04 JANUARY 2007
It's one of the seven wonders of the human brain: When we pay attention to something,
it seems to become brighter, more distinct, more memorable. We pay attention
to a passing billboard, and notice the smiling lady holding a paint brush,
and even the name of her paint store. Otherwise, we may absently observe a
human silhouette on the billboard, without seeing the face or absorbing the
all-important marketing message. 
We see a car stalled at the roadside, but
don't notice whether it's a mauve Mazda or a maroon Mercury.
A dense array of electrodes helps scientists understand electrical activity in the brain. The Northwestern University researchers used 64 separate data points. Photo: Waisman Center, University of Wisconsin-Madison
Our decision to pay attention, in other words, changes how our brains work. How?
Obviously, pointing our eyes at a scene will make us more likely to register details. But that's not the whole story. Simply making a decision to pay attention, without moving the eyes, also changes the sensory machinery inside the brain.
But how? Scientists have suggested that paying attention may make a given stimulus as effective as a larger stimulus, or cause it to evoke a stronger response, with the neurons firing faster. In a new study, neuroscientist Marcia Grabowecky of Northwestern University and her colleagues found that paying attention affects neurons primarily by making them fire more synchronously, effectively producing a stronger response to the stimulus.
In the "stimulus contrast" model (a), the neuron starts
responding at a lower level of stimulus, but eventually reaches the same level.
In the "response gain" or "activity gain" models (b), the starting point does
not change, but the maximum firing rate is higher. Courtesy Yee Joon Kim
et al (see below).
Not shifty-eyed
To probe how paying attention changes neurons, Northwestern researchers Yee Joon Kim, Grabowecky, Ken Paller, Krishnakumar Muthu and Satoru Suzuki sat human guinea pigs in front of a screen with side-by-side, bulls-eye images that each pulsed at a particular rate.
The different pulsing rates allowed the researchers to track the brain's response: When we watch a flashing light, neurons in the visual cortex fire in rhythm with that light. "We are taking advantage of the fact that when you regularly flicker a stimulus, you get evoked activity in the brain at the same rate as the stimulus," Grabowecky says.
The
Experimental Setup
With their eyes focused on
the same spot in the center, subjects were asked to pay attention to one
specific pulsating bulls-eye.
The rate of pulsation affects how neurons fire in the brain. Courtesy
Yee Joon Kim et al (see below)
The subjects were told to keep their eyes on a mark between the bulls-eyes while paying attention to one specific bulls-eye. Using an electroencephalograph (EEG) machine to monitor brain electrical activity, the researchers assessed the effects of paying attention by observing how synchronously the visual cortex was firing in unison with each flashing image.
As expected, paying attention did make the brain more responsive to the selected image, although curiously, the results offered something to each side of the debate mentioned above: Paying attention made the neurons sensitive to a weaker stimulus (that is, images with lower contrast), but increased the response even more to higher-contrast stimuli.
Wearing a stylish skullcap full of electrodes and wires,
a lab assistant "models" the
experimental apparatus. Question: Do they heat the lab? Photo:
courtesy Marcia Grabowecky
Doin' the wave
Previous research into the effects of attention have focused on individual neurons,
but EEGs measure the electrical activity of groups of neurons, not individuals.
By looking at groups, the EEG showed an increased synchrony of nerve pulses in
the brain visual centers concerned with the attended stimulus. In a press release,
co-author Ken Paller said, "When you need to dig deep to summon that extra ounce of attention, it's as if you engage a symphony of brain activity that can come to your rescue as millions of neurons together make the music that represents a vivid conscious experience."
12 hertz electrical activity in the brain while the subject is attending to (top) and ignoring (bottom) the 12-hertz flashing bulls-eye. Red indicates highest activity, blue is lowest. Courtesy Yee Joon Kim et al (see below).
The research is a basic effort to understand the brain, and it examined an unusual situation: paying visual attention to something outside the center of vision. Still, there's a potential payoff to paying attention to attention, Grabowecky observes. "As society gets more complex and technological, we are bombarded with information all the time, so understanding the how and why of paying attention gets more critical." Whether it's cars, planes, or computer screens, "How do you set up an interface so you can draw attention to the critical factors, and draw less attention to the less important stuff?"
— David Tenenbaum
Bibliography
• Attention
induces synchronization-based response gain in steady-state visual evoked potentials,
Yee Joon Kim et al, Nature Neuroscience vol. 10, #1, Jan. 2007.
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