that [is] possible."
The results, Ben-Jacob says, set the stage for the creation of a neuromemory chip that could be paired with computer hardware to create cyborglike machines capable of such tasks as detecting dangerous toxins in the air, allowing the blind to see or helping someone who is paralyzed regain some if not all muscle use.
Ben-Jacob points out that previous attempts to develop memories on brain cell cultures (neurons along with their supporting and insulating glial cells) have often involved stimulating the synapses (nerve cell connections). So-called excitatory neurons, which amplify brain activity, account for nearly 80 percent of the neurons in the brain; inhibitory neurons, which dampen activity, make up the remaining 20 percent. Stimulating excitatory cells with chemicals or electric pulses causes them to fire, or send electrical signals of their own to neighboring neurons.
According to Ben-Jacob, previous attempts to trigger the cells to create a repeating pattern of signals sent from neuron to neuron in a population—which neuroscientists believe constitutes the formation of a memory in the context of performing a task—focused on excitatory neurons. These experiments were flawed because they resulted in randomly escalated activity that does not mimic what occurs when new information is learned.
This time, Ben-Jacob and graduate student Itay Baruchi, who led the study, targeted inhibitory neurons to try to bring some order to their neural network. They mounted the cell culture on a polymer panel studded with electrodes, which enabled Ben-Jacob and Baruchi to monitor the patterns created by firing neurons. All of the cells on the electrode array came from the cortex, the outermost layer of the brain known for its role in memory formation.
Initially, when a group of neurons is clustered in a network, merely linking them will cause a spontaneous pattern of activity. Ben-Jacob and Baruchi sought to imprint a memory by injecting a chemical suppressor into a synapse between inhibitory neurons. Their goal: to disrupt the restrictive function of those cells, essentially causing the brakes they put on the excitatory members in the network to loosen. "This is like teaching by liberation," Ben-Jacob says. "We liberate the excitatory neurons to do what they want to do."
The pair chemically treated inhibitory neurons by injecting them with droplets of picrotoxin, an antagonist of gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the brain. The chemical suppression of the inhibitory neuron created a pattern kicked off by a neighboring excitatory neuron that was now free to fire. Other neurons in the culture began to fire one by one as they received an electrical signal from one of their neighbors. This continued in the same pattern, which repeated for over a day. This new sequence of activity coexisted with the electrical pattern that was spontaneously generated when the neural culture was initially linked.
A day later, they imprinted a third pattern starting at a different inhibitory synapse. Again, it was able to coexist with the other motifs. "The surprising thing is it doesn't affect the other patterns that the network had before," Ben-Jacob says.
The bottom line, the authors wrote: "these findings hint chemical signaling mechanisms might play a crucial role in memory and learning in task-performing in vivo networks."