Heightened susceptibility to interference in an animal model of amnesia: impairment in encoding, storage, retrieval--or all three?

There has recently been a resurgence in the idea that amnesia may be characterized by an increased susceptibility to interference. In the present study we tested this idea using a well-controlled and well-established animal model of amnesia: impairment in object recognition following perirhinal and postrhinal cortical (PPRh) damage. We used this paradigm to test whether memory impairment was exacerbated by the interpolation of a potentially interfering item either before (proactive interference) or after (retroactive interference) the to-be-remembered item. Rats with PPRh damage were impaired in object recognition memory, with a minimal delay, when the interfering stimulus was perceptually similar to the test stimuli. When the interfering stimulus was less perceptually similar to the test stimuli, the PPRh-lesioned rats performed similarly to Controls. Both proactive and retroactive interference were observed, and both depended on the similarity of the interfering item to the test items. These findings provide support for the idea that amnesia can indeed be characterized by increased vulnerability to interference, and we illustrate, using simulations generated by a computational model of amnesia, how the mechanism for this vulnerability to interference can be understood, not in terms of an impairment in encoding, storage or retrieval, but in terms of an impairment in encoding, storage and retrieval.