The substrate of consciousness

How does the brain generate consciousness? Recent research showed that a common assumption has to be revised.

Consciousness is a state ascribed to awake (or dreaming) humans (and other animals), it is the capacity to have subjective experience. Every sight, sound or feeling is a constituent of our subjective, conscious experience. Consciousness have long been an important topic in philosophy and in the last few decades it also became a subject of experimental psychology and neuroscience. Most researchers in this field believe that consciousness is produced by the brain, as certain ailments of the brain have pronounced effects on consciousness. Thus, a central question of this endeavor is, how does the brain generate consciousness?

Studies aiming to answer this question mostly use the same strategy: the stimuli are presented for a very short time (30-60 ms) and the subjects have to indicate if they perceived them. Due to the short stimulus presentation time, most of the subjects only preceive some of the stimuli, and so there are trials in which there is conscious experience of the stimulus and there are trials in which there is no conscious experience of the stimulus. There are some trials in which there is no stimulus, so the validity of responses can be assessed for each subject. Current data suggests that cortical areas (mainly the parieto-temporo-occipital association cortex and some frontal regions) have a central role in generating consciousness.

File:Temporo-parietal junction.svg

The approximate location of the parieto-temporo-occipital association cortex By Henry Vandyke Carter, via Wikimedia Commons

Some studies focused on the temporal dynamics of the underlying neural activity instead of the location. The basic idea is that brain function is based on the firing of its neurons. For example, to perceive a touch, the appropriate receptor cell starts firing, or in other words, generates an action potential, which is a state change that can be relayed to other neurons. This change travels through a probably incredible amount of other nerve cells and this journey somehow generates the conscious experience of being touched. The journey is actually a pattern of patterns: the temporal pattern of the spatial pattern of active neurons at a certain time point.

spatiotemporalpattern.png

This could be a spatiotemporal activity pattern of neurons: the graphs symbolize the same set of interconnected neurons at two distinct time points, with the red nodes marking active neurons (Source: http://www.math.u-szeged.hu).

Previous research showed that the spatiotemporal activity pattern generated by consciously perceived or seen trials, is different from that elicited by unseen trials. Unseen trials generate small, variable and short lived patterns, which means that the number of neurons involved is small, the pattern is changing considerably in time and it subsides shortly after stimulus presentation. On the other hand, consciously perceived stimuli elicit stable, reproducible and longer lived patterns, which means that the pattern is highly similar in different time points, it is usually similar at another presentation of the same stimulus, and it is upheld even after the stimulus is no longer visible. However, recent studies using magneto-encephalography (MEG) showed that neural activity accompanying conscious perception is not stable. MEG records the magnetic fields generated by the ionic currents involved in the firing of the neurons and the studies found that these signals were more variable in the trials which subjects have seen.

Thus, it seems that a basic assumption concerning the neural correlates of conscious experience has to be rejected. Neural activity accompanying conscious perception is not stable, instead it changes rapidly. This suggests, that the computational mechanism behind conscious experience is more complex than what was suggested before and thus the temporal aspects of neural activity will have to be given more emphasis in further studies.

Here are the research articles (1, 2) and a recent review on the neuroscience of consciousness by a pioneer of the field.