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Towards a true neural stance on consciousness (Lamme, 2006)

Consciousness is a complex phenomenon. Why do some processes in the brain evoke conscious experiences, but others do not? A difficulty with traditional behavioural measures of consciousness is that a demonstration of the presence or absence of consciousness is dependent on other cognitive functions such as language (e.g., for reporting), memory and attention. This makes it difficult to dissociate consciousness from these other cognitive functions. Victor Lamme argues that, in order to truly address the mind–brain relationship in consciousness, neural and behavioural measures need to be put on equal footing. He uses the example of visual consciousness to illustrate this point.

When a new image hits the retina, it is processed through successive levels of the visual cortex by means of feedforward connections. In only around 100–150 ms, the brain “knows” about the new image. This feedforward sweep enables rapid extraction of complex and meaningful features from the visual scene and potential motor responses are prepared. Studies in humans and monkeys seem to indicate that, no matter what area of the brain is reached by the feedforward sweep, this in itself does not produce reportable conscious experience. What seems necessary for conscious experience is that neurons in visual areas engage in recurrent processing where high- and low-level areas interact.

Lamme argues that it is this widespread, recurrent processing that forms the key neural ingredient of consciousness. Crucially, he argues that recurrent processing is fundamentally different from feedforward processing in that it creates a condition that satisfies the Hebb rule: pre- and postsynaptic neurons are active simultaneously. This triggers the activation of synaptic plasticity processes that are the neural basis of learning and memory. If one of the functions of consciousness is to allow for learning, then recurrent processing fits the criteria necessary for neural learning to occur.

By defining visual consciousness in such “neural terms”, several testable predictions ensue. First, recurrent processing should be necessary for conscious experience. Second, learning should follow the phenomenal aspects of stimuli (e.g., colour) rather than their physical features (e.g., wavelength), even when what is learned is not reportable. An advantage to adopting such a “neural stance” to consciousness is that consciousness may now be dissociated from other cognitive functions such as attention, working memory and reportability. We would also be able to measure the presence or absence of consciousness without resorting to behavioural measures, opening avenues to investigate consciousness in coma, anaesthesia or animals. Rather than inferring consciousness from behavioural measures, our understanding of consciousness should emerge from arguments of neuroscience.

Reference

Lamme, V.A.F. (2006). Towards a true neural stance on consciousness. TRENDS in Cognitive Sciences,10(11):494–500 [doi: 10.1016/j.tics.2006.09.001].

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