A central issue of cognitive neuroscience is to understand how a large collection of coupled neurons combines external signals with internal memories into new coherent patterns of meaning. An external stimulus localized at some input spreads over a large assembly of coupled neurons, building up a collective state univocally corresponding to the stimulus. Thus, the synchronization of spike trains of many individual neurons is the basis of a coherent perception. Based on recent investigations of homoclinic chaotic systems and their synchronization, (...) a novel conjecture for the dynamics of single neurons and, consequently, for neuron assemblies is formulated. Homoclinic chaos is proposed as a suitable way to code information in time by trains of equal spikes occurring at apparently erratic times. In order to classify the set of different perceptions, the percept space can be given a metric structure by introducing a distance measure between distinct percepts. The distance in percept space is conjugate to the duration of the perception in the sense that an uncertainty relation in percept space is associated with time-limited perceptions. This coding of different percepts by synchronized spike trains entails fundamental quantum features which are not restricted to microscopic phenomena. It is conjectured that they are related to the details of the perceptual chain rather than depending on Planck's action. (shrink)