ABSTRACT

Association is what most people mean by memory. Conditioning, involving arbitrary stimulus-response association, is found in species as diverse as snails, wasps, flatworms, birds, and mammals generally. Thus its origins extend back 520 million years. Jellyfishes may not be capable of conditioning, likely indicating its mediation by developed nervous systems.

Sensory memory is a short-lived trace of a stimulus's physical form. In vision it lasts less than half a second in daylight, and up to four seconds at nighttime. There is a peripheral component in the retina and a central component in the brain. As an aftereffect of neural stimulation, it has likely existed since the evolution of either flatworms or cnidarians 500-600 million years ago.

Short-term memory lasts about 20 seconds, or longer if refreshed through rehearsal. Its capacity depends on what is remembered: E.g., 8 colors but only 3 nonsense syllables. The concept of working memory adds an executive control component, with similar frontal lobe involvement in Old World monkeys and humans. That suggests an origin at least 32 million years ago. A short-term memory exists in birds through convergent evolution.

Of several ways to parse long-term memory, one fruitful way distinguishes between episodic and semantic memory. Episodic memory is of specific past events. Though difficult to demonstrate in animals, a lowland gorilla can remember, over several minutes, a single-trial feeding, something not attributable to mere familiarity. Chimpanzees and rats also show evidence of episodic memory. It involves the hippocampus, a 400-million-year-old structure, although it is doubtful that the hippocampus has supported recollection over that entire span.

Semantic memory is knowledge independent of recollected events. A modality-free version engaging the lateral temporal lobe may be human-specific. Other primates rely on vision- or hearing-specific memories involving the medial temporal lobe and other local areas.