John Maunsell knows what he likes when he sees it. Even as an undergraduate zoology student at Duke University, he knew he wanted to study neuroscience and was quickly drawn to studying visual processes in the brain.
"I just decided at one point to step back and think about what the really interesting questions in biology would be for the next 20 or 30 years. And it didn't take long to come to the brain and mind and how it all works," says Maunsell, now a professor of neurobiology at Harvard Medical School. "I really wanted to know how it is that this tissue can make thoughts and perceptions and do the things that it does." Those questions have fascinated him ever since.
When he was starting his research career as a grad student at the California Institute of Technology, little was known about the mechanisms behind brain functions like vision. But through the years, Maunsell has never shied away from some of the most daunting questions in the field, such as how the brain can select the most pertinent information among all the visual information coming in from the outside world. He decided that focusing on attention was a good place to starting looking for answers.
"Attention makes a big difference in what you see or what you hear. The things that you're not paying attention to more or less don't register," Maunsell says. So he set out to find what changes occur in the sensory regions of the brain that could explain why we're so much better at handling visual information when we're paying attention to it and where in the brain those changes happen.
To do that, he measured the activation of individual neurons in the brains of monkeys and looked at how those signals change depending on whether the monkeys are paying attention to certain visual stimuli.
He found that the neuron signals are stronger when the animal is paying attention. "Every cell in the visual part of the cerebral cortex is specialized for representing a different type of stimulus. The neurons' preferences don't seem to change much as attention shifts from one to the other, but their overall responsiveness goes up. They respond to every stimulus in a proportionally stronger way," based on how much they are focusing on it.
Maunsell says the challenge is to figure out how all these signals are being used. His most recent research asks how the cerebral cortex—the outermost part of the brain that is responsible for higher brain functions such as memory, attention, and consciousness—selects information and sends it to other parts of the brain for further processing. For example, when you see a car approaching, that information is sent to other parts of the brain so that you can decide not to cross the street—Maunsell wants to know how that happens.
By stimulating monkeys' brain cells in different regions of the cortex, he found that all parts of the cortex are equally important. It makes sense, he says, because you never know what piece of information about the environment you'll need next, so it's best to have everything available to you.
He was surprised, though, to find that this research had some interesting implications for how memories are formed.
In his experiments, he electrically stimulated neurons in the monkeys' brains, creating false sensations, such as a spot of light. At first the monkeys couldn't tell that their brains had been stimulated, even with a relatively large current of about 50 microamps. But with a couple of weeks of training, they were sensitive enough to see the spot with as little as 5 microamps. "It was as if they were learning to detect these stimuli," he says.
There were consequences of this learning, though. When the monkeys were shown actual bright spots on a screen, they could no longer see them. This "blindness" was not due to damage—after another few weeks of training with real spots, they could see them again.
Maunsell thinks that may be what occurs when we learn something new. "Our hope is that this is what actually happens every time you learn a new face or new building, that what you're doing is pushing around the representations in your cerebral cortex, and with practice you can get extremely good at that." The question is, he says, "what price are you paying for that learning?"
Yet another daunting question, and once again Maunsell isn't shying away from trying to answer it.
