NONLINEAR BRAIN DYNAMICS IN PERCEPTION: THREE 'CODES' OF THE BRAIN

A major challenge for neuroscientists is to deduce and explain the neural mechanisms of the rapid transposition between stimulus energy and abstract concept - between the specific and the generic - in both material and conceptual aspects. Brain researchers are attempting three explanations of perception in terms of neural codes. Cellular neurobiologists find rate and frequency codes for stimulus features in trains of action potentials induced by stimuli and carried by topologically organized axons. Cognitivists correlate grouped firings of nerve cell assemblies with generalizations over classes of stimuli (faces, objects, odorants, words, etc.). Dynamicists correlate 2-D spatial patterns of brain waves (EEG) with the meanings of stimuli, which contain the knowledge about the stimuli. The patterns self-organize by trajectories through high-dimensional brain state space. This multivariate code is expressed in landscapes of non-convergent ('chaotic') attractors, which form the memory bank of the brain. Each pattern is formed by the dissipative dynamics of the cortical system. Its formation is preceded by a discontinuity in the oscillation of the EEG when a stimulus directs a search trajectory into a basin of attraction. Convergence to an attractor implements the cognitive process of generalization and abstraction. The attractor constructs the memory of the stimulus, and expresses it in the spatial pattern of amplitude modulation of a carrier wave. I propose that by its attractor dynamics the sensory cortex constructs knowledge from the information that is provided by the senses. Whereas sensory information is expressed in the action potentials of neurons and networks of neurons, knowledge is expressed in the continuous field dynamics that is indirectly observed in the EEG, in which the synapse and the action potential that support mental function are treated not as bits of information but as infinitesimals in differential calculus.