When the brain snaps to attention, individual neurons don’t necessarily work harder, but clusters of them form cooperative units, a new study suggests.
This unifying brain process, in which nerve cells briefly align the peaks and valleys of their electrical outbursts, may underlie an animal’s shifting of attention to a particular sight, sound, or other sensation, according to a team of neuroscientists led by Peter N. Steinmetz of the California Institute of Technology in Pasadena.
In the past decade, several scientists linked synchronized electrical activity in groups of neurons to perception and memory in cats and other nonhuman animals (SN: 2/21/98, p. 120). Preliminary evidence also implicates coordinated neural firing in human perception and learning (SN: 2/20/99, p. 122).
Some investigators hold that groups of neurons must fire in aligned patterns to generate thoughts and consciousness. Others regard such synchronized activity as a byproduct of more crucial processes that occur within densely connected webs of neurons.
Taking the former position, Steinmetz and his colleagues have focused on the allocation of mental resources. “Change in synchrony may be an essential neural mechanism of selective attention,” they conclude in the March 9 Nature.
The researchers recorded the electrical activity of neurons in a monkey brain area near the middle of the cortex. Cells in this region, the secondary somatosensory cortex, emit discharges most rapidly when the animal touches an item while paying close attention to it.
Each monkey was trained to perform the same visual task and one of three tactile maneuvers. They also learned to switch from one task to the other at assigned times while the researchers presented visual and tactile stimuli simultaneously.
The visual task required each monkey to detect, in a series of trials, which of three white squares on a computer screen dims slightly. As for tactile tasks, one monkey touched a finger pad over which a series of raised letters moved. Upon finding a match for a letter shown on a computer screen, the animal pressed a key. Another monkey completed a more difficult version of this task, in which the letter to be matched changed after each correct response. The third animal had to indicate repeatedly whether raised bars on a finger pad had the same or different orientations.
The monkeys switched between tactile and visual tasks every 7 to 8 minutes as microelectrodes implanted in their brains recorded cell activity in the secondary somatosensory cortex. The scientists analyzed responses of 436 individual neurons and 648 pairs of neurons.
Nearly 80 percent of individual neurons discharged different numbers of electrical pulses during the two tasks, an encouraging sign that the change of attention altered their firing rates. Moreover, two-thirds of all neuron pairs displayed synchronized activity during one or both tasks.
Most importantly, the researchers say, synchrony in cellular duos reached higher levels during tactile trials—and in particular during the more difficult letter-matching challenge—than during the visual task. Coordinated activity often rose without accompanying hikes in firing rates by individual neurons.
“Whatever the underlying mechanism, these results are strong evidence that neuronal [pulse] synchrony correlates with a cognitive process, namely, shifting of the attentional focus,” assert Emilio Salinas of the Howard Hughes Medical Institute at the Salk Institute for Biological Studies in La Jolla, Calif., and Ranulfo Romo of the National Autonomous University of Mexico in Mexico City in a comment in the same issue of Nature.
It’s unclear whether increased synchrony in the somatosensory cortex improves subsequent stages of tactile processing in the brain, Salinas and Romo say.