Basic processes learning and memory

Learning is generally defined as a relatively permanent change in behavior as a function of experience. Most organisms show the capacity to learn. Early studies of intelligence in animals used learning tasks to attempt to characterize species differences in capacity.

In a series of studies investigating the learning skills of rhesus monkeys (Macaca mulatto), Harry Harlow showed in 1949 that, following extensive experience with large numbers of individual problems, monkeys were able to solve a novel problem in only one trial. The problems involved making a choice between two objects that differed from each other in several physical dimensions such as color, shape, material, size, and position. A reward such as a peanut was hidden under one of the objects. The monkeys gradually learned to choose consistently the rewarded object. At that point Harlow would introduce a new problem with novel objects. Over the course of many individual problems, the monkeys took fewer trials to reach a high level of performance.

Following a few hundred such problems, the monkeys were consistently correct on the second trial of each new

A goat uses a tree stump so she can reach high vegetation. (Photo by Antony B. Joyce/Photo Researchers, Inc. Reproduced by permission.)
An African leopard (Panthera pardus) stores its kill (an impala) in a tree. (Photo by Tom Brakefield. Bruce Coleman, Inc. Reproduced by permission.)

problem. That is, the monkeys no longer required a period during which they learned to choose the rewarded object through trial-and-error. Rather, their response on the first trial of a new problem (whether it was correct or incorrect) informed them which object to choose on subsequent trials. This understanding of the solution to the problem based on one experience with two novel objects was called "learning set," or "learning to learn" by Harlow. It is a good example of cognitive flexibility. Learning set is still used to study aspects of learning and cognitive flexibility in humans and non-humans. Animals of a multitude of species are capable of learning set, including cats, rats, squirrels, minks, sea lions, and several species of monkeys. The investigation of learning set in rats demonstrates the importance of considering the species-typical sensory capacities of an animal when studying cognition. Rats, who have very poor vision but excellent olfactory ability, have some difficulty with visual learning set but easily achieve high levels of performance with olfactory discriminations.

Memory provides mental continuity across time by allowing information from one point in time to be used at a later point in time. That time span can be in seconds or minutes (short-term or working memory), hours, days, or longer (long-term or reference memory). It forms the basis for learning, since without memory the influence of past experiences would not exist. Memory involves three processes: encoding, storage, and retrieval. Encoding refers to the form or code in which items or events are stored; storage involves the way that memories are stored in the brain including where, how, and how long; and retrieval refers to the act of remembering, or accessing information that was previously stored in memory. Two ways of studying retrieval are through recall and recognition measures. In recall, an individual is asked to reproduce the items or information stored at a previous time. In humans recall usually involves verbal reports. Recognition measures simply ask "Have you seen (experienced) this item before?" and involve presenting various items that were and were not in the memory set being tested. The individual is required to respond in a way that implies "yes" or "no." Because it can be nonverbal, this form of retrieval is most commonly used with nonhuman animals.

The above distinctions and methods are relevant to laboratory research with animals, and they are addressed specifically below. However, memory clearly plays a role in an animal's daily life. Food-related behaviors such as foraging or storing food for later use require memory. To what extent memory is used in foraging, and how that process is used, has been addressed in field studies of nonhuman primates. The question is whether monkeys and apes use memory to guide

their travel when foraging. For animals like gorillas (Gorilla gorilla) or leaf monkeys (Presbytis spp. and Trachypithecus spp.) whose sources of food are easily found and available in large quantities at one site ("banqueters"), remembering the location of particular food sources may not be a crucial aspect of their foraging activity. For species like orangutans and capuchin monkeys (Cebus spp.) whose food is distributed in patches that change in availability as fruits mature ("foragers"), memory of where particular trees are located as well as when they fruit may be important. Researchers have investigated whether foraging paths in social groups can be better explained by opportunistic searching or by memory-guided paths to sites that are seasonally plentiful. They have found evidence that not only do monkeys remember where plentiful sources of fruit are located, they also remember when the trees will be fruiting.

