The Long and the Short of It

The ability to form and retain new long-term declarative memories for facts and events requires the integrity of medial temporal lobe structures including the hippocampus and surrounding entorhinal, perirhinal and parahippocampal cortices (Figure 1; Squire and Zola 1996). A fundamental goal of memory research is to define the neural signals underlying memory function in these areas. This minireview focuses on the patterns of mnemonic activity that have been described throughout the monkey medial temporal lobe. Most such studies have examined patterns of neural activity as animals performed various short-term memory tasks (i.e., tasks requiring memory for seconds to minutes). These studies show that neurons in the perirhinal and entorhinal cortices convey robust short-term memory signals. The evidence for short-term memory signals in the hippocampus has been mixed with some studies reporting memory-related signals and some not. Little is known about mnemonic signals in the parahippocampal cortex. A fundamental function of the medial temporal lobe is the formation and maintenance of long-term memory (i.e., memory lasting from minutes to hours or more). Thus, an important question is: how do the short-term memory signals described in medial temporal lobe neurons relate to the long-term memory functions of these areas?