Long-lasting and context-specific freezing preference is acquired after spaced repeated presentations of a danger stimulus in the crab Chasmagnathus

A visual danger stimulus elicits an escape response in the crab Chasmagnath us that declines after repeated presentations. Previous results report that such waning may be retained as context-signal memory (CSM) or signal memor y (SM): CSM is long lasting, associative, and produced by spaced training, while SM is an intermediate memory, nonassociative, and produced by massed training. The performances of both spaced and massed trained crabs are here examined, using video analysis to determine topographic changes in the beh avioral response during and after training. During spaced training, escape vanishes and is mainly replaced by freezing, while during massed training, escape decreases over trials without being replaced by any defensive respon se. After 24 h, the marked proclivity to freezing persists in spaced traine d crabs, while a high level of escaping is shown by massed trained crabs. T he long-lasting freezing preference of spaced trained crabs proves to be co ntext-specific and appal ent fi om the very first presentation of the dange r stimulus at testing, though freezing is not triggered by the sole exposur e to the context. We conclude (a) that freezing preference is the acquired response of the CSM process; (b) that CSM can be properly categorized as an instance of contextual conditioning and SM of classical habituation; (c) t hat CSM and SM are not two phases of a memory processing but two distinctly types of memory; and (d) that therefore, the temporal distribution of trai ning trials has a drastic effect on crabs memory, more dramatic than that p reviously described. The possibility that massed and spaced presentations o f the same stimulus may represent two different stimulus types is discussed . (C) 2000 Academic Press.