The Index of Intrusion Control (IIC): A guide to capturing individual variability in intentional intrusion control in the laboratory

Intrusive memories can be downregulated using intentional memory control, as measured via the ‘Think/No-Think’ paradigm. In this task, participants retrieve or suppress memories in response to an associated reminder cue. After each suppression trial, participants rate whether the association had intruded into awareness. Using linear modelling, previous research has found that repeatedly exerting intentional control over memory intrusions reduces their frequency. This paper discusses the differences in methodology and analytical approaches used by studies in the past 10 years investigating intrusion control via the T/NT paradigm. In addition, by reanalysing published datasets, we demonstrate large individual variability within studies in the number of intrusions reported within each block of the T/NT task. Results revealed that the change in intrusion frequencies across time is not linear, but quadratic. We propose the Index of Intrusion Control (IIC) to measure the change in intrusion frequencies across time. Furthermore, we demonstrate how this metric has superior predictive value for subsequent forgetting effects, when compared to approaches used in previous studies. We conclude with recommendations for improving the validity of the design and results of future studies and streamlining methodology and analyses between studies investigating intrusion control in the laboratory.