How standardized are "standard protocols"? Variations in protocol and performance evaluation for slow cortical potential neurofeedback: A systematic review

Abstract

Neurofeedback (NF) aims to alter neural activity by enhancing self-regulation skills. Over the past decade NF has received considerable attention as a potential intervention option for many somatic and mental conditions and ADHD in particular. However, placebo-controlled trials have demonstrated insufficient superiority of NF compared to treatment as usual and sham conditions. It has been argued that the reason for limited NF effects may be attributable to participants' challenges to self-regulate the targeted neural activity. Still, there is support of NF efficacy when only considering so-called "standard protocols," such as Slow Cortical Potential NF training (SCP-NF). This PROSPERO registered systematic review following PRISMA criteria searched literature databases for studies applying SCP-NF protocols. Our review focus concerned the operationalization of self-regulatory success, and protocol-details that could influence the evaluation of self-regulation. Such details included; electrode placement, number of trials, length per trial, proportions of training modalities, handling of artifacts and skill-transfer into daily-life. We identified a total of 63 eligible reports published in the year 2000 or later. SCP-NF protocol-details varied considerably on most variables, except for electrode placement. However, due to the increased availability of commercial systems, there was a trend to more uniform protocol-details. Although, token-systems are popular in SCP-NF for ADHD, only half reported a performance-based component. Also, transfer exercises have become a staple part of SCP-NF. Furthermore, multiple operationalizations of regulatory success were identified, limiting comparability between studies, and perhaps usefulness of so-called transfer-exercises, which purpose is to facilitate the transfer of the self-regulatory skills into every-day life. While studies utilizing SCP as Brain-Computer-Interface mainly focused on the acquisition of successful self-regulation, clinically oriented studies often neglected this. Congruently, rates of successful regulators in clinical studies were mostly low (<50%). The relation between SCP self-regulation and behavior, and how symptoms in different disorders are affected, is complex and not fully understood. Future studies need to report self-regulation based on standardized measures, in order to facilitate both comparability and understanding of the effects on symptoms. When applied as treatment, future SCP-NF studies also need to put greater emphasis on the acquisition of self-regulation (before evaluating symptom outcomes).

Systematic review registration: https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42021260087, Identifier: CRD42021260087.

Keywords: brain computer interface (BCI); neurofeedback; self-regulation; slow cortical potentials (SCP); systematic review.

Figure 1

PRISMA flow-diagram. ^ae.g., Conference abstracts, Books and Book-chapters; ^be.g., Reviews, Editorials and Non-relevant experiments.

Figure 2

Visualization of the variation of SCP-nf outcomes. The red-line illustrates mean amplitude for all positivation (deactivation) trials, the blue-line illustrates mean amplitude of all negativation (activation) trials. The gray-area indicates last 3 seconds of active phase (5–8 s). (A) Profile of a “regulator,” with both negativation and positivation on correct side in relation to baseline. (B) Profile of a “inverted-regulator.” There is a clear differentiation between activation and deactivation, but in the opposite direction as instructed. (C) Profile of a “non-regulator.” There are barely any differences between activation and deactivation, as both are fluctuating around the baseline. (D) Profile of successful deactivation, and differentiation. Althou, activation trials have a lower amplitude than the deactivation trials, their mean is not negative (i.e., wrong side of baseline). (E) Profile of successful activation. However, the deactivation trials mimic the activation trials, hence there is merely any differentiation. (F) Profile of successful activation and differentiation based on the time measure 5–8 s. However, based on the time measure 3–6 s (dark gray area), as used in most studies not using the NeuroConn system, the profile would show an inverted-differentiation at best.