The adults with speech sound disorders had difficulty identifying tokens of the words “say” and “stay” when the contrast was cued solely by variations in first format transition onset frequency. For both the 20 ms and 40 ms continua the speech-impaired participants had significantly shallower slopes on their identification functions, suggesting greater difficulty with using spectral cues to develop categorical boundaries between the two words. Furthermore, two speech-impaired adults were unable to perceive “stay” for any of the stimuli when the first formant onset frequency was varied, while none of the control participants had similar difficulties. While the speech-impaired participants needed a greater frequency change to identify a stop consonant such as “t,” even when the size of the frequency change increased at onset frequencies around 211 Hz, the speech-impaired adults still had a lower percentage “stay” responses than the controls.
There were methodological shortcomings of this study; only nine speech-impaired adults were studied and some of the adults came from the same families. We selected the adults very carefully to assure that they represented a homogenous sample. However, because three pairs were related it could be argued that they were not independent; however, we found no relationship between pairs from the same family. For example, the most severely affected participants on the speech perception tasks were participants 3 and 5, who came from different families. Moreover, we only studied speech perception using one particular set of words, “stay” and “say”. We do not know whether the same results would have been found on other types of speech discrimination tasks such as differentiating “ba” from “da”.
The results presented here are the first to indicate that adults with persistent familial speech disorders with normal language functioning also have difficulty with word identification using spectral cues such as formant transitions. Similar to Nittrouer (1992)
, we found fewer perceptions of “stay” when the stop gap was reduced from 40 to 20 ms in the adult controls. Nittrouer (1992)
found that children were more sensitive to the formant transition cue than the stop gap duration cue. Furthermore the children’s identification functions were shifted to the right of the adult functions, towards higher onset frequencies. Therefore, our speech-impaired adults did not demonstrate patterns similar to unimpaired children and the concept of a developmental delay does not fit the pattern of perceptual deficits displayed by the speech-impaired participants.
The control and speech-impaired participants performed similarly on identification of “say-stay” when using a temporally-based cue, i.e., stop gap duration. This may suggest that these adults needed both stop gaps in addition to spectral cues to perceive the categories correctly. Stop gap detection in speech stimuli has been less well studied than place of articulation as a speech perception cue in language and speech-impaired children. Only a few studies have manipulated this temporal cue independently of spectral changes. One study found adults with familial dyslexia required longer silence durations than controls to identify stimuli as [sta] (Steffens, Eilers, Gross-Glenn, & Jallad, 1992
). Tallal and colleagues (Tallal & Stark, 1981
) reported that while significant differences were not observed between controls and language-impaired children on discriminating [sa]-[sta], three times as many language-impaired children as compared to controls failed to reach training criteria using these stimuli.
Some differences between our findings and those of previous investigators may reflect differences in the underlying populations and/or methodologies. For example, others used a synthesized [sa]-[sta] stimuli, which are non-meaningful, compared to the meaningful “say-stay” stimuli used in this study (Steffens et al., 1992
; Tallal & Stark, 1981
). However, our speech-impaired participants did not benefit from the meaningful stimuli when the perception involved brief formant transitions as in the “say-stay” continua varying first formant transitions. Thus, these results suggest that small, spectral cues may be difficult for these participants. The ability of the speech-impaired adults to categorize the stimuli similarly to controls using the stop gap durations indicates that their difficulty was not with categorical perception per se
, although they may need redundant cues to identify speech sounds. Further, because the task demands for all three continua were exactly the same, the stop gap results indicate that both speech-impaired and control participants understood the task equally well and could perform the task.
The differences between the speech-impaired and control participants were not due to differences in language or nonverbal intelligence, which were controlled through participant selection. However, the results of the Rhythm Test and Tonal Memory Test indicated that the speech-impaired adults had poor short-term auditory memory skills compared to controls. The Tonal Memory Test (Seashore et al., 1939
) is based on memory for tones that are sufficiently long in duration (250 ms) to provide an adequate window for encoding (Tallal & Piercy, 1973
), but differ in frequency.
