Neurochemistry
A majority of the studies for ADD involve research on the metabolic activity of certain chemicals. Yet, only a few of these studies include locations of this activity with some of this research mixing in SLDs. The main distinction made by researchers between ADD and SLD is that of attention versus cognitive, respectively. Therefore, this project focused on the SLD research but found that a majority of the ADD metabolic activity was lower than the norm. In fact, the activity is 8% lower in the premotor and superior prefrontal cortex (Elmer-DeWitt, 1990).
As for the SLD metabolic studies, the majority of research relied on PET scans to trace neurochemicals while performing tasks, such as reading. Some of the chemicals being traced are glucose in Zametkin's study, testosterone and androgens in Tallal's study, and blood flow in Petersen and Fox's study. For RD, the metabolic activity while reading varies greatly from the norm according to Petersen and Fox. This study showed that the Wernicke's area of the temporal lobe establishes semantic and phonological associations which was not activated in people with dyslexia. Later researchers found the activity in the prefrontal and inferior visual areas was different from the norm (Gross-Glenn et al.). The difference was that dyslexics were activating the prefrontal and frontal lobes while reading unlike the normal reader using the temporal lobe. Some of the activity levels for the thalamus differed per sensory input (i.e. auditory) to the cerebral cortex (Shaywitz et al., 1991). Tallal used Shaywitz and Wood's research to discover that an important part of these sensory input differences was found in the prefrontal cortex near the thalamus for speech-sound processing. The thalamus helps this process through speed and accuracy as a timekeeper. This research is important to understanding SLD with auditory perception deficits in the reading process of seeing a word but not comprehending it unless given time to analyze for speech-sound.
Neurophysiology
Another fascinating area of SLD research is brain imaging. As previously mentioned, PET scans were the main technology used to trace and record metabolic activity while performing tasks, such as reading. Yet, functional MRIs (fMRI) are the most common technology used in morphological studies of SLD, which is the most common neurological study for SLDs.
These fMRI studies have brought in new insights for visual and auditory processing, hemisphere symmetry, and timing required for performing reading tasks. The visual morphology has shown a difference in the parvocellular and magnocellular areas for detecting motion and patterns (Zeki and Shipp, Livingstone and Hubel). The visual motion detection area called V5 in the extrastriate cortex near the occipital and temporal lobes was inactive when reading. Additionally, Eden et al. found that only Broca's area activates in RD while reading unlike normal readers that also activate Wernicke's area and the insula. In the fMRI scans, the left temporal lobe is abnormal by being either symmetrical or having reversed asymmetry (i.e. R>L) in comparison to normal readers being asymmetrical (i.e. L>R) (Frith, C. and U., 1996). Also, contrast sensitivity is reduced in SLD which is required in reading to distinction between text and background (Gross, K. and S. Rothenberg, 1979).
For the auditory processing in reading, McCroskey and Kidder found this process to be 1.5 times slower in people with SLD. The auditory perception is an input to the auditory nerves known as the LGN and MGN (medial geniculate nuclei) near the temporal lobe. As previously mentioned, Eden and Paulesu found the LGN of the left temporal lobe to be abnormal with fewer neurons. Also, Galaburda found the MGN to be smaller than normal symmetry. Thus, a dyslexic person takes .1 seconds to process sounds, as compared to a normal reader processing sounds in .04 seconds.
As far as symmetry of the hemispheres, the majority of the neurological research for SLDs, especially RDs, has been these studies of hemisphere symmetry through the morphology of the corpus callosum and the planum temporale region. Galaburda et al.found differences in language function areas for people with SLDs, such as symmetry or reversed asymmetry of Wernicke's area and chaotic patterns of the planum temporale region. This data may correlate with Gross-Glen et al. and Duarain finding metabolic activity differences in the prefrontal and inferior visual/lingual areas of the brain for RD and the temporal area for normal readers.
The corpus callosum has been studied for symmetry morphology and transmission activity with SLDs having more symmetrical (Rosen et al.)or reverse asymmetry(Heir et al., 1978). Reverse asymmetry is opposite the norm of the left hemisphere being larger than the right. Heir et al. found reverse asymmetry in SLD with a wider posterior right hemisphere. In addition to this reverse asymmetry, Filipek et al. also found the right hemisphere to be wider in the posterior and narrower overall with the left hemisphere being the same as the norm. Gross-Glen et al. and Duara et al. had the most information on the corpus callosum being more shallow in SLD with little separation. Yet, the right hemisphere was larger than the left in the midposterior angular gyrus region of the inferior parietal lobe. Also, people with RDs had a large splenum and a more narrow callosum allowed for more interhemispherical activity. The meaning through function of these areas will be analyzed in further depth for the conclusion.
The other major symmetry morphology study was the planum temporale region which was different without the normal asymmetry of a larger left side (Galaburda et al., Hynd, Geschwind). Hynd also found that language areas were shorter for SLDs and had reverse asymmetry (left side longer than the right) in posterior temporal with the planum temporale. Galaburda was most notable for finding heterotopias, abnormal locations of cerebral cortex collections) like the planum temporale as a surface area in the visual cortex which is the superior surface of the temporale lobe.
Overall, these studies have shown perceptual deficits: in vision related to impaired motion detection and in phonology related to impaired coding. This impairment is related to symmetry, reversed asymmetry, and lower numbers of neurons in, predominantly, the temporal lobes and visual motion areas of the occipital lobe. Also, the auditory deficits may be closely related to temporal abnormalities that impede speed and accuracy for speech-sound relationships in reading of the planum temporale region, as well as the thalamus near the corpus callosum.
For memory of letter sequence problems in dysgraphia, Cipolotti and Warrington (1996) noticed a stroke victim of left hemisphere wrote each letter as read to him for word recognition and comprehension. Similarly, SLDs are cross-cultural as damage to corpus callosum has effected Japanese patients abilities to write Kana (vowel-like) and Kanji (consonant-like) (Carlson 1998 p504).
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