Characteristics of Associative Potentiation/Depression

Abstract

Studies of the monosynaptic, bilateral entorhinal cortex (EC) — dentate gyrus (DG) responses are being used to choose among the numerous synaptic modification rules suggested in neurodynamic theories of concept formation and pattern recognition. The EC-DG system has many advantages, including (a) numerous available controls [1], (b) the wealth of anatomical and physiological knowledge about the system [2], and (c) the use of bilaterally placed, widely separated stimulating electrodes [1]. In addition, there is the fact that the requisite stimulation intensity and response amplitude of the contralateral EC-DG pEPSP have no lower limits that restrict the phenomenology of associative potentiation/depress ion. That is, so long as a contralateral response is measurably above noise levels and a powerful converging ipsilateral response is available, then all of our reported results for long-term potentiation and depression hold for the weak contralateral response. These results encourage an extrapolation in which only a single EC-DG fiber is activated. This extrapolation, the specificity and individuality of contralateral modifiability [1,3,4,5], the synaptic modification correlates of LTP revealed by electron microscopy [6,7], and the existence of a suitable molecular mechanism to explain associative potentiation (i.e. the voltage-controlled NMDA receptor [8]), further encourage us to study synaptic modification in the EC-DG system in order to properly formalize the rule of associative modification.