Our results of impairments on the Digit Span Subtest in speech-impaired adults are consistent with a previous study finding deficits in short-term verbal memory in speech-impaired children (Smith, 1967
). However, the short-term memory deficits of these speech-impaired adults, were not limited to phonological stimuli based on finding deficits with non-speech stimuli (i.e., the Rhythm Test and Tonal Memory Test results). The phonological working memory model does not separate phonological memory from perceptual processing skills required for accurate identifying the phonological representations, thus confounding measurement of memory and perceptual abilities (Bowey, 1997
). It would appear that limitations in both verbal and nonverbal auditory memory and speech perception are affected in these speech-impaired adults.
These results in speech-impaired adults are similar to some reports of phonologically-impaired children having speech perception difficulties (Bird & Bishop, 1992
; Broen et al., 1983
; Hoffman et al., 1985
; Rvachew & Jamieson, 1989
). In each of these studies of speech-impaired children, however, the authors noted differences among the speech-impaired children with some having perceptual difficulties and others performing normally. None of these studies determined whether children had family histories of speech disorders and we do not know whether some or all of these children eventually recovered from their speech disorders later in childhood. Neither verbal nor nonverbal memory was assessed in the children with speech perception difficulties so it is unknown whether short-term memory deficits co-existed with speech perception difficulties in these children. On the other hand, Stark and Tallal (1988)
compared children with speech articulation disorders with normal children and language-impaired children on a variety of speech, language, auditory processing, memory, speech perception, motor and sensorimotor skills. Many of their speech-impaired children previously had delays in language development, and 21/36 came from families with histories of speech and/or language disorders. When compared with the controls, the speech-impaired participants did not differ from the normal control group on speech discrimination testing on the/ba/-/da/stimuli pair for formant transitions for 40 ms. However, the speech-impaired participants was significantly impaired relative to the control group on the Serial Memory subtest for cross-modal stimuli incorporating three elements (Stark & Tallal, 1988
). Although the authors profiled the speech-impaired participants as primarily having deficits in motor performance, the children were not impaired on the diadochokinetic speech test suggesting that the children did not have speech apraxia. The description of the childrens’ speech articulation errors was limited to the Templin Darley test scores with no description of the types of speech errors. This was the only study incorporating both speech discrimination and short-term memory in children with speech errors and only found deficits on short-term serial memory. By finding both speech perception and verbal and nonverbal deficits in adults with residual speech errors, we hypothesize that our participants may have been those most impaired on both speech perception and verbal and nonverbal memory as children.
Other studies have examined short-term memory for verbal material in speech-impaired children (Saxman & Miller, 1973
; Smith, 1967
), although Saxman and Miller (1973)
concluded that diminished linguistic ability could better account for differences between the speech-impaired and control groups rather than short-term memory. Bergendal and Talo (1969)
examined short-term memory skills using nonverbal stimuli in the Seashore tests, similar our study. They studied severely speech delayed children with multiple speech errors, normal vocabularies and significant grammatical errors in their sentences, suggesting that they were language-impaired. The estimated effect sizes of differences found between the normal and speech-impaired children were 2.2 on the Tonal Memory Test and 1.45 on the Rhythm Test of 1.45 (Bergendal & Talo, 1969
), similar to found here in our speech-impaired adults.
Thus, speech perception difficulties and verbal and nonverbal memory deficits have been reported separately in studies of speech-impaired children, some of whom may have had language delays. However, both sets of skills (speech perception and short-term memory) have not been assessed together in most studies and have not been found to co-exist in speech-impaired children as a group. Furthermore, none of the studies in children selected those with family histories or followed such children to determine if they continued to have residual speech disorders into adulthood. Therefore, because children with familial speech disorders that persist into adulthood have not been studied on both speech perception and short-term memory skills, we can only hypothesize based on our results in adults that the co-existence of both speech perception and verbal and nonverbal memory deficits may be associated with residual speech errors continuing into adulthood.
Our finding that speech-impaired adults with residual deficits have difficulties in both speech perception and short-term memory may suggest that these deficits may be part of this syndrome. Of course, this does not determine whether or not the presence of deficits in speech perception and short-term memory for spectral cues played a role in the persistence of the speech impairment in this syndrome. Prospective studies of children with familial speech impairments are needed to determine whether associated deficits in speech perception and short-term memory are predictive of the degree to which these children’s deficits can be remediated with therapy.