The authors developed a recurrent connectionist model of single word reading including visual,orthographic, phonological,and semantic processing. The model differentiated words from nonwords by integrating measures of polarity across four key processing layers.The contribution of semantics depended on the type of nonword foils. The model was more reliant on semantic information when the nonword foils were pseudowords and pseudohomo phones rather than consonant strings. The results support the view that semantic involvement in lexical decision is graded by the difficulty of the decision task.