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Human Memory

We’ve all had the experience of opening a website, or starting an application, and then finding ourselves staring dumbly at the screen having completely forgotten what we were about to do once it loaded. Believe it or not, the design of the interface actually has a bearing on this. Humans have cognitive limitations, strengths and weaknesses which affect their ability to recall items from memory. This directly impacts on their performance when interacting with computers, and if we recognise this fact, we can accommodate it when designing systems.

Psychological knowledge about the nature of memory is patchy, but improving with research. This research has led to the development of some generally accepted principles and best-practice guidelines for interface design. These can be used to develop systems which make better use of the user’s ability to recall items from memory effectively. Therefore, understanding the psychological principles underlying the workings of human memory is a central part of the HCI practitioner’s role.

Memory & HCI: implications

It is generally agreed that there are three types of human memory:

• Sensory buffers: momentary stores for stimuli received by the senses. This information, unless encoded in the short-term memory, is quickly lost.
• Short-term memory (or working memory): short-term memory acts as a store for information required fleetingly. An example of this would be recalling a telephone number long enough to write it down. Short-term memory degrades quickly, and has a limited capacity.
• Long-term memory: this forms the main resource for memory. Here we store everything we ‘know’. Long-term memory is characterised by huge capacity, slow access time and relative accuracy over time.

In 1983, psychologists Card, Moran and Newell developed a model of the various types of memory with a view to demonstrating how memory-retrieval affects interaction with computers. The Model Human Processor (MHP), as it was termed, shows how the three types of memory interact with perceptual, motor and cognitive subsystems.

What this model describes is how users are able to retrieve task-related information from different types of memory store simultaneously in response to stimuli from various senses. For example, this is how we are able to undertake tasks such as driving, which require us to access all forms of memory at once in order to recognise the information incoming from the senses. In the case of driving, we have:

• Haptic (tactile) feedback from the steering wheel and pedals.
• Perceptual information such as road signs and other cars.
• Auditory feedback such as horns and the increasing volume of approaching cars.

Long-term memory, for example, stores the skills required to drive, directions and the ability to recognise objects such as cars. Short-term memory is required to process road signs.

This tells us that humans are extremely effective multi-modal processors: we access memory efficiently when we receive information from more than one sense, especially when we respond using different channels. Because of this, we know that users’ responses to feedback from a computer system are quicker when they utilise more than one channel of communication. Examples of this include:

• A user pressing a button in response to a beep from a computer.
• Systems with vibrating handsets to provide haptic feedback (e.g. games, flight training simulations).
• A user giving a voice command to their mobile phone in response to a visual cue (display screen message).

There is no need to restrict systems to one mode of feedback. Visual, haptic (things picked up through the sense of touch) and auditory feedback provide users with more efficient means of accessing memory, especially in complex tasks with numerous sub-components.

In addition, psychological knowledge about the nature of memory allows HCI practitioners insight into the constraints this places on the design of interactive systems. Short-term memory holds information that is actively being used (thought about, reasoned with). It is thought to be limited to somewhere in the region of seven ‘chunks.’ A chunk can be thought of as a single object that conveys a larger amount of information (like a Chinese ideogram). Examples of these include words, shapes and colours. However, the information decays in seconds as items are displaced by new items coming in. Icons are an example of chunked information on a desktop which allows users to distinguish between the various programs available to them.

Users perform tasks best when all the information they require from a system can be stored in short-term memory, because accessing items is fast and requires little work. A good HCI principle is to avoid designing displays that present irrelevant information or distractions (like those found on an overly ‘busy’ web page), because this displaces important information from short-term memory. As workload increases, the performance of memory drops. A good example of this is the Microsoft Office Assistant. The infamous animated paper-clip character makes suggestions to the user about the best use of Word, Excel, etc. The suggestions are often good, but the Assistant appears unsolicited during tasks, and the distraction can displace information from the short-term memory.

Long-term memory, on the other hand, is huge and potentially permanent. However, retrieval time is slower than other forms of memory, depends on recency of access and can be confused by similar information. For example, a password that is only used a few times a year is likely to be ‘forgotten’ because it is not rehearsed, or confused with a password for another system. HCI professionals have to design tasks that do not demand the user to recall rarely-used information.

Nevertheless, because long-term memory stores vast scripts for all the learnt tasks in our lives, it becomes a huge reservoir of rich metaphors that designers can tap into to help users accomplish tasks. An example of this is the Windows ‘desktop’ metaphor, which utilises our ‘real world’ knowledge of the office environment (in/out trays, filing cabinets, notepads, etc.) to help us understand the computer environment we are working with.

Jargon buster

Sensory buffers
Momentary stores for stimuli received by the senses, this information, unless encoded in short-term memory, is quickly lost

Short-term memory (or working memory)
Short-term memory acts as a store for information required fleetingly

Long-term memory
Forms the main resource for memory

Model Human Processor
Psychological model showing buffers, short term and long-term memory interacting with information from the senses to produce responses

Multi-modal
adjective often used to describe systems in which input and/or output use more than one of the human senses

Haptic
Anything to do with touch, tactile senses

Further Reading

An introduction to the field: Chapter 7: Memory and Training in;
Wickens, C.D. & Hollands, J.G. (2000) Engineering Psychology and Human Performance, 3rd Edition, Prentice Hall, New Jersey

Seminal HCI work on the subject:
Card, S.K., Moran, T.P. & Newell, A. (1983). The Psychology of Human-Computer Interaction, Lawrence Erlbaum, New Jersey