Discussion: Memory

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Session Overview

How do we carry information forward in time? This discussion session complements the prior lecture sessions Memory I and Memory II.


Before jumping into the discussion of topics related to memory, try this brief demonstration. Here's what to do: Click the link below, spend exactly 15 seconds looking at the faces that pop up, and then click the link again to hide the image. Then continue on with this discussion on this page. We'll check back with you in a bit...

View/Hide Image

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What is memory for? What does it do for us? The classic example of the importance of memory is touching a hot stove. The first time you touch a hot stove you realize it was a bad deal. You store that away for the future, so if you see a stove again, you don't touch it. So, memory guides our future behavior based on our past experiences, ideally to increase our chances of survival.

So, if the essential task of a memory system is to carry information forward in time, what properties should that system have? Think about the memory devices you use in everyday life: A USB stick, a post-it note, your mind, etc. What do they need to be able to do?

Sample Answer

It should record what’s going on now (encoding).

It should retain the information (storage).

It should provide access at a later time (retrieval).


There are several different types of memory systems in the brain which accomplish this task of carrying information forward in time in very different ways. The terms used by psychologists include sensory memory, short-term memory, and long-term memory, and each of these terms have been refined by neuroscience research. We'll talk in lecture about, for example, the degree to which certain kinds of memory are hippocampus-dependent, and about how encoding, storage, and retrieval are accomplished in these types of memory.

A good example of sensory memory would be if I showed you a bunch of letters just for a second and asked you to report as many of them as you could.


Most people can report the first four or five, and it seems like they just can't take in any more information so quickly. But a carefully-designed experiment can show that sensory memory has much more capacity than you'd think. In one study, the same letters were flashed, but a high-, medium-, or low-pitched tone was played immediately after, indicating which of the three rows to report. As it turns out, you can report any of the rows with pretty good accuracy. The idea is that our sensory memory is very good. It can store a lot of information, but it fades very rapidly. The transmission of information from sensory memory to short-term memory (which enables you to say the names of the letters) is constrained. You just can't say all of the letters you saw before that sensory memory decays.

Short-term memory is the kind of memory that requires active maintenance, like looking up a phone number and repeating it to yourself until you can dial it. It's transient, it requires maintenance, and it only lasts a couple of seconds. Long-term memory, though, brings us to the topic of forgetting, and to Ebbinghaus, who invented and memorized word lists. On the first day he'd remember most of them; on the second day, not as many; on the third day, not as many; and so on and so on. And he discovered this power law relationship (the "forgetting curve") between what you need to know and how much time has elapsed, which seems to be an intrinsic property of human memory.

Graph, image, word list.

Image by MIT OpenCourseWare.

Critical Thinking

The graph below depicts the results of an experiment in which participants had to remember 15 items. Take a look at the graph and describe the results for both the 0-second delay condition and the 30-second delay condition.


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Based on what you know about memory processes, explain the results for the first three items presented and the last three items presented.

Sample Answer

If you’re prompted to recall items in a list immediately after learning it, you often remember the last few items. However, if you wait 30 seconds, you probably won't remember the last few items. Regardless of the delay period, the first few items are, on average, recalled most accurately.

We remember earlier items better because we have more time to put them into long-term memory, a phenomenon known as the primacy effect. In addition, the items seen more recently are also remembered better because they are still in short-term memory — this is the recency effect.

If you’ve been rehearsing the items, the first few have had the opportunity to enter long-term memory, where they persist, even after a 30-second delay.



Another way to classify memory is by whether it's implicit or explicit. Explicit memory is stuff that we could recall if we were asked to recall it, whereas implicit memory is stuff that you do without any active representation or self-description. Implicit memory includes habits (stepping on the brake when you see a red light), skills (riding a bike), priming, and classical conditioning.

Now might be a good time to review the difference between classical conditioning and operant conditioning. (Return to Discussion: Learning if you need a refresher.) Why is classical conditioning considered implicit memory, while operant conditioning is explicit?

Demonstration: Implicit Memory

Listen as Tyler introduces the demonstration: (MP3) (0:00:40)

Then click to reveal a game.

Play the Game

Having just listened to Tyler's story, here are some word stems. What words first come to mind?

CAR ___
MET ___
PRO ___
PER ___
THI ___
POT ___


After you've played the game, listen to Tyler's explanation: (MP3) (0:03:12)

Spoiler (what you heard first)

The chairman of the committee usually opens the meeting with one of his boring and sometimes even perverse lectures. Today he started with some stupid proverb: I can't even remember it – something about not planting thistle seeds. Then he went on a pointless tangent about how to guard a political leader from a potential assassination in a crowded place such as a carnival. Is there any method behind the madness of his talks?


Demonstration: Explicit Memory

Remember those faces that you saw at the beginning of the discussion? Let's test your memory. Click the link below and try to determine which of these men you've seen before.

View/Hide the Test

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How did you do?

View/Hide the Answer

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