Primacy versus recency in a quantitative model: Activity is the critical distinction

Behavioral and neurobiological evidence shows that primacy and recency are subserved by memory systems for intermediate- and short-term memory, respec tively, A widely accepted explanation of recency is that in short-term memo ry, new learning overwrites old learning. Primacy is not as well understood , but many hypotheses contend that initial items are better encoded into lo ng-term memory because they have had more opportunity to be rehearsed. A si mple, biologically motivated neural network model supports an alternative h ypothesis of the distinct processing requirements for primacy and recency g iven single-trial learning without rehearsal. Simulations of the model exhi bit either primacy or recency, but not both simultaneously. The incompatibi lity of primacy and recency clarifies possible reasons for two neurological ly distinct systems. Inhibition, and its control of activity, determines th ose list items that are acquired and retained. Activity levels that are too low do not provide sufficient connections for learning to occur, while hig her activity diminishes capacity. High recurrent inhibition, and progressiv ely diminishing activity, allows acquisition and retention of early items, while later items are never acquired. Conversely, low recurrent inhibition, and the resulting high activity, allows continuous acquisition such that a cquisition of later items eventually interferes with the retention of early items.