Anodal Transcranial Direct Current Stimulation (TDCS) at Fp3-Fp2 Reduces the Face Inversion Effect: Evidence Against an Expectancy Account.

BACKGROUND: Anodal transcranial direct current stimulation (tDCS) delivered through the Fp3-Fp2 montage reduces the face inversion effect (FIE), a result now used to argue that perceptual expertise underlies face recognition. Because beliefs about brain stimulation can change behaviour in the absence of any current, the literature rests on a premise that has never been examined: its sham condition provides a neutral baseline. The objective of this study was to test that premise, by establishing whether taking part in a tDCS study, as against no such study, changes performance in this paradigm. METHODS: Participants (n = 96, 32 per group) completed an old/new recognition task with upright and inverted faces. The anodal and sham groups were run under a double-blind procedure, so that participants in both knew they might be receiving active stimulation; a third, no-tDCS group completed the identical task on the same timetable, with no electrodes applied and no mention of stimulation. RESULTS: Anodal tDCS halved the FIE relative to both control groups (M = 0.23 vs. 0.55 and 0.56), where F(2, 93) = 9.99, p < 0.001, and ηp2 = 0.18, through impaired recognition of upright faces, where p < 0.001; sensitivity to inverted faces was unaffected, where p = 0.983. The sham and no-tDCS groups were equivalent on every measure taken, and every Bayes factor for that comparison favoured the null (BF = 0.18-0.27). CONCLUSIONS: Within the paradigm and procedure employed here, the beliefs that accompany taking part in a tDCS study did not change performance. The reduction in the FIE by anodal tDCS at Fp3-Fp2 is therefore unlikely to be an expectancy effect, and the sham control used throughout this literature study can be treated as a valid baseline.