Properties of Sensory Channels

A Taxonomy, and Connections with User Interface Design


Regardless of what display technologies or other output devices are used, a human ultimately perceives information from a computer through the familiar senses of sight, hearing, touch, etc. This paper explores some of the basic properties of the different sensory channels, classifies them within a taxonomy, and tries to find useful analogies in user interfaces. Finally, speculations on how aliens might sense their environment and interact with each other are given. The observations made offer a new way of thinking about certain types of interaction. Note that these observations are not limited to just interaction between a human and computer, but touch on more general kinds of interaction between abstract entities.

Introduction and Motivation

At an anatomical level, there are many details that make each sensory channel different. However, at a more abstract level, some basic differences and similarities are apparent. For example,

It is also interesting to think about some of the physical quirks of our senses. For example, if both light and sound are waves, why is it that our eyes can form images from light waves, but our ears cannot form images from sounds waves ? Or, why is it that our eyes only sense 3 primary colours, but our ears can sense thousands of different frequencies of sound ? Even more, why is it that we have 2 eyes, 2 ears, and 2 nostrils ? What are the advantages of having pairs of sensory organs, and are the reasons the same in all cases ?

Thinking about these and similar issues, I was motivated to try and enumerate the basic, abstract properties of senses and sensory information, and to create a taxonomy. In so doing, I hope to (i) find analogies between the properties of sensory channels and aspects of user interfaces that may help inform the understanding and design of user interfaces and virtual worlds, and (ii) speculate on what possible sensory channels may be relevant to alien beings on another planet, and what the consequences might be on their way of interacting with their environment, and perhaps even what their computer output devices may be like.

Concepts and Terminology

Figure 1. Imagine an observer, equipped with sensors, who goes out exploring a world full of sources of information. What are the different ways that the observer may acquire information ? What different types of sensors might the observer be equipped with ?

Figure 2. Different possibilities for sensors. A: an omnidirectional sensor that does not discriminate between different directions. B: a parabolic mirror enables the creation of a unidirectional sensor that can be rotated to detect information coming from a specific direction. C: an array of sensors, each of which is "tuned" to a different information component. (Examples of this arrangement of sensors is in the human ear, where cilia in the cochlea are tuned to detect different frequency components of sound, and in the human nose, where odours are detected by receptors tuned to various different molecules.) D: an array of sensors combined with a lens, enabling the formation of an image. (Cones on the human retina work like this, and in addition there are three different types of cones, tuned to red, green, and blue frequencies of light respectively.) It would also be possible to form an image using an array of unidirectional sensors, such as the one in B, each pointing in a slightly different direction.

Taxonomy of Sensory Channels

The below table is a taxonomy showing different properties of information and sensors, and shows where the human senses (and some other instruments) exist within the taxonomy. Objections could be raised at some of the classifications, and many details and possible extra dimensions have been omitted. However, this taxonomy is proposed as a useful compromise between excessive detail and uninformative simplicity.

The colour coding of the table gives an indication of the effort involved in each sensory activity.

information is spatially localized (it can't travel far from its source) information travels or propagates across space 1
information diffuses or meanders across space information travels in straight line (rectilinear propagation) 2
sensors are omnidirectional (orientation of sensor doesn't matter) sensors are unidirectional (they detect information coming from a given direction) 3
small number of sensor(s) large array of sensors that form an image
sensors cannot distinguish different components (e.g. frequencies) within the information cutaneous sensing of texture;
cutaneous sensing of temperature
  light meter;
noise meter;
ocelli (simple eyes)
different components (e.g. frequencies) can be filtered or independently detected in the information a small number of different components are sensed by different sensors         human eye
sensors can be tuned to detect different components     tunable radio receiver radio telescope  
large array of sensors that each detect a different, fixed component (so an entire range of components is perceived simultaneously) human taste;
faint odours (e.g. scent of a flower);
faint sounds (e.g. whisper)
human smell human ear "parabolic"-shaped ear;
echolocation in bats (?)

Colour Legend:
Active sensing: To obtain information, the observer must use locomotion to displace herself (or an appendage, such as a finger or antenna). She must go to the information.
Semi-active sensing: Although information flows to the observer, the observer may have to scan in different directions or across different components to find the information.
Passive sensing: The information is ambient or pervasive, constantly flowing to the observer, who can perceive it (as if it were always in the background) without effort.

