Developments in knowledge about the brain are revolutionizing every aspect of society—the last decade of the 20th century, in fact, has been called the "decade of the brain." Positron emission tomography (PET), magnetic resonance imaging (MRI), and functional magnetic resonance imaging (fMRI) have greatly increased our understanding of how the brain functions.
Such technologies and the massive research efforts of the human genome studies sponsored by the National Institutes of Health provide a wealth of new insights into brain activity as it relates to communication. Foundas (2001) gives readers an overview of current technologies and their application to study of the anatomical and functional relations for speech and language.
From Science to Practice
Probable links between biology and behavior are reported in the research literature. With increased numbers of applied research investigations, we are beginning to infuse neuroscience discoveries into daily practice. Our discipline, like others in allied health and education-related fields, is in a nascent stage in brain-behavior research and examining outcomes related to specific intervention processes. As Aram and Eisele (1994) point out, language-processing models derived from research with adults do not match the patterns of children. A need for further study of developmental changes and evidence-based intervention practices to document outcomes is essential.
Research reports also underscore the limitations of the findings. Results must be generalized with caution because of the small numbers of participants in most of the studies. There is agreement that the heterogeneity of participants and discrepancies in defining the criteria for group membership also confound the research. There is evidence as well that some investigations may not have been sufficiently specific in the measurements taken when defining the neuroanatomical areas of interest and the appropriate measurement parameters.
It is generally agreed that multiple factors must be carefully defined and controlled. These include such factors as categorical membership (e.g., typical/atypical communication; language/speech associated with aphasia, TBI, dysarthria; dyslexia/dysgraphia; SLI; LD/LLD; ADD/ADHD). Other factors include gender, IQ, handedness, socioeconomic status, and genetics, to name a few. An overview of the evolving research related to developmental language disorders can be found in Lane, Foundas, and Leonard (2001).
Plante (1991) provided evidence of a genetic component associated with developmental language disorders in families. More recent insights into the genetic contributions to learning are remarkable. In many ways, the pendulum has swung back from the experiential (nurture) to the pre-programmed (nature). Overall intelligence is clearly a polygenic trait (the additive effect of several genes that contribute to the phenotype) with an estimated heritability (percent of the trait due to genetic factors) of 80%–90%. It is important to note, however, that this strongly imprinted learning capacity does not negate the effects of learning, training, or experience. Even so, it is still true that much of an individual's overall learning potential is predetermined by genetic factors.
Genetics and Learning Disabilities
The last five years have seen tremendous advances in the genetic influences on learning disabilities. One of the key elements in any search for genetic linkage is the ability to accurately define a specific phenotype. Early studies in the search for genes that influence dyslexia were largely unsuccessful because of the inability to define a quantitative phenotype. However, advances in neuropsychologic testing allowed researchers to more clearly define pure reading disabilities that were familial.
Using the combined power of targeted neuropsychological testing and whole genome linkage screening, at least three distinct loci—sites—have currently been identified that play a major role in reading disabilities. The next steps will be for research to go from linkage to detailed mapping, identification of candidate genes, and ultimately sequencing of contributing genetic loci.
Genetic conditions such as Williams syndrome lend insight into genetic influences on behaviors. Individuals with Williams syndrome consistently have interesting behavioral characteristics. They are extremely personable individuals and are often described as pleasant or polite. Their expressive language skills usually greatly exceed all other performance measures. They usually exhibit little stranger anxiety and show problems with recognition of things such as "personal space" and other socially important clues. The discovery of a specific genetic marker for Williams syndrome (a deletion on the long arm of Chromosome 7) indicates the location of a major gene influence on behaviors.
In addition, linkage studies are beginning to identify genetic loci specific for other behaviors. In ADHD, humans with a certain variation of their SNAP-25 gene are 50% more likely than those without the variation to be hyperactive. Also, linkage with dopamine receptor 5 (DRD5), alpha adrenergic receptor, dopamine hydroxylase have been reported. A genetic basis for addictions also has been claimed. Two alleles (alternative forms of a given gene; A1 and A2) of the dopamine receptor (DRD2) appear to be related to an individual's success in quitting smoking.
A tremendous amount of attention has recently focused on the genetic basis of autism. The concept that specific functions such as socialization and communication are genetically regulated is a relatively new idea. Many still suggest that the remarkable increases in the numbers of reported cases of autism are caused by environmental factors.
However, the bulk of the evidence suggests prenatal influences on brain development, that is, cerebral dysgenesis (malformations of the brain).
A distillation of the vast body of literature in this realm suggests that autism may be a "final common pathway" of brain expression. Within this context, a common physiologic link may be excessive cellular communications. Currently, the advances in genetic research have clearly documented one fact: Autism is a disorder that does not have a single genetic cause.
In the clinical realm, current technology allows the identification of the etiology (specific cause) of individuals with autism in approximately 30% of the cases. Given these results, it is advisable to offer a formal neurogenetic evaluation to every individual/family affected with one of the autism spectrum disorders. Although current technology does not yet allow for the therapeutic application of this technology, the hope is that it will soon.
It has been well established that sickle cell anemia in the African American population accounts for many incidences of neurologic impairment and hearing loss in those who have the disease. It has been reported that approximately "one in every 10–12 African Americans can expect to carry the disease trait, and one in 600 will have the disease" (Salas-Provance, 1996; p. 166). The effect on hearing is a sensorineural hearing loss that is progressive and degenerative. With each sickle-cell crisis there appears to be worsening of the hearing, perhaps associated with the decreased blood supply to the cochlea.
The Large Picture
It is important for any health care provider these days to be familiar with the status of current genetic technology. No longer is genetics an isolated, small part of clinical medicine. Rather, it permeates all of clinical services. Practitioners should be aware of current methods of genetic testing, as well as the ethical, legal, and social implications of such testing.
The practitioner should be aware of the major indications for referral for genetic evaluations. Many practitioners and patients/families rightfully ask for the rationale for testing for many conditions that are potentially untreatable. It is important for the family and the practitioner to recognize that an etiologic answer is important for many reasons. Identifying a cause lends insight into issues such as prognosis, associated medical concerns, recurrence risk, and access to services. Most importantly, the clinical geneticist can be an extremely helpful member of the multi-disciplinary team taking care of children with special health care needs.
In the future, continued advances in genetic technology raise the hope of true genetic therapies. Currently, there are precious few such applications. Despite all of the optimism that appears in the popular press to this end, this optimism should be cautiously stated. It is important for practitioners to convey to families that the hope of such technology is real, but not yet available. Several major hurdles need to be overcome before true "gene therapy" will be clinically helpful to most patients. Practitioners do have sufficient information about how the brain develops and "learns," however, to guide service providers in modifying the environment to ensure that it is linguistically enriching. Genetics and neuroimaging hold promise for continued expansion of our knowledge and understanding that will further shape evidence-based practice.