Dorsal stream areas

The magnocellular system provides the main visual input to the dorsal ‘where’ corticofugal stream, so that tests focusing on these higher cortical areas can also provide evidence about M impairments. However, one always has to bear in mind that higher dorsal stream tasks depend on the whole dorsal stream. Although 90% of its visual input is provided by the M system, 10% is provided by P and K sources. Therefore higher area visual tasks provide only circumstantial evidence for deficits in the subcortical visual magnocellular system.

Nevertheless there is enough direct evidence from studying the subcortical visual M stream specifically, together with the more circumstantial evidence from higher areas that receive large M-cell input, to conclude that many dyslexics do indeed have impaired M-cell function.

Visual motion sensitivity

The best way of assessing the M sensitivity of neurons in individuals at a crucial stage of the dorsal stream, the middle temporal visual motion area (MT/V5), is to measure their responses to visual motion in ‘random dot kinematograms’. Clouds of dots moving in the same direction, ‘coherently’, are progressively diluted with noise dots moving in random directions until the subject can no longer detect any coherent motion in the display. This threshold therefore defines the sensitivity of that individual’s visual dorsal stream system for the detection of motion. Several researchers have shown that this is reduced in many dyslexics (Cornelissen et al., 1995; Talcott et al., 2000b; Hill & Raymond, 2002; Downie et al., 2003; Samar & Parasnis, 2005). Over the whole range of reading abilities from good readers to dyslexics, the lower a person’s motion sensitivity the worse is his reading (Talcott et al., 2000b). Other work has shown reduced velocity discrimination (Eden et al., 1996; Demb et al., 1998a) and elevated speed thresholds for motion-defined form (Felmingham & Jakobson, 1995) in dyslexics.

Furthermore Fischer (Chapter 2, this volume) demonstrated that many visual and optomotor tasks which are dependent for their speed and accuracy upon M-cell input, are weak in dyslexics at the age of 7 and become progressively more deficient compared with those of good readers as the children grow older. Indeed cohort studies have shown that children’s motion sensitivity measured before they start reading predicts their reading progress by the time they are 8 years old (Boets et al., 2011). Thus it is clear that M-cell function plays an important, though not exclusive, role in the acquisition of reading skills.

Nevertheless some people with low motion sensitivity can still be good readers (Skoyles & Skottun, 2004), since visual motion sensitivity is by no means the only influence on their reading. Yet individual differences in motion sensitivity explain over 25% of the variance in reading ability over a whole population (Talcott et al., 2000b). In other words individuals’ dorsal stream performance dominated by M-cell input probably does help to determine how well their visual reading skills develop, and this is true of everybody, not just dyslexics.

 
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