A paper that recently appeared in the journal Neuron sets up an interesting dichotomy in describing how to view the function of the brain. One option it presents is that the brain is an input-output device: give it a stimulus, and it will process it and respond. The alternative view is that the brain is simply doing its own thing, and stimuli act to modulate its activity, rather than direct it. Since the first perspective is an easier one to approach experimentally, it has received most of the attention, but the paper presents evidence that the alternative view shouldn't be ignored.

The experiments in the paper are built around two observations. The first is that just about every measure of brain function detects spontaneous, organized activity even when the owner of the brain doesn't appear to be doing anything—in fact, this kind of activity has been detected when people are under anesthesia. The second key observation is that, even on the simplest tests, the same individual will perform differently when the test is repeated. The authors simply asked if these two were linked: is human action influenced by spontaneous brain activity?

The researchers focused on the somatomotor cortex, or SMC, as previous work has shown that the left and right SMC undergo coordinated spontaneous activity. The experiments involved a test that only required activity in the left SMC: right-handed individuals were asked to watch a symbol, and press a button with their right hand when it vanished. Due to the way the brain is wired, only the SMC on the left side will be active during this test.

As expected, there was a lot of variability in the results. The standard deviation in the time for the button press was about 15 percent of the average time, and the force used to press the button also varied, with a standard deviation of about 20 percent of the average. Using MRI imaging of blood flow, the researchers were able to detect both the activity involved with the button press, as well as the spontaneous background activity of the brain.

Because the left and right SMCs have coordinated spontaneous activity, the researchers were able to subtract the right's activity from the left, which should (in theory) leave nothing but the signal involved in the button press behind. The theory appeared to work. The subtraction provided a much cleaner connection between the button press and brain activity in the left SMC. Once spontaneous activity was accounted for, noise was down by 60 percent, and the signal to noise ratio in the experiments doubled. Putting this another way, spontaneous activity accounted for about 60 percent of the variation between tests.

The authors say that these results show that spontaneous brain activity is more than simply a physiological artifact; it helps account for some of the variability in human behavior. In that sense, they argue for a greater acceptance of the view that our brain may have some intrinsic activity that's somewhat independent of sensory input.

Neuron, 2007. DOI: 10.1016/j.neuron.2007.08.023