Patric Stanton, Terrence J. Sejnowski
In modeling studies or memory based on neural networks, both the selective enhancement and depression or synaptic strengths are required ror effident storage or inrormation (Sejnowski, 1977a,b; Kohonen, 1984; Bienenstock et aI, 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 brier, high-frequency activation or excitatory synapses in the hippocampus produces an increase in synaptic strength known as long-term potentiation, or L TP (BUss and Lomo, 1973), that can last ror 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 or synaptic strength that could balance LTP has not yet been demonstrated. We stu(cid:173) died the associative interactions between separate inputs onto the same dendritic trees or hippocampal pyramidal cells or field CAl, and round that a low-frequency input which, by itselr, does not persistently change synaptic strength, can either increase (associative L TP) 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 or synaptic strength is Hebbian, and LTD is anti-Hebbian since it is elicited by pairing presynaptic firing with post(cid:173) synaptic hyperpolarization sufficient to block postsynaptic activity. Thus, associa(cid:173) tive L TP and associative L TO are capable or storing inrormation contained in the covariance between separate, converging hippocampal inputs •
• Present address: Dep~ents of NeW'Oscience and Neurology, Albert Einstein College of Medicine, 1410 Pelham Parkway South, Bronx, NY 10461 USA.
tPresent address: Computational Neurobiology Laboratory, The Salk Institute, P.O. Box 85800, San Diego, CA 92138 USA.
Storing Covariance by Synaptic Strengths in the Hippocampus