Perception (German: Wahrnehmung) is the first stage of human information processing: taking in signals through the senses, such as reading a display, hearing an alarm, or feeling a pressure point. What does not get through at this stage cannot be processed at any later one.
Place in information processing
In the perception-cognition-action model, perception comes first, and a consequence follows that is regularly overlooked in practice: A perception problem cannot be fixed by measures that only start at the level of comprehension. If a warning message is not seen, neither better wording nor more extensive training helps. Both measures assume that the information has arrived in the first place.
When assessing an observed use error, the first question to clarify is therefore whether the user perceived the relevant information. Only if the answer is yes are comprehension or action problems the right explanation. Skipping this sequence leads to measures that miss the actual problem, typically an additional note in the instructions for use that leaves the cause untouched.
Visual perception
The majority of information on medical devices is conveyed visually. Whether it arrives depends on measurable variables that can be influenced deliberately through design:
- Contrast and luminance: decisive not on the data sheet but under real lighting conditions: a darkened operating room, glaring daylight at a window, emergency lighting in an ambulance.
- Character size relative to viewing distance: What matters is not the absolute font size but the visual angle. A display that is easily legible at a desk can be illegible from the foot of a bed.
- Viewing angle dependence: Many display technologies lose a great deal of contrast at a shallow viewing angle, exactly the position from which a standing user looks at a device lying down.
- Color vision deficiency: A relevant proportion of the male population cannot reliably distinguish red and green. Color alone therefore never carries safety-relevant information.
- Age-related changes: Declining accommodation ability and an increased need for light are the rule rather than the exception in most user groups.
- Light-dark adaptation: Switching between bright and dark environments takes time. In the first moments after such a change, recognition performance is markedly reduced.
In addition, there is the distinction between central and peripheral vision: The peripheral visual field is sensitive to movement and changes in brightness but unsuited to detail and color. A status change that is meant to be noticed peripherally must therefore be signaled through movement or brightness, not through a color change on a small symbol.
Auditory and haptic perception
Auditory signals have the advantage that they require no visual attention, a considerable gain in situations where attention must stay with the patient. Their effectiveness, however, depends on the margin above the ambient noise: A signal that is unambiguous in a quiet test room can be masked in an intensive care unit with several active devices.
Two effects deserve particular attention. First, sound localization is poor for pure tones: With several similar devices in a room, it is often unclear which device is currently alarming. Second, a high number of alarms, especially false alarms, leads to habituation: Signals are attended to with delay or no longer at all. This alarm fatigue is a known safety problem in everyday clinical practice, and it arises not at the individual device but in the interplay of all devices at the workplace.
Haptic feedback (a tangible pressure point, a vibration signal, a mechanical detent) is valuable where neither sight nor hearing is available. It confirms an input without diverting attention and is an effective means against unnoticed failed inputs, especially when operating with gloves.
Attention and expectation
Perception is not a passive process. It is selective and strongly guided by expectations: People preferentially see what they expect to see and reliably overlook what lies outside their current focus of attention, even if it is clearly visible. This effect is described as inattentional blindness and explains a large share of the cases in which it seems incomprehensible in hindsight how a conspicuous message could have been missed.
For design, the following results: Safety-relevant information must not depend on the user happening to look at the right place at the right moment. And changes that occur while the user looks away are especially easy to miss, one reason why state changes after interruptions should be displayed permanently and not only briefly. The higher the cognitive load, the narrower the focus of attention becomes and the stronger these effects are.
Design consequences
A few robust rules follow from the mechanisms described: Safety-relevant information is coded redundantly, that is, through at least two channels or features. Critical states are displayed permanently rather than only announced when they occur. Alarms are differentiated by urgency and deliberately limited in total number to prevent habituation effects.
For critical tasks, the perceptibility of safety-relevant information must be checked with particular care, since even small perception deficits there can lead to safety-relevant use errors. Most important, however, is the test condition: Perceptibility must be demonstrated under real environmental conditions, not in a quiet, well-lit test room. The relevant conditions are provided by the survey of use environment factors; whether they are taken into account in the test setup determines whether the summative evaluation can uncover perception problems at all.
Regulatory reference
Perception-related requirements are spread across several levels. IEC 62366-1 addresses them indirectly: Use errors that result from information that was not perceived are to be considered in the usability engineering process and minimized through design, in practice in the use-related risk analysis.
More concrete requirements are added at the product level: IEC 60601-1-8 sets requirements for alarm systems of medical electrical equipment, including the distinguishability of priority levels. For the ergonomic design of displays, the ISO 9241 series (adopted in Germany as DIN EN ISO 9241) is relevant. For symbols on the labeling of medical devices, ISO 15223-1 provides a dedicated standards basis that supports the use of established and therefore expectation-conforming symbols.
Perception is the first stage of information processing and a prerequisite for everything that follows. What is not seen, heard, or felt can be neither understood nor answered correctly.
Perceptibility is a measurable property, not a matter of the user's diligence. It must be demonstrated under real environmental conditions and should always be backed up redundantly for safety-relevant information.
Frequently asked questions (FAQ)
Why does an additional warning notice not help if a message is overlooked?
Because the problem lies at the perception stage, and a notice only takes effect at the comprehension stage. If information is not perceived, better wording changes nothing. Effective are measures that improve perceptibility itself: contrast, size, position, permanence of the display, or a second sensory channel.
What role does color vision deficiency play in medical technology?
A considerable one. A relevant proportion of the male population cannot reliably distinguish red and green, exactly the color combination most often used for status displays. Safety-relevant distinctions should therefore always be additionally coded through shape, position, text, or symbol.
What is meant by alarm fatigue?
The habituation to frequent, often not action-relevant alarms, which leads to signals being attended to with delay or no longer at all. It arises not at the individual device but through the sum of all alarms at a workplace and can therefore be addressed only by considering the entire context of use.
Must perceptibility be tested under real lighting conditions?
Yes. Contrast, viewing angle, and legibility behave fundamentally differently in a well-lit test room than in a darkened operating room, with incident daylight, or under emergency lighting. Which conditions have to be recreated follows from the survey of use environment factors.
Do you want to make sure that safety-relevant information from your product arrives even under real conditions? We test perceptibility where your product is used.
More about our usability engineeringSources
- IEC 62366-1:2015+AMD1:2020, Medical devices, Part 1: Application of usability engineering to medical devices
- IEC 60601-1-8:2006+AMD1:2012+AMD2:2020, Medical electrical equipment, Part 1-8: General requirements for basic safety and essential performance, Collateral standard: General requirements, tests and guidance for alarm systems in medical electrical equipment and medical electrical systems
- ISO 15223-1:2021, Medical devices, Symbols to be used with information to be supplied by the manufacturer, Part 1: General requirements