درک محرک گفتار و غیرگفتاری توسط کودکان با و بدون اختلال خواندن و بیش فعالی و نقص توجه

The auditory temporal deficit hypothesis predicts that children with reading disability (RD) will exhibit deficits in the perception of speech and nonspeech acoustic stimuli in discrimination and temporal ordering tasks when the interstimulus interval (ISI) is short. Initial studies testing this hypothesis did not account for the potential presence of attention deficit hyperactivity disorder (ADHD). Temporal order judgment and discrimination tasks were administered to children with (1) RD/no-ADHD (n=38), (2) ADHD (n=29), (3) RD and ADHD (RD/ADHD; n=32), and (4) no impairment (NI; n=43). Contrary to predictions, children with RD showed no specific sensitivity to ISI and performed worse relative to children without RD on speech but not nonspeech tasks. Relationships between perceptual tasks and phonological processing measures were stronger and more consistent for speech than nonspeech stimuli. These results were independent of the presence of ADHD and suggest that children with RD have a deficit in phoneme perception that correlates with reading and phonological processing ability.

Children with specific reading disability (RD) consistently exhibit deficits in phonemic awareness, or the ability to manipulate the elements of spoken language (Blachman, 2000; Fletcher et al., 1994; Shankweiler & Crain, 1986; Stanovich, 1988). These deficits may be detected in children as young as 5 years of age, are predictive of future reading ability (Hatcher & Hulme, 1999; Hulme, Muter, & Snowling, 1998; McDougall, Hulme, Ellis, & Monk, 1994; Muter, Hulme, Snowling, & Taylor, 1997; Scarborough, 1998), and may have a direct role in the etiology of RD (Bradley & Bryant, 1983; Fletcher et al., 1994; Foorman, Francis, Fletcher, & Lynn, 1996; Shankweiler & Crain, 1986; Stanovich & Siegel, 1994; Wagner & Torgesen, 1987; Wagner, Torgesen, & Rashotte, 1994). While the association of deficits in phonemic awareness and RD is well established, the etiology of these deficits is not. The auditory temporal deficit hypothesis suggests that at least a subgroup of children with RD have difficulty processing rapidly presented acoustic stimuli, regardless of whether they are speech or nonspeech in nature. This deficit results in difficulty in the perception of brief portions of the speech signal, such as formant transitions, so that speech perception is inaccurate. Inaccurate speech perception, in turn, may lead to more elaborated difficulties in phonological processing, including poor phonemic awareness (Tallal, 1980; Tallal et al., 1993 and Tallal et al., 1997). In initial studies that led to the formulation of the auditory temporal deficit hypothesis, Tallal and Piercy, 1973a and Tallal and Piercy, 1973b found that children with specific language impairment (SLI), who have more general receptive and expressive language deficits than most children with RD (Tomblin & Zhang, 1999), performed normally on discrimination and temporal order judgment (TOJ) tasks utilizing nonspeech stimuli (steady-state complex tones) when the interstimulus interval (ISI) was relatively long (428 ms). However, as ISI decreased, performance deteriorated relative to controls. Tallal and Piercy (1973b) interpreted these findings as suggesting a deficit in the speed of auditory processing, as lengthening the stimulus or the ISI improved performance. In another series of experiments, children with SLI were found to be deficient in discrimination and TOJ tasks for stop consonants (/ba/ vs /da/), while performance for steady-state vowels (/æ/ vs /ε/) was similar to that of controls, unless the number of stimulus elements was large (five) ( Tallal & Piercy, 1974). Again, these results were interpreted as supporting a deficiency in the speed of processing of acoustic stimuli, as the differentiation between /ba/ and /da/ is largely dependent on perception of a brief (40- to 50-ms) spectral sweep that occurs after the initial noise burst before the onset of the steady-state vowel. Subsequently, Tallal and Piercy (1975) found that lengthening these brief transitional events improved performance, which they interpreted as further support for the hypothesis that children with SLI have a deficit in discriminating brief auditory events due to a limitation in auditory processing speed. Tallal (1980) extended the auditory temporal processing deficit hypothesis to children with RD by employing the same nonspeech stimuli and tasks as Tallal and Piercy (1973b) and demonstrating that a subgroup of children with RD performed worse than non-reading-impaired (NI) children on discrimination and TOJ tasks at short (average of performance at an ISIs of 8, 15, 30, 60, 150, and 305 ms), but not long (428 ms), ISIs. Performance at short ISIs was correlated with performance on a phonics test.These findings regarding perception of nonspeech stimuli were interpreted as indicating a “primary perceptual deficit” affecting “rapid acoustic analysis” that was related to “difficulty in learning the sound–symbol relationships that are the basis of phonics rules” (Tallal, 1980). Tallal (1980) distinguished between this perceptual deficit and an ordering deficit on the basis of similar deficits on TOJ and discrimination tasks, as the latter do not require ordering. Reed (1989) replicated the Tallal (1980) findings regarding nonspeech stimuli in children with RD, also finding deficits for stop consonants but not vowels on TOJ tasks. The presence of a deficit in rapid processing of nonspeech acoustic stimuli, and a potential role for such a deficit in phoneme perception deficits observed in children with RD, remain a matter of controversy. Information critical to the perception of some phonetic contrasts is present in brief acoustic events such as formant transitions that precede the onset of the periodic portion of a stop consonant–vowel, and many studies indicate deficits in phoneme perception in children with RD (e.g., Godfrey, Syrdal-Lasky, Millay, & Knox, 1981; Lieberman, Meskill, Chatillon, & Schupack, 1985; Manis et al., 1997; Mody, Studdert-Kennedy, & Brady, 