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

Intrusive memoriescan be downregulated usingintentional 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 ratewhether 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, wedemonstrate large individual variability within studies in the number of intrusions reported withineach block of the T/NT task. Results revealedthat the change in intrusion frequencies across time is not linear, but quadratic. We propose theIndex of Intrusion Control (IIC) to measure the change in intrusion frequencies across time. Furthermore,wedemonstrate how this metric has superiorpredictive valuefor subsequent forgetting effects, when compared to approaches used in previous studies.We concludewithrecommendations for improving the validity of the design and resultsof future studiesand streamliningmethodology and analyses between studiesinvestigating intrusion control in the laboratory.