Impaired morphological processing: insights from multiple sclerosis

Multiple Sclerosis (MS) is a neurological disease characterised by damage affecting large bundles of white matter fibres. Morphological segmentation of complex words (e.g. walked) into stems (walk) and suffixes (∼ed) is thought to depend on intact white matter. We tested the hypothesis that Arabic speaking patients with MS may lose the ability to segment morphologically complex words in a primed lexical decision task using word pairs that shared either a root and a semantic relationship (+R + S, e.g. “AnzAl”–“nuzwl” lowering-landing), a root without semantics (+R–S, e.g. “rtAbp”–“trtyb” monotony-tidying up),a semantic relationship (–R + S, e.g. “xyr”–“nEmp” good-grace), or a phonological relationship (–R + Phon, e.g. “mEdn”–“mEAnd” mineral-stubborn). While healthy controls showed priming by root regardless of semantics and inhibition by phonology, the patients showed facilitation by semantics (+R + S and –R + S), and inhibition by phonology (–R + Phon). These findings are used to adjudicate three contending models of lexical processing.