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Stuttering has been considered a heritable disorder since the 1930s. There have been different models of transmission that have been proposed most involving a polygenic component with or without a major locus. In spite of these models, the characteristics being transmitted are not known. This study used two different tasks—a tapping task that is thought to probe hemispheric differences and a Stroop task, which appears to create interferences in speech motor programming and/or execution. The 48 participants in this study included individuals who stutter, high risk family members and controls for each group. Results indicated that for tapping at a comfortable rate, the experimental groups were significantly different from their control groups and for tapping at a fast rate, the stuttering and high risk groups were different from each other. The results of the Stroop test were not statistically significant.

Studies related to the familial nature of stuttering have been conducted since the 1930s (Berry, 1938, Bryngelson, 1935 and Bryngelson, 1939). There have been many reports indicating that stuttering clusters in families (see an early review by Sheehan & Costley, 1977). All studies, past and contemporary, suggest that individuals who stutter are more likely to have family members who stutter when compared with non-stuttering individuals. In a more recent review of the literature, Yairi, Ambrose, and Cox (1996) examined at least 26 studies related to familial history in stuttering. They too concluded that in spite of different findings in terms of the percentages, familial incidence of stuttering is consistently greater for individuals who exhibit stuttering than for the general population. Specifically, familial incidence of stuttering in very young children who stutter was found to be as high as 65%. In addition, twin studies examined by these authors indicated a high stuttering concordance rate of 63–83% among monozygotic twins. Andrews and Harris (1964) were among the first to put forth a genetic model that explains the transmission of stuttering within families. They postulated either a purely polygenic model or a polygenic model with a possible component of major locus or loci. Using segregation analysis Kidd (1980), and later Cox, Kramer, and Kidd (1984), offered a polygenic multifactorial transmission model wherein genetic susceptibility is believed to be transmitted through the cumulative contribution of multiple unspecified genes, with some role assigned to multiple environmental factors. Later, Ambrose, Yairi, and Cox (1993) also used segregation analysis and were the first to report statistically significant evidence for both single major locus and polygenic components for stuttering. Recently, Viswanath, Lee, and Chakraborty (2004) conducted a segregation analysis on 56 pedigrees of adults with persistent developmental stuttering. Using a regression model, they found evidence for an autosomal dominant major gene effect. Two co-variates – sex and affection status of parents – influenced the major gene. Although these findings and the emerging models have strengthened the conclusion that there is a strong genetic component to the familiality of stuttering, what exactly is being transmitted is not known. It could include abnormalities in biochemical pathways, structural differences, brain processing for speech and language, motor skills, and/or others (Cox, 1993 and Yairi, 1998). Quite likely, the postulated major gene(s) and other genetic components transmit a number of traits, some of which contribute to the expression of stuttering. This constellation of genes may vary from one family to another. Furthermore, a particular characteristic that increases the susceptibility for stuttering may not, by itself, cause the stuttering. But, when it co-occurs with certain other characteristics, stuttering may be expressed. As stated, however, the nature of these genes and their expression is not currently known. Some of the traits carried by genes underlying susceptibility to stuttering might also be expressed in non-stuttering family members. Susceptibility to stuttering is frequently transmitted by a non-stuttering parent having a family history of stuttering. Because the parents, as well as non-stuttering siblings, share some of the genes with the stuttering child, they may also share some of the fluency-related traits possessed by their child who stutters. Many studies that have as their focus the identification of traits that are related to the primary liability to a complex disorder. For example, offspring in families in which both parents have type 2 diabetes have been studied longitudinally to identify traits that precede (and may predict) the onset of the disease in these individual who are clearly at high risk of developing diabetes (Warram, Sigal, Martin, Krolewski, & Soeldner, 1996). In general, studies like this are challenging to design and implement. It is not always easy to isolate with precision a subset of individuals who must have high genetic liability to the disease without actually having become affected; this can be particularly challenging in disorders with late age at onset as it is difficult to obtain all the information before