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.

The effects of transcranial direct current stimulation on perceptual learning for upright faces and its role in the composite face effect.

“…Experiment 1a and Experiment 1b each included 48 subjects randomly assigned to either sham or anodal tDCS groups (24 in each group). The sample size was decided based on previous studies that used the same tDCS experimental procedure (double-blind, between subjects) and montage to modulate perceptual learning and face recognition (Civile et al, 2019; Civile et al, 2020; Civile, Elchlepp, et al, 2018). All the subjects were students from the University of Exeter and were selected according to the safety screening criteria approved by the Research Ethics Committee at the University of Exeter.…”

“…In recent years, a new line of research developed by Civile, Verbruggen, et al (2016) first, and then extended by Civile, McLaren, and McLaren (2018); Civile, Obhi, and McLaren (2019); and Civile et al (2020), has provided additional evidence for the role of perceptual learning in face recognition skills by strengthening the analogy between the inversion effect for checkerboards (Civile, Zhao, et al, 2014) and that for faces (Civile, McLaren, et al, 2014; Civile et al, 2016; Yin, 1969). Through the use of a particular transcranial Direct Current Stimulation (tDCS) procedure, they were able to provide evidence that the inversion effect for checkerboards and that for faces shared at least some of the same causal mechanisms.…”

“…To test the correspondence between the inversion effect for checkerboards and that for faces, Civile, Elchlepp, et al (2018); Civile, Colvin, Siddiqui, and Obhi (2019); and more recently Civile et al (2020), extended the same tDCS procedure to the face inversion effect. Thus, tDCS stimulation (anodal or sham) was delivered while the subjects were asked to memorize a set of upright and inverted faces presented one at a time in random order.…”

“…Based on the MKM model, Civile et al (2019); Civile et al (2020); Civile, Elchlepp, et al (2018); and Civile, Verbruggen, et al (2016) suggested that the tDCS procedure affects perceptual learning by disrupting the salience modulation mechanism that would normally produce perceptual learning for upright stimuli. Figure 1 below gives a schematic representation of the salience modulation in normal circumstances and under the tDCS procedure.…”

“…This prediction is based on the fact that our tDCS procedure would maintain the salience of the common elements shared among all the upright faces at a relatively high level. Thus, it would harder for the participants to learn about the unique elements typical of each face causing more difficulty at detecting whether the “target” face is same or different from the “test” face despite the task being easier than the usual old/new recognition task used in previous work (Civile et al, 2019; Civile et al, 2020; Civile, Elchlepp, et al, 2018; and Civile, Verbruggen, et al, 2016). If confirmed, these results would provide further evidence in support of this tDCS procedure being able to influence our life expertise in being exposed to upright faces manifested as perceptual learning.…”