Physiology&Behavior, Vol. 32, pp. 1027-1031.Copyright©PergamonPress Ltd., 1984. Primedin the U.S..A 0031-9384/84$3.00 + .00 Facilitation of Generalization Performances in Spatial Learning Problems by Posttrial Stimulation of the Mesencephalic Reticular Formation MARTINE AMMASSARI-TEULE,*t ANNE-MARIE FOMBON* AND VINCENT BLOCH* *D~partement de Psychophysiologie, Laboratoire de Physiologie Nerveuse, CNRS 91190 Gif-sur- Yvette, France tlstituto di Psicobilogia e Psicofarmacologia del CNR, Via Reno, 1, 00198 Roma, Italy Received 9 May 1983 AMMASSARI-TEULE, M., A.-M. FOMBON AND V. BLOCH. Facilitationof generalization performances in spatial learning problems by posttrial stimulation of the mesencephal&reticular formation. PHYSIOL BEHAV 32(6) 1027-1031, 1984.--The effects of low posttrial stimulation of the mesenphalic reticular formation (MRF) on spatial learning problems were studied in two consecutive stages. First, the performances of stimulated (S) and no-stimulated (NS) subjects were assessed in a reconvergent three-arm maze. Secondly, the same animals were tested in a generalization paradigm, since they had to solve a similar task in a radial four-arm maze, each group being divided into newly stimulated (S-S, NS-S) or no-stimulated (S-NS, NS-NS) subjects. In the first experiment, no differences were found between the acquisition proc- esses of groups S and NS. Conversely, in the second experiment, previously stimulated animals (S-S and S-NS) showed better generalization of the experimental rule. The results are discussed in terms of perseveration of the neural activity after the early MRF stimulation. From a functional point of view, our data indicate that an enhancement of the neural activity during acquisition facilitates not only a high accessibility of memories concerning the experimental rule in a subsequent similar situation but also the capability to apply a previously elaborated response system in a more complex situation. Memory consolidation Generalization Spatial learning Reticular stimulation THE model of memory consolidation is based upon the as- sumption that the neural activity triggered by the registration of information continues over time and that this persevera- tion of impulses in neuronal networks is a necessary condi- tion for an effective storage of information. Previous studies have demonstrated significant facilitation of learning and memory performance when the brain was aroused by mild posttrial electrical stimulation of the mesencephalic reticular formation (MRF) during the so-called consolidation period. This facilitative effect has been observed in different species and in a number of experimental situations [5, 10, 11, 12, 13, 20]. At the neurophysiological level, this improvement of per- formance rests upon a similarity of brain conditions induced by MRF stimulation and those normally required for learning and memorizing. For example, it has been observed that post-trial MRF stimulation suppresses the characteristic in- crease of paradoxical sleep which is normally consecutive to learning [6]. Moreover, such stimulation has been found to compensate for the deleterious effect of experimental depri- vation of paradoxical sleep on acquisition performance [7]. Later investigations have also shown the functional rela- tionship existing between low MRF stimulation and the ac- tivation of limbic structures generally assumed [27] to be more directly involved in the control of learning and memory processes [17,28]. With a classical conditioning paradigm it has been shown that the US-CS association elicited modifi- cations of the multi-unit activity responses in CA3 hip- pocampal cells; successively, it was observed that these modifications appeared earlier when MRF stimulation was delivered after each US-CS presentation; moreover, this cel- lular conditioning persisted after a long time interval in sub- jects which received MRF stimulation during training [8]. Finally, it has been suggested that this facilitative effect could rest upon an enhancement of long term potentiation of the synaptic transmission [19], since this phenomenon is also facilitated by post tetanizing reticular stimulation [19]. Therefore, this set of data suggests that MRF stimulation accelerates and intensifies the working of a system which is normally active in acquisition and retention processes. At a higher level of integration, i.e., behavioral level, modification of a more general process such as arousal occurring at critical stages of information processing have been invoked to explain the observed facilitation of perform- ance when MRF stimulation was applied. However, if we more particularly consider acquisition, 1027