Word type and frequency effects on lexical decisions are process-dependent and start early

When encountering letter strings, we rapidly determine whether they are words. The speed of such lexical decisions (LDs) is affected by word frequency. Apart from influencing late, decision-related, processing stages, frequency has also been shown to affect very early stages, and even the processing of non-words. We developed a detailed account of the different frequency effects involved in LDs by 1) dividing LDs into processing stages using a combination of Hidden semi-Markov models and multivariate pattern analysis applied to EEG data and 2) using generalized additive mixed models to investigate how the effect of continuous word and non-word frequency differs between these stages. We discovered six stages shared between word types, with the fifth stage consisting of two sub-stages for pseudo-words only. In the earliest stages visual processing was completed faster for frequent words, but took longer for word-like non-words. Later stages involved an orthographic familiarity assessment followed by an elaborate decision process, both affected differently by frequency. We therefore conclude that frequency indeed affects all processes involved in LDs, and that the magnitude and direction of these effects differs both by process and word type.