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Memory is fascinating because it isn’t a single thing. There are multiple types of memory that serve different functions and are supported by different parts of the brain. In humans, different types of memory are often differentiated on the basis of peoples’ commentary about the subjective experiences associated with memory, such as “I remember what it was like to be there” or “I just knew I had seen them somewhere before.” Because we cannot get these sorts of reports from nonhuman animals, we have to come up with other clever ways of distinguishing one type of memory from others. Studying memory in monkeys helps us answer questions about which aspects of memory are general characteristics of primate memory and which are uniquely human (or uniquely monkey).
Watch any animal interact with the environment and you’ll see that it reacts to some objects as familiar and others as unfamiliar. But how would you know if an animal were remembering something absent, far away, or out of sight? To address this question, we’ve developed a recall test for monkeys in which they reproduce part of a remembered shape that is not present. This is similar to some human visual recall tests, in which subjects draw what they remember. We compared performance in this recall task to that in a recognition test, in which the image to be remembered is re-presented at test, and can be responded to as familiar. Performance by monkeys in these two tests shows a similar pattern to known human data. This suggests that the ability to recall absent information doesn't depend on language or anything else uniquely human. To research some of the innovative methods other labs have developed to tap the same aspect of memory, please visit Charles Menzel, Howard Eichenbaum, and Richard Morris.
The image and video to the right show a monkey reproducing part of a three-block shape.
The distinction between explicit memories (those memories we are aware of) and implicit memories (those we are unaware of) is central in taxonomies of human memory. In the lab, we study whether monkeys can access some aspect of their memory through several metacognitive paradigms. For example, monkeys demonstrate that they may "know when they know" by declining memory tests when they have forgotten or by seeking information only when ignorant. Other labs are working on related problems in nonhumans, for example: Sara Shettleworth , Herb Terrace , David Smith , David Washburn. The figure and video to the left depict a prospective test where the monkey can opt out of trials on which he has forgotten the studied image.
Think of the last time you met a group of people and were told their names one-by-one. Which names you remember likely depends on the order in which you heard them, the number of names, and your prior familiarity with those names. One common finding is that our ability to remember items from a list of sequentially presented items follows a “U” shaped function, such that we remember the first and last items better than middle items. Indeed, most of the things we remember from our day-to-day lives happen in similar sequences, so understanding how memory for lists works is important for understanding normal memory. In the lab, we model this common memory situation in monkeys using lists of photographs. Other labs working on this problem in nonhumans include those of Herb Terrace and Tony Wright. The image to the right shows some of our findings about the effects of list position and image novelty on memory in monkeys, compared to typical human results.
- Memory for what, where, and when
A hallmark of episodic memory in humans is the experience of remembering having personally been there at the time the remembered event occurred. To model this kind of memory in monkeys, we are testing whether or not monkeys remember various aspects of the context in which memories were formed (begin a search for more information at the following labs: Nicky Clayton, Jon Crystal). The picture on the left shows a monkey looking for hidden food in one experiment that tested whether monkeys know what food they found, where they found it, and most importantly how long ago they had that experience.
- Future planning
Memory did not evolve in order to allow us to reminisce about the past. It evolved to allow past experience to guide current and future behavior. Humans use memory for planning all the time; we plan to repeat dinners we enjoyed, or to take an umbrella with us when we remember hearing rain predicted in the morning forecast. Our lab has explored whether monkeys can engage in simple planning that requires them to take an action now to satisfy a future need. Other researchers are also studying this same issue, including Nicky Clayton and Bill Roberts.