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Giulio Tononi* and Gerald M. Edelman
Conventional approaches to understanding consciousness are generally concerned with the contribution of specific brain areas or groups of neurons. By contrast, it is considered here what kinds of neural processes can account for key properties of conscious experience. Applying measures of neural integration and complexity, together with an analysis of extensive neurological data, leads to a testable proposal-the dynamic core hypothesis-about the properties of the neural substrate of consciousness.
What is the neural substrate of conscious experience? While William James concluded that it was the entire brain (1), recent approaches have attempted to narrow the focus: are there neurons endowed with a special location or intrinsic property that are necessary and sufficient for conscious experience? Does primary visual cortex contribute to conscious experience? Are brain areas that project directly to prefrontal cortex more relevant than those that do not (2)? Although heuristically useful, these approaches leave a fundamental problem unresolved: How could the possession of some particular anatomical location or biochemical feature render some neurons so privileged that their activity gives rise to subjective experience? Conferring this property on neurons seems to constitute a category error, in the sense of ascribing to things properties they cannot have (3).
Here, we pursue a different approach. Instead of arguing whether a particular brain area or group of neurons contributes to consciousness or not, our strategy is to characterize the kinds of neural processes that might account for key properties of conscious experience. We emphasize two properties: conscious experience is integrated (each conscious scene is unified) and at the same time it is highly differentiated (within a short time, one can experience any of a huge number of different conscious states). We first consider neurobiological data indicating that neural processes associated with conscious experience are highly integrated and highly differentiated. We then provide tools for measuring integration (called functional clustering) and differentiation (called neural complexity) that are applicable to actual neural processes. This leads us to formulate operational criteria for determining whether the activity of a group of neurons contributes to conscious experience. These criteria are incorporated into the dynamic core hypothesis, a testable proposal concerning the neural substrate of conscious experience (4).
General Properties of Conscious Experience
Consciousness, as...