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| 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 |
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