Recent research into the causes and nature of reading problems has revealed more about the neural processes involved in reading. Recall from Chapter 2 that successful reading involves two basic processes— (1) coordination between the language and visual recognition processing systems that leads to (2) decoding and comprehension—that are generated by three neural systems. Figure 5.4 is a simplified illustration of how these three systems interact. Decoding written text into sounds that represent words results when the visual recognition and auditory processing systems see and sound out the words in the reader’s head. The frontal lobe interprets the meaning conveyed by those word form representations.
Problems with any one or more of these systems can cause reading difficulties. In some children, the problems occur during early brain development and affect their ability to process the sounds of language and, eventually, to decode written text. This developmental deficit appears to be the most common cause of reading difficulties, and usually results in a lifelong struggle with reading. Less common are problems with reading caused by organic impairments in hearing and vision that can occur at any time in a person’s life.
Most research studies on reading have focused primarily on developmental reading problems that scientists refer to as developmental dyslexia. (There are several other types of dyslexia, such as trauma dyslexia, which is caused by trauma during childhood affecting the brain’s reading areas.) In developmental dyslexia, the child experiences unexpected difficulty in learning to read despite adequate intelligence, environment, and normal senses. It is a spectrum disorder, varying from mild to severe, that has a genetic component. Estimates of the percentage of U.S. schoolchildren with dyslexia vary widely, from 5 to 15 percent. This range seems high, but that may be because there is not full agreement on the threshold used to define the impairment. But neuroscience is helping with this dilemma. Neuroimaging studies have established that there are significant differences in the way normal and dyslexic brains respond to specific spoken and written language tasks. Furthermore, there is adequate research evidence that these differences may lessen with appropriate instructional interventions.
Figure 5.4 Successful reading requires the coordination of three systems: (1) Visual recognition and (2) auditory processing to decode the words, and (3) frontal lobe processing to determine meaning.
Scientists have long been searching for the causes of reading problems. This has not been an easy task because of the large number of sensory, motor, and cognitive systems that are involved in reading. Struggling readers may have impairments in any one or more of these systems, but not all struggling readers have dyslexia. Deficits in auditory processing, low IQ, the complexity of English orthography, or a poor educational environment can also explain reading problems in some children.
Linguistic Causes
Several potential linguistic causes of reading problems and developmental dyslexia have emerged from recent research studies, including phonological deficits, differences in auditory and visual processing speeds, the varying sizes of brain structures, memory deficits, genetics, and brain lesions. It is possible that several of these causes are related to each other and can coexist in the same individual.
Phonological Deficits
The ability to sound out words in one’s head plays an important role in reading familiar words and sounding out new ones. Phonological information is used by the working memory to integrate and comprehend words in phrases and sentences. Numerous studies continue to show that phonological operations are impaired in many dyslexics (e.g., Vellutino, Fletcher, Snowling, & Scanlon, 2004), but not all (van Ermingen-Marbach, Grande, Pape-Neumann, Sass, & Heim, 2013). But the exact causes of the impairment were not clear. Because many people with dyslexia have average or above-average intelligence, researchers suspected that the phonological processing deficits appeared only when the brain was trying to decode writing. Exactly why that happens is not fully known. However, studies of the differences in auditory and visual processing speeds as well as functional magnetic resonance imaging (fMRI) scans of the brain during reading are shedding new light on the possible causes of the phonological impairments.
Differences in Auditory and Visual Processing Speeds
One of the more intriguing explanations of some reading difficulties, including dyslexia, has come from research studies using magnetoencephalography (MEG), a technique for measuring the electric signals emitted during brain activation as a result of mental processing. These studies noted abnormal auditory activation but normal visual activation during reading (Helenius, Salmelin, Richardson, Leinonen, & Lyytinen, 2002; Renvall & Hari, 2002; Schulte-Körne & Bruder, 2010; Tallal et al., 1996; Temple et al., 2003). Sometimes referred to as temporal processing impairment, the differences in the processing speeds could explain some of the symptoms common to dyslexia.
The explanation goes like this: When reading silently, our eyes scan the words on the page (visual processing), and we sound out those words in our head. This sounding out represents the auditory processing necessary for us to decode and interpret what we are reading. To read successfully, the visual and auditory processing systems have to work together—that is, be in synchrony.
When a child begins to learn to read, it is essential that the letter (grapheme) the child sees corresponds to what the child hears (phoneme) internally. In Figure 5.5, the child with normal auditory processing (left) is looking at the letter d, and the auditory processing system is simultaneously sounding out /d/ or duh. As the eye moves to o, the phoneme /ô/ or awh will sound out, and then g produces the phoneme /g/ or guh. Later, when this child is asked to write dog, the /d/ phoneme will recall the letter d, and so on.
