Abstract
The memory system in the brain is conventionally divided into three categories: declarative memory, emotional memory, and procedural memory. We propose yet another type of memory that is one level higher than these normal memories, namely, memory of the “temporal organization of behavior”. This type of memory is thought to be formed and stored in the prefrontal cortex, taking at the cellular level the form of synaptic plasticity. Synaptic plasticity in rat prefrontal cortex is indeed powerfully modulated by dopamine, the critical neuromodulator for the function of this cortex, where abnormalities in the dopamine level lead to abnormalities in prefrontal synaptic plasticity. Abnormal synaptic plasticity is also seen in the prefrontal cortex after injection of the psychoactive drug MK801, which is known to cause schizophrenia-like cognitive dysfunction in humans. MK801 spontaneously induces in rats synaptic potentiation in hippocampus-prefrontal cortex projection, and this abnormal plasticity is indeed accompanied by deficits of hippocampus-prefrontal cortex projection-dependent cognition that involves memory of the temporal organization of behavior. These results show that synaptic plasticity in the prefrontal cortex correlates with cognitive processes that involve this cortex.
