I continue to explore the implications and ramifications of the idea that memory takes the form of different systems all of which have some properties in common and every one of which differs from others in fundamental ways. My special interests are still focused on episodic memory--the kind of memory that allows us to "mentally travel" in time, and thus recollect our own past experiences, events we have observed and participated in. I first proposed, mostly on theoretical grounds, that episodic memory is different from other kinds of memory back in 1972, but since that time the initial conceptualization has been considerably elaborated and modified. In the beginning there was a great deal of skepticism about, and resistance to, the idea that episodic memory is fundamentally different from other kinds of memory. Over the intervening years, however, more and more scientists have come to accept the idea, and episodic memory has become a regular theme of clinical, experimental, and theoretical research.

One feature that distinguishes episodic from other forms of memory is what I have called "autonoetic" ("self-knowing") consciousness. When you remember a past happening you have a familiar kind of mental experience that you recognize as "remembering." It is clearly different from the kinds of mental experiences that you have when you see the sights and hear the sounds around you, or when you think about things in the world "out there," or when you solve problems, or imagine what people, objects or scenes look like. The autonoetic kind of conscious awareness does not accompany the use of any other kind of memory-based skill, it is a feature of only episodic memory.

Another distinguishing feature of episodic memory is "chronesthesia" ("subjective sense of the past, present, and future time.") Contrary to popular opinion, no other kind of memory (or memory system) has any special relationship to time. The subjective sense of time ("chronesthesia") and the self-knowing and self-centered consciousness ("autonoetic" consciousness) are closely related concepts that have been difficult to study in the past, but which now are being investigated by using the techniques of functional neuroimaging (scanning of activity in living brains).

A third (hypothetical) feature that makes episodic memory different from other kinds is that it is of relatively recent evolutionary origin, and that as such, at least in its full-fledged form, it exists in human beings only. Many animals--mammals such as mice, squirrels, dogs, elephants, and chimpanzees, as well as most if not all birds--have excellent "semantic" memory. That is, they are capable of conscious learning of facts about the world. However, there exists no evidence that they can mentally travel in time in the same was as humans do, to remember the past and to plan for the future. It is in this sense that remembering the past and envisioning the future can be thought of as uniquely human brain/mind capacities.

A special difficulty in objectively studying episodic memory in nonhuman animals lies in the fact that animals do not have language. When, in a behavioral test, animals make use of knowledge that they have learned in the laboratory, we cannot ask them, as we can ask verbally competent humans, whether they remember learning it (episodic memory) or whether they just know what to do in a given situation (semantic memory). Inspired by others, I have described a kind of a possible test of autonoetic episodic memory for animals without language (chapter in a 2005 book edited by Terrace and Metcalfe). Young human children past a certain age, being human, have no difficulty passing the test. It remains to be seen whether any of our furry or feathered friends can do likewise.