Ordered Retinotectal Projections and Brain Organization

Abstract

The analogy of brain to computer has been sporadically popular. This chapter pursues instead the analogy between brain function and cooperative effects. As an example, the cooperative theory for the establishment of topologically ordered fiber projections is discussed, according to which fibers growing from the retina to the optic tectum retain or recover the geometric relations that they had in the retina by sensing signals transmitted via the optic axons to the tectal cells. By comparison between a retinal cell’s signal and that which it senses in tectal cells it contacts, its synapses are either reinforced (eventually to become permanent) or extinguished (in which case the retinal cell’s projection withdraws from that tectal cell). This proposal has been tested by extensive computer simulations, which correctly describe normal and experimentally perturbed development in many different situations by means of a single algorithm and one set of parameter values.

The above simulation results illustrate four features common to cooperative systems: (1) the processes are based on systems with a large number of microelements in an initially undifferentiated state; (2) they contain self-amplifying fluctuations; (3) the fluctuations may compete in Darwinian fashion; and (4) fluctuations may cooperate by enhancing the “fitness” of other fluctuations. The result is the emergence of ordered modes, in which all interactions have come to a global equilibrium. The system of ordered modes constitutes a new “macro”-level of complexity. The relationship between micro-level and macro-level is not trivial. No deterministic dynamics can, in general, be formulated for the ordered modes.

The chapter continues on the assumption, prevailing today, that the brain activity relevant for thought processes conforms to the scheme of cooperative phenomena. This conceptual framework is shown to be natural to the discussion of a number of important issues on brain function: the autonomous nature of organization in the brain (in distinction to the computer, which requires a programmer); the perceived unity of our thought processes in a system composed of myriads of elements; causality and determinism, discussed as nonissues; and perception as an active process, rather than a passive intake of information. —The Editor