Shriberg (1994) has conceptualized developmental phonological disorders as speech disorders that originate during the developmental period. In most cases the cause of such disorders cannot be attributed to significant involvement of a child's speech or hearing processes, cognitive-linguistic functions, or psychosocial processes (Bernthal and Bankson, 1998), but causal origins may be related to genetic or environmental differences (Shriberg, 1994; Shriberg and Kwiatkowski, 1994). Children with developmental phonological disorders are heterogeneous and exhibit a range in the severity of their phonological disorders. Generally, the expected developmental period for speech sound acquisition ends at approximately 9 years of age, thus encompassing birth through the early school years. In sum, it is posited that children who exhibit phonological disorders differ with regard to the etiology and severity of the disorder and include both preschool and school-age children (Deputy and Weston, 1998). Some individuals with developmental phonological disorders acquire normal speech, while others continue to exhibit a phonological disorder throughout the life span, despite having received treatment for the phonological disorder (Shriberg et al., 1997).

Residual phonological errors are a subtype of developmental phonological disorders that persist beyond the expected period of speech-sound development or normalization (Shriberg, 1997). They are present in the speech of older school-age children and adults. Individuals with residual errors can be further classified into subgroups of those with a history of speech delay and those without a history of speech delay (i.e., individuals in whom a speech delay was diagnosed at some time during the developmental period and those who were not so diagnosed). It is postulated that the two groups differ with respect to causal factors. The residual errors of the first group are thought to reflect environmental influences, while nonenvironmental causal factors such as genetic transmission are thought to be responsible for the phonological errors of the second group.

Most residual errors have been identified as distortions (Smit et al., 1990; Shriberg, 1993) of the expected allophones of a particular phoneme. Distortions are variant productions that do not fall within the perceptual boundaries of a specific target phoneme (Daniloff, Wilcox, and Stephens, 1980; Bernthal and Bankson, 1998). It has been hypothesized that distortions reflect incorrect allophonic rules or sensorimotor processing limitations. That is, such productions are either permanent or temporary manifestations of inappropriate allophonic representation and/or the sensorimotor control of articulatory accuracy. It has been suggested that children initially delete and substitute sounds and then produce distortions of sounds such as /r/, /l/, and /s/ when normalizing sound production; however, investigative study has not supported this hypothesis as a generality in children who normalize their phonological skills with treatment (Shriberg and Kwiatkowski, 1988). Ohde and Sharf (1992) provide excellent descriptions of the acoustic and physiologic parameters of common distortion errors.

Smit et al. (1990) conducted a large-scale investigation of speech sound acquisition and reported that the distortion errors noted in the speech of their older test subjects varied with respect to judged clinical impact or severity. Some productions were judged to be minor distortions, while others were designated as clinically significant. Shriberg (1993) also noted such differences in his study of children with developmental phonological disorders. He classified the errors into nonclinical and clinical distortion types. Nonclinical distortions are thought to reflect dialect or other factors such as speech-motor constraints and are not targeted for therapy. Clinical distortions are potential targets for treatment and have been categorized by prevalence into common and uncommon types. The most common and uncommon types are listed in Figure 1. The most common residual errors include distortions of the sound classes of liquids, fricatives, and affricates. Uncommon distortion errors include errors such as weak or imprecise consonant production and difficulty maintaining nasal and voicing features. In most cases, residual errors constitute minor involvement of phonological production and do not have a significant impact on intelligibility, but research indicates that normal speakers react negatively to persons with even minor residual errors (Mowrer, Wahl, and Doolan, 1978; Silverman and Paulus, 1989; Crowe Hall, 1991).

Figure 1..  

Common and uncommon distortion errors as reported by Shriberg (1993).

Treatment for persons with residual errors is generally carried out using approaches that have been used with younger children. The treatment approaches are based on motor learning or cognitive-linguistic concepts (Lowe, 1994; Bauman-Waengler, 2000); however, in most cases a motor learning approach is utilized (Gierut, 1998). Although most individuals normalize their residual errors with intervention, some individuals do not (Dagenais, 1995; Shuster, Ruscello, and Toth, 1995). The actual number of clients in the respective categories is unknown, but survey data of school practitioners reported by Ruscello (1995a) indicate that a subgroup of clients do not improve with traditional treatment methods. Respondents indicated that children either were unable to achieve correct production of an error sound or achieved correct production but were unable to incorporate the sound into spontaneous speech. The respondents did not list the types of sound errors, but the error sounds reported are in agreement with the residual errors identified by both Shriberg (1993) and Smit et al. (1990).

In some cases, specially designed treatments are necessary to facilitate remediation of residual errors. For example, principles from biofeedback and speech physiology have been incorporated into treatments (Dagenais, 1995; Ruscello, 1995b; Gibbon et al., 1999). Different forms of sensory information other than auditory input have been provided to assist the individual in developing appropriate target productions. Shuster, Ruscello, and Toth (1995) identified two older children with residual /r/ errors who had received traditional long-term phonological treatment without success. A biofeedback treatment utilizing real-time spectrography was implemented for both subjects, and the results indicated that the two subjects were able to acquire correct production of the former residual error.

In summary, residual errors are a distinct subtype of developmental phonological errors that are present in the speech of older children and adults who are beyond the period of normal sound acquisition. Most residual errors are described as distortions, which are sound variations that are not within the phonetic boundaries of the intended target sound. Generally, residual errors are minor in terms of severity and do not interfere with intelligibility, but normal speakers do react negatively to such minor speech variations. An exact estimate of children and adults with residual errors is unknown, but it is thought that there are substantial numbers of individuals with such a phonological disorder.