Perceptual learning of synthetic speech by individuals with severe mental retardation

Date

2002-05

Journal Title

Journal ISSN

Volume Title

Publisher

Texas Tech University

Abstract

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. This study compared the performance of a group of individuals with severe mental retardation (n=14) with a matched control group (n=14) on word identification accuracy and latency tasks. Specifically, the effects of training on novel versus repeated words produced by the DECtalk synthesizer were analyzed. 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 (Adain.son, 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 would hear a series of words and that their job was to touch the picture on the computer screen depicting the stimulus item. A touch screen mounted on the 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 effects; 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 while 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 they were to point to the drawing depicting the word. Additionally, they were told to make their best guess if they were uncertain. Immediately prior to each experimental session, practice items were run to ensure that 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 identification latency. Data for word identification accuracy and latency were analyzed using a repeated measures (2X2X3) ANOVA in which group served as a between factor variable while type of task, type of stimuli, and listening sessions served as within subject variables. Analysis revealed a significant main effect for group [F (1, 52) = 7.523, p < .05] on the word identification accuracy task indicating that individuals with severe mental retardation had significantly lower word identification accuracy scores (mean = 80.95) than the control group (mean = 91.19). A non-significant trend toward improved word identification accuracy across sessions [F (2, 104) = 2.635, p > .0765] was noted. The most interesting finding of this study was the lack of a significant effect [F (1, 52) = 0.199, p > .05] for stimulus type (i.e., repeated vs. novel) across groups on the word identification accuracy task. The presence of a significant interaction between word identification latency and group [F (2, 104) = 8.53, p< .01] indicated that individuals with mental retardation were processing synthetic speech more quickly as a result of repeated exposure.

In summary, current results indicated 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 a significant effect for stimulus type indicated that individuals with mental retardation generalized their knowledge of the acoustic-phonetic properties of synthetic speech to novel stimuli. These results were significant because they indicated that individuals with mental retardation became more skilled at recognizing synthetic speech whh repeated exposure. This was an important finding in the context of increased use of VOCAs by individuals with significant communicative and cognitive impairments.

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