Activity in Cognitive Elements 48

6 TRACE has been simulated with a variety of input equations with negligible difference in results. Activity in Cognitive Elements 47 sequence (and likewise for the relationship between the phase sequence and the phase cycle). Since Hebb considered the cell assembly to be the smallest unit of conscious thought, our simulation, which is oriented to perception, cognition, and memory for objects, would be more properly considered as at the phase sequence level. For the purposes of our model, however, these distinctions may not be of great importance. We assume that the basic functions characteristic of these networks are likely to be the same across the three different levels. 2 The embedding fields theory of Grossberg (1969,1971), in which the unit activation function has positive feedback and decay, provides a unique exception to this generalization. 3 We present here a system of difference equations. In fact, solutions to the corresponding system of differential equations were obtained by several approximation methods. These solutions were quite close to those obtained by employing Euler's method with a step size of 1. So, to simplify the exposition and reduce computational requirements, we present and simulate the difference equations. 4 In the remainder of the exposition we replace the functional notation X(t) with the simpler notation X. It should be understood that a variable X which appears in the right hand side of an equation is evaluated at time t. 5 I is an instantaneous function, not a state variable, so it is evaluated at time t+1 rather than at time t as are the state variables in these equations. Memory storage as a function of arousal and time with homogeneous and heterogeneous lists. Footnotes 1 We have adopted the expression "cell assembly" because of its common usage in descriptions of Hebb's work. Hebb's analysis of cognitive processes actually included three levels of organization: cell assemblies, phase sequences, and phase cycles. Cell assemblies served as elements in the higher level network that constituted the phase Pillsbury, W. B. (1913). "Fluctuations of attention" and the refractory period. (1956). Tests on a cell assembly theory of the action of the brain, using a large digital computer. Differential recall of paired associates as a function of arousal and concreteness-imagery levels. (1989). Oscillatory responses in cat visual cortex exhibit inter-columnar synchronization which reflects global stimulus properties.