many relatives die. It is sometimes difficult to design an appropriate battery of tests that can be meaningfully completed by affected individuals, controls, and unaffected individuals with high genetic liability. Nevertheless, such research offers unprecedented opportunity for gaining a better understanding of the primary etiological factors underlying a complex disorder. Stuttering presents two advantages. First, it is a disorder of early onset and most relatives are still alive. Second, it is comparatively easy to identify with confidence relatives at high risk for the disorder who do not stutter. If it were possible to identify traits or characteristics that occur in the families of people who stutter, it would bring us considerably closer to an understanding of what general domains or aspects are involved in conditions that may lead to stuttering. Traits that non-stuttering members of stuttering families have in common with their stuttering relatives may help us: (a) determine what else is being transmitted with the major locus and narrow down areas in which to search for information about the functions of genes that underlie stuttering; (b) in addition, the traits may provide information pertaining to the polygenic components. The current investigation is an attempt to identify some of these characteristics by examining a certain class of non-stuttering relatives of individuals who stutter. It is assumed that these non-stuttering relatives have necessary, but not sufficient, factors for stuttering. The three groups of participants – individuals with stuttering, high risk for stuttering, and controls – can be considered to have varying degrees of factors relevant to stuttering. The individuals who stutter possess factors that are both necessary as well as sufficient to cause stuttering. In genetic terms, these factors can be thought of as components of a single major locus and a number of polygenic ones. Some environmental factors can also be present in this group, thereby allowing for multifactorial models. The high risk (but not stuttering) group can be thought of as having some necessary factors that are not sufficient to cause stuttering. The control subjects may, or may not, possess some scattered polygenic factors, which are certainly insufficient to cause stuttering. The polygenic-multifactorial model provides impetus to such research by predicting that, on the average, performance of high risk non-stuttering family members of the stuttering probands will be poorer on fluency related tasks than control individuals. Felsenfeld (1997) discusses this idea of reduced ‘fluency competence’ among the family members of the stuttering proband when compared to randomly selected control speakers. Although reduced fluency competence is generally expressed as disfluencies in speech, family members who do not stutter may exhibit other related characteristics. For example, inasmuch as production of fluent speech consists of linguistic and motor programming, neurological signals to initiate the movement, and the actual motor execution of the movements, reduced fluency competence may be reflected at any of these stages. Based on the considerations discussed above, the present study focused on the family members of individuals who stutter. Specifically, individuals with a high risk for stuttering who do not express stuttering were the prime targets. Although these individuals do not stutter overtly, they are considered ‘carriers’ with a potential to stutter. For example, a person who does not exhibit stuttering may have siblings or parents who stutter. If such a person transmits stuttering to his/her child, he/she is considered at high risk for stuttering or a carrier of stuttering. A battery of tasks assumed to tap different aspects of fluency discussed above were administered to these individuals. Potentially, such tasks could have included those revealing hemispheric functions as reflected in various finger tapping tasks ( Webster, 1986, Webster, 1990a and Webster, 1990b), interferences in speech programming and initiation ( Postma & Kolk, 1993), word interference ( Bosshardt, 1999), discoordination in speech production ( Van Leishout, Hulstijn, & Peters, 1991), acoustic differences in speech production ( Robb & Blomgren, 1997), and cognitive and language related differences ( Wingate, 1988). In the present investigation we elected to examine tasks assumed to involve neuromotor components of speech motor programming and execution, as well as hemispheric differences along with fine motor sequencing. Thus, we used: (a) a finger tapping task that assesses fine motor control requiring the processing by both hemispheres; (b) the Stroop test that assesses interferences in word retrieval and word encoding in programming for speech. The specific questions asked were: 1. Are there differences in performance between individuals who stutter and non-stuttering controls on tasks (finger tapping) reflecting cerebral dominance and/or interferences in word retrieval and encoding (Stroop test)? 2. Are there differences in performance between the high risk group and its control group on the above-mentioned tasks? 3. Are there differences in performance between the high risk group and the individuals who stutter on the same tasks?