Long Term Memory
Primates have an astounding range of behaviors and cognitive abilities. This flexible range of behaviors is supported by a large capacity for long-term memory. Humans undoubtedly possess an extensive ability for long-term memory formation and retention. If we limit our study of long-term memory to declarative memory (memory of facts or events that can be consciously recalled) (http://www.human-memory.net/types_declarative.html), then we can attribute the formation of these memories to the medial temporal lobe in the brain. One very famous human case study that supports the idea that the medial temporal lobe is responsible for declarative memory formation is that of Patient H.M. H.M, following a childhood accident, had medically intractable epilepsy. At age 27, he had surgery to remove the majority of his medial temporal lobe to eliminate the epilepsy; as a result, he was incapable of forming new declarative memories, demonstrating the importance of the medial temporal lobe for long-term memory formation.
Much of what we know about human long-term memory has actually resulted from non-human primates studies. Neuronal recording methods in non-human primates such as macaques, have demonstrated that different areas of the prefrontal cortex (PFC) are specialized for different learning functions (Simons & Spiers, 2003); for example, the medial PFC is thought to be responsible for stimulus-response association learning based on reward while the lateral PFC supports goal-directed control functions. Additionally, one seminal lesion study in non-human primates contributed to the identification of the role of the hippocampus in memory consolidation (Zola-Morgan & Squire, 1990). The researchers trained monkeys to discriminate between 100 pairs of objects in a temporally graded fashion prior to hippocampal removal; monkeys with hippocampal lesions demonstrated severe memory impairments for more recently learned objects. However, they showed no memory impairments, relative to unlesioned monkeys, for the distantly learned objects, demonstrating that the hippocampus is required only for the consolidation of new long-term memories (Zola-Morgan & Squire, 1990).
Additionally, this Fagot and Cook (2006) used a large set of picture-response associations to test the limits of memory capacity in both baboons and pigeons. They demonstrated that baboons could learn 3,500-5,000 items and still hadn’t reached their upper limit after 3 years of testing. Miyashita (1988) showed that neurons in the anterior ventral temporal cortex of monkeys display selective activity for specific visual stimuli after training with a visual association task.
Research done in captive great apes demonstrates that they do have episodic-like memory (Martin-Ordas et al., 2010). Episodic memory refers to memory of what, where, and when information, accompanied by conscious awareness. Researchers used a food storage memory task that required temporal encoding and found that great apes could distinguish between different events, where the same food items were hidden in different places at different times. Other studies in bonobos, chimpanzees, gorillas, and orangutans demonstrate that these primates can use feature and spatial cues in a food retrieval task (Kanngiesser & Call, 2010), demonstrating episodic-like memory. Great apes also display deferred imitation (displaying a copied behavior over a variable delay). A two-year longitudinal study in chimpanzees and orangutan demonstrated that both species could display deferred imitation and improved over a two-year period (Bjorklund, Bering, & Ragan, 2000). This is a form of social learning (Albert Bandura, 1977), which is highly evolutionarily advantageous, as it does not require trial and error, and enables an individual to learn skills and new knowledge from others in his/her social group.
While these studies suggest that long-term episodic-like memory is present in great apes, it is difficult to determine whether these apes have conscious awareness, a tenet of human episodic memory. Perhaps as more research is done in captive great apes or other primates, a better, less anthropomorphized definition for long-term declarative memory will arise.
In the Atkinson-Shiffrin memory model, long-term memory refers to the final stage of memory formation, in which unlimited quantities of information can be stored indefinitely (Atkinson & Shiffrin, 1968). Short-term memory, on the other hand, is a limited capacity buffer. Long-term memory can be broken down into explicit and implicit memory. Explicit memory, or declarative memory, refers to consciously recollected memories for the "what, where, and how." Explicit memory encoding is dependent on the medial temporal lobes (Meulemans & Van der Linden, 2003). However, implicit memory refers to habit memory or skill learning (such as riding a bicycle); this form of memory is largely independent of the medial temporal lobe, and relies instead on the basal ganglia as well as other brain structures (Foerde & Poldrack, 2009). Here, we will focus on explicit memory in our discussion of long-term memory.
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