Alterations in the dynamics of semantic activation in Alzheimer's disease:Evidence for the gain/decay hypothesis of a disorder of semantic memory

While it has been commonly assumed that a deficit in semantic memory underl ies many of the clinical and cognitive features of early Alzheimer's diseas e (AD), there has been little agreement on what constitutes a proper theore tical description of this impaired system. Currently, most theories of this disorder fall into two categories: those that posit degraded semantic repr esentations, and those that posit impaired retrieval operations with relati vely intact semantic representations. It is argued that each position has b oth empirical and logical disadvantages that have prevented the development of a consensus on how to describe the pathology of semantic memory in AD. In this paper we present the details and supportive evidence for an alterna tive account of the semantic memory deficit of AD: the Gain/Decay hypothesi s, The core claim of the Gain/Decay hypothesis is that a reduction in the t ime constant of spreading activation in AD produces dynamic changes in the availability of semantic representations that depend on the time frame in w hich this information must be accessed. The implications of this hypothesis for a range of experimental and clinical phenomena are discussed, as are p ossible biological correlates of the hypothesized alterations in the modula tion of activation.