Almost all we know about the neurophysiology of vision comes from animal research but converging evidence from different sources, principally psychophysical experiments, strongly implies that the human visual system is organised in a similar manner to that of the rhesus macaque monkey. The visual system of this species is divided into two main streams: the magnocellular or M pathway and the parvocellular or P pathway. Cells of the M sub-division are larger than those of the P sub-division and the two classes of cell show different neurophysiological responses (for review in relation to reading see Breitmeyer, 1993). For example, M system cells respond to the onset and/or offset of a stimulus whereas P system cells respond with a more sustained response. M cells respond best to stimuli of low luminance and low spatial frequency (coarse detail) but high temporal frequency (rapidly changing). They are relatively insensitive to colour. Cells of the P system respond best to static stimuli of high spatial frequency (fine detail)
at high luminance levels and are sensitive to colour. Given the distinctive neuro-physiological properties of the two systems, it is generally considered that the M system is specialised for the perception of movement whereas the P system is specialised for the analysis of form and colour. The cells of these two subdivisions are physically segregated in different layers of the lateral geniculate nucleus (LGN) and to some extent, though perhaps less than was at one time thought (see Yabuta, Sawatari, & Callaway, 2001), at the primary visual cortex (area V1) and perhaps even beyond.
A considerable amount of experimental work has accumulated to suggest that there is an impairment of the M system in dyslexia (for reviews, see Beaton, 2004; Lovegrove, 1991, 1996; Lovegrove, Martin, & Slaghuis, 1986; Stein, 2000; Stein & Walsh, 1997). Much of this work originated from William Lovegrove's laboratory in Australia and involved presentation of stationary stimuli such as gratings (patterns of alternating black and white bars). Differences between reading-disabled and control children were found in their ability to detect subtle differences in contrast between the luminance of the alternating light and dark bars (Lovegrove, Bowling, Badcock, & Blackwood, 1980; Lovegrove et al., 1982; Spafford, Grosser, Donatelle, Squillace, & Dana, 1995; but see Gross-Glenn et al., 1995). This is known as contrast sensitivity and it varies with the width of the light and dark bars (in technical terms, with spatial frequency). Differences between reading impaired and control children in contrast sensitivity functions have been reported to vary with overall luminance level (Martin & Lovegrove, 1984) and spatial frequency (Lovegrove et al., 1982) in a manner suggestive (to some) of a defect in the M pathway of dyslexics. This view is supported by the findings of one study showing that the difference in sensitivity to temporal frequency between dyslexics and control subjects increased with the frequency of flicker of a sinusoidal grating (Felmingham & Jakobson, 1995).
Recent experiments have involved presentation of moving stimuli (usually random dots), sometimes in combination with neuroimaging techniques. Participants are required to makejudgements about the relative speed or direction of movement of different sets of dots. It turns out that at least some dyslexic children (Cornelissen, Richardson, Mason, Fowler, & Stein, 1995) and adults (Eden et al., 1996; Talcott et al., 1998) are impaired in various aspects of movement processing.
The M system carries information from the primary receiving area of the visual cortex (V1) to other cortical areas and in particular to an area in the middle temporal lobe known as MT/V5. Neuroimaging studies with humans indicate that this area is active in response to certain aspects of movement (Tootell et al., 1995; Watson et al., 1993; Zeki et al., 1991). It is widely thought of as being a "center" for motion detection and perception. However, some degree of motion processing probably also takes place at cortical areas which provide input to MT/V5.
Eden et al. (1996) reported that six dyslexic adults were significantly worse than eight control subjects in discriminating between the relative velocities of two sets of moving dots. Using fMRI it was also found that area MT/V5 was not activated in the dyslexic subjects though it was in the controls. In contrast to these results, using the new technique of MEG, Vanni, Uusitalo, Kiesila, and Hari (1997) found that similar responses were evoked in area V5 of Finnish adult dyslexic and control subjects by a low-brightness stimulus which slightly shifted position back and forth every 45 ms. Lack of activation in area MT/V5 of the dyslexics studied by Eden et al. (1996) cannot have been due to an overall problem with their visual system since they showed normal responses to stationary stimulus patterns in other visual areas. It is therefore possible that the different methodologies (MEG vs fMRI) account for the discrepancy in findings reported by Eden et al. (1996) and Vanni et al. (1997), or that faster movement of the stimulus in the study by Vanni et al. than in the one by Eden et al. led to equal degrees of activation of MT/V5 in dyslexics and controls.
Exactly what leads to poor motion processing in dyslexia is not known— there are a number of conceivable mechanisms (Walther-Muller, 1995); some studies have provided evidence linking movement-related (putatively magno-cellular) functions to reading performance. Cornelissen, Hansen, Hutton, Evangelinou, and Stein (1998) presented a motion detection task to a group of 58 unselected children aged from 9 to 11 years. Performance on this task correlated with the proportion of certain kinds of errors made in reading a set of regular words. It was argued that this kind of error reflected faulty coding of the position of letters within words and that this information is carried by the M system. Impaired magnocellular function might lead to a degraded encoding of letter position during reading such that "positional uncertainty of this kind could cause letters or parts of letters to be lost or duplicated, or even incorrectly bound together, leading to a scrambled or nonsense version of what is actually printed on the page" (p. 473).
In another study of motion detection thresholds, adult dyslexics and control participants were given tests of non-word reading (Talcott et al., 1998). Within each of the participant groups, poorer performance on putative tests of M pathway function was associated with a greater number of non-word reading errors. Furthermore, velocity discrimination thresholds have been reported to correlate with the reading rate for words on a standard test of reading (Demb, Boynton, Best, & Heeger, 1998a).
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