Footnotes (regarding the determination of direction and distance to a source of information):
1 In this case, an observer equipped with a pair of sensors could detect the approximate gradient, i.e. the direction from which the information is coming. This is one reason, for example, humans have two ears, and may be the reason many animals have two nostrils (although dogs are often seen waving their snout to and fro when sniffing the air, suggesting how gradient detection can done with a single sensor).
2 In this case, if the information travels with a constant speed, then an observer equipped with three sensors could conceivably estimate the direction and distance to a source of information. Seismologists use this technique to determine the epicentre of an earthquake. It is also possible emit a pulse of information, and use a single sensor to measure the time for the pulse to return, to determine the distance to an object. This principle is used in radar, sonar, and many auto-focus cameras.
3 In this case, the direction to a source is of course implied by the orientation of the sensor. Furthermore, an observer equipped with two sensors could use triangulation to estimate the distance to the source of information. Humans use their two eyes this way.

Analogies in User Interfaces

The active-passive scale

Sullen et al. [sellen1992] identity five different dimensions for characterizing feedback. One of these is "demanding" versus "avoidable" feedback, denoting whether the user can choose to monitor or ignore the feedback. This is very similar to the active-passive scale (see definitions of Active, Semi-Active, and Passive Sensing in the colour legend) used in the above taxonomy of sensory channels. In this paper, however, rather than limiting our attention to feedback, we wish to describe any information that may be "out there", e.g. on a computer, and of potential interest to a user. The active-passive (or demanding-avoidable) scale serves as a measure of the effort required to retrieve a given piece of information. From a design perspective, we would like important or frequently accessed information to be retrievable with as little effort as possible. We now consider different schemes for achieving this within a user interface.

A good example of passively-sensed (or demanding, or ambient) information is audio output. Sullen et al. [sellen1992] reference work by Monk [monk1986] where "Monk argued that sound is a good choice for system feedback in that users do not constantly look at the display while working." [sellen1992, p. 142] Indeed, as already indicated by our taxonomy, auditory information from a computer can be perceived without effort on the part of the user (assuming the user has not become so habituated to the audio output that she is simply ignoring it). Baecker et al. agree, using the words "ubiquitous" and "localized" [baecker1995, p. 526] to describe audio and visual channels, respectively. Another potential channel for delivering ambient information is smell. Although there are practical problems, devices that output different odours do exist [kaye2001].

Some sensory channels which our taxonomy classifies as active (i.e. requiring effort) can be made more passive in a user interface setting by virtue of the limitations of the output devices involved. For example, if a computer is equipped with a small, low-resolution screen, and the user has no reason to look away from the screen, we may assume that the user can passively sense all information on the screen without ever changing their general direction of gaze. Similarly, if the user's hand never leaves a mouse, the user can passively sense the kinesthetic feedback of pressing a mouse button down without having to first move their finger to the button.

One scheme, then, for output of visual information that can be sensed passively (or almost passively) is to make the information occupy a large area of the screen, so it is visible wherever the user looks. This scheme does not preclude the display of other information. An anecdotal example helps explain this. Users of window managers with multiple virtual desktops can often keep track of the currently active desktop using a small, zoomed-out "map" of all desktops. This map is typically located in the corner of the screen. I once encountered a user who, to avoid having to glance at such a map, had configured his system to change the background image whenever he changed desktops. The background images were chosen to make it easy to see which desktop he was currently in (Figure 3), wherever his attention happened to be focused on the screen.

Figure 3. A screenshot showing a background "wallpaper" image that reminds the user they are in virtual desktop #2. Wherever the user happens to look, the numeral 2 is easily perceived in the background image, which creates a kind of ambient (or passively sensed) information.

Sellen et al. [sellen1992] used a similar strategy to try and make visual feedback as salient as possible. In their studies of mode errors, the visual feedback used to indicate a mode change was a change in the background colour of the entire screen.

Another possibility for making visual information easily perceived is to place it at a common focal point of the user, e.g. the mouse cursor, so it is obtrusive and unlikely to go unnoticed. Many user interfaces exist which change the mouse cursor depending on the system's current mode. McGuffin et al. [mcguffin2002b] describe the design of an interaction technique in which a slider widget temporarily becomes attached to the mouse cursor. They suggest that this creates "visual tension" [mcguffin2002b, p. 41], analogous to kinesthetic tension [buxton1986, sellen1992], which reminds the user of what is occurring.

As we move along the scale from passive sensing to active sensing, we encounter the notion of displaying information in a corner of the screen or in a status bar. Although such information is always visible, the user must make a (small) effort to look at it, and may easily forget or otherwise fail to do so. In keeping with the taxonomy of sensory channels, we may say that such information is sensed semi-actively. This partly motivates the idea of using gaze as a channel for input [ware????,zhai????,vertegaal2002]: it arguably requires less effort to "point" with one's eyes than with one's hand or finger. Indeed, Argyle and Cook point out that gaze "must be treated both as a channel and a signal" [argyle1976, p. xi].

