How not to study spontaneous activity.

Brains are restless. We have long known of the existence of a great deal of uninterrupted brain activity that maintains the body in a stable state--from an evolutionary standpoint one of the brain's most ancient tasks. But intrinsic, ongoing activity is not limited to subcortical, life-maintaining structures; cortex, too, is remarkably active even in the absence of a sensory stimulus or a specific behavioral task. This is evident both in its enormous energy consumption at rest and in the large, spontaneous but coherent fluctuations of neural activity that spread across different areas. Not surprisingly, a growing number of electrophysiological and functional magnetic resonance imaging (fMRI) studies are appearing that report on various aspects of the brain's spontaneous activity or "default mode" of operation. One recent study reports results from simultaneously combined electrophysiological and fMRI measurements in the monkey visual cortex (Shmuel, A., Leopold, D.A., 2008. Neuronal correlates of spontaneous fluctuations in fMRI signals in monkey visual cortex: implications for functional connectivity at rest. Hum. Brain Mapp. 29, 751-761). The authors claim to be able to demonstrate correlations between slow fluctuations in blood-oxygen-level-dependent (BOLD) signals and concurrent fluctuations in the underlying, locally measured neuronal activity. They even go on to speculate that the fluctuations display wave-like spatiotemporal patterns across cortex. In the present report, however, we re-analyze the data presented in that study and demonstrate that the measurements were not actually taken during rest. Visual cortex was subject to almost imperceptible but physiologically clearly detectable flicker induced by the visual stimulator. An examination of the power spectral density of the neural responses and the neurovascular impulse response function shows that such imperceptible flicker strongly suppresses the slow oscillations and changes the degree of covariance between neural and vascular signals. In addition, a careful analysis of the spatiotemporal patterns demonstrates that no slow waves of activity exist in visual cortex; instead, the presented wave data reflect differences in signal-to-noise ratio at various cortical sites due to local differences in vascularization. In this report, assuming that the term "spontaneous activity" refers to intrinsic physiological processes at the absence of sensory inputs or motor outputs, we discuss the need for careful selection of experimental protocols and of examining the degree to which the activation of sensory areas might influence the cortical or subcortical processes in other brain regions.