Consciousness in the Brain

Even at the stage of the retina, a division arises between the dorsal and the ventral stream. The dorsal stream is responsible for movements in relations to objects, and is unconscious. The ventral stream has specialised modules for different aspects of vision. This processing happens at a still unconscious stage, with consciousness only found at the end of the process. Consciousness-related stimuli pick up strength as they progress through the cortex. After 200-300 ms unconscious processing fades, but consciousness-related signals move towards the front of the brain.

The ventral stream culminates in the temporal region. From here neurons feed back to the earlier cortex, disambiguating the signal by comparing it with memories of past stimuli. This reiteration takes about 100-150 ms., in part explaining the 300-500 ms. time lag between initial stimuli and conscious perception.

The outcome of this reiteration is the stage at which processing become conscious. The brain goes through what can be described as a phase change to support stable, conscious images. The shift involves increases in activity in regions supporting consciousness, with activity rising by as much as twelve-fold. The areas involved in consciousness have especially large pyramidal neurons with long axons, large dendrites and abundant spines. Such neurons are concentrated in layers II and III of the cortex, which are much thicker in areas of the brain related to consciousness.

Consciousness also involves processing of images in single-neurons. A few hundred active neurons arranged in patches in the visual cortex may each specialise in the production of particular images. Conscious perceptions correlate both to the global gamma synchrony and to high-frequency activation in single-neurons. The link between these single-neurons and the gamma synchrony comes when individual neurons are supported by the intense activity of a clump of neighbouring neurons.

Some regions of the brain are viewed as hubs, on which multiple sensory inputs converge, to give a single interpretation or evaluation, which may be fed back so as to alter processing in the earlier unconscious cortex. Thus projections go from the temporal cortex to the orbitofrontal, which evaluates the reward/punisher characteristics of stimuli. The orbitofrontal deals with a variety of reward values, and this gives rise to the idea of a common neural currency for comparing rewards that have little or nothing in common.

The orbitofrontal projects to the anterior cingulate, which evaluates possible actions in response to stimuli, in particular their costs. This includes selecting for choices of action with short-term negative outcomes, but longer term rewards. The evaluations of these two regions are projected to other brain regions that plan or initiate behaviour. So preferences can be projected from the orbitofrontal to the dorsolateral, and thus have an influence on the content of working memory, longer-term planning and reasoning.

The brain’s reward circuit may also direct voluntary attention. Top-down voluntary attention runs from the reward circuit and the frontal-eye-field to the earlier unconscious areas of the cortex, and is the driver in giving unconscious preference to sensory inputs that are relevant to goal-directed behaviour.

The neuroscience of the last two decades has shown us where and under what circumstances consciousness arises in the brain, and how it has adaptive functions. However, this still does not explain what consciousness actually is. Consciousness is not found in normal physics, which functions without requiring it, or giving rise to it. This leads us to the conclusion that consciousness must derive from a fundamental property of the universe, to which the distinctively conscious areas of the brain provide a link or gate.

by Simon Raggett