LONG-TERM POTENTIATION - WHATS LEARNING GOT TO DO WITH IT

Long-term potentiation (LTP) is operationally defined as a long-lastin g increase in synaptic efficacy following high-frequency stimulation o f afferent fibers. Since the first full description of the phenomenon in 1973, exploration of the mechanisms underlying LTP induction has be en one of the most active areas of research in neuroscience. Of princi pal interest to those who study LTP, particularly in the mammalian hip pocampus, is its presumed role in the establishment of stable memories , a role consistent with ''Hebbian'' descriptions of memory formation. Other characteristics of LTP, including its rapid induction, persiste nce, and correlation with natural brain rhythms, provide circumstantia l support for this connection to memory storage. Nonetheless, there is little empirical evidence that directly links LTP to the storage of m emories. In this target article we review a range of cellular and beha vioral characteristics of LTP and evaluate whether they are consistent with the purported role of hippocampal LTP in memory formation. We su ggest that much of the present focus on LTP reflects a preconception t hat LTP is a learning mechanism, although the empirical evidence often suggests that LTP is unsuitable for such a role. As an alternative to serving as a memory storage device, we propose that LTP may serve as a neural equivalent to an arousal or attention device in the brain. Ac cordingly, LTP may increase in a nonspecific way the effective salienc e of discrete external stimuli and may thereby facilitate the inductio n of memories at distant synapses. Other hypotheses regarding the func tional utility of this intensely studied mechanism are conceivable; th e intent of this target article is not to promote a single hypothesis but rater to stimulate discussion about the neural mechanisms underlyi ng memory storage and to appraise whether LTP can be considered a viab le candidate for such a mechanism.