Perceptual learning in this project was indexed by the face inversion effect. This refers to reduced performance when we try to recognise a familiar face turned upside down compared to its usual upright orientation. The main results from the project can be grouped by the key findings that emerged from the series of studies we conducted.
a) Transcranial Direct Current Stimulation modulates perceptual learning indexed by the inversion effect
TDCS uses a constant low current delivered to the brain area of interest via electrodes on the scalp. When anodal stimulation is delivered, the current causes a depolarisation of the resting membrane potential which modulates neural excitability. Sham (control) stimulation lasts for a brief time (30sec) causing no change to neural excitability, but leaving participants believing they were stimulated.
We are now able to show that a particular tDCS paradigm (anodal stimulation delivered for 10 mins at 1.5mA over the left DLPFC at Fp3) can affect perceptual learning by reducing the inversion effect for chequerboards and faces by impairing recognition for upright stimuli. Our findings suggest that our tDCS procedure can affect the perceptual learning acquired for upright stimuli drawn from a familiar category that gives us a benefit when asked to recognise and discriminate between new upright stimuli taken from that category. Critically, no effect of tDCS is found for inverted stimuli, suggesting that after inversion there is no perceptual learning to be lost (we do not usually experience faces turned upside down) and so the tDCS is ineffective. Importantly, the tDCS paradigm is able to entirely eliminate the inversion effect for chequerboards, but that’s not the case for faces. Despite the face inversion effect being significantly reduced in the anodal condition compared to sham, the inversion effect is still significantly present. This is a very important result because it suggests that the face inversion effect may be due to two main components. One component is based on perceptual learning and we can modulate and disrupt it by applying our tDCS paradigm. The other component may have to do with face “specificity” and is unaffected by tDCS.
b) TDCS combined with EEG and fMRI can help us to control and influence perceptual learning and face recognition skills
In order to investigate further the neurocognitive bases of perceptual learning and the mechanisms that control this phenomenon, we then investigated the inversion effect using the Electroencephalography (EEG), and then combined this with our tDCS procedure.
An ERP is a measured brain response to a specific event/stimulus. ERPs are calculated by averaging EEG activity time-locked to the presentation of a stimulus. The N170 ERP peak component is a negative deflection occurring at 170ms after a stimulus is presented and it is usually found to be largest at parietal-occipital regions. It is reliably found with faces and is significantly affected by inversion which results in a larger peak amplitude and a delayed latency.
The most striking finding from our work on the N170 is that obtained by using the combination of tDCS and EEG to investigate the inversion effect on the N170. Importantly, we provided the first evidence in the literature for tDCS being able to modulate the electrophysiological correlates of perceptual learning (indexed by the inversion effect on the N170).
Furthermore, we combined tDCS and fMRI to explore the brain areas activated by the specific tDCS stimulation. For this specific study we used chequerboard stimuli so as to fully control the development of expertise during the pre-exposure phase (a categorization task). Through a region of interest analysis, we found that brain areas in the Precentral Gyrus, Brodmann Area 6 (BA6), showed significantly higher activation in the anodal condition vs sham.
c) TDCS can improve face recognition skills by eliminating the negative effects of generalisation
We demonstrated how the same tDCS procedure can improve performance in circumstances where generalisation is already making face recognition harder. Hence, we showed that the tDCS procedure was able to eliminate the harmful generalisation from the manipulated faces to other faces in the experiment, leading to an increased inversion effect in the anodal group compared to sham by means of an improvement in recognition performance for upright normal faces.
Overall, this project was extremely productive, resulting in 9 papers published so far, 2 papers currently under review and 2 currently in preparation. Moreover, the results from this project were showcased in 14 poster/talk presentations at international conferences and workshops. In addition, the M-C researcher organised an International Workshop of Perceptual Learning at the University of Exeter (June 2019).
