Communication modalities

Each participant completed the stimulability tasks for all 5 morpheme categories. Each participant was randomly assigned a target communication modality (spoken or graphic symbol) for each morpheme category. Random assignment was completed without replacement for each

months were assigned to each communication modality in each morpheme category. All responses in both modalities were recorded using the score sheet in Appendix C. Video recordings were used to verify the accuracy of online data coding as needed.

In the spoken modality, participants used verbal speech to generate responses. The researcher used verbal speech for all cues and for demonstrating correct morpheme use. All tasks for morpheme categories assigned to the spoken modality were completed as a block before any tasks assigned to the graphic symbol modality were completed. Tasks in the spoken modality were completed in the numbered order shown in the Task column of Table 3.2.

In the graphic symbol modality, participants used an experimental SGD application program on an iPAD Mini 4 platform to generate responses. A series of 5 custom-made vocabulary pages with 15 key locations each was used to assess stimulability of all tense markers in the graphic symbol modality. Each task was completed using a single vocabulary page, as summarized in Table 3.2. Participants were not expected to navigate across multiple pages to demonstrate stimulability of any given tense marker. Some vocabulary pages were used for multiple tasks. All vocabulary pages are shown in Appendix D, with text glosses added to each symbol location for readability. The pages presented to the participants did not include text glosses.

Keys on each page were arranged in a grid with 3 rows and 5 columns. All words and morphemes necessary to selectively generate correct responses to each corresponding practice and probe item were represented on these locations by single meaning pictures with no text gloss. On some pages, some symbols were used to represent 2-morpheme subject noun phrase, such as “the bird.” All other symbols will be used to represent 1 free morpheme or 1 bound morpheme. A custom set of single meaning pictures was used to represent each of the 15 tense

markers graphically, as shown in Table 3.3. These symbols were designed so that a rationale could be used to explain the association between the symbol and corresponding tense marker.

The -3s and –ed symbols are used in an existing set of sequenced multi-meaning icons (Baker, 1986) with a text gloss. Each page included symbols for multiple contrasting tense markers.

Participants were always required to selectively generate a target tense marker as part of a correct response. For example, participants were required to differentiate between three possible forms of AUXILIARY DO.

In the graphic symbol modality, the target response on each item required participants to select at least two of the available graphic symbols, with one symbol representing a subject or verb stem and a second symbol representing a tense marker. The experimental SGD was configured to speak and add text to a message window as symbols were selected. The message window was configured to trigger the SGD to speak all displayed text when it was selected. This allowed participants to produce longer messages after selecting multiple symbols. One location of each vocabulary page was be used as a “clear display” control key, which was used to clear the contents of the message window. Participants were allowed to select the clear display control key if they made an error and then begin generating their intended response a second time. Other locations included single meaning pictures representing language content necessary for generating correct responses on a corresponding task. If all necessary locations for a task were used, any remaining locations were left blank.

The researcher provided explicit models of targets in the graphic symbol modality throughout the administration of stimulability tasks in the graphic symbol modality, consistent with Binger and Light’s (2008) suggestion that strategies used for assessing expressive morphology in verbally speaking children can be adapted for AAC speakers by incorporating

Table 3.3. Single meaning pictures corresponding to 15 English tense markers

aided language stimulation. Any time models were provided in the graphic symbol modality, the researcher selected graphic symbols and used the experimental SGD to produce speech output.

These models were used to familiarize participants with the pre-stored vocabulary on each page at the single word level and to model correct use of tense markers in obligatory contexts.

Each time a new task was started in the graphic symbol modality, aided language stimulation was used to familiarize the participant with the pre-stored vocabulary on the current page at the single-word level. The researcher elicited imitations of the words corresponding to each new symbol in isolation by prompting the participant to select the symbol and generate speech output. The researcher also verified that participants recalled familiar symbols from previous vocabulary pages. The familiarization procedure was not repeated if the same vocabulary page was used for two consecutive tasks.

This is an established familiarization process. Sutton and Morford (1998) used aided language stimulation to model each symbol on an array of 20 single meaning pictures once for typically developing kindergarteners. After this rapid familiarization process, these kindergarteners used their array of 20 single meaning pictures to generate simple sentences in an elicitation task (Sutton & Morford, 1998). Similar procedures have been used to familiarize preschoolers to smaller displays before eliciting repetitions of subject-verb-object sentences (Sutton, Trudeau, Morford, Rios, & Poirier, 2010).

The researcher explained and modeled correct use of the message window and clear display control key while familiarizing the participant to the first page by making and correcting an error during utterance generation. Participants were not evaluated for their accuracy using the clear display control key or message window.

The researcher used verbal speech for scripted verbal cues and explicit models of graphic symbol use for demonstrating correct morpheme use in responses. All tasks for morpheme categories assigned to the graphic symbol modality were completed as a block in the numbered order shown in the Task column of Table 3.2.

Stimulability tests from two children ages 30-42 months and two children ages 43-54 months were randomly selected for inter-rater reliability testing. A second, trained rater independently reviewed video recordings of these selected stimulability tests and independently scored each test item as either correct or incorrect. Interrater reliability for scoring individual test items on the stimulability test was found using the kappa statistic (Cohen, 1960). Excellent agreement was found for scoring individual test items (κ = 0.90, p < 0.001).