A neuronal correlate of consciousness?

In a previous article in this series, Lewis Wolpert [1] introduced the good fairy godmother of science (GOOFGOOS) who, like a Greek oracle, will answer one specific question. My question to her would address the heart of cognitive neuroscience: the relationship between our perceptions, thoughts and memories and the activity of nerve cells that must somehow be responsible for the associated subjective experience. Understanding this relationship entails pinpointing the neuronal populations in the brain that express the correlates of consciousness. So my question is, “are there specific neurons whose activity mediates consciousness?” That is, neurons whose firing gives rise to the current content of my consciousness? Rather than consciousness being expressed by some global, neuronal assembly that includes all neurons in some area of the brain, this hypothesis is based on the assumption that the firing activity of a specific cell class is solely responsible for mediating consciousness [2]. If — with some yet to be invented technique — one could directly stimulate such neurons in an awake human, the subject should have the experience associated with the features encoded by these nerve cells. Since the days of Penfield’s experiments with awake patients, we have known that direct cortical stimulation can lead to conscious experience. But is this is caused by a cell class with unique properties — pyramidal cells projecting directly to the frontal lobe, for example, or cells located in a specific cortical layer that make specific local connections and express a unique set of ion channels? A hint of such neurons can be found in recordings from the brain of an awake monkey while it performs a binocular rivalry task, in which the two eyes see different images. The vast majority of cells in the cortical motion area MT that follow the changing perception of the monkey — as reported by its key presses — rather than the constant visual stimulus, are located in lower layers and are not spread equally across all layers [3]. Most interestingly from the point of view of molecular biologists, if such neurons exist, it is only a question of time before specific markers, such as antibodies that label them, are found. (This is not as unlikely as it sounds: at least one monoclonal antibody is known to bind to antigens on the cell surface of neurons in the magnocellular ‘motion’ pathways in the cortex of primates, including humans [4].) The next step will be to transiently inactivate all such cells. The existence of such neurons would be a further confirmation of the grand theme of specificity that runs through modern biology — for example, the specificity of a protein that will let a sodium but not a potassium ion pass through the cell membrane, or in the genetic sequence for a voltage-dependent sodium channel that encodes a calcium channel instead if a single amino acid is replaced; in terms of cells, this is the specificity of a neuron in the monkey brain that is strongly triggered by viewing a particular face but is indifferent to most other faces. Descartes told us 300 years ago that the seat of the “soul” was in the pineal gland, whereas we now think it might be in a subpopulation of pyramidal cells in a specific layer in cortex. Where’s the big difference? The GOOFGOOS’s affirmative answer would be just the beginning of our quest to understand consciousness. If there are such cells, where they project to will reveal a lot. Do they, for instance, have axons that exclusively contact neurons in the higher, motor-planning stages of the brain? Or will the ‘consciousness neurons’ in the lower part of the visual hierarchy project to the lower parts of the motor hierarchy, and neurons toward the top of the visual hierarchy project to the highest stages of the motor-planning hierarchy? And what does the connectivity of these neurons tell us about tying together activity from different sensory modalities into a unique, perceptual experience? What about the crucial role of short-term memory in consciousness? Will its neural correlate be found exclusively at synapses associated with ‘consciousness neurons’? And how does the time-scale of activation of these neurons correlate with perceptual time-scales? There are many mental diseases that affect consciousness, such as schizophrenia, or autism: can they be related to specific loss of these neurons? At the moment, we know of no really compelling arguments for the existence of ‘consciousness’ neurons. It is possible, after all, that any one neuron in the cortical system can, at any given time, express some correlate of consciousness. However, if specific neuronal correlates of consciousness do exist, it would be a great pity to miss them for want of looking.