Magnocellular deficit hypothesis: 2 visual pathways: (1) Magnocellular pathway: Processes visual info about where objects are, assessed using tests of motion coherence. (2) Parvocellular pathway: Processes visual info about what objects are, assessed using tests of form coherence. Stein & Walsh (1997) stated that a deficit in dorsal, magnocellular, visual system leads to dyslexia. Evidence of deficit of M-cells in post-mortem brains (Livingst1 et al, 1991). Deficits on M-cell tasks: Contrast sensitivity (Martin & Lovegrove, 1984). Flicker detection (Lovegrove et al, 1986). Frequency doubling illusion (Pammer & Kevan, 2007). Coherent motion detection (Talcott et al, 2000). Extremely controversial. Not all studies find diffs on M-cell tasks (e.g. White et al, 2006; Hill & Raymond, 2002). May be P-cell deficits too (Amitay et al, 2002). Unclear mechanism: Poor eye-movement control (Stein et al, 1987; but see Liversedge’s more recent work)? Poor mapping of graphemes to phonemes (Cornelissen & Whitney, 2006)? Whitney & Cornelissen (2005) postulated exis10ce of ‘graph1mes’: where grapheme & ph1me info stored. Either deficit route is sufficient, but neither necessary