However, if the auditory processing system is impaired and lags behind the visual processing system, then the child’s eye is already scanning to the letter g while the phoneme /d/ or duh is still being processed in the auditory system. As a result, the child’s brain incorrectly associates the letter g with the phoneme sound of /d/ or duh. Now, when we ask the child to write the word dog, the child hears the first phoneme /d/ or duh but incorrectly recalls the letter g, perhaps eventually writing the word god.
If this notion is correct, then finding a way to bring the auditory and visual processing systems in closer synchrony should help to remedy the problem. That is exactly what several researchers tried. They developed a computer program, known as Fast ForWord, designed to help poor readers slow down visual processing to allow the auditory processing sufficient time to recognize the sound of the initial phoneme. Using this program with children who had reading problems produced surprisingly successful results. The program and process are discussed in greater detail in Chapter 6. Recent studies suggest that temporal processing impairment may lessen as children with reading problems, including dyslexia, mature into adults (Vandermosten et al., 2011).
Figure 5.5 In normal auditory processing (left), the phoneme that the child hears correctly matches the letter that the eyes see—in this case, the sound duh corresponds to the letter d. If the auditory processing system is delayed, however, the child’s eyes are already on the letter g while the duh phoneme is still sounding in the child’s head. The brain errs in matching duh with g.
Structural Differences in the Brain
Some MRI studies have found that the brains of people diagnosed with dyslexia are structurally different from nondyslexic brains. In one study, the researchers noted that the dyslexic brains of 16 men had less gray matter (surface of the cerebrum) in the left temporal lobe, frontal lobe, and cerebellum than the brains of 14 nondyslexic subjects (Brown et al., 2001; Steinbrink et al., 2008). Having less gray matter (and thus fewer neurons) in the left temporal lobe (where Wernicke’s area is located [see Chapter 1]) and in the frontal lobe (where comprehension occurs) could contribute to the deficits associated with dyslexia.
Many brain imaging studies in recent years are yielding similar results regarding how the brain activity in people with dyslexia differs from that in typical r
eaders. One common finding is the reduced level of activity in the left temporal lobe where the visual word form area (see Chapter 2) is located (e.g., McCrory, Mechelli, Frith, & Price, 2005), as well as reduced connectivity to other language processing areas (van der Mark et al., 2011). This could be the result of a lesser amount of white matter in this region compared to the amount in typical readers, as some studies have reported (e.g., Rimrodt, Peterson, Denckla, Kaufmann, & Cutting, 2010). Another abnormality frequently found in dyslexia relates to the left frontal cortex where Broca’s area, one of the brain’s language processing regions (see Chapter 1), is located. This area is frequently overactive when people with dyslexia attempt to read or carry out various phonological tasks (Georgiewa et al., 2002; Grande, Meffert, Huber, Amunts, & Heim, 2011). It may be that this hyperactivity in Broca’s area is an attempt to compensate for the insufficient activation in the brain’s decoding sites.
A third anomaly common to people with dyslexia deals with the visual recognition system. Apparently, they are not able to recall the letters that comprise a word simultaneously, which explains their slow reading time. To compensate, they often activate regions in the right hemisphere, hoping to access the word’s phonology—an inefficient and often unproductive endeavor (Zoccolotti et al., 2005). In sum, as the research evidence accumulates, it seems clear that most children with developmental dyslexia have visual analysis and phonological decoding areas of the brain that are insufficiently active and dysfunctional.
Phonologic Memory Deficits
Skilled reading requires an ability to retain verbal bits of information (phonemes) in working memory. Studies have shown distinct deficits in this phonologic memory among poor readers. The deficits more commonly involve serial tasks, such as holding a string of phonemes to make a word and a sequence of words to generate a sentence (Carretti, Borella, Cornoldi, & De Beni, 2009; Howes, Bigler, Burlingame, & Lawson, 2003). In addition, some studies suggest that the memory deficits could result from difficulties with cognitive processing in the frontal lobe (Wang & Gathercole, 2013). However, for some children, this may be a symptom of a developmental delay that could correct itself as they mature.
Genetics and Gender
Studies of genetic composition have long shown strong associations between dyslexia and genetic mutations in twins and families (Pennington, 1990), and recent investigations have actually identified some half-dozen of the specific genes involved (e.g., Benítez-Burraco, 2010; Kaminen et al., 2003). Preliminary studies seem to indicate that the genetic mutations disrupt the migration of neurons in the fetal brain as they attempt to travel to areas of the cortex to their final position. Instead, they accumulate in disorganized tangles, often around the visual word form area (Meng et al., 2005; Paracchini et al., 2005). These tangles prevent this brain region from carrying out its normal functions of recognizing and decoding written text. As a result, the brain is forced to construct alternate and less efficient pathways. Thus, dyslexia is a lifelong condition and not just a “phase.”
Nationwide, three to four times more boys are identified with reading problems than girls. Although this was once thought to be the result of genetic deficits, the true reason may be because boys are overidentified (often due to their rambunctious behavior) and girls are underidentified (if they sit quietly in class and obey the rules). Studies show that many girls are affected as well but are not getting help (Shaywitz, 2003).
