11 June 2009
Our brain makes use of two codes for the perception of one sensory experience. Cyriel Pennartz, Professor of Animal Physiology and Cognitive Neurobiology at the University of Amsterdam (UvA), makes this claim as part of a newly-developed theory. Before now, it was not clear how the brain recognised a certain 'clump' of electrical pulses as belonging to a specific sense (touch, sight, etc.). The theory is intended to act as a basis for new experiments and computer simulations. Pennartz' research was published last month on the website of the scientific journal Consciousness and Cognition. It will be published in the paper edition of this journal later this year.
During his research, Pennartz focused on the Qualia problem. This concerns the philosophical question of why our conscious, subjective perception is characterised by highly different, specific sensations (such as seeing, smelling, hearing, touching), while the brain processes that lead to these sensations are actually of the same sort.
Previously, the labelled-lines hypothesis was used to explain this problem. This hypothesis states that the nature of the sensory perception a person experiences is determined by the type of sense and corresponding nerve fibre that are stimulated at that moment. For example, the exertion of pressure on the skin gives a tactile sensation via stimulation of touch sensors, but strong pressure on the eye leads to a visual sensation (stars, flash of light). Pennartz claims that there is something strange about this theory. Firstly, all nerve fibres operate between sense and brain in the same way: they all transfer information with electrical impulses (spikes). Secondly, the brain itself is not informed of the nature of the sensors that emit the impulse streams, nor about the anatomic origin of the nerve fibres (e.g. the skin). So the question of how the brain recognises a certain 'clump' of electrical pulses as belonging to touch, sight or another sense remains. In comparison: a series of 'zeroes and ones' does not lead to an experience of the colour ‘blue'. A new theory is necessary to answer this question.
In his new theory, Pennartz describes the problem by first shifting the perspective to networks of brain cell groups that exchange information from different modalities and compare these. The nature of a sensory perception is explained by the relationships that a particular sensation has with other sensations, which match up or do not match up with the sensation in question.
Although promising, this approach on its own does not prove sufficient to give electrical signals a meaningful content in our perceived world (e.g. the colour ‘red'). An electrical brain signal means nothing in itself for the owner of the brain, even if the signal is exchanged with other brain areas. Pennartz has therefore suggested a second, more radical idea: brain networks make use of two methods of encoding a conscious experience. The first method relates to the speed with which a brain cell emits impulses. This classical method is used to 'detect' a characteristic, but without an experiential component (compare it with a thermostat that switches on when it gets cold). The second method concerns the phase during which one brain cell is active compared to another (two pendulums that swing in synchronisation have a different phase than if they swing in opposite directions). The relative phase of cell activity can be used to encode the relationships between the sensory characteristics that are detected by individual cells. Pennartz therefore claims that it is the combination of impulse speed and phase that is sufficiently powerful to generate an integrated and meaningful sensory experience at the level of brain cell networks.
Pennartz, C.M.A. Identification and integration of sensory modalities: Neural basis and relation to consciousness. Consciousness and Cognition (Online edition, 5 May 2009).