Episodic-like memory is the memory system in animals that is comparable to human episodic memory. The term was first described by Clayton & Dickinson referring to an animal's ability to encode and retrieve information about 'what' occurred during an episode, 'where' the episode took place, and 'when' the episode happened. This ability in animals is considered 'episodic-like' because there is currently no way of knowing whether or not this form of remembering is accompanied by conscious recollection—a key component of Endel Tulving's original definition of episodic memory.
Distinction from human episodic memory
In order for an animal's memory to qualify as 'episodic-like,' it must meet three behavioural criteria reflecting the content, structure and flexibility of the memory. Content refers to recalling what happened, where it happened, and when it happened on the basis of a specific past experience. Structure refers to forming an integrated what-where-when representation, and flexibility refers to being able to use the information in a versatile way. In order for an animal to meet these behavioural criteria then, they must be able to not only form what-where-when memories, but integrate the what, where and when of the event into an integrated framework. The assessment of episodic-like memory must rely on these behavioural characteristics because animals do not have the language capabilities to participate in the episodic memory tests developed for humans.
Evidence of an episodic-like memory system was first provided by Clayton & Dickinson (1998). They demonstrated that western scrub jays (Aphelocoma californica) remember where they cache different types of food, and discriminately recover them depending on the perishability of the item and the amount of time that has passed since caching. Recent research has further investigated episodic-like memory in these birds, but has also explored evidence of this memory system in other species such as the rufous hummingbird, pigeons, primates, rats and honeybees.
The overall organization and brain structures involved in human episodic memory have also been compared to what exists in the animal brain. Many similarities have been found including the role of the medial temporal lobe, a structure including the hippocampus. Nonetheless, many have cautioned making comparisons to human episodic memory. Many criticisms of this area of research have been made including questioning how accurately we can attain this information simply based on behavioural criteria and non verbal tests.
Neural mechanisms
The ability to encode and retrieve past experiences relies on the circuitry of the medial temporal lobe, a brain structure that includes the hippocampus and other para-hippocampal cortical areas. The organization of these brain areas is largely conserved across mammalian species as are the major pathways that information travels between the medial temporal lobe and the neocortex. Because of this strong parallel between animal brain structures and the structures involved in human episodic memory, there is reason to believe that not only do some animals possess the ability to form what-where-when memories, but that this episodic-like system may be more comparable to human episodic memory than once thought.
According to Eichenbaum (2000), information that is projected from nearly all neocortical areas come together onto one or more subdivisions of the para-hippocampal region which includes the perirhinal cortex, the postrhinal cortex and the enotrhinal cortex. This information is then projected onto subdivisions of the hippocampus which are connected by a path which begins with the dentate gyrus, continuing to area CA3, then CA1, and finally through to the subiculum. Information is then directed back to the para-hippocampal region and finally to the same areas of the cerebral cortex that were the source of input to this region. This organization has been observed in primates, rats, and other mammalian species.
The hippocampus in particular has an evolutionarily well conserved structure. It contains similar cell types and comparable structural design in humans and non-human primates. Its structure in rodents is also remarkably similar although in rodents it contains approximately ten times fewer cells and more connections between hemispheres.