Two people sitting at a sports bar watching a game may notice very different things around them. While one may see the couple next to him arguing, the other may see a small fire starting in the kitchen. How they direct their attention, whether consciously or not, could lead to important decisions: for example, whether to evacuate the bar or to keep watching the game.
In the past, researchers have investigated brain attention mechanisms in humans and animals with a variety of tools and approaches but, until now, have not had a fine-grained view of the processes that control attention. Now, scientists have revealed how neurons communicate to enable us to detect important events in our sensory-rich environments. The research could eventually inform everything from how we make decisions to treatments for ADHD and Alzheimer’s disease.
Publishing online last month in Nature, the research team described how paying attention to something strengthens the signaling between neurons in the cerebral cortex, boosting the ratio of signal to noise. They discovered these results by measuring neuron activity in macaques who were performing an attention-related cognitive task.
The researchers trained animals to look at a dot on a monitor and direct their attention to one of two visual stimuli presented a short distance from the dot. “When the visual stimulus at the attended location brightened slightly, animals moved a joystick to indicate they detected the change in the visual stimulus,” says the paper’s lead author Farran Briggs, of the Geisel School of Medicine at Dartmouth, who conducted the study in Marty Usrey’s lab at the University of California, Davis.
“Because animals learned to shift the location of their attention on alternating trials, we were able to compare neuronal activity on trials where attention was directed toward versus away from the visual stimulus that activated the recorded neurons, thereby establishing a measurement of the influence of attention on neuronal activity,” Briggs says. The researchers took measurements on each side of a synapse leading into the cerebral cortex to measure when neurons were firing, the strength of the signal, and the signal-to-noise ratio.
“In our study, we developed a high resolution assay for communication between connected neurons, and by combining this with cognitive methods, could provide new information about attention mechanisms,” says co-author Usrey. The interdisciplinary study is a great example of how collaborations can lead to new discoveries, says co-author Ron Mangun of the Center for Mind and Brain at the University of California, Davis. “Cognitive theory coupled with neuroscience methods opens the door on understanding the mechanisms of attention and other higher brain functions,” he says.
Mangun spoke with CNS about the study and its implications for human attention and attention-related disorders.
CNS: In the Nature paper, you describe training macaques to “covertly” focus their attention on drifting stimuli while you recorded neuron-level processes. Can you explain what you mean by “covert”?
Mangun: Covert attention means paying attention to something in the visual field without moving the eyes to look at it. The old saying – “I saw it out of the corner of my eye” – is sort of a reference to covert attention.
We all know that we do not have to look directly at something to pay attention to it—for example, when paying attention to the attractive person sitting across from you on the train, without looking directly at them because that would be rude. Or when driving, it can be dangerous to look away too long from the path you are traveling, so one learns to covertly attend to nearby cars, for example on the freeway without necessarily glancing at them.
CNS: Did you find any of the results surprising?
Mangun: One fascinating result was that attention influences the strength of signaling between neurons, even if those neurons show little or no changes in their firing rates, which is the typical sign of enhanced processing with attention. This means attention can operate on subtle mechanisms of information processing.
CNS: To what extent do your results (with macaques) mimic the way human attention works, or are there some limitations?
Mangun: They both behave similarly, and indeed, when well-trained, the monkeys can do the task with even greater efficiency than humans (although one never trains humans for months on such tasks). We believe the neural mechanisms that are used by humans evolved in primates over millions of years, and therefore it is likely we share similar mechanisms. But, one has to keep in mind that we did not evolve “from” macaques; they and we are both highly evolved primates from a common ancestor in the ancient past. So, it is entirely possible that they or we have specialized mechanisms the other does not possess in the same way. That being said, the macaque monkey is a wonderful visual animal and a good model for human vision and visual attention.
CNS: Why is it important to study attention?
Mangun: Attention frames our momentary experience. Two people viewing the same scene, say a busy New York street corner, or challenged with the same task, say to detect a threat, may actually focus attention on different things, and therefore have different experiences, leading to different decisions (e.g., “no threat here,” even though a bomb was in fact in the suitcase). So, in everyday life, attention is critical to survival. Since it is a critical cognitive brain function, it is even more apparent when disease, brain damage, or developmental disorders damage the delicate attention system, altering one’s functioning and creating a tremendous medical and financial burden for society. Attention is affected in diseases such as ADHD, Autism, Schizophrenia, Alzheimer’s, stroke, and so on. If we can understand the complicated attention system, or systems, in the brain, we will have the information needed to develop effective diagnostic tools and then treatments.
CNS: How can this work ultimately help people with attention disorders?
Mangun: There are many ways. For example, understanding how attention works will enable us to develop training methods and to assess these to ameliorate attention problems, say for example, in ADHD. Learning what brain neurochemical systems support attention mechanisms will help us target treatment, say in the case of Alzheimer’s disease.
CNS: What are some of the next steps for your research?
Mangun: We developed a highly sensitive measure of attention that we can now use to answer some fundamental questions about how attention operates in early vision. We would like to extend this to higher order stages of vision. As well, we would like to understand how neurochemical systems influence the mechanisms we have uncovered. Finally, developing methods to test these mechanisms in humans will be an important goal – we won’t do this with the same methods, but the mechanisms we have revealed make predictions about how human attention should work, and we hope to test these in humans using psychophysical approaches, and other methods such as EEG and fMRI that can be used in humans.
“Attention enhances synaptic efficacy and the signal-to-noise ratio in neural circuits,” Farran Briggs, George R. Mangun, W. Martin Usrey, Nature (2013), online June 26, 2013.
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