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.