1997; Tallal, Stark, Kallman, & Mellitis, 1980). It is not clear, however, that children with RD have a deficit in the perception of these specific events within the speech signal itself. The initial studies upon which the auditory temporal deficit hypothesis was based, as reviewed above, used nonspeech stimuli that were steady-state in nature. There was no attempt in either Tallal (1980) or Reed (1989) to assess perception of nonspeech stimuli with portions modeled after formant transitions. One study that did present nonspeech stimuli modeled after the formant transitions in a place contrast (/ba/–/da/) found differences between RD groups for speech stimuli only (Mody et al., 1997). This study provided support for an alternative hypothesis suggesting that deficits in phoneme perception are the result of deficient phonological coding and are therefore specific to speech stimuli (Brady, Shankweiler, & Mann, 1983; Liberman & Mattingly, 1989; Lieberman et al., 1985; Mody et al., 1997). This “speech specific” hypothesis is also supported by studies that fail to find evidence for a general auditory temporal processing deficit in children with either SLI or RD (Bishop, Carlyon, Deeks, & Bishop, 1999; Helzer, Champlin, & Gillam, 1996; Mody et al., 1997; Nittrouer, 1999; Schulte-Korne, Deimel, Bartling, & Remschmidt, 1998), as well as those that find a deficit for speech, but not nonspeech stimuli (Adlard & Hazan, 1998; Brady et al., 1983; Mody et al., 1997). Initial studies testing the auditory temporal deficit hypothesis had a range of limitations, including small group size, widely varying and at times ill-defined criteria for group membership, lack of control for the high comorbidity of RD and attention deficit/hyperactivity disorder (ADHD), and inadequate control of stimulus parameters. For example, Tallal (1980) and Reed (1989) delivered their stimuli-free field. Therefore, there is no way to know what the stimulus intensity was at the ear or whether this intensity was the same for all participants. In addition, many of the studies in this area do not specifically exclude children with SLI in order to insure that findings reflect deficits in children with RD rather than children with SLI who may be included in RD groups. The current study was designed to address these issues. Relatively large groups of children with and without RD were identified using definitions similar to those in previous studies by our group (Breier et al., 2001; Fletcher et al., 1994; Foorman et al., 1996) and others (Joanisse, Manis, Keating, & Seidenberg, 2000; Manis et al., 1997; Post, Swank, Hiscock, & Fowler, 1999; Stanovich & Siegel, 1994). Children with SLI were identified and excluded. In addition, as reading ability is a continuous variable that is normally distributed in the population (Rodgers, 1983; Shaywitz, Escobar, Shaywitz, Fletcher, & Makuch, 1992), we repeated all analyses treating reading ability as a continuous rather than a categorical variable in order to examine the generalizability of results across reading disability definitions. In order not to exclude children with RD whose word recognition skills may be within normal limits due to past treatment, the criteria for assignment to the RD group included deficits in spelling and nonword reading, which are hallmarks of the disorder (Torgesen & Wagner, 1999). Aggregating scores by averaging across tests also attenuates the error of measurement, improving the reliability of estimates of whether a child is above or below a particular cutoff. Attention deficit hyperactivity disorder was also included as a factor in a factorial design with RD by identifying children with and without ADHD and RD. Although ADHD is a common behavioral disorder that exhibits substantial comorbidity with RD, affecting 30–70% of the RD population depending on the sample and how the disorder is defined (Shaywitz & Shaywitz, 1994), many studies of perception in children with RD do not take the potential presence of ADHD into account in a systematic fashion. Behavioral deficits associated with ADHD include an inability to sustain focused attention (Barkley, Grodzinsky, & DuPaul, 1992), an impulsive response bias (Barkley, 1997a and Barkley, 1997b), and reduced working memory (Barkley, 1997b), all of which potentially affect performance on perceptual tasks. We specifically include a group with both RD and ADHD to account for the possibility of a synergistic interaction between the two disorders. The core experimental paradigms used by Tallal and colleagues in assessing auditory function in their initial studies were (a) same/different discrimination tasks, in which participants indicate whether two sequentially presented acoustic stimuli are the same or different; and (b) TOJ tasks, in which participants repeat the sequence of presentation of acoustic stimuli (Tallal, 1980; Tallal and Piercy, 1973a, Tallal and Piercy, 1973b, Tallal and Piercy, 1974 and Tallal and Piercy, 1975). The inclusion of a same/different discrimination task serves as a control for the TOJ task, which has a sequencing as well as a perceptual component. Stimuli were either synthetic syllables that differed in only one phonetic feature (e.g., /ba/–/da/) or steady-state complex tones modeled after the formant content of vowels. The rationale was that speech and nonspeech stimuli would be similar in acoustic content, but the latter would have no linguistic referent. We used stimuli similar to those described by Tallal (1974) and Reed (1989). The factors manipulated were ISI, stimulus type, and task. The auditory temporal deficit hypothesis predicts an interaction between the effects of RD group membership and ISI on performance, with children with RD exhibiting increasing deficits relative to controls with decreasing ISI regardless of whether tasks involve speech or nonspeech stimuli or whether discrimination or temporal order judgment are required. While we expected a general lowering of ability for children with ADHD across experimental factors, we were specifically interested in the interaction of RD and ADHD group membership suggesting potential synergistic effects when the two disorders are comorbid.