Storing Covariance by the Associative Long-Term Potentation and Depression of Synaptic Strengths in the Hippocampus

In modeling studies or memory based on neural networks, both the selective enhancement and depression or synaptic strengths are required ror efficient storage or inrormation (Sejnowski, 1977a,b; Kohonen, 1984; Bienenstock et al, 1982; Sejnowski and Tesauro, 1989). We have tested this assumption in the hippocampus, a cortical structure or the brain that is involved in long-term memory. A brief, high-frequency activation or excitatory synapses in the hippocampus produces an increase in synaptic strength known as long-term potentiation, or LTP (Bliss and Lomo, 1973), that can last for many days. LTP is known to be Hebbian since it requires the simultaneous release or neurotransmitter from presynaptic terminals coupled with postsynaptic depolarization (Kelso et al., 1986; Malinow and Miller, 1986; Gustatrson et al, 1987). However, a mechanism ror the persistent reduction of synaptic strength that could balance LTP has not yet been demonstrated. We studied the associative interactions between separate inputs onto the same dendritic trees or hippocampal pyramidal cells or field CA1, and round that a low-frequency input which, by itself, does not persistently change synaptic strength, can either increase (associative LTP) or decrease in strength (associative long-term depression or LTD) depending upon whether it is positively or negatively correlated in time with a second, high-frequency bursting input. LTP of synaptic strength is Hebbian, and LTD is anti-Hebbian since it is elicited by pairing presynaptic firing with postsynaptic hyperpolarization sufficient to block postsynaptic activity. Thus, associative LTP and associative LTD are capable or storing information contained in the covariance between separate, converging hippocampal inputs.

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