Human Memory

Introduction

In the previous lessons, we studied the human capability to perceive information through the senses. In this lesson, we will learn how human memory uses and processes this information.

Human memory is the ability to gain, store, retain, and retrieve information. Memory is the second phase of the Model Human Processor. In order to explain the basic function of memory, we can divide the memory process into three memory stages, or we can say that there are three types of memory:

• Sensory memory (SM): This type of memory stores information received through sensory stimuli. Some of the information is passed onto the short-term memory and the remaining information is lost.
• Immediate memory (IM): This type of memory stores information acquired by sensory memory for a short period of time and processes it. If the information is rehearsed or repeated several times, it becomes part of the long-term memory.
• Long-term memory (LTM): This type of memory stores information received from immediate memory after several repetitions. This information gets stored for a long period of time and it gets lost over time if it isn’t accessed again.

Example

Before diving into the details of each stage, let’s first understand the general process with a simple example. Imagine you are driving. While you drive, you are receiving sensory input from many different sources: visual input from seeing the other vehicles around you, people walking on the sidewalk, traffic signals, and auditory input like a car honking. You pay attention to some inputs and ignore the others. Then the vision of a red circle right in front of you comes into your SM. You pay attention to it and realize it is a stop light. This information is now in your IM. You’ve recognized the stop light, but what are you expected to do with this information? Your LTM tells you to stop driving until the red signal becomes green because that’s what you have rehearsed every time you see a red signal.

Suppose you encountered a long sequence of red and green signals. The sequence might stay in your mind for a short period of time but, eventually you will forget it. The sequence of signals was in your IM, and your actions to stop at the red light and resume driving once it turns green are in your LTM.

Now let’s dive into learning some interesting capabilities of each memory stage.

Sensory memory

Sensory memory acts as a buffer between sensory inputs and your IM. Sensory memory is constantly overwritten by new incoming information. Only the information you pay attention to gets stored. Each sensory channel has a separate SM. Thus, we can modify the previous model of memory as follows:

The capacity and retention time of SM is as follows:

• Capacity: large
• Retention time: 25 - 500 milliseconds

Which information gets attention?

We pay attention to the things we are interested in at that time. For example, in the driving scenario, your concentration would be focused on the other vehicles around you, the traffic signals, your vehicle’s speed and gas, etc.

Immediate memory

Immediate memory can be classified into two categories: short-term memory and working memory.

Short-term memory

Short-term memory (STM) acts as a notepad to store information for a short period of time. If it isn’t needed, it is forgotten. For example, when we dial a phone number written on a piece of paper, we temporarily store the number in our memory. Once we have dialed the number, it will vanish from our memory because the task is done and we don’t need the information anymore. Or, while reading, we keep each sentence in our mind from start to finish in order to understand it. Once we move onto the next sentence, the previous one vanishes from our memory.

STM features are as follows:

• Capacity: limited
• Access time: ~70 milliseconds
• Retention time: 15-30 seconds

Measuring STM capacity

The STM capacity can be measured using two methods:

• Free recall: By determining how many elements can be recalled freely in any order.
• Ordered recall: By determining how many elements of a sequence can be recalled in order.

Let’s make a comparison between these two methods using a simple example. Read the number given below only once, then hide it.

Now, try to recall as many numbers as possible from the numbers you just saw, irrespective of their order. This is the free recall method.

Let’s read another number. But this time, try to remember the numbers in sequence.

As you might notice, you were probably able to remember more numbers in the free recall method. If you were able to recall 5 to 9 numbers of the sequence, your digit span falls in the average category. According to George A. Miller, a Professor of Psychology, an average adult can remember 7±2 items in STM. We can use this ability to enhance our memory capacity. For example, look at the sequence below only once, hide it, then try to recall it.

Were you able to recall more numbers this time? Basically, we divided the sequence into chunks of numbers, and according to the 7±2 rule, we were able to store 7±2 chunks of numbers rather than 7±2 numbers individually.

Working memory

Working memory is used to retrieve the information stored in LTM for a short period of time, and use this information in conjunction with any information currently stored in the STM. Working memory, like STM, has a limited capacity.

Let’s look at an example to demonstrate this concept. When you are attending a lesson, you are perceiving the information being displayed on the projector or the words being conveyed by your teacher in STM. Meanwhile, you are also retrieving information already stored in your LTM and comparing it with the information stored in your STM by using your working memory.

Long-term memory

Long-term memory (LTM) is basically everything we know. This includes all the factual information, our behavioral information, experimental knowledge, and more. In the example of STM where we had to dial a number, we read it from paper and stored it in our STM. But how did we know how to dial this number on the phone or which button to click? This knowledge was in our LTM.

Before diving into the details of how we store this information in LTM, let’s first look into some features that differentiate LTM from STM:

• Capacity: Huge, nearly unlimited
• Access time: ~100 milliseconds
• Retention time: unlimited

LTM types

LTM can be divided into three types depending upon the nature of the stored information.

• Episodic memory: Stores memories of past experiences and events. For example, the memory of an accident, first day of college, birthday parties, etc.

• Semantic memory: Stores information of the things you know and factual knowledge. For example, words and their meanings, country names and people’s names, etc.

• Procedural memory: Stores information about how to do things. For example, how to move the body (motor controls), how to use a computer, brush your hair, etc.