Research
Neurobiological Basis of Learning Disabilities:
An Overview
by Dr. Christina Fiedorowicz
Learning disabilities (LD) are neurobiological in nature. How do we know this? Over the last decade, particularly, compelling scientific evidence from genetic research and studies of the brain has clearly demonstrated the neurobiological basis of learning disabilities.
Genetic Link to Learning Disabilities
A genetic basis for learning disabilities has been confirmed through twin studies, sibling analysis and family pedigree analysis. Twin studies have shown that if one twin has a reading disability, the probability of the other twin also having a reading disability is 68 per cent for identical twins (monozygotic) and 40 per cent for fraternal twins (dizygotic). Familial transmission of LD has been investigated, and has shown that if there is a family history (parents, siblings, and extended family) of reading disabilities, the probability of having a reading disability is significantly increased. Several modes of transmission have been investigated. Although there are, as yet, no definitive conclusions, a possible linkage to chromosomes 6 and 15 has been identified.
In the past, the reported incidence of reading disabilities was disproportionately higher for males than females (4:1 to 2:1). Current findings indicate that the ratio is 1.5:1. The reason for this change is that, in the past, there was an under-identification of females. With better diagnostic processes, more females have been identified in recent years.
Early Development of the Brain
Fetal brain development is complex; cells grow, multiply, migrate, and establish interconnections and a communication system. Growth continues over a prolonged period of time, and the first three years of brain development are critical. Cells in the early stage reproduce at an astonishing rate of 250,000 a minute. The human brain has approximately 100 billion neurons and one trillion glial cells (supporting cells). The cells assemble themselves in a series of tightly choreographed steps, with clockwork precision. The cells migrate to distant locations. Wiring the brain involves trillions of connections between neurons linking one part of the brain to another. The developing brain undergoes an incredible metamorphosis through a series of extraordinary changes.
Nature is the dominant factor driving this phase of development, but nurture plays a vital role. Changes in the environment can interfere with the precision of development. Sensory experiences contribute to shaping, thereby determining which connections develop and which ones are pruned. Normal growth can be disrupted by a wide variety of factors. Environmental events, such as toxin exposure, can significantly alter outcome. Early development as well as later cognitive and behavioural development can be affected. Negative effects on brain development include: prenatal factors; toxic and teratogenic agents; poor nutrition; very low birth weight; gestational age; oxygen deprivation; early oxygen dependence; neonatal seizures; hemorrhages; resistance to thyroid hormones; PCBs and other dioxins; alcohol, cigarettes, marijuana, and cocaine; lead and cadmium; and iron and chloride deficiencies.
Measuring the Brain
A variety of methods are now available to measure the physical structure as well as the function of the brain. Neuroanatomical techniques include autopsy studies; neuro-imaging techniques include CT scan, MRI, PET, rCBF, and SPECT; electrophysiological measures include EEG, ERP, and AEP; and neuropsychological assessments evaluate brain/behaviour relationships.
A number of studies of brain structure and function have been carried out on subjects with LD. One method to look at structural differences in the brain is through the microscope in postmortem or autopsy studies. Postmortem findings have indicated that the normal brain has asymmetries. For example, one side of the brain is not exactly the same as the other. These asymmetries are expected and considered normal (just as it is quite ordinary or typical for one foot to be longer than the other).
Important research efforts have focused on reading disabilities, since they represent the most common and frequently identified type of LD. Studies have shown that brains of subjects with reading disabilities have no asymmetry in brain structures where there should be asymmetry, that is, there is an absence of ordinary asymmetry. For example, the temporal lobe (planum temporale area) in the left hemisphere has been found to be typically larger than the temporal lobe (planum temporale area) in the right hemisphere in subjects without LD (asymmetrical), whereas, this area in the left hemisphere has been found to be the same size as in the right hemisphere in subjects with LD.
Another technique for studying the brain is the CT scan (computed tomography (roentgen-ray)) With this technique, a beam of x-rays is shot through the brain, identifying bone, grey matter, and fluid. A computer then reconstructs an image of each slice or brain section, allowing abnormalities in structure to be detected. CT scans of the occipital lobe, for example, have shown asymmetry of the occipital pole in subjects without LD and symmetry in subjects with LD.
