A new study offers new evidence of an effect that the brain often doesn’t realize: that a simple change within neurons can make them behave differently.
An article published Monday in Science by a team led by University of California, San Diego psychology professor and neuroscientist J. Andrew Napier examines how electrical stimulation of a neuron in the brain affects the behaviour of other neurons.
In certain circumstances, the brain has a “memory” of how that neuron responds to a given combination of electrical signals. It’s called “synaptic plasticity” and seems to enable the brain to adapt to changing situations much like how a human remembers how to drive a car. The ability to switch the activity of individual neurons is an essential part of learning to walk.
Napier and his colleagues have demonstrated this ability by stimulating one neuron using electricity in the form of an alternating current; the current induces changes in how the other neurons fire. The paper’s authors used this technique to look more specifically at how a nerve in the rat brain reacts to different combinations of electrical signals, which they call random-to-mixed-signal stimulation.
In this case, they stimulated one neuron at a time with alternating current pulses ranging along a spectrum of intensities from 7 to 36 volts. The rat neuron was stimulated with voltage changes ranging from 10 to 36 volts on one side of the neuron. Then the rat was replayed scenes from a “go” trial in which a rat was allowed to explore a cage before being rewarded. Some scenes were “random” — the rat was allowed to explore a cage and then another cage — while others were “mixed” — the rat was rewarded for exploring a single cage. In “mixed” condition, more of the “mixed” stimuli (the rat) were eliminated from the mix before being presented.
Mixed trials were less likely for both male and female rats, and the rat was more likely to explore each cage (more) during mixed trials, which the authors called “mixed-response” trials. When the same patterns of electrical stimulation were applied to a different brain region of the rat — in this case, a part of the medial forebrain bundle which connects limbic regions with the prefrontal lobe — the results were consistent with the idea that mixed-response stimulation results in the rat becoming more attracted to the “mixed” stimuli — the rats were significantly less fearful and wary of mixed-response trials.
They also noted, however, that the results did not indicate that the neurons in this area of the brain are more “attracted” to the mixed stimulus — it’s more likely that a change in electrical activity causes the neurons to be attracted to the mixed stimulus — rather than being “sensitized” to it.
“The brain is more likely to adjust its function to adapt to new experiences,” explained Napier. “This new stimulation effect does not directly alter neuronal activity, but has the effect of changing which neurons are activated and, thus, changing the way the brain integrates those inputs.”
It’s not clear exactly what this will mean for people, he said. “The finding suggests that mixed-response stimulation has important utility for studies of learning and memory,” he added. “For example, studies of the effect of different tasks on the brain have not focused on the effect of different tasks on the specific neurons in a brain region.”
In previous lab/animal experiments, Napier’s group had already shown that the brains of rats and mice respond in different ways to mixed-signal stimulation and to repeated stimulation. But these new results add to the evidence by showing that this effect also has biological effects on neurons in the animal brain.
“More importantly, the findings have significant implications for psychiatric disorders in which the brain is hypersensitive,” Napier said. “For example, people can suffer anxiety, depression and even psychosis when the brain is hypersensitive to novel situations and stimulation. For the first time, this work shows that it is possible to modulate the level of arousal in the brain, even while it is not the case that the brain needs to be exposed to any external stimuli in order to respond appropriately on a given day.”
The paper, “Random-To-Mixed-Response Stimulation Enhances Learning,” was co-authored by Jonathan S. Bloch of the UC San Diego Jacobs Center for Integrative Neuroscience, and Daniel J. Wexler, UC San Diego Department of Psychology.