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Horizontal versus vertical associative memory concepts are denned. Vertical associative memory involves chunking: the specification of new (previously free) nodes to represent combinations of old (bound) nodes. Chunking is the basis of semantic memory, configuring in conditioning, and cognitive (as opposed to stimulus-response) learning. The cortex has the capacity for chunking, but the hippocampal (limbic) arousal system plays a critical role in this chunking process by differentially priming (partially activating) free, as opposed to bound, neurons. Binding a neuron produces negatively accelerated repression of its connections to the hippocampal arousal system, consolidating the memory by protecting the newly bound neuron from diffuse hippocampal input and thus retarding forgetting. Disruption of the hippocampal arousal system produces the amnesic syndrome of an inability to do new chunking (cognitive learning)-anterograde amnesia-and an inability to retrieve recently specified chunks-retrograde amnesia.With a time period of development spanning over two thousand years of human history, the doctrine of the association of ideas is to this day the dominant theory of the mind. I imagine there were always protesters who claimed that the mind was too complex to be encompassed by such associationism. However, until recently, there was no comparably elegant and general alternative theory. Now there is such a theory, though as is so often the case, the new theory is more of an extension than a repudiation of classical associationism.The basic idea derives from the concept of chunking. Chunking was originally formulated by G. Miller (1956), but subsequently chunking has been implicated in a wide variety of

In single-trace fragility theory, forgetting is produced by two factors, time and interference. Memory traces are assumed to have two partially coupled dynamic properties, strength and fragility. Strength determines the probability of correct recall and recognition, while fragility determines the susceptibility of the trace to the time-decay process but not to the interference process. Consolidation is assumed to be a continual reduction in the fragility of the memory trace rather than any change in strength or availability. Decreasing fragility accounts for the continually decreasing forgetting rate, the temporal character of retrograde amnesia and recovery therefrom, and the type of internal clock necessary for nonassociative recency judgments. Data are presented to indicate that interference is independent of the interval between original and interpolated learning, that nonassociative recency discriminability approaches a limit at about 30 min, and that the decay rate of long-term retention in amnesic patients is the same as in normal Ss.For some years now it has been fashionable to assume at least two d~namically distinct memory traces or processes, short term or long term. An enormous number of memory phenomena have been interpreted as providing support for the two-trace hypothesis. I think that all of these phenomena are equally consistent with a single-trace hypothesis, though this single trace must be viewed as undergoing a consolidation process that progressively reduces its rate of forgetting and its susceptibility to amnesic agents with increasing trace age.Wickelgren (1973) discussed all the frequently cited psychological evidence for distinguishing two dynamic types of memory traces and demonstrated that all of this evidence was equally consistent with a single-trace theory, with three exceptions. The three phenomena alleged to support the distinction between short-term and long-term memory were (a) the different forms of retention function, (b) the different effects of similarity on storage interference (as measured by recognition tests), and (c) evidence from amnesic patients such as the classic H. M. of Scoville and Milner (1957). I now believe that a single-trace theory can account for even these three phenomena and one such theory is described in this paper. Instead of two traces, the theory proposes two mechanisms that produce decay of the memory trace: (a) a storage interference process dependent on the processing of subsequent similar material and (b) an interference-free time-decay process. The theory also contains a new hypothesis regarding consolidation based on the assumption that memory traces have two partially coupled dynamic properties, strength and fragility. Greater strength implies more accurate recognition and recall. Greater fragility implies greater *This research was supported by Grant 3-0097 from the National Institute of Education, by Grants MH 00890 and MH 17958 from NIMH, and by Contract F 44620-73-e-0056 from ARPA to Ray Hyman. I thank Adam Reed for influencing me to seri...

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