The Effects of transcranial Direct Current Stimulation on Perceptual Learning for Upright Faces and its Role in the Composite Face Effect

Abstract

In the 3 experiments reported here we show that a specific neurostimulation method, whose influence can be understood in terms of a well-known theory of stimulus representation, is able to affect face recognition skills by impairing participants’ performance for upright faces. We used the transcranial Direct Current Stimulation (tDCS) procedure we have recently developed that allows perceptual learning, as indexed by the face inversion effect, to be modulated. We extended this tDCS procedure to another phenomenon, the composite face effect, which constitutes better recognition of the top half of an upright face when conjoined with a congruent (in terms of the response required) rather than incongruent bottom half. All three experiments used the Face-Matching task traditionally used to study this phenomenon. Experiment 1a (n 48) showed that anodal tDCS (using a double-blind between-subjects design) delivered at Fp3 (10 mins at 1.5 mA) affected overall performance for upright faces compared with sham but had no effect on the composite face effect itself. Experiment 1b (n 48) replicated our usual tDCS-induced effects on the face inversion effect but this time using a Face-Matching task instead of the old/new recognition task previously used to obtain the effect. Experiment 2 (n 72) replicated the findings from Experiment 1a, and, using an active control group, showed that the Fp3 anodal tDCS effects on performance to upright faces are not obtained when a different brain area is targeted. We interpret our results in the light of previous literature on the tDCS effects on perceptual learning and face recognition and suggest that different mechanisms are involved in the face inversion effect and the composite face effect.