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Paying Attention to Attention

All kids know that parents have "eyes in the back of their heads." John Reynolds, associate professor in the Systems Neurobiology Laboratory, post-doctoral researcher Jude Mitchell, and graduate student Kristy Sundberg are starting to understand how people and other higher animals manage to pay attention to certain objects while keeping an eye on others.

In their study, published in the journal Neuron, the researchers report two classes of brain cells with distinct roles in visual attention, and highlight at least two mechanisms by which these cells mediate attention.

In the experiments, animals learned how to play a sophisticated attention-demanding video game. Throughout the game, Reynolds and Mitchell measured electrical activity from individual neurons involved in mediating visual attention.

During the video game, moving objects appeared on a computer monitor, occasionally falling at a location that elicited a response from the neuron, in the form of a volley of electrical spikes known as action potentials. The researchers examined this neuronal response to see if it changed when the animal either ignored or attended to the stimulus. They found that neurons typically produced stronger electrical signals when the animal attended to the stimulus.

But they also noticed that different neurons produced different shaped action potentials: "broad spikes" or "narrow spikes." Other researchers had previously identified two different types of brain cells that produce these two waveforms. The most common neuron type, called pyramidal cells, produce broad spikes. These neurons carry signals between distant areas of the brain. The other type, called fast-spike interneurons, produce narrow spikes. These neurons only connect to nearby neurons.

By sorting the neurons according to the waveform they produced, the researchers discovered that attention had different effects on the two different types of brain cells. The narrow-spiking cells usually fired more rapidly when attention was directed to the object than when it was ignored. What's more, attention caused the stream of spikes produced by these neurons to be much more even-paced. Broad-spiking neurons, on the other hand, were less influenced by attention. Some fired faster, while others fired more slowly when attention was directed to the moving object.

This is the first study of attention to distinguish between different classes of neurons. By making these distinctions, Reynolds and Mitchell are beginning to piece together the biological underpinnings of attention. This will improve scientists' understanding of neurological diseases in which attention is impaired, such as schizophrenia and autism.