Another reason that boys are overidentified as poor readers may be that teachers confound competence in early reading and writing skills. They may assume that poor writing skills also mean poor reading skills. But studies show that boys are almost as skilled as girls in reading, but do, in fact, display significantly poorer writing skills (Berninger, Nielsen, Abbott, Wijsman, & Raskind, 2008). By assessing reading and writing skills separately, fewer boys may be identified as poor readers.
Lesions in the Word Form Area
Researchers using positron emission tomography (PET) scans have noticed that people with developmental dyslexia have lesions in the left occipitotemporal area of the brain. You may recall from Chapter 2 that this area is identified as the visual word form area most used by skilled readers to decode written text. Another discovery in these studies was that the amount of blood flowing to this brain region predicted the severity of the dyslexia (Rumsey et al., 1999). Regardless of the cause, a lesion and reduced blood flow would likely hamper the ability of this patch of neurons to decode written text. In this case, the individual may display a condition known as alexia, which is difficulty in identifying a string of letters, although speech production and comprehension remain intact. However, demonstrating its impressive plasticity, other studies show that when a lesion occurs in the brain’s visual word form area during childhood, an area exactly symmetrical to the VWFA, located in the right hemisphere, can take over the functions, albeit not as efficiently (Cohen et al., 2004).
Nonlinguistic Causes
Some people, who are otherwise unimpaired, have extreme difficulties in reading because of deficits in auditory and visual perception not related to linguistic systems. This revelation was somewhat of a surprise because conventional wisdom held that impairments in reading (and also in oral language) were restricted to problems with linguistic processing. The following are some possible nonlinguistic causes found in the research literature.
Perception of Sequential Sounds
The inability to detect and discriminate sounds presented in rapid succession seems to be a common impairment in individuals with reading and language disorders. These individuals also have difficulty in indicating the order of two sounds presented in rapid succession. This particular deficit is related to auditory processing of sound waves in general and is not related directly to distinguishing phonemes as part of phonological processing. Hearing words accurately when reading or from a stream of rapid conversation is critical to comprehension (Wright, Bowen, & Zecker, 2000).
Sound-Frequency Discrimination
Some individuals with reading disorders are impaired in their ability to hear differences in sound frequency. This auditory defect can affect the ability to discriminate tone and pitch in speech. At first glance, this may seem like only an oral language–related impairment. However, it also affects reading proficiency because reading involves sounding out words in the auditory processing system (Wright et al., 2000). A longitudinal study showed that children who had problems with pitch discrimination as newborns had more difficulties than typical children when learning to read. This was especially evident for children in the study with a family history of dyslexia (Leppänen et al., 2010).
Detection of Target Sounds in Noise
The inability to detect tones within noise is another nonlinguistic impairment that seems to affect learning to read. These students have difficulty hearing the differences between tones and noise. This makes reading very challenging because the child’s language processing system cannot distinguish phonemic tones from all the incoming auditory information (Chait et al., 2007; Wright et al., 2000). When added to the findings in the two deficits mentioned above, this evidence suggests that auditory functions play a much greater role in reading disorders than previously thought. It also suggests that the evaluation of children who are having reading difficulties should include a thorough assessment of their auditory processing skills.
Visual Magnocellular-Deficit Hypothesis
The interpretation of some research studies has led to a hypothesis about the functions of the visual processing system. This proposes that certain forms of reading disorders are caused by a deficit in the visual processing system, which leads to poor detection of coherent visual motion and poor discrimination of the speed of visual motion. This part of the visual system involves large neurons and so is referred to as the magnocellular system. Impairment in this system may cause letters on a page to bundle and overlap, or appear to move—common complaints from some struggling readers and dyslexics. Current studies continue to show a strong correlatio
n between deficits in the visual magnocellular system and developmental dyslexia (e.g., Laycock, Crewther, & Crewther, 2012). This correlation is also found in Chinese dyslexics, lending further credence to the notion that deficits in detecting visual motion and outlines affect one’s ability to read different orthographic symbols (e.g., Wang, Bi, Gao, & Wydell, 2010).
Motor Coordination and the Cerebellum
Several imaging studies show that many people with dyslexia have processing deficits in the cerebellum of the brain (Baillieux et al., 2009; Nicolson, Fawcett, & Dean, 2001). Another study determined that nearly 75 percent of the subjects with dyslexia had smaller lobes on the right side of the cerebellum compared to nondyslexic participants (Eckert et al., 2003). The cerebellum is located at the rear of the brain just below the occipital lobe (Figure 5.6). It is mainly responsible for coordinating learned motor skills. Deficiencies in this part of the brain could result, according to researchers, in problems with reading, writing, and spelling. Problems in reading may result if cerebellar deficits delay the time when an infant sits up and walks, and begins babbling and talking. Less motor skill coordination can mean less articulation and fluency in speech. This, in turn, leads to less sensitivity to onset, rime, and the phonemic structure of language.
How the Brain Learns to Read Page 17