Striatum Neurons Helped Switch Attention Between Stimuli
Researchers have shown that fast-firing striatum neurons play an important role in switching attention between stimuli and selecting relevant signals. Some of the neurons of this type in the experiment with rhesus macaques were activated when the stimuli were replaced, and some responded to the appearance of distracting signals, the response to which had to be suppressed. The article is published in the proceedings of the National Academy of Sciences.
Every second our sense organs receive information about various stimuli. Adaptive behavior consists in not reacting to everything in a row but highlighting events that are important at the moment, skipping the rest. This function is attributed to the front of the striatum, which receives projections from various areas of the brain that are responsible for choosing behavioral goals and evaluating rewards. However, it is not clear exactly how the striatum neurons process this information, increase the activity of relevant stimuli and suppress unnecessary ones.
Scientists from Canada and the United States, led by Thilo Womelsdorf of Vanderbilt University, studied the activity of neurons in the striated body of two rhesus macaques in the following paradigm. Two striped circles appeared on the screen in front of the monkeys, one with an up arrow and the other with a down arrow. Either before or after the arrows appeared, the circles became colored (red and green). The color indicated which circle was the signal for action: if the "active" circle was green and an up arrow appeared on the green circle, then the monkey had to raise its eyes — for this it received a reward.
After 30 attempts or more, the color values changed — red became active, and then green again. Each time, the macaques needed to relearn how to respond to the correct color. At this time, researchers recorded the electrical activity of 350 neurons in the anterior striatum. According to the characteristics of impulses, these neurons were divided into two types: fast-discharge interneurons and spike projection neurons.
Neurons in the striated body responded to the appearance of color (it suggested which circle to focus on), but not the arrow (a signal to act). Fast — firing neurons were the first to respond-some of them increased their activity immediately after the circles turned green and red, and some of them, on the contrary, reduced the number of impulses. Spiked projection neurons responded second, after the first class of rapidly discharging cells ceased activity.
After changing the color values, the proportion of correct responses (eye movements) first decreased sharply, and then gradually recovered — so the monkeys retrained to pay attention to the correct stimulus. And the frequency of discharges of neurons in the striatum was inversely correlated with the number of correct responses — at first, these cells were highly active, but as they retrained, they decreased the frequency of impulses.
Two opposite processes can be involved in selecting an active stimulus: activating attention to the desired object and suppressing activity in response to an irrelevant stimulus. To find out which of these two tasks the striatum neurons perform, the researchers looked at their activity in response to "distracting stimuli" — the temporary disappearance of an inactive color that signaled that there would be no reward in this attempt, and no need to look up or down. In these cases, it was necessary to pay attention to the inactive color and suppress the action.
Most striatum neurons responded only to action stimuli, but one of the subgroups of fast-firing neurons was activated when a distracting stimulus was presented. This means that different cells of the striatum are responsible for the opposite processes: suppression of irrelevant stimuli and activation of the response to the desired signals.
The researchers conclude that fast-firing neurons play an important role in redirecting attention to important stimuli, including in a constant change of reinforced signals.
Disorders of the striatum are considered a key factor in the development of schizophrenia. Scientists have even learned to diagnose this disorder by the activity of the striatum with an accuracy of more than 80 percent.