Questions surrounding memory in nonhuman animals refer to capacity, duration, and organization. These questions are addressed in laboratory experiments, some of which are designed to provide similar challenges to those provided in the animal's natural environment. A good example of this is the radial maze, developed to study memory in rats. The maze consists of a center circular area to which are attached straight alleys or runways (arms) in a manner like spokes attached to the hub of a wheel. Goal boxes at the end of each runway provide incentives. This apparatus simulates the foraging challenge presented to rats in their natural ecology. The rat is released into the center area and observed to see how it will obtain all the food. The rat's task is to run down an arm, eat the food in that goal box, return to the center area, and choose another arm to enter, repeating this until all the food has been found. Returning to an arm that was previously visited is considered an error. The most efficient solution is for the rat to run down each arm only once. To do that, the rat has to rely on memory of where it has been. Rats show effective use of memory in the task, and researchers have demonstrated that this memory is based on the formation of a spatial map of the maze. Cues from the room containing the maze are used to remember the locations of the arms that have been explored.

Research with humans has shown that humans have a large capacity for remembering items and events over long periods of time. Monkeys and orangutans have shown that they remember photographic stimuli over a delay of at least a year. Even more impressive, a chimpanzee (Pan troglodytes) showed memory for symbols learned 20 years before. Memory for concepts has been shown by squirrel monkeys (Saimiri spp.) over a five-year period, by rhesus monkeys across seven years, and by a sea lion over a ten-year period. These findings are limited only by the test intervals imposed by researchers; the limitations of specific or conceptual memory in animals have

After running a maze several times, a mouse takes less time to find the cheese. (Photo by © Don Mason/Corbis. Reproduced by permission.)

not yet been demonstrated. It is likely that animals have very long-duration memory capacity, especially for conceptual information.

Many experimental studies with nonhuman primates have demonstrated memory phenomena similar to those shown by humans. Historically, human memory has been studied in the laboratory by providing people with lists of items to remember. When humans are provided with an ordered list of items to be remembered, they will show what psychologists term the "serial position effect." The nature of the serial position effect is that items early in the list and late in the list are remembered better than those in the middle of the list. Monkeys show this same effect with visual stimuli. The tests of monkey memory involve recognition memory while the typical procedure with humans involves recall memory. However, Lana, a chimpanzee who had had language symbol training, was able to perform a recall memory task with a list of symbols by choosing the list items from her entire vocabulary of symbols. She, too, showed a serial position effect. The serial position effect is one of the most stable phenomena in memory research with humans. To find the same phenomenon in nonhuman primates suggests that memory processes are similar across species notwithstanding the difference in language ability.

A current area of investigation in nonhuman animals is the question of episodic memory. Memory can be categorized in many different ways. The distinction between working and reference memory was discussed previously. Reference memory can also be divided into declarative (explicit, or conscious) and non-declarative (implicit, or unconscious) aspects. Declarative memory is further subdivided into episodic and semantic memory. Semantic memory refers to memory for information, in other words, generic knowledge. Episodic memory refers to memory for particular events or experiences and implies that the memory involves revisiting that event or experience. This aspect of episodic memory can be characterized as the distinction between knowing something versus recalling the specific event that provided the knowledge. Knowing that an incentive is located in a particular location without remembering the experience of seeing it hidden illustrates the distinction between semantic memory (knowledge) and episodic memory (memory for the event). Panzee, a language-trained chimpanzee, uses language symbols to indicate to an uninformed human caretaker that a particular item has been hidden at some previous time (as long as 16 hours before). Further, she will guide the human to the point where the item (that is outside of Panzee's enclosure and hence unavailable to her) is hidden. Panzee has shown the first attribute of episodic memory; the question is how to determine whether

Chimpanzees (Pan troglodytes) are common subjects for memory and other research. Humans and chimpanzees have 98% of their genes in common. (Photo by ©Renee Lynn/Corbis. Reproduced by permission.)

this memory involves more than simple knowledge that a particular object is hidden outside the enclosure. That is, does Panzee's memory include "time traveling" back to the experience of seeing the object being hidden? At the present time, the answer is unclear. However, the limitations on demonstrating episodic memory in nonhuman animals are likely to be more those of procedure (how do we clearly demonstrate evidence for this capacity in nonverbal organisms?) than capacity (do animals have episodic memory?).

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