The effect of tDCS on recognition depends on stimulus generalization: Neuro-stimulation can predictably enhance or reduce the face inversion effect

Abstract

This paper reports results from three experiments that investigate how a particular neurostimulation procedure is able, in certain circumstances, to selectively increase the face inversion effect by enhancing recognition for upright faces, and argues that these effects can be understood in terms of the MKM theory of stimulus representation. We demonstrate how a specific transcranial Direct Current Stimulation (tDCS) methodology can improve performance in circumstances where error-based salience modulation is making face recognition harder. The three experiments used an old/new recognition task involving sets of normal vs Thatcherised faces. The main characteristic of Thatcherised faces is that the eyes and the mouth are upside down, thus emphasizing features that tend to be common to other Thatcherised faces and so leading to stronger generalization making recognition worse. Experiment 1 combined a behavioural and ERP study looking at the N170 peak component, which helped us to calibrate the set of face stimuli needed for subsequent experiments. In Experiment 2 we used our tDCS procedure (between-subjects and double-blind) in an attempt to reduce the negative effects induced by error-based modulation of salience on recognition of upright Thatcherised faces. Results largely confirmed our predictions. In addition, they showed a significant improvement on recognition performance for upright normal faces. Experiment 3 provides the first direct evidence in a single study that the same tDCS procedure is able to both enhance performance when normal faces are presented with Thatcherised faces, and to reduce performance when normal faces are presented with other normal faces (i.e. male vs female faces). We interpret our results by analyzing how salience modulation influences generalization between similar categories of stimuli.