T he human/technology interface is a major part of the assistive technology component of the Human Activity Assistive Technology (HAAT) model (see Figure 2-5). Bailey (1996)³ defines an interface as “the boundary shared by interacting components in a system” (p 173) in which “the essence of this interaction is communication or the exchange of information back and forth across the boundary.” The human/technology interface is the boundary between the human and the assistive technology, across which information is exchanged. In practice, the human/technology interface describes the way in which the human controls the device.

If the individual has good fine motor control, she may use a keyboard and/or mouse to control a computer or assistive technology device. This control would also let her drive a powered wheelchair using a joystick. If another individual has poor fine motor control, it may be necessary to find alternative ways for him to control assistive technologies or mainstream devices, such as computers or cell phones, by using gross motor movements.

In this chapter we discuss the possible movements that can be used to control an assistive or mainstream technology electronic device, and the most common ways of accommodating for lack of motor control.

Anatomic sites for control of assistive technologies

The Human/Technology Interface (HTI) for assistive technologies described by the HAAT model has two major components: the human and the technology (see Figure 2-5). The human capabilities that are required to use the control interface (e.g., keyboard or joystick) are described in this section.

Figure 5-1 shows the body sites that can be used to control a device. These are called control sites. Control sites include hand or finger, arm, head, eye, leg, foot, and mouth (for switches based on respiration or phonation). Each control site is capable of performing a variety of movements or actions. When the interaction between a person with a disability and an assistive device involves relatively fine control (e.g., using a keyboard), the hand and fingers are the preferred control sites because they are typically used for manipulative tasks. Even if hand control is limited, there are control interfaces that can accommodate for limitations in fine motor control. It is also possible to improve the existing function by using control enhancers (described later in this chapter).

Figure 5-1 Anatomical sites commonly used for control of assistive technologies. (From Webster JW, Cook AM, Tompkins WJ, Vanderheiden GC: *Electronic Devices for Rehabilitation,* New York, 1985, John Wiley and Sons, p 207.)

If fine motor control limitations prevent hand use, then the use of the head as a control site is preferred. Using pointers of various types (e.g., a head pointer) as control enhancers, it is possible to obtain relatively precise control using head movements such as tilting side to side, horizontal rotation, and linear forward and backward movement. Very few functional movements are purely horizontal, vertical, or rotational.

If both hand and head control are poor, then control interfaces—generally switches—can be used to detect movements of the shoulder, elbow, forearm, hand, or finger. The use of the arm or leg is less desirable for precise tasks because these represent naturally gross movements controlled by large muscle groups, and therefore they are less desirable for manipulative functions such as keyboard use. Shoulder movements include elevation, flexion, extension, abduction (away from the body), and adduction (toward the body). The movements of the elbow are flexion and extension. The movements of the forearm consist of pronation (turning the palm down) and supination (turning the palm up). The wrist can flex or extend or move from side to side (radial deviation or ulnar deviation). The fingers can individually flex and extend, or together perform a grasp and release movement. The thumb can flex and extend, abduct and adduct, and oppose each of the fingers. Each of these types of movements can be detected by an appropriate control interface.

Another control site is foot movement. For fine manipulative tasks, the foot is less desirable than the hand or head because visual monitoring can be difficult and the foot is generally not as finely controlled as the hand. However, some individuals are able to develop fine control of the foot for typing (Figure 5-2). Control movements used in the lower extremities include raising and lowering of the leg at the hip (e.g., hip abduction and adduction), knee flexion and extension, foot plantar flexion (toes point down) or dorsiflexion (toes point up), and foot inversion or eversion (rotary movements, similar to pronation and supination). Switches of various types can be controlled by these movements.

Figure 5-2 Child using her foot to control an expanded keyboard.

Finally, respiratory air flow can be detected and used as a control site by sip (inhaling) or puff (exhaling) switches. Phonation may produce sounds (including whistling) or speech. There are control interfaces that can detect sound, and speech recognition can also be used as a control interface. Tongue movements can also be used for control.

Case Study – Comparative Evaluation

Max is an 18-year-old male who has cerebral palsy. He lives in a residential facility and attends a work program through United Cerebral Palsy. Max has been referred to ABC Assistive Technology Center for a communication device. He currently communicates with others using a manual communication board and eye blinks for “yes” and “no.”

Through evaluation of Max’s range and resolution, it has been determined that his best control sites are his right hand and his head. However, he does not have fine enough control at either site to use direct selection. You decide to perform comparative interface testing using a tread switch with his hand and a lever switch at the side of his head. Data collected during the comparative testing phase of the evaluation show that Max is more accurate and faster activating the switch with his head (versus his hand). However, Max has indicated a preference for using his hand instead of his head.

Questions

1. Given Max’s limited verbal communication, how would you gather information from him regarding his opinion on the hand and the head switches?

2. What type of subjective information would you want to gather from Max regarding his use of and preference for each of these two switches?

3. Your data indicates that Max is faster and more accurate using the head switch. However, Max has indicated to you that he prefers the hand switch. What would your recommendation be and why?

Connecting the user to the technology

There are three technology elements of the human/technology interface that contribute to the operation of a device: the control interface, the selection set, and the selection method. These three elements are interrelated, and careful attention must be given to each element to have an effective human/technology interface.

Control Interface

The control interface is the hardware by which the human in our assistive technology system operates or controls a device. It is sometimes also referred to as an input device. Examples of control interfaces include a keyboard, one or more switches, a touch screen or touch pad, a mouse, and a joystick.

When the control interface is activated by the user, information is sent via a signal to the processor. The processor interprets the information and generates two signals that are converted (1) into feedback to any display that is being used and (2) into an activity output, depending on the functions of the assistive technology system. For example, the power wheelchair (Chapter 12) joystick is typically setup so that the signal for the UP input is transformed into forward movement of the wheelchair, DOWN into reverse movement, LEFT into movement to the left, and RIGHT into movement to the right. That same joystick can be used to control a television set (Chapter 14) in which the same four movements of UP, DOWN, LEFT, and RIGHT control television volume up, volume down, channel up, and channel down. The selection set must include an element corresponding to each function of the device.

Selection Set

Each control interface allows the user to choose one or more items that provide input to the assistive technology device or control its operation in some way. The group of items available from which choices are made is called the selection set.¹¹ For example if a person wants to use a power wheelchair, the selection set might be forward, back, left, right, and stop. For typing on a computer with a special control interface, the selection set would be the entire computer keyboard. Selection sets can be represented by traditional orthography (e.g., written letters, words, and sentences), symbols used to represent ideas, computer screen icons, line drawings, or pictures. The modalities in which the selection set is presented can be visual (e.g., letters on the keyboard or icons on the screen), tactile (e.g., Braille), or auditory (e.g., spoken choices in auditory scanning).

The size, modality, and type of selection set chosen are based on the user’s needs and the desired activity output (see Figure 2-5). Activity outputs in the HAAT model include communication (replacing or augmenting speech or writing), mobility, manipulation (e.g., things we would normally do with our hands and arms) and cognition (assisting with mental activities). An electronic aid to daily living (EADL) (Chapter 14) or a power wheelchair (Chapter 12) typically have fewer choices in the selection set than an augmentative communication device (Chapter 11) or computer (this chapter). The size of the selection set may also vary according to the user’s skills and age. For example, an individual who spells and has good physical control has the skills to use the selection set of a standard keyboard, which consists of all the letters and function keys. Another individual who is working on developing language and communication skills may have a selection set consisting of only two picture symbol choices displayed on a lap tray.

Selection Methods: Direct and Indirect Selection

There are two basic selection methods that an individual with a disability can use to make selections with a control interface: direct selection and indirect selection. Direct selection methods generally have one interface for each selection that can be made. For example, each letter on a keyboard has a separate key. Indirect selection methods include scanning, directed scanning, and coded access.

Direct selection allows the individual to use the control interface to randomly choose any item in the selection set. The person indicates her choice by using voice, finger, hand, eye, or other body movement. In this method of selection the user identifies a target and goes directly to it.¹⁵ At any one time, all the elements of the selection set are equally available for selecting. Typing on a keyboard or picking a flower from the garden is direct selection. Direct selection is the most difficult method physically because it requires refined, controlled movements. Because there is an immediate, direct result from the selection made, it is more intuitive and easy to understand and the cognitive demands are not great. Figure 5-3 shows the input that is made using direct selection to obtain the letter S. The various types of control interfaces that allow the individual to use direct selection are described later in this chapter.

Figure 5-3 This figure shows the input required to obtain the letter S using direct selection. (From Smith RO: Technological approaches to performance enhancement. In Christiansen C, Baum C, editors: *Occupational therapy: overcoming human performance deficits*, Thorofare, N.J., 1991, Slack.)

When an individual’s physical control does not support direct selection, indirect selection methods are considered. Indirect selection involves intermediary steps in order to make a selection. The most common indirect selection methods are scanning, directed scanning, and coded access. Most electronic assistive technology devices can be accessed by more than one type of control interface and selection method. The selection set on most devices also can be varied to match the user’s needs. From a manufacturing perspective, the versatility of a device allows it to be applicable to a wider population, which helps to contain the cost of the device and makes it possible to adapt to changing user needs and skills.

Scanning

With scanning, the selection set is presented on a display and each item in the selection set is sequentially lighted and/or indicated by sound or speech. When the particular element that the individual wishes to choose is presented, the user activates a control interface to select that item. The control interface used for scanning is typically a single switch or an array of two or more switches. Depending on the needs of the user, scanning can vary in format (i.e., the type of symbols and the way they are presented). The way that the control interface signal is used to make the selection can also vary. Scanning requires good visual tracking skills, a high degree of attention, and the ability to sequence. The advantage of scanning is that it requires very little motor control to make a selection.