Magnetic resonance imaging (MRI) is a technique that involves picking up the electromagnetic energy of brain protons and constructing an image by superimposing magnetic fields. MRI research has shown that subjects without LD showed leftward asymmetry in the angular gyrus of the parietal lobe, whereas subjects with LD did not show the expected asymmetry.
It has been demonstrated through autopsy, CT Scan, and MRI studies, that there are structural differences in the brains of subjects with LD in comparison to subjects without LD. It has also been demonstrated that there are differences in brain function in the subjects with LD, that is, how the brain works. Functional neuroimaging techniques, including PET (positron emission tomography), rCBF (regional cerebral blood flow), fMRI (functional magnetic resonance imaging), and SPECT (single photon emission computed tomography), are used to measure brain activity while subjects are engaged in a task such as reading. An fMRI is a non-invasive method that measures blood flow, while PET and SPECT methods involve the injection of radioactive materials. SPECT scan results have indicated that subjects with LD show underfunctioning in the occipital lobe while reading, in comparison to subjects without LD.
Electroencephalograms (EEGs), event related potentials (ERPs), and averaged evoked potentials (AEPs) record electrical activity of the brain through electrodes. Research has shown that subjects with LD (dyslexia) showed less electrical activity in the parietal lobe, in comparison to subjects without LD.
Neuropsychological assessments include a variety of tests of cognitive/intellectual, language, visual-perceptual, academic, motor, sensory, and emotional/behavioural abilities and functions. A profile of strengths and weaknesses is then correlated with known brain functions. The neuropsychological research has indicated significant findings as well. Deficiencies in language/verbal learning, reading, written language, verbal reasoning, verbal memory, arithmetic computation, and processing speed have been associated with left hemispheric dysfunction. Deficiencies in spatial function, nonverbal reasoning, nonverbal cues, social skills, and social/emotional information have been associated with right hemispheric dysfunction. Phonological processing deficits have been identified as a primary difficulty in subjects with language and reading disabilities, and structural and functional abnormalities in the medial geniculate nuclei have been associated with these findings.
Through the application of these investigative procedures, anomalies in brain structure and associated dysfunction have been implicated in subjects with LD. Some of these include: the planum temporale, medial geniculate nuclei, perisylvian regions, frontal cortex, parietal operculum, inferior parietal lobe, temporal gyrus, corpus callosum, insular region, angular gyrus, occipital-striatal region, and the brainstem reticular activating system.
Conclusion and Implications
There is significant scientific evidence from genetic research and studies of the brain that has demonstrated the neurobiological basis of learning disabilities. There are differences in both brain structure and brain function, and these findings have important implications. Educators must recognize and accept the scientific evidence, establish policies, develop effective educational programs, and match the instructional goals, content, and pace of teaching specifically to the learning needs of individuals with LD so that these individuals can achieve maximum success.
It is important to emphasize that individuals with LD can learn, but the process may be inefficient as a result of the specific differences in brain structure and function. Inefficiency refers to either low accuracy or low speed in learning or performing a task and is quite distinct from inability or incapacity. Information can be processed, but at a slower rate and/or by different methods as compared to individuals without LD. The educational process, learning strategies, compensatory techniques, and remedial intervention can significantly impact the learning process. Therefore, effective and efficient learning and teaching methods are needed to specifically meet the needs of individuals with LD.
These findings have been reviewed in more detail in a paper prepared by the Learning Disabilities Association of Canada (Ottawa, Ontario), entitled Neurobiological Basis of Learning Disabilities by C. Fiedorowicz, E. Benezra, W. MacDonald, B. McElgunn, and A. Wilson, 1999.
Christina Fiedorowicz, PhD, CPsy, is president of Achieve Educational Services Inc. and has a private practice in neuropsychology in Ottawa, Ontario. Her clinical and research interests include various aspects of brain functions, learning disabilities, and attention deficit disorders. She is a member of the Professional Advisory Board for both the Learning Disabilities Association of Canada and Children & Adults with Attention Deficit Disorder.
This article was published by the Canadian Child Care Federation in the Linking Research to Practice: Second Canadian Forum Proceedings Report. November 1999. . © CCCF Posted by LDAC with permission.
