Synaptic strengthening and continuum activity-wave growth in temporal sequencing during cognitive tasks

Sequencing, the distinction between "before" and "after," is a basic feature of cognition, and is exemplified most simply by classical or Pavlovian conditioning. It is generally accepted that such learned behavior is connected anatomically to synaptic strengthening, as first postulated by Hebb. However, the original Hebbian formulation has difficulties, most prominently the assumption that synaptic connections between individual afferent and efferent neurons while apparently modified by learning, has no mechanism that is presently understood. There is also the problem that there is, in general, a long synaptic path between the cortical areas associated with the conditioned and unconditioned reflexes. Neural continuum theory, which deals with elements containing large numbers of neurons, provides an alternative explanation, namely that synaptic strengthening occurs on a larger scale, involving the neurons in regions of consistently substantial activity. The neural continuum has the property of amplifying waves of wavelength large compared with synaptic connection ranges, which propagate over distances comparable to the physical separation between the cortical areas associated with the components of a cognitive task. In this example, propagating waves generated by the conditioned and unconditioned reflexes interfere, producing a standing wave in an intermediate geometric region concave toward the (temporally later) unconditioned reflex. Synaptic strengthening in this region has the effect of a "lens" focusing the activity generated by the conditioning stimulus toward the motor region associated with the conditioned response, producing the Pavlovian reflex.