ernst_p_ppel's picture
Head of Research Group Systems, Neuroscience and Cognitive Research, Ludwig-Maximilians-University Munich, Germany; Guest Professor, Peking University, China
Continuity of Time

Principia Mathematica Philosophiae Universalis by Isaac Newton is one of the fundamental works of modern science, and this is true not only for physics, but also for philosophy and the foundations of reasoning. Newton gives in "Scholium" the following definition: "Absolute, true, and mathematical time, of itself, and from its own nature, flows equably without relation to anything external." The underlying concept of continuity of time is expressed in mathematical formula describing for instance physical processes. This concept of continuity is almost never questioned.

The Newtonian concept of continuity of time is also implicitly assumed by Immanuel Kant, when he refers to time as an "apriori form of perception" in his Critique of Pure Reason. We read in the translation: "Time is not an empirical conception. For neither coexistence nor succession would be perceived by us, if the representation of time did not exist as a foundation à priori... Time is a necessary representation, lying at the foundation of all our intuitions."  

The concept of continuity of time is also hidden in another famous quotation ín psychology; William James writes in Principles of Psychology when he refers to the present: "In short, the practically cognized present is no knife-edge, but a saddle-back, with a certain breadth of its own on which we sit perched, and from which we look into two directions into time. The unit of composition of our perception of time is a duration, with a bow and a stern, as it were—a rearward—and forward-looking end...We seem to feel the interval of time as a whole, with its two ends embedded in it." Here we are confronted with the idea of a traveling moment, i.e. a temporal interval of finite duration is moving gradually through physical time (and not jumping), again assuming continuity of time. But is it really true and can it be used to understand neural and cognitive processes?

This theoretical concept of continuity of time in biological and psychological processes—usually appearing as an implicit assumption or as an "unasked question"—is wrong. The answer is very simple if one takes a look at the way organisms process information to overcome the complexity and temporal uncertainty of stimuli in the physical world. One source of complexity comes from stimulus transduction, which is principally different in the sensory modalities like audition or vision, taking less than one millisecond in the auditory system and more than twenty milliseconds in the visual system. Thus, auditory and visual signals arrive at different times in central structures of the brain.

Matters become more complicated by the fact that the transduction time in the visual modality is flux-dependent, since surfaces with less flux require more transduction time at the receptor surface. Thus, to see an object with areas of different brightness or to see somebody talking, different temporal availabilities of local activities within the visual modality and similarly different local activities across the two modalities engaged in stimulus processing must be overcome. For intersensory integration, aside from these biophysical problems, physical problems also have to be considered. The distance of objects to be perceived is obviously never pre-determined. Thus, the speed of sound (not of light) becomes a critical factor.

At a distance of approximately ten to twelve meters, transduction time in the retina under optimal optical conditions corresponds to the time the sound takes to arrive at the recipient. Up to this "horizon of simultaneity," auditory information is earlier than visual information; beyond this horizon, visual information arrives earlier in the brain. Again, there must be some kind of mechanism that overcomes the temporal uncertainty of information represented in the two sensory modalities. How can this problem be solved? The best way of the brain is to step out of the mode of continuous information processing.

The brain has indeed developed specific mechanisms to reduce complexity and temporal uncertainty by creating system states (possibly using neuronal oscillations) within which "Newtonian time" does not exist. Within such system states temporally and spatially distributed information can be integrated as experimental evidence shows. These states are "atempora" because the before-and-after relationship of stimuli processed within such states is not defined or definable. This biological trick implies that time does not flow continuously, but it jumps from one atemporal system state to the next.