Processing of Synthetic Speech
by Persons With Severe Mental Retardation
Kasey Hester (NSSLHA Member)
Texas Tech University Health Sciences Center
Department of Communication Disorders
2A-300 HSC, Stop 6073
Lubbock, TX United States 79430
Rajinder Koul (Member)
Texas Tech University Health Sciences Center
Department of Communication Disorders
2A-300 HSC, Stop 6073
Lubbock, TX United States 79430
Abstract
This study evaluated the magnitude and type of practice effect as a result of repeated exposure to synthetic speech in persons with severe mental retardation. Results indicated significant (p<.01) practice effect as a result of repeated listening. Implications for clinical practice are discussed.Narrative Summary
Individuals with little or no functional speech as a result of mental retardation frequently rely on non-speech communication systems to augment or replace natural speech. These systems include electronic voice output aids (VOCAs) that provide synthetic speech output upon activation. However, very little is known to what extent individuals with mental retardation comprehend synthetic speech produced by VOCAs. Specifically, the purpose of this study was to evaluate the magnitude and type of practice effects in individuals with severe mental retardation as a result of systematic exposure to synthetic speech. Subjects included 15 individuals who met the following criteria: (1) classified as individuals with severe mental retardation according to IQ and adaptive behavior measures; (2) no uncorrected hearing and/or visual deficits; (3) demonstrated reliable pointing skills; (4) passed a nonidentity matching task involving live natural voice and line drawings/pictures; (5) had no history of VOCA use. Receptive language and non-verbal cognitive performance were obtained using the standardized tests. Subjects who were unable to use direct selection as an access technique were excluded. Stimulus materials included 4 lists of 10 words each. These words were selected from a list of the first 50 words used by typically developing preschoolers (Nelson, 1973) and a dictionary of symbol vocabulary used by youth with severe mental retardation (Adamson, Romski, Deffebach, & Sevcik, 1992). One list was designated as repeated and the remaining three as novel. Within each list, 20% of the words were repeated to judge intra-subject reliability. The synthetic speech used was DECTalk Betty (i.e., simulated adult female voice). A Microsoft Visual Basic program was developed to present the stimuli and the prompts, and to record responses. The experimental stimuli were presented using a laptop computer and external speakers that were placed approximately 12 inches in front of the subject. The experimental stimuli were presented at 75 dB SPL as determined by a sound level meter. Subjects’ were instructed that they will hear a series of words and their job is to touch the picture on the computer screen depicting the stimulus item. A touch screen mounted on a computer screen in conjunction with the Visual Basic program automatically recorded responses. The touch screen was calibrated to ignore “miss hits” (i.e., the subject slid his hand across the screen and activated a wrong selection) by using a timed activation direct selection strategy. The computer screen displayed one target picture, a visual representation of the synthetic word, and three unrelated foils. The position of the pictures within each experiment was randomized to avoid position effect; the order of presentation of the lists was randomized to avoid order effects; and a constant inter-stimulus interval of 10 seconds was maintained during presentation of the words within each list. All subjects had to pass a pretest in order to participate in this study. This pretest was designed to exclude subjects who were unable to obtain 100% correct scores for experimental stimuli, presented via live natural speech. In the absence of perfect scores on the pretest, it would be difficult to determine whether the performance demonstrated by individuals with mental retardation was due to the difficulty in processing synthetic speech or due to lack of conceptual knowledge of the stimulus items. The pre-experimental procedures were conducted at least one week prior to the beginning of the experiment. There were a total of 3 experimental sessions, each separated by a period of at least 24 hours. During each session, subjects were presented with a list of novel words, and a list of repeated words. The same repeated word list was presented across all sessions. In contrast a new novel word list was presented in each session. Subjects were instructed that they would hear a series of words preceded by a carrier phrase and that their task was to point to the drawing depicting the word. Additionally, they were told to make their best guess if they were uncertain about the word. Immediately prior to each experimental session, three practice items were run to ensure that the subjects were familiar with the task. The practice items were different from those used in the experimental task. Data were analyzed using a repeated measure design. The two dependent measures were (1) word identification accuracy and (2) word latency. Raw scores for each subject were entered into a 2 (Stimulus type: novel and repeated) X 3 (Listening trial: trial 1, trial 2, trial 3) X 2 ANOVA. Results indicated that there was a significant effect for word latency [F (2, 56) = 7.83 (p < .01)]. No significant difference was observed between novel and repeated stimuli across word identification [F (1, 56) = .245 (p> .01)] and word latency tasks [F (1,56) = .438 (p > .01)]. Although there was a trend towards increased word identification scores across trials, it did not reach significant levels [F (2, 56) = .232 (p > .01)]. There was no significant interaction between stimuli and repeated listening trials across word identification [F (2,52) = .517 (p > .01)] and word latency [F (2,52) = .149 (p > .01)] tasks. In summary, current results indicate that perception of synthetic speech in individuals with mental retardation was enhanced (i.e., significant decrease in latency) as a result of systematic exposure to synthetic speech. Also, the absence of significant effect for stimulus type indicates that individuals with mental retardation were able to generalize their knowledge of the acoustic-phonetic properties of synthetic speech to novel stimuli. These results are significant because they indicate that individuals with mental retardation become more skilled at recognizing synthetic speech with repeated exposure. This is an important finding in the context of increased use of VOCAs by individuals with significant communicative and cognitive impairments.