Because scanning is inherently slow, there have been a number of approaches used to make it more efficient and faster for the user.⁵ The major method for improving scanning efficiency is to use techniques that are efficient in that they allow the user to select groups of entries (e.g., letters) as opposed to entering them singly. Approaches that do this are called rate enhancement and they are discussed later in this chapter.

Directed Scanning

Directed scanning is a hybrid approach in which the user activates the control interface to select the direction of the scan, vertically or horizontally. There is typically one switch for each direction of movement. This is often four directions, but it can be as many as eight. The user first selects the direction in which he wishes to scan. The cursor continues to move in the selected direction by the user holding the switch down. When the switch is released, the cursor stops and the user either waits for an acceptance time interval or hits an additional switch. The selected item is sent to the device.

A joystick or an array of switches (two to eight switches) is the control interface used with directed scanning. Figure 5-4 gives an example of the input required to select the letter S using directed scanning with a four-position joystick. Directed scanning requires more steps than direct selection but fewer steps than single-switch scanning. The user needs to be able to activate and hold the control interface and to release it at the appropriate time. If the individual can produce the movements required to use this method, the outcome is faster entry of the desired selections into the device.

Figure 5-4 Directed scanning showing the input required to select the letter S. The user selects the direction of the scan and the items in the selection set are scanned sequentially by the device. When the desired item is reached, the user makes the selection. (From Smith RO: Technological approaches to performance enhancement. In Christiansen C, Baum C, editors: *Occupational therapy: overcoming human performance deficits,* Thorofare, N.J., 1991, Slack.)

Selection Techniques for Scanning

The action required by the user to activate the control interface to make a selection during scanning and directed scanning usually can be varied to accommodate the user’s skills. Table 5-1 lists the three scanning techniques and the level of motor skill required by each technique. This table is helpful in matching the scanning technique to the user’s skills. For example, some techniques depend more on the ability to react quickly to activate a switch. Others require vigilance and the ability to wait until a choice appears. Still others require the user to hold a switch until the choice appears and then release.

Table 5-1

Importance of Techniques for Scanning and Directed Scanning

	Automatic Scanning	Step Scanning	Inverse Scanning
Wait	High	Low	Medium
Activate	High	Medium	Low
Hold	Low	Low	High
Release	Low	Medium	High
Motor Failure	Low	High	Low
Sensory/Cognitive Vigilance	High	Low	High

Modified from Beukelman D, Mirenda P: Augmentative and alternative communication, ed 3, Baltimore, 2005, Paul H Brookes, p 184.

Automatic scanning sequentially presents items that the user may choose. The rate of presentation (scan rate) can be set and adjusted according to how fast the user can respond. When the desired selection is presented, the user selects the choice by activating the control interface and stopping the scan. Automatic scanning requires a high degree of motor skill by the user to wait for the desired selection and then to activate the control interface in the given time frame. It also requires a high degree of sensory and cognitive vigilance for attending to and tracking the cursor on the display.

In step scanning, the user activates the control interface once for each item to move through the choices in the selection set. When the user comes to the desired choice, there are two possibilities for selecting it. Either an additional control interface is used to give a signal to select that choice or an acceptance time is used. Step scanning allows the user to control the speed at which the items are presented. The ability to wait is not required for the scan, but it may be for the acceptance of the selection. The ability to activate the control interface repeatedly, however, is important for step scanning. Motor fatigue can be high because of repeated control interface activation.

Inverse scanning is initiated by the individual activating and holding the control interface closed. As long as the control interface is held down, the items are scanned. When the desired choice appears, the individual releases the control interface to make the selection. Inverse scanning requires holding the control interface and releasing it at the proper time. Inverse scanning may be easier for some people than automatic scanning, which requires activation of the control interface within a specified time frame. For individuals who require lots of time to initiate and follow through with movement, inverse scanning can be helpful. Like automatic scanning, motor fatigue is reduced over step scanning because of fewer control interface-activations; however, sensory and cognitive fatigue are higher because of the vigilance required to attend to the display.

Selection Formats for Scanning

There are a number of formats in which the items in the selection set can be presented to the user for selection in scanning (Box 5-1). In a linear scan format, as shown in Figure 5-5, the items in the selection set are presented in a vertical or horizontal line and scanned one at a time until the desired selection is highlighted and selected by the user. Circular scanning (or, rotary scanning) (Figure 5-6) presents the items in a circle and scans them one at a time. Because of the slowness inherent in both these types of scanning, Vanderheiden and Lloyd (1986) recommend that the array be limited to 15 choices.¹⁶