Finally, although a user sitting at a desktop computer need not displace their body very much, they nonetheless do navigate and travel through virtual worlds, which motivates the notion of active sensing on a computer. Any time a user must, for example, click on a button to bring up information, or scroll to a desired area, or fly through a virtual world to find something, they are actively retrieving information. This is analogous to an observer who actively goes out into their environment to acquire tactile information.

An interesting scheme for making such navigation easier was suggested by Pierce et al. [pierce1999]. In 3D virtual worlds, the user often needs to "look around" their point of view to orient themselves. This is relatively easy and natural to do in the real world, however, in many interfaces for navigating 3D, this can be cumbersome. Pierce et al. introduced the notion of "glances" for better supporting panning in a virtual 3D world: the user can quickly look away from their current direction, and when they are done glancing in the new direction, their view snaps back to the previous direction.

In summary, we have seen how the active-passive scale for classifying sensory channels has direct analogies within user interfaces.

Speculations on Extra-terrestrial Life

It is interesting to imagine alien beings in alien environments whose sensory organs are variations on human senses, or that fill in some of the blanks in the taxonomy. This exercise can lead to thought experiments akin to Thomas Nagel's oft cited "What Is It Like to Be a Bat ?" [nagel1974] [dennett1991, pp. 441--448] (see Dawkins [dawkins1986, Chapter 2: Good Design] for a description of the auditory system of bats, and how they can "see" with sound, in some sense).

Many of us are taught from an early age about the 3 primary colours of light. Mixing together these 3 colours allows any visible colour to be synthesized. What is perhaps not as emphasized, however, is that these 3 primary colours are merely an artifact of human biology: the colour associated with a given distribution of light is determined by the excitation levels of the 3 types of cones in our retina. These cones effectively sample a continuous distribution at only 3 points: red, green, and blue. Alien beings with 5 different kinds of cones, for examples, would be able to see a richer range of colours, and could perceive differences between colours that are indistinguishable to a human. Even more difficult to imagine is an alien capable of directly sensing hundreds of different frequencies of light (much like how the cilia in a human's inner ear are tuned to hundreds of different frequencies of sound). Would such beings experience light as a kind of music, with rich melodic and tonal qualities ? Would such beings "sing" luminous songs with special organs ?

Another bizarre possibility is that some aliens may have sensory organs which can be tuned to detect different narrow bands of frequencies, not unlike how a radio receiver is tuned. If an alien species had, for example, tunable acoustic sensors ("ears"), and could also tune the pitch of their voice to fall within one of many corresponding bands of voice, it might be possible for a room full of such aliens to carry on multiple, simultaneous conversations, without separate conversational groups being audible to each other. (This is akin to how many conversations can take place over radio waves all at once, but without interfering with each other because they take place on different frequency bands.)

One could even imagine two aliens who are carrying on a private conversation within a secret frequency band, and a third "spy" alien who sees their mouths moving, and decides to try scanning different audio frequencies with her ears until she finds the right band for eavesdropping. This example shows how communication can involve multiple, interacting sensory channels: although the first two aliens tried to keep their conversation private by using a secret audio band, they did not hide the visual cue of their moving mouths from the eyes of the third alien.

Gaze and mutual gaze in aliens

In humans, eyes serve both to take in visual information from the environment, and also to send signals out to others. In the very act of gazing somewhere, we cannot help but reveal (to anyone looking at us) where we are looking (unless we are wearing sunglasses). We are all familiar with the social interaction this can result in, e.g. when we "catch" someone else staring at us, we may feel threatened, challenged, flattered, etc. -- and we quickly decide if we should look away or return the stare. It is interesting that humans are one of the few primates with a dark pupil surrounded by a white sclera; this may be to enhance the visibility of our direction of gaze [kobayashi2001a,kobayashi2001b].

The state of mutual gaze, or eye contact, between two people is particularly interesting. Not only do the two individuals become aware of each other's gaze, they are also aware that each other is aware of this. We might even be tempted to suppose that they are each aware of the fact that each other is aware of this latter statement, ad infinitum. Of course, such an infinite sequence of deductions can't really be what happens in our mind. Somehow, it seems sufficient that the two parties involved be aware that their knowledge of the situation is shared. In any case, mutual gaze is a psychologically significant situation: we know that one effect of being looked at is heightened arousal [argyle1965][vertegaal2002]. It has also been of interest more generally: the famous philosopher Jean-Paul Sartre [sartre1943] regarded mutual gaze "as the key to `inter-subjectivity'" [argyle1976, p. x]. It is probably true that mutual gaze constitutes a kind of "strange loop" or "tangled hierarchy", as defined by Hofstadter [hofstadter1979]. When one eye looks at another eye, the former becomes a meta-eye: an eye that sees other eyes, like a sentence talking about other sentences, or a vacuum cleaner that cleans other vacuum cleaners.

We can represent gaze abstractly using arrows. Figure 4 shows one individual gazing at another, and Figure 5 shows two individuals engaged in mutual gaze. It is easiest to think about these figures in terms of the human visual channel, however the notion of gaze can be generalized to other sensory channels. Imagine an alien species with a sensory organ that can be somehow adjusted (either in orientation, or tuning, or otherwise) to perceive different information, and that this sensory organ reveals the state of its own adjustment, emitting information about itself that can be perceived by other individuals. In the case of humans, the eyes are adjusted (in orientation) to perceive information coming from different directions, and the eyes themself emit a (visual) signal of where they are looking. By analogy, one can imagine an alien whose ears can be tuned to a given frequency band, and that may also (due to evolution?) give off a sound of a specific frequency to signal their current tuning. Two such aliens can be engaged in mutual "acoustic" gaze, by simultaneously perceiving each other's tuning.

Thus, in the notation used in Figures 4 and 5, if an arrowhead touches the sensory organ of individual X, this means that the originator Y of the arrow can perceive the tuning/orientation/adjustment of X's sensory organ.

Figure 4. A can see where B is looking, but B cannot see A. Furthermore, A knows that B is oblivious to A's gaze. If asked to draw a diagram representing this situation, only A would be able to draw the above diagram.

Figure 5. Mutual gaze, or eye contact, as it occurs (for example) in humans. A can see that B is looking at A, and B can see that A is looking at B. Their knowledge of the situation is symmetric, and furthermore A and B each know that their knowledge is shared. If asked to draw a diagram representing this situation, A and B would each be able to draw the above diagram.

Things become more interesting if we allow for a second sensory channel to play a role in gaze. In Figures 6, 7, and 8, each individual is equipped with a second sensory organ which senses information through a second channel (dashed lines). We might imagine that the first and second channels are visual and acoustic (e.g. perhaps the aliens emit a sound whose frequency indicates the orientation of their eyes), or perhaps both channels are visual, with one involving "visible" light and the other infrared light.

Figure 6. A can see where B is looking. However, unbeknownst to A, B can also sense where A is looking, through a second sensory channel (represented by dashes). So B knows that A knows where B is looking, but A does not know this.

Figure 7. Quasi-mutual gaze. Through a second sensory channel (represented by dashes), A and B can each sense where the other is looking. However, neither A nor B are aware that the other can sense where they are looking. Hence, although the situation is symmetric, and the knowledge that A and B each have of the situation is symmetric, neither of them would be able to draw the above diagram if asked to. Each of A and B thinks that they are covertly spying on the other. It would seem, then, that none of the normal phenomena associated with mutual gaze (e.g. signalling a threat, dominance, interest, or love) could occur in this situation. (Note that the quasi-mutual gaze represented here is somewhat akin to two humans looking at each other through sunglasses, neither of them realizing that their stares are being returned. Of course, this would only involve one sensory channel, whereas in the diagram we have two channels.)

Figure 8. Mutual "gaze" involving two sensory channels. This is an elaborate situation where A and B each use their two sensory organs to gain awareness of the other's two sensory organs. Each of A and B can deduce symmetric knowledge of the situation, and also deduce that this knowledge is shared. If asked to draw a diagram of the situation, each of A and B would be able to draw the above diagram.

Finally, we may note that all the scenarios of mutual gaze considered so far have involved only two individuals. Of course, if the "gaze" is done using unidirectional sensors, such as the human eye, then a "line of sight" is necessarily involved, limiting mutual staring to dyadic groups. However, were a different, omnidirectional channel involved, it could be possible for N parties to be locked in a mutual, shared, and simultaneous sensing of each other. I leave it to the reader to wonder what kind of social interaction such N-way mutual gaze could lead to.


Thanks to Brian Wong for providing some initial reactions to this work.


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Copyright ( C ) Michael McGuffin, 2002
Written between July 31 and August 9, 2002
Based on ideas developed in 1999
Minor updates performed Jan 11, 2003
Minor update performed Mar 12, 2003