DYNAMIC VS STATIC DISPLAYS: WHAT ARE THE ISSUES?

Author: Walter Woltosz, President
Presenter: Philip Lawrence, Vice-President, Operations
Words+, Inc.
Palmdale, California, USA
Telephone (800)869-8521

Web Posted on: December 12, 1997

"The purpose of a communication display is to arrange language in space so individuals can, by selecting from the available options, say what they wish to say as quickly as possible, and can do so with a minimal amount of effort." (Blackstone, 1993)

The above quotation from Sarah Blackstone is profound. For AAC users with sufficient visual skills to use visual feedback, the display of language symbols is a primary factor in determining both the speed of communication and the amount of language available to the user in a practical way.

Static displays are those on which the symbols do not change automatically. Dynamic displays are those on which the language symbols change automatically as a normal part of operating the device. Dynamic displays have been in common use in AAC since 1980 for text, and since 1986 for pictographic symbols. Static displays have been in use much longer, with their original use probably lost in antiquity.

This paper attempts to summarize the advantages and disadvantages of both types of display, to discuss the factors that need to be considered with both display types, and finally to recommend a decision-making approach.

For either type of display, the overall size of the display, and the size of individual symbols within the overall space, will be determined by a combination of factors, including environmental factors, as well as the user's visual, motor, and language skills.

The overall size of the display area will place a limit on how many symbols of a certain individual size can be displayed at one time. This is the "space" referred to in the quotation from Sarah Blackstone above.

The overall size of a "paper" communication board will often be limited by the environment in which the board will be used, i.e., the practicalities of supporting and transporting the display. For persons using direct selection input methods, overall display size may be dictated by the user's range of motion rather than by support and transportation issues.

Several factors affect the attributes of symbols. The size of individual symbols employed may be dictated by:

• (1) the user's visual acuity,
• (2) the user's pointing accuracy,
• (3) the communication partner's visual acuity) and
• (4) the available sizes of ready-made symbols.

The number of symbols included in a particular user's display is affected by

• (1) the overall display space available for symbols,
• (2) the minimum size of symbols that can be put into the available space,
• (3) the size of the user's vocabulary, and
• (4) the user's visual perceptual skills.

The number of symbols desired on the display will usually be proportional to the size of the user's vocabulary. Although the user's visual perceptual skills may limit the number of symbols that can be included at any one time, the use of colored backgrounds to group symbols has been seen to allow a greater number of symbols to be included on a display without sacrificing search time.

Language is dynamic, and most AAC users experience the need for changing vocabularies. The ability to modify displays as needed can depend on:

• (1) the ability of the user and/or support person to identify needed changes,
• (2) the availability of symbols with which to make those changes in a timely manner, and
• (3) the ability of the programming person to perform the operations needed to incorporate the modifications into the device.

As noted by Beukelman (1991), the learning costs of AAC services need to be weighed against other demands on the users time. Time to learn complex language systems is time taken away from other learning activities. Recent studies suggest that learning times on dynamic display pictographic systems may be much shorter than those on recall-based static display systems (Lawrence & Woltosz, 1994), (Spiegel et. al., 1993)

STATIC DISPLAYS: Static display systems provide a fixed set of symbols which are mechanically affixed to an underlying layer of plastic or paper material.

Non-electronic static display devices: Paper communication boards and books, which are the oldest visual-symbol-based AAC devices, offer the following advantages: low cost of materials, portability, durability, and reduced display preparation time. Many methods like this were used for the communication partner to interpret the message.

Electronic static display devices: The earliest electronic AAC devices employed static displays. These devices generally provided a single language element for each symbol on the display (levels and sequences were not used). Messages were often generated by selecting a single symbol, and the message was then printed on a narrow paper strip, if at all.

In the early 1980's the strategies of levels and sequencing of symbols were introduced as methods to allow more language with fewer symbols. The use of levels means that each symbol on the display may represent a different language item on different levels. The device typically uses one or more of the display's symbols to enable the user to change levels. Symbol sequencing takes advantage of the many ordered combinations of symbols possible with a given symbol set. With pictographic symbol sequencing, the user might select a symbol for SUN followed by a symbol for DRINK to represent their favorite hot drink. Abbreviation expansion is the use of sequences of traditional orthography symbols (letters) to represent larger amounts of text.

DYNAMIC DISPLAYS: A dynamic display is one on which the visual symbols presented to the user are generated electronically, on which the symbols change as a normal part of operating the device (Woltosz, 1984).

Let us now turn our focus to significant user considerations affecting display choice.

VISUAL: For users with good visual abilities and limited language, a simple static display device may be able to include all of the symbols needed to represent an appropriate working vocabulary. For users with large language requirements, and/or those with limited visual acuity, visual field, and/or perception, display size is a critical consideration. The use of color in symbols is usually beneficial to users with good color vision. For non-electronic static displays, the use of color may also be of benefit to the communication partner, who must see, recognize, and interpret the symbols. As the number of symbols that can be employed on a static display becomes smaller and smaller, the number of levels and/or length of sequences required to represent a given amount of language becomes greater and greater.

MOTOR: Users with limited range of motion and/or limited pointing accuracy may encounter a situation where the number of symbols that they can reasonably access on a display is far less than the size of their vocabulary. Dynamic displays can provide a large number of pages with relatively few symbols per page, alleviating the language-motor and language-vision tradeoffs, but requiring different skills on the part of the user.

Cognitive factors include the ease of learning, ease of use, and dependence on recognition and recall memory. The learning costs of AAC devices need to be weighed against other demands on the user's time. The dependence on recognition memory and recall memory affect the learning time required for more advanced concepts. Recognition is a lower cognitive skill than recall(Light & Lindsay, 1991a), so that strategies that are based on recognition produce a lower cognitive load than those based on recall.

Motivation and energy are two of the factors we have observed that are difficult to quantify, but which experienced clinicians know are very important. Users who are highly motivated tend to have more successful AAC interventions than those who are not.

Communication rate with any AAC device is slower than natural speech. The speed necessary to meet the user's communication needs may vary from one user to another. Communication rate is affected by certain user "times" when making selections while operating a device. These are decision times, visual search times, number of selections /language item, device response time, and volume of language /selection.

The user of any AAC device must make repeated decisions. Decisions are made about what to do next at each step during the message-generating process. With static display devices that do not employ multiple levels or symbol sequences, decisions are straightforward.

For static display devices that employ levels or sequences, each selection step is based on a combination of recall and recognition decisions. With considerable practice, the motor patterns required to make some such selections can become automatic, in which case the decision process becomes subconscious for those who can perform these strategies.

For dynamic display users, operation of the device is usually through a sequence of recognition-based decisions. This tends to reduce the learning time of the language and has been seen to result in some automaticity benefits when display configurations are consistent.

Error correction decisions often are complex. For static display systems employing levels or sequences, the fact that an error has occurred at all may be masked until the final message is output, making the correction process more difficult. With dynamic displays, the fact that the display goes to the wrong page is an instant clue that an error has been made, and the user can usually select a symbol and try again, minimizing the time required to correct such errors.

Visual search times for the user to search the visual display will depend on a complex set of interacting factors, including at least the number of symbols on the display, the size and spacing of the symbols, the user's familiarity with the display(s), the organization of the display an the transparency of symbols.

If a single selection requires 30 seconds to make, but it produces a complete sentence of 10 words, then a rate of 20 words/minute has been achieved. If two selections are required for every word, and the average time per selection is five seconds (10 seconds per word), then a rate of 6 words/minute will be achieved. Thus, quantity of language/selection is an important consideration.

In addition to the user-related considerations described above, there are four major factors that are related to the device itself that are affected by the kind of static or dynamic display it employs. These are

• 1)device size and weight,
• 2) convenience,
• 3) durability and reliability, and
• 4) cost.

The size and weight of an AAC device must be suitable for the user and their environment. Paper communication boards are very light, but are often large. At this point, there seems to be no clear size or weight advantage to either electronic static or dynamic display devices of comparable language capacity. Dynamic display devices have seen significant weight reductions in the last year or so, with the lightest full-capability systems weighing in at about 4.5-6.5 pounds (2-3 kilograms).

AAC devices can experience a harsh environment. Although electronic static display AAC devices are generally constructed from less fragile components than dynamic display devices, they have not always experienced greater durability and reliability. The hard drives in today's portable PC's are capable of withstanding typical loads of 5 G's operating and 50 G's nonoperating. The LCD display is generally considered to be the most fragile part of a dynamic display device.

For any sophisticated electronic device, a backup device or strategy should be readily available for times when the primary AAC device is unavailable, whether due to batteries that did not get charged when needed, or to the time required to effect needed repairs.

COST - While cost should not be a major factor in selecting the most appropriate AAC device for a particular user, real world constraints sometimes make it so. The costs associated with static and dynamic display devices depend primarily on:

• (1) the materials used in making the device,
• (2) the labor required to gather the materials, plan the layout, and construct or program the device,
• (3) the cost of training (both user and support personnel) required to enable the device to be properly and efficiently used, and
• (4) the cost of maintaining the device in operating condition over its useful life.

RECOMMENDED SELECTION CRITERIA: We now take the bold step of recommending a set of decision-making criteria for selecting static and/or dynamic displays for users of AAC devices. These are presented in the form of factors that favor one type of display over the other.

Factors favoring non-electronic static displays: Immediate need - can't wait to buy, need to use in wet environments, very light weight, low cost, available materials, limited language needs (some) and need for backup device.

Factors favoring electronic static display devices: very long battery life(some), very small size and weight (some), built-in printer (some), durability (some), limited language needs (some) and need for backup device.

Factors favoring dynamic displays: ease of learning/ease of use, ease of programming, large number of symbols, photographic images, multimedia output, visual-language tradeoff, motor-language tradeoff and built-in computer functions (some).

REFERENCES

Beukelman, D. (1991) Magic and Cost of Communication Competence, Augmentative and Alternative Communication, 7, 2.

Blackstone, S. (1993) For Consumers. Augmentative Communication News, 6(1), 1-6.

Lawrence, P. & Woltosz, W. (1995) Case Studies of Individuals Using Pictographic Dynamic Display AAC Devices: Practical Applications and Strategies. 1995 Technology and Persons with Disabilities Conference, Los Angeles, California, USA

Light, J., and Lindsay, P. (1991a) Cognitive Science and Augmentative and Alternative Communication, Augmentative and Alternative Communication, 7, 186-203.

Spiegel, B., Benjamin, B., and Spiegel, S. (1993) One method to increase spontaneous use of an assistive communication device: case study. Augmentative and Alternative Communication, 9, 111-118.

Woltosz, W. (1984) Personal Computers as Augmentative Communication Systems. Words+, Inc., Palmdale, California, USA.