needed (Section 3.2.3, [MacLean 2009]). The latter presents information in an ambient manner while the former can interrupt the user’s current state or action to convey the information. Interesting designs are possible by moving between these two ends. For example, a posture correcting chair provides awareness of the user’s posture with ambient pressure sensations at the back of their seat which can gradually move into the user’s attentional foreground when necessary [Zheng and Morrell 2012].
A designer can engineer the properties of an individual stimulus to create different sensations. In addition to signal amplitude, haptic signals commonly use temporal and/or spatial parameters. For example, vibrotactile signals have several temporal parameters including frequency, rhythm, and pulse envelope (specified by attack, decay, sustain, and release parameters [MacLean 2008b, Ternes and MacLean 2008, Choi and Kuchenbecker 2013] as well as spatial parameters such as location (x,y) and direction when several actuators are combined over a surface (e.g., a haptic seatpad) [Schneider and MacLean 2014, Schneider et al. 2015b]. Variable friction and force feedback devices can provide different signals over space and time depending on the user’s interactions [Levesque et al. 2011, Levesque et al. 2012].
3.3.2 The Sensation-Human Connection
In devising effective interactions, designers must consider a device’s connection to the user’s body and the range of haptic sensations perceptible in a given context.
Physical Connection
A haptic device’s connection to its user’s body varies with technology and use case, and impacts perception.
Contact mode can vary. Location, surface area, and tightness are part of the body-device connection; prototypes for wrist, belts, jackets, shoe insoles, or handheld devices vary these parameters. The contact can be persistent (e.g., a wristband) or occasional and on-demand (e.g., a haptic keypad on an ATM or a haptic door knob) [Karuei et al. 2011, MacLean and Roderick 1999].
Bodily distance can vary. Haptic signals can be felt through an internal mechanism (such as vibrating tattoos [Radivojevic et al. 2014]), an external but contacting device (smartwatches, game controllers), or an external, noncontacting device (ultrahaptic devices [Carter et al. 2013]). The current norm is to feel the sensations through an external and contacting device.
On the human side, contact can be active or passive. Human-active touching is generally done for a reason. Active or passiveness of user touch is influenced (afforded) by device and interaction design. For example, sensations rendered by today’s variable friction technology can only be felt with active (sliding) finger movement. Conversely, users commonly receive vibrotactile sensations passively as event-based notifications; finger movement yields no additional information.
Effective Size of the Sensation Space (Signal Set Size)
The number of sensations that humans can perceptually distinguish is a function of hardware, bodily connection, perceptual capability, and context of use.
Hardware specifications such as actuator frequency range determine the rendering limitations and provide an upper bound for the number of perceptually distinct stimuli. These specifications can be used to compare expressive capability among hardware elements (e.g., VT actuators).
Connection characteristics—body location, prototype assembly and materials, and contact mode (orientation, grip, tightness)—impact sensation distinguishablity [Gallace et al. 2007]. Karuei et al. [2011] reports differences in vibration detection thresholds on 13 different body locations and 2 different bodily states (e.g., walking vs. sitting).
Differences in perceptual and processing capabilities for those of different ages, visual acuity, profession, and simply genetics (Section 3.1.4) impact signal distinguishability [Goldreich and Kanics 2003]. Stevens and Choo [1996] report that the decline in tactile acuity with age affects all body locations, but has a larger impact on fingers and toes compared to more central body locations such as lips and tongue.
Context of use can impact haptic perception and processing capabilities, through parameters such as environment, body state (running vs. resting), and sensory and cognitive load and involvement (listening to music, driving) [Karuei et al. 2011, Blum et al. 2015]. This in turn determines the effective set size for distinguishable stimuli. For example, the number of different vibration notifications an individual can discern while driving a car (with its environmental vibrations, high sensory, and cognitive involvement) is smaller than when seated at an office desk.
3.3.3 The Meaning
Sometimes haptic signals are able to directly represent a meaning, e.g., through adequately high fidelity representation of a real physical sensation. More often, abstraction is required: perhaps the sensation being represented is beyond the capacity of the haptic device to display, or the information itself is abstract (“speed up”). Mapping haptic sensations to intended meaning—encoding the information—is a crucial design task that needs to be done in a consistent and compatible way across the full vocabulary used in an application, and sometimes more broadly [MacLean 2008b].
In this section, we discuss users’ cognitive meaning-mapping frameworks, then present encoding and vocabulary-development approaches that have been used by haptic designers.
Interpretive Schemas and Facets
To interpret haptic signals, people employ a number of conceptual or translational schemas, often combining them. We might compare a haptic sensation to a natural one (“This is like a cat purring”), to emotions and feelings (“This is boring”), or consider its potential usage (when a quickening tactile pulse sequence is described as a “speed up”). The meaning someone chooses is typically influenced by the sensation itself but also by the context of use and the user’s background and past experiences [Seifi et al. 2015, Schneider and MacLean 2014, Obrist et al. 2013].
Facets are a concept originating from the domain of library and information retrieval which nicely capture the multiplicity and flexibility of users’ sense-making schemas for haptic sensations. A facet is a set of related properties or labels that describe an aspect of an object [Fagan 2010]. Five descriptive facets have been proposed and examined for haptic vibrotactile stimuli (Figure 3.2, [Seifi et al. 2015]):
Figure 3.2 People use a variety of cognitive frameworks to make sense of haptic signals. Bottom left image (from Schneider et al. [2016]). Bottom right image courtesy of Anton Håkanson.
Physical properties that can be measured—such as duration, energy
Sensory properties—roughness, softness
Emotional connotations—pleasantness, urgency
Metaphors or familiar examples to describe a vibration’s feel—drumbeat, cat purring
Usage examples or types of events where a vibration fits—speed up, time’s up
If a designer neglects a consistent consideration of these meaning assignment facets the result is likely to be confusion and bad user experience. Leveraged properly, facet-driven mappings can be lead to more intuitive, consistent results and
