II.

Types of Memory

Over the last decades, psychologists have generated a taxonomy of normal human memory, distinguishing first between so-called procedural and declarative memory, that is, memory for skills versus memory for facts. The former is memory for how to do something—to ride a bicycle, for instance; the latter is memory for what things are—that a bicycle is called a bicycle, for instance. Declarative memory may further be subdivided into semantic (for example, knowing the names of days of the week) and episodic, or autobiographical (for example, remembering that I went on a bicycle ride last Tuesday). Procedural memory persists even in disease states such as Alzheimer’s where episodic and semantic memories are progressively lost, implying that the ways in which procedural and declarative memories are made and stored may differ. But this is not the only memory classification possible. For instance, events with a high emotional content are remembered much better than purely cognitive ones. Some memories are held for only a very short time—minutes or even seconds (working, or short-term memory). Others may persist for a lifetime (long-term memory).