William Loftus

Electrophysiological activity propagating along forebrain neurons is widely regarded as essential for the generation of consciousness. However, it has never been demonstrated conclusively that neuronal activity per se is sufficient to generate consciousness, to the exclusion of concomitant extraneuronal phenomena. Accordingly, it is reasonable to consider whether consciousness depends upon an alternative substrate capable of representing the information carried by neurons. Ion waves in the interstitial fluid of the white matter are an extraneuronal phenomenon that, according to the ion wave hypothesis introduced here, may constitute the neural substrate of consciousness. Because the interstitial fluid forms a single continuous physical medium rather than a collection of discrete cells, its dynamics may support forms of global information integration that are difficult to achieve within networks of discrete neurons alone. This proposal is motivated by two considerations: First, by virtue of the positioning of the interstitial fluid within the white matter, information transmitted between forebrain processing centers might be transformed during transmission into ion waves. And second, the hypothesized ion wave system occupies a single contiguous macroscopic space and may be capable of holistically integrating all information within the system. The transformation of information from cell-based electrophysiological activity to fluid-based ion waves is proposed to occur as follows. The specification of conscious content is encoded as action potentials conducted along white matter fibers. As an action potential, or "spike," propagates along a single fiber, different species of ions enter and exit at successive nodes of Ranvier. It is therefore hypothesized that multiple fibers with temporally coordinated oscillating spike trains induce longitudinal waves of oscillating ion concentration within the interstitial fluid. As this process occurs simultaneously across all fibers, the interstitial fluid contains a three-dimensional system of ion waves that evolves over time. A broad implication of the ion wave hypothesis is that the brain has a two-stage architecture for the generation of consciousness. The first stage, specification, determines the content of conscious experience—but not the experience itself—through the electrophysiological activity of forebrain neural networks. The second stage, realization, generates conscious experience itself, with its specific content determined by the preceding neuronal specification. The prevailing view assigns both functions of specification and realization to neurons; in contrast, the ion wave hypothesis assigns these functions to neurons and ion waves, respectively. It is worth noting that the proposed functional division of specification and realization parallels the distinction between the so-called "easy" and "hard" problems of consciousness. Under the ion wave hypothesis, the hard problem may be reformulated from “how and why action potentials of forebrain neurons are accompanied by consciousness” to “how and why ion waves in the forebrain interstitial fluid are accompanied by consciousness?”