These lecture notes were not written as a course handout, but as a resource for lectures. Therefore, references and comments will not always be complete.
(part 1 of 6)
Revised 10/97-FER
The last part of the course should have raised your awareness of the ways in which design affects performance. I ended by thinking about some design relevant user characteristics. This part of the course will continue with that theme and should further raise awareness of the characteristics and capabilities of the human operator.
To recap some of the issues I raised in the last part of the course:
A problem for human factors is optimising the functionality and usability of devices and systems so that they match the requirements of users and their tasks.
We need to understand the users' limitations, processing capabilities and characteristics, so that we can allocate functions between the user and the device for maximum effect. This involves knowing:
Ideally, for the purpose of engineering design, it would be desirable to model the human part of the overall system in the same way as engineering style design specifications present clear and precise instructions. Unfortunately, as we have seen in the last few weeks, humans are less predictable, consistent and deterministic than computers and thus defining a general model of the human part of the system is not yet possible.
Instead, a number of more fragmentary and incomplete models of the human information processor have been proposed by cognitive psychologists and reinforced by empirical exploration in the field of experimental psychology. These models are predictive and not prescriptive. Although they provide good first order effects, analysis at a more detailed level usually reveals limitations and inconsistencies. The models are therefore useful at predicting gross behaviour, but often suppress detail.
In general, the most accurate, detailed and specific models relate to those aspects of human performance that can be most easily tested. Thus, characteristics of the senses are well established (particularly vision and hearing), whereas those aspects of the human processor that can only be observed are less clearly understood. You will find later on when we talk about models of learning and memory that there are many more contested issues.
OHP of human body: there are many issues to consider when dealing with the human operator, but today we will focus on sensation and perception, particularly paying attention to features that could be important for design.
We have 5 senses: hearing, vision, taste, smell and touch. Today we will focus on the visual system and on the auditory system (the 'distal senses'), but I will mention issues related to olfaction (smell), taste and touch later. I will also tell you a little about haptic perception. Whilst hearing, vision, taste, smell and touch deal with unsolicited as well as solicited stimuli, haptic perception refers to obtained sensory input, e.g. when you touch a fabric to see how it feels.
Note that we tend to privilege vision and hearing but the other sensory modalities can be extremely important considerations in many designs. The different shaped knobs were an example of using tactile sensations to impart information. For handicapped users we may want to exploit a different sense, for example, touch for the blind.
If we wanted to create the ultimate reality machine, sensory psychology would provide the key data in order to ensure that the world is not over designed. It could tell us exactly where we could make compromises and which information we could dispose.
If you read this, you may wish to ask in class about a 6th sense that is little mentioned, a bit disappointing, but exists non-the-less.
Note that there is an important distinction between sensation and perception.
Whatever sensory system is considered (vision, hearing, taste, touch, or smell), the particular organ receives energy from the outside world, converts it into small electrical potentials and passes these along neurons to the brain. This is the process of sensation (or reception). It is determined by the stimulus quality and the particular organ and part of the nervous system, and is an 'objective' process. Our senses are biological detectors which have a restricted range of operation.
Human colour vision is a good way to illustrate this. Our sensitivity to spectral radiation is not constant through the electromagnetic spectrum. Rather we sample the electromagnetic spectrum through a pair of filters.
OHP: diagram of sensitivity in nanometres (nm) of rods and cones.
One filter is provided by the spectral sensitivity of the rod receptors, which have a maximum sensitivity around 500 nm. The second provided by the pooled responses of the three cone receptors has maximum sensitivity around 555 nm. Below 400 nm and beyond 700 nm we are effectively blind. Therefore, there is a wide range of information that meets our eyes but which is ignored because it falls outside our "window of visibility".
Another example is that if our eyes were more sensitive in the infrared band, we would be able to see the heat produced by our bodies in the dark . Further, Bekesy (1957) has argued that if our ears were more sensitive we would be able to detect the individual molecules or air bouncing off the eardrum. If our ears were more sensitive to low frequencies we would hear the vibrations of our bodies. As it is, we hear the noises produced by the muscles of the arm and hand when we stick a finger in our ear.
In vision filtering occurs not just in the colour domain but also in space and in time. For example, fast but discrete changes are perceived as continuous (e.g. watching movies); we don't perceive the inter-stimulus blanks.
The following table gives some representative thresholds for the various senses and some examples (taken from Galanter, 1962).
Vision A candle flame seem from 50 km on a
clear dark night (100 quanta to the eye,
or 10 quanta absorbed by the rods)
Audition The tick of a watch from 6m in very
quiet conditions (.0002 dynes/cm2)
Taste One gram of tale salt in 500 litres of
water (.0001M)
Smell One drop of perfume diffused throughout
a three room apartment or 1 X 10-12
moles/litre of ethyl mercaptan
Touch The wing of a bee falling on your cheek
from a height of 1cm (10 mg force)
(Some terms: quanta = smallest discrete amount; nanometer = 9 decimal places behind 0 meters, i.e. 10-9; dyne = unit for measuring force, force acting upon a gram for a second that generates a velocity of one cm per second.)
On entering the brain, the nerve impulses are interpreted to produce a recognisable pattern of sight, smell, sound, etc. This process is influenced by each individual's past experience, expectations, feelings and wishes. This is the stuff of perception.
The process of perception is entirely subjective in nature. Simply presenting stimuli in such a fashion that they will be received accurately does not mean that they will be perceived 'accurately' (i.e. as intended).