Several theories of consciousness agree that the cerebral cortex in the brain plays a key role, but they disagree on which areas and layers of the cortex are critical. To answer these questions, we may have to study the details of how those parts of the brain operate, which cannot be done to satisfaction in humans. Some work has been attempted on animal models, but those studies lack reliable measures of consciousness which have so far hampered research.

This project, from a team led by Chiara Cirelli at the University of Wisconsin, seeks to, first, validate in freely moving mice and rats a measure of consciousness, called the Perturbational Complexity Index (PCI). The PCI involves two components. First, a part of the brain is stimulated with a magnet or an electrical signal. Then, brain activity is recorded to measure the extent to which that stimulation affects other parts of the brain. Conscious patients normally show a more complex response, whereas unconscious patients normally show a less complex response. This has been validated in humans so successfully, that it has been adopted in clinical settings.

Studying perturbational complexity in rodents provides an advantage over the use of PCI in humans because the stimulation and measurements can be done at a much higher resolution. The resulting data can therefore provide more detail about the complex mechanisms involved in consciousness. PCIst is obtained by electrically and/or optogenetically stimulating different cortical areas and layers, while simultaneously recording with high-density probes hundreds of cortical and subcortical neurons.

The team will use recently introduced high-density electrodes that, for the first time, will allow scientists to record the activity of thousands of different brain cells, including in brain areas that have rarely, if ever, been studied in consciousness research. With these tools, the team will be able to address some long-standing questions in consciousness research, such as whether our capacity to have an experience – i.e. to be conscious – depends on one part of the cerebral cortex more than another and if so, why.

Preliminary results indicate that PCIst reliably distinguishes among conscious and unconscious states – waking, NREM sleep, REM sleep, anesthesia. The project seeks to use PCIst to test whether some cortical areas and/or cortical layers more reliably track consciousness levels. This analysis will provide critical new data to understand the mechanisms of consciousness and unconsciousness and help adjudicate between competing theories.