The Roles Played by External Input and Synaptic Modulations in the Dynamics of Neuronal Systems

The framework within which Tsuda proposes his solution for transitory dynamics between attractor states is flawed from a neurological perspective. We present a more genuine framework and discuss the roles that external input and synaptic modulations play in the evolution of the dynamics of neuronal systems. Chaotic itinerancy, it is argued, is not necessary for transitory dynamics. Appears in The Behavioral and Brain Sciences, 24, pp. 811-812 The dynamics of Hopfield Networks (Hopfield, 1982) are a far cry from that of systems of neurons in the brain. The existence of the energy function ensures that under the guidance of an asynchronous update rule, such networks relax to fixed point attractors. This behavior is not in conformance with that observed in systems of neurons in the brain where limit cycles, let alone stable fixed points, are not encountered. Tsuda’s efforts at introducing complex dynamics into such model networks are commendable. His solution, however, is suspect. Tsuda’s system (Tsuda, 1991, 1994) is an otherwise standard Hopfield Network without the symmetric coupling constraint, endowed with an additional class of specialized nodes that by his own account, is primarily responsible for the system’s unconventional dynamics. It therefore stands to reason that we take a closer look at these nodes. Roughly speaking, the nodes in the noted class stay dynamically inactive (imparting a constant bias) as the remainder of the system approaches an attractor. If the remainder of the system settles on