Separating the roles of orthographic and semantic information in the neural correlates of evidence accumulation for lexical decision-making

Making decisions requires the accumulation of evidence, which is described quantitatively by the drift diffusion model (DDM). In most of the DDM applications, it is assumed that such evidence is driven by a single process. Yet, in reality, this accumulation could be driven by multiple different sources of information that drive the decision. Here we examine the situation where the accumulation process is driven by orthographic and semantic information, in the service of making lexical decisions. We can separate those factors neatly by using Chinese characters. The DDM was fit to the behavioural data to obtain estimates of its model parameters. We found decreased drift rate, which reflects the strength of evidence, for non-words relative to actual words. There was a negative correlation between the drift rate and subjective word-likeness and familiarity. Although the amplitude of the N1 (which is related to orthographic processing) and N400 (which is related to semantic processing) did not differ across word types, after fitting the ERPs components to separate models as the regressors, the N1 and the N400 did help to better estimate the trial-by-trial estimates of the drift rate in the conditions relevant for orthographic and semantic processing, respectively. Taken together, our study shows how different sources of evidence for lexical decisions are reflected in brain activity and inform the decision making process.