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Introduction

This article discusses how we perceive the world through filters (our senses) and how modelling these filters can be useful when designing human-computer interfaces. It goes on to attempt to classify modes of interaction using the criteria of sense and dimension for application in Human-Computer Interaction.

Our Filtered World

In Douglas Adams' book Mostly Harmless, a new version of the Hitchhiker's Guide to the Galaxy has been invented. The new guide is a black disc which opens out like a piece of oragami into a black bird. When the guide is first used by a new user it "callibrates" itself with the user by experimenting on the user to find the limits of their senses and what dimensions they perceive.

In the story, the guide callibrates itself to a human being, Arthur Dent's daughter. When it has finished it says:

"I can tell you that in your Universe you move freely in three dimensions that you call space. You move in a straight line in a fourth, which you call time, and stay rooted to one place in a fifth, which is the first fundamental of probability. After that it gets a bit complicated, and there's all sorts of stuff going on in dimensions 13 to 22 that you really wouldn't want to know about. All you really need to know for the moment is that the Universe is a lot more complicated then you might think."

The guide later explains why the callibration was necessary:

"Your universe is vast to you. Vast in time, vast in space. That's because of the filters through which you perceive it. But I was built with no filters at all, which means I perceive the mish mash which contains all possible universes but which has, itself, no size at all."

So human beings perceive the world through very specific filters. Our eyes can safely detect certain levels of light and our ears can detect certain levels of sound. We perceive certain colours of light more than others (e.g. red appears brighter) and sounds of certain frequencies and amplitudes can mask other sounds. Our senses have limitations, we have blind spots, we can only perceive within certain limits and we can only process a certain amount of information at a time. Often the brain substitutes gaps in our perception with theory driven perception to form a complete but sometimes inaccurate picture in our minds.

When designing the interface between a human being and a machine, it is useful to model our perception and take advantage of this knowledge. Compression technology for images and sounds takes advantage of the limitations of our senses. For example, mp3 compression removes frequencies we can't hear, to reduce the amount of data required to represent information. Have you ever wondered what an mp3 sounds like to a dog?

We can classify interaction by different criteria. Two possible criteria are "sense" and "dimension", that is which of our senses we perceive something with and in how many dimension we perveive it.

Senses

The five human senses we all know about were first listed by Aristotle:

• sight
• hearing
• touch
• taste
• smell

There are also four other commonly agreed senses:

• thermoception (sense of heat)
• equilibrioception (sense of balance)
• nociception (sense of pain)
• proprioception (sense of body awareness)

As well as these are other senses which could be argued to exist such as hunger and thirst, but the main senses currently of the most interest in Human-Computer Interaction are those of sight, hearing and increasingly the sense of touch for haptic interfaces. The other senses may have uses in the future, but these are currently regarded the most useful.

Dimensions

A dimension is a parameter used to describe the position and characteristics of a conceptual object within a conceptual space. In physics, string theory and m theory predict that space actually has 10 or 11 dimensions respectively, but four dimensions are commonly discussed in everyday life.

The first three dimensions are spatial dimensions, a line is a "one dimensional" shape, a plane is a "two dimensional" shape and a cube is a "three dimensional" shape. Movement in any direction can be described in terms of movement in three dimensions: up/down; left/right and forwards/backwards.

The "fourth dimension" often refers to the temporal dimension of time. In maths, the fourth dimension is actually another spacial dimension, but we will use time here as it has more relevance to HCI. Time is unusual in that we perceive it at a constant speed, in a single direction.

So we can say that we perceive the world through four information dimensions, including three spacial dimensions (space) and one unidirectional temporal dimension (time).

Classification

This is a rather loose interpretation of dimensions, but one which serves the purpose of the classification of modes of interaction. For example, a line of text is not one dimensional as such, but it serves a useful distinction in computing. A single line of text can be displayed on an LCD screen or a sequence can be flashed on an LED, which is different to displaying two dimensional shapes on a computer screen.

Note that where sound is involved, it always varies along the fourth dimension. Of course, we can only perceive an image if it is displayed for a non-zero amount of time, but a static image represents more meaning than a static sound so it is more important to distinguish a static image from a moving one.