The emergency of "dissociated state" under the drug influence is quite specific failure of memory retrieval without essential changing of perception, consolidation and storage a new information. Indeed, in spite of the salient disturbances of task performance in drugged state, memory storage processes and task executive processes are nearly intact. It may be concluded that the emergency of dissociated state is specific disturbance of retrieval process. Therefore, state-dependent learning and retrieval of dissociated responses is helpful behavioral model for the study of memory retrieval mechanisms.

Among drugs resulted in memory dissociation are glutamate-, cholin-, anticholin-, GABA-ergic / Arkhipov, 1992; Arkhipov, Azarashvili, 1988; Nacagawa, Iwasaki,1995; Overton, 1984/. Memory dissociation that appeared from these drugs demonstrates that the blockade of task performance after drug treatment is specific reorganization of the retrieval process. The retrieval of long-term memory traces is possible in spite of amnesic influence of the drugs, but retrieval process is state-dependent. Memory traces are accessible for retrieval in the brain state, if they was encoded in this brain state. Here we emphasize one side of memory dissociation phenomenon: to demonstrate practically separate retrieval of long-term memory traces in dissociated states.

Thus, dissociated responses in animals can be obtained by training in pharmacologically induced dissociated states. It was shown that more than two dissociated responses may de obtained. For example, learning and retrieval of one response from three previously learned in experimental chamber may be seen when animal placed in this chamber after treatment by saline, pentobarbital (15 mg/kg, i.p.) or sodium oxybutirate (250 mg/kg, i.p.) /Arkhipov, Azarashvili, 1981/. This experimental situation gives opportunity for separate retrieval of several responses. The study of the retrieval of dissociated tasks is possible if another drugs are administrated before the retrieval test. For example, if animals have learned two tasks (R1 in the normal state and R2 in the drugged state), during test for performance the animals can retrieve after pharmacological treatment: i.) R1; ii.) R2; iii.) R1 and R2 during test session, this situation may be considered as the disruption of memory dissociation, because of the animals remembered two tasks simultaneously; iiii.) R1 or R2 does not retrieve but animals are capable for new learning, this situation may be considered as the emergence of new dissociated state. Besides, we does not consider the case when retrieval is not possible from toxic effect of administrated drug.

It is possible to summarize some results from the study of memory dissociation. The phenomenon demonstrates:
1) The possibility of separate memory retrieval in dependence on functional state. The choice of the R1 or R2 for the retrieval can be regulated by pharmacological influences.
2) The clasterization of engrams. Some engrams are accessible for the retrieval, while others are repressed.
3) Regulation of retrieval processes by cholinergic and GABA-ergic systems.
4) The key role of hippocampus in memory dissociation processes. Some evidence of this assertion is the possibility for long-lasting dissociated state after cholinergic stimulation of hippocampus /Arkhipov, Azarashvili, 1985/ and relation of memory dissociation to declarative form of memory.
From the necessity to inhibit some part of memory traces for state dependent retrieval of responses it may be supposed that declarative (attention-dependent) form of memory /Squire, Zola, 1996/ is capable to dissociate. Therefore experimental expressiveness of memory dissociation is critically determined by the involvement of attention-dependent memory in used task. It was suggested the key role of hippocampus in the emergence of dissociated states /Arkhipov, Azarashvili, 1985; Arkhipov, Shchipakina, 1992; Zharikova, Arkhipov, 1994/. Possible role of hippocampus in the retrieval of declarative memory traces is to estimate the suitability of activated memory traces for the current situation. It is known that exploratory activity and attention critically depend on cholinergic processes and hippocampal activity. Hippocampus may estimate the stimulus novelty and filter external and internal informational signals /Vinogradova, 1995/. Thus, attention-dependent memory is the foundation of memory dissociation processes. It may be suggested that the retrieval of not only dissociated responses occurs with participation of cholinergic and GABA-ergic systems of hippocampus. Such processes are controlling the attention-dependent retrieval of memory traces in widespread circumstances.