Episodic memory involves retrieval of what items comprised the event, where the event took place, and when the event occurred. Retrieval of such detailed information requires mentally travelling back in time to the previously experienced event. Such mental time travel is a key component of episodic memories and is associated with the subjective experience of “remembering”. Humans can report whether they “remember”, and such responses can be assumed to reflect episodic memory. One problem with interpreting the mental states of animals is that they cannot tell us about their subjective experience. Just over a decade ago, Endel Tulving, who introduced the term episodic memory and linked this process to mental time travel, concluded that the evidence indicated that animals do not have episodic memory (Tulving, 2005). Tulving stated there was no evidence that the hippocampus, a region of the human brain associated with context memory, was associated with context/temporal memory in animals.
More recent research with rats and monkeys showed that context memory is associated with the hippocampus (Eichenbaum et al., 2007). Moreover, the anatomic organization of the hippocampus is highly conserved across rats, cats, and monkeys (Manns & Eichenbaum, 2006). The elephant hippocampus is of similar complexity to humans (Patzke et al., 2014). Elephants are also known to have incredible spatial memory (Hart, Hart, & Pinter-Wollman, 2008). For instance, during times of drought, the matriarch will lead the herd for hundreds of miles toward water holes, and memory for the location of a specific water hole can last for decades. The similarity between the elephant hippocampus and the human hippocampus coupled with the remarkable spatial memory of elephants provide compelling evidence that they have episodic memory.
Hippocampal time cells, which have been observed in rats and monkeys, are active at specific periods following the onset of an event. For instance, time cells are active when a rat is running on a treadmill, but only if there is a subsequent to-be-remembered task (e.g., if the rat is supposed to turn left at the next fork in a previously learned maze). This dependence on a to-be-remembered task suggests these cells are associated with imagery of a future action, which requires mental time travel, the key characteristic of episodic memory. Such time cells provide a brain mechanism that is likely utilized during episodic memory in rats and other mammals.
Some of the most compelling evidence that animals can have episodic memory stems from the discovery of memory replay in the hippocampus. Memory replay refers to the reactivation of brain activity associated with a previous experience in the correct temporal order and, interestingly, this activity is speeded up in time. Memory replay, which has predominantly been observed in rats, has been shown to be coordinated by hippocampal sharp-wave ripples (activity at 200 cycles/second). A study of bottlenose dolphins also showed evidence for memory replay (Kremers, Jaramillo, Böye, Lemasson, & Hausberger, 2011). The dolphins heard recorded humpback whale sounds that were broadcast at the beginning of shows. The whale sounds are very different from the whistles and burst-pulsed vocalizations typically made by dolphins. Sounds from the dolphins were subsequently recorded and it was found that the dolphins made whale-like sound productions, mostly at night but also during quiet restfulness while swimming slowly or floating. Furthermore, these whale-like productions were speeded up in time, which is reminiscent of the speed up of memory replay that has been observed in rats. A set of human observers classified actual humpback whale sounds, dolphin whistles, the dolphin whale-like productions played at normal speed, and the dolphin whale-like productions played at half speed. Of most importance, a similar percentage of dolphin whale-like productions played at half speed and humpback whale sounds were classified as produced by a whale. These findings suggest that dolphins, like rats, have memory replay.
Based on the evidence of memory replay during sharp-wave ripples in the hippocampus in rats, one of the champions of the view that animals do not have episodic memory (Suddendorf & Corballis, 2007) has made a surprising reversal of position and has stated “it seems highly likely from an evolutionary perspective that this activity is homologous to that involved in mental time travel in humans” (Corballis, 2013, p. 5). This underscores the strength of the memory replay evidence in support of the view that mammals have the capacity for mental time travel and episodic memory.
Do all animals have episodic memory? Given that hippocampal sharp-wave ripples coordinate memory replay and have been observed in all mammals that have been tested, it can be concluded that all mammals have episodic memory. Future work will be needed to assess whether there is other evidence for episodic memory in birds.
The preceding brain evidence satisfies Tulving’s criterion for episodic memory in mammals. Given that episodic memory is one of the highest forms of cognitive processing, the conclusion that mammals have episodic memory has implications for the way that humans treat them. Although invasive experiments in mammals have provided novel insight into the mechanisms of memory, their advanced cognitive abilities must be weighed against the potential benefits of animal research.
Check out the rest of the memory series blogs:
Part 3: The Brain Basis of Forgetting
Part 4: Episodic Memory in Mammals
Part 5: The Consolidation Debate