Automatic Networking: Brain systems charge up in unconscious monkeys

Anesthetized monkeys may be dead to the world, but their brains remain surprisingly lively. Organized patterns of activity continually course through neural networks that during waking life control the animals’ eye movements and other critical functions, a new brain-scan investigation finds.

Unconscious monkeys also display a type of spontaneous brain activity that until now had been observed only in people at rest, say neuroscientist Marcus E. Raichle of Washington University in St. Louis and his colleagues. Some researchers suspect that this so-called default network supports the capacity to imagine the future, daydream, and think about oneself and others (SN: 2/17/07, p. 104: Net Heads).

“These findings are consistent with the perspective that the [primate] brain is governed primarily by internal dynamics,” the researchers conclude in the May 3 Nature. If they’re correct, events external to the individual play only a supporting role in ongoing brain activity.

Raichle’s team used functional magnetic resonance imaging in a new way to investigate spontaneous fluctuations of neural activity in the brains of 11 anesthetized macaque monkeys. This technology measures blood-flow changes in the brain, which reflect cells’ activity.

The scientists first looked for correspondences between spontaneous neural activity in a section of the frontal brain, known as the frontal eye field, and in regions in the rest of the brain. A few areas displayed rises and falls in activity that correlated with those in the frontal eye field.

Earlier studies had indicated that this set of correlated regions is anatomically interconnected and belongs to a system that controls eye movements in alert monkeys. The network is active when monkeys perform learned eye movements.

Raichle and his coworkers also observed two other networks active in anesthetized monkeys. One network is within the somatomotor system, which contributes to movement and touch. The other is in the visual system. Earlier studies showed that conscious animals activate these networks when performing tactile or visual tasks.

The researchers also found correlated activity among three parts of the outer brain layer, or cortex, in the anesthetized animals. These three interconnected areas resemble the default network reported for people at rest. It’s not yet known whether this network performs the same mental functions in nonhuman primates as it does in people.

The new report shows for the first time that spontaneous, organized activity in specific brain networks is “neither restricted to the human brain nor tied to a conscious state,” say Mark A. Pinsk and Sabine Kastner, neuroscientists at Princeton University, in a comment published with the new report.

Unconscious forms of such activity may bolster brain-cell connections needed for effective network functioning, Pinsk and Kastner theorize.

“The demonstration that anatomical connections among brain regions powerfully shape spontaneous fluctuations in neural activity is a major advance,” remarks neuroscientist Olaf Sporns of Indiana University in Bloomington.

In related research, Sporns and his coworkers have developed a computational model of neural activity in macaques at rest, including simulated blood-flow alterations in interconnected structures.

When experimentally prodded, the model generates fluctuating, correlated activity patterns in related structures that resemble those previously reported for conscious macaques. This coordinated activity disintegrates when the researchers alter structural links in the model. The work from Raichle’s group may send Sporns’